A SHORT HISTORY OF SUSPENSION  

Part Six


A PERPLEXING PHENOMENON: LEVEL VARIATION

We know that the softer the spring (and this is desirable), the more sensitive it is to load variations (this is undesirable). The softer the spring, the more it will give under the load, thus decreasing the space between the car floor and the ground level (road clearance). There will also be a corresponding decrease between the wheels and the body.

When a driver is alone in his car, the only load is the driver's weight, say 170 lbs. But if five people and their luggage are carried, the load will be about 900 lbs. The springs will give under this weight. Thus the clearance will vary according to whether the car is driven empty or with a full load. Now, for good road holding qualities, the car should always have the same position. Conventional suspensions are prevented from being made softer than they are by the excessive variations in position that would result. On the other hand, a suspension of the decreasing flexibility type restricts the clearance variations and guarantees the essential clearance the wheels require.

But it is possible to do better still, by adopting a suspension which corrects the car's position due to static and dynamic variation in height. 

Static variations are due to load increase which, of course, causes a spring deflection. Dynamic variations in height pose another problem for suspension engineers. Let us take an example: a car running on a flat road.  It comes to a steep hill. The front wheels will reach the hill first and start climbing up the slope. But the suspended part of the car, which was so far horizontal, will tend to remain in this position, owing to the force of inertia. The lay of the land has changed, causing a dynamic variation.  The faster the car is going and the softer the springs, the greater it will be. This phenomenon will occur whenever the lay of the land changes significantly: hills, hollows, humpbacks. Also, when going up a hill, the car's centre of gravity is displaced towards the rear, and when going down hill towards the front. These changes in the centre of gravity will also affect the ground clearance. If these ups and downs are close together - if, instead of being hills to drive up and down, they are holes and bumps in the road, or if the road surface is corrugated, constant dynamic variations will occur and give the car an unpleasant pitch. It will behave like a ship on a rough sea, heading into the crest of the waves to lurch down again afterwards. Like a ship, it will pitch.

If the ship sails along the crest of the waves, they will make it sway from side to side. It is said to roll. This occurs when a soft sprung car is driven over a road with holes and bumps staggered on both sides if and when sharp corners to the left and right are taken in rapid succession. Other influences must also be taken into account: side winds, centrifugal force on corners; a highly cambered road, making the car look lopsided; the rearing of the car in a quick getaway; or the way it puts its nose down when the brakes are jammed on. To counteract these phenomena, a certain number of devices have been designed, for example stabilizer bars, to prevent the car from rolling.

Several manufacturers, including Citroën, have also adopted independent wheels, with each wheel reacting individually to the state of the road. Watch a mule on a mountain track; its four legs are independent It can bend its left legs to walk along the hill-side or climb on a stone with a joint bent, without unbalancing the load it is carrying. In the same way, the independent wheels of a car respond separately to the state of the road, each wheel reacting on its own to the bumps of the surface.

A good suspension, then, requires four independent wheels. But, clearly, this would not be sufficient. It would also require an automatic clearance corrector which, as we have seen, would not only reestablish the car's position if the load were altered, but also react immediately to road surface conditions and to any change in the centre of gravity.

We shall see that this rare bird - the automatic height corrector - really does exist. But first let us continue our review of suspension problems. Unfortunately, we have not finished with them. How are shocks to be counteracted?

THE DAMPING PARADOX

The elasticity of a spring works both ways, as we have seen. Thus any stress on the spring will lead to a series of oscillations comparable in theory to those of the pendulum of a clock.

Take your favorite clock and stop the pendulum at a standstill. Then push it gently to the left. It will start moving in this direction, to return through its position of equilibrium and swing over to the right, and so on.  Your action has produced a series of oscillations on either side of the position of equilibrium. This is a pendulum motion.

Similar things happen in an automobile. At the standstill, the spring is in the equilibrium position. In motion, the variations of terrain create series of oscillations similar to the pendulum, i.e. series of movements going from one side to the other of the equilibrium position. This has two tiresome results: first, the repeated, upward oscillations are transmitted to the body and thus to the passengers; secondly, downwards, the same oscillations transmitted to the wheels make them bounce on the ground, thus losing adhesion -- the basic factor of road holding.

This is all the more marked in that the wheel has its own highly elastic suspension, the tyre, which reacts to road surface conditions immediately.  It is vital to dampen these oscillations, both in the suspension and in the wheels. This is all the more necessary when the road surface is corrugated in such a way that the frequency of the bumps coincides with the period of the suspension (time between two complete oscillations on either side of the position of equilibrium). This will produce the phenomenon of synchronous resonance, which will increase the oscillations beyond limits.

It is the same phenomenon that enables children to reach dizzy heights on a swing, merely by coordinating the impulsions of their legs with the period of oscillation of the swing.

As any impulsion on the spring makes it store up energy which is gradually released by oscillation, part of this energy must be removed in order to decrease the oscillations.

This is a problem schoolchildren are familiar with. If a tap fills a bath in a given time, how long will the drain require to empty it before it overflows ? In the motor car, the drain is the shock absorber. The shock absorber has to eliminate part of the energy the spring has stored.  Friction, an excellent energy consumer, is used for this purpose.

There are various types of shock absorbers: the friction or solid type (friction damper) and the hydraulic type, in which the speed of the movement is reduced by the effect of lamination of a fluid. This is also called viscous dampening and is preferable to the solid type because it is more sensitive and responsive, its action is continuous and progressive in all circumstances.

On most cars, the shock absorbers are added to the suspension proper, as an extra. However, it would obviously be preferable for the shock absorbers to be built into the suspension. Anyway, the result is a paradox. We are trying to obtain a suspension as soft as possible and yet we are doing everything in our power to prevent it from being too soft.

If we exaggerate, we may say that a very powerful shock absorber would stop entirely the motion of the spring. This would amount to doing away with the suspension altogether. If the shock absorber is very weak it will have no effect at all. Here again, the difficulty consists in reaching a happy medium between a suspension that is sufficiently soft to absorb the bumps of the road and a shock absorber firm enough to brake the oscillations of the body and wheels quickly. According to the type of vehicle, manufacturers have to choose between two solutions:
 

 
 
     

  • for a sports car, where road holding is the vital factor, the shock absorbers will be powerful; thus, the springing will be hard. 

     

  • for a luxury Sedan, on the other hand, the shock absorbers will be weak: the suspension will therefore be very soft, but the road holding will not be so good. 

Lieutenant de la Besse's sledge.  1909
 

Westinghouse car.  1908


For years, attempts have been made to combine these two points of view: comfort and road holding. For years, people have put up with a more or less satisfactory compromise.

© 2000 Julian Marsh


 

A SHORT HISTORY OF SUSPENSION  

Part Six


A PERPLEXING PHENOMENON: LEVEL VARIATION

We know that the softer the spring (and this is desirable), the more sensitive it is to load variations (this is undesirable). The softer the spring, the more it will give under the load, thus decreasing the space between the car floor and the ground level (road clearance). There will also be a corresponding decrease between the wheels and the body.

When a driver is alone in his car, the only load is the driver's weight, say 170 lbs. But if five people and their luggage are carried, the load will be about 900 lbs. The springs will give under this weight. Thus the clearance will vary according to whether the car is driven empty or with a full load. Now, for good road holding qualities, the car should always have the same position. Conventional suspensions are prevented from being made softer than they are by the excessive variations in position that would result. On the other hand, a suspension of the decreasing flexibility type restricts the clearance variations and guarantees the essential clearance the wheels require.

But it is possible to do better still, by adopting a suspension which corrects the car's position due to static and dynamic variation in height. 

Static variations are due to load increase which, of course, causes a spring deflection. Dynamic variations in height pose another problem for suspension engineers. Let us take an example: a car running on a flat road.  It comes to a steep hill. The front wheels will reach the hill first and start climbing up the slope. But the suspended part of the car, which was so far horizontal, will tend to remain in this position, owing to the force of inertia. The lay of the land has changed, causing a dynamic variation.  The faster the car is going and the softer the springs, the greater it will be. This phenomenon will occur whenever the lay of the land changes significantly: hills, hollows, humpbacks. Also, when going up a hill, the car's centre of gravity is displaced towards the rear, and when going down hill towards the front. These changes in the centre of gravity will also affect the ground clearance. If these ups and downs are close together - if, instead of being hills to drive up and down, they are holes and bumps in the road, or if the road surface is corrugated, constant dynamic variations will occur and give the car an unpleasant pitch. It will behave like a ship on a rough sea, heading into the crest of the waves to lurch down again afterwards. Like a ship, it will pitch.

If the ship sails along the crest of the waves, they will make it sway from side to side. It is said to roll. This occurs when a soft sprung car is driven over a road with holes and bumps staggered on both sides if and when sharp corners to the left and right are taken in rapid succession. Other influences must also be taken into account: side winds, centrifugal force on corners; a highly cambered road, making the car look lopsided; the rearing of the car in a quick getaway; or the way it puts its nose down when the brakes are jammed on. To counteract these phenomena, a certain number of devices have been designed, for example stabilizer bars, to prevent the car from rolling.

Several manufacturers, including Citroën, have also adopted independent wheels, with each wheel reacting individually to the state of the road. Watch a mule on a mountain track; its four legs are independent It can bend its left legs to walk along the hill-side or climb on a stone with a joint bent, without unbalancing the load it is carrying. In the same way, the independent wheels of a car respond separately to the state of the road, each wheel reacting on its own to the bumps of the surface.

A good suspension, then, requires four independent wheels. But, clearly, this would not be sufficient. It would also require an automatic clearance corrector which, as we have seen, would not only reestablish the car's position if the load were altered, but also react immediately to road surface conditions and to any change in the centre of gravity.

We shall see that this rare bird - the automatic height corrector - really does exist. But first let us continue our review of suspension problems. Unfortunately, we have not finished with them. How are shocks to be counteracted?

THE DAMPING PARADOX

The elasticity of a spring works both ways, as we have seen. Thus any stress on the spring will lead to a series of oscillations comparable in theory to those of the pendulum of a clock.

Take your favorite clock and stop the pendulum at a standstill. Then push it gently to the left. It will start moving in this direction, to return through its position of equilibrium and swing over to the right, and so on.  Your action has produced a series of oscillations on either side of the position of equilibrium. This is a pendulum motion.

Similar things happen in an automobile. At the standstill, the spring is in the equilibrium position. In motion, the variations of terrain create series of oscillations similar to the pendulum, i.e. series of movements going from one side to the other of the equilibrium position. This has two tiresome results: first, the repeated, upward oscillations are transmitted to the body and thus to the passengers; secondly, downwards, the same oscillations transmitted to the wheels make them bounce on the ground, thus losing adhesion -- the basic factor of road holding.

This is all the more marked in that the wheel has its own highly elastic suspension, the tyre, which reacts to road surface conditions immediately.  It is vital to dampen these oscillations, both in the suspension and in the wheels. This is all the more necessary when the road surface is corrugated in such a way that the frequency of the bumps coincides with the period of the suspension (time between two complete oscillations on either side of the position of equilibrium). This will produce the phenomenon of synchronous resonance, which will increase the oscillations beyond limits.

It is the same phenomenon that enables children to reach dizzy heights on a swing, merely by coordinating the impulsions of their legs with the period of oscillation of the swing.

As any impulsion on the spring makes it store up energy which is gradually released by oscillation, part of this energy must be removed in order to decrease the oscillations.

This is a problem schoolchildren are familiar with. If a tap fills a bath in a given time, how long will the drain require to empty it before it overflows ? In the motor car, the drain is the shock absorber. The shock absorber has to eliminate part of the energy the spring has stored.  Friction, an excellent energy consumer, is used for this purpose.

There are various types of shock absorbers: the friction or solid type (friction damper) and the hydraulic type, in which the speed of the movement is reduced by the effect of lamination of a fluid. This is also called viscous dampening and is preferable to the solid type because it is more sensitive and responsive, its action is continuous and progressive in all circumstances.

On most cars, the shock absorbers are added to the suspension proper, as an extra. However, it would obviously be preferable for the shock absorbers to be built into the suspension. Anyway, the result is a paradox. We are trying to obtain a suspension as soft as possible and yet we are doing everything in our power to prevent it from being too soft.

If we exaggerate, we may say that a very powerful shock absorber would stop entirely the motion of the spring. This would amount to doing away with the suspension altogether. If the shock absorber is very weak it will have no effect at all. Here again, the difficulty consists in reaching a happy medium between a suspension that is sufficiently soft to absorb the bumps of the road and a shock absorber firm enough to brake the oscillations of the body and wheels quickly. According to the type of vehicle, manufacturers have to choose between two solutions:
 

 
 
     

  • for a sports car, where road holding is the vital factor, the shock absorbers will be powerful; thus, the springing will be hard. 

     

  • for a luxury Sedan, on the other hand, the shock absorbers will be weak: the suspension will therefore be very soft, but the road holding will not be so good. 

Lieutenant de la Besse's sledge.  1909
 

Westinghouse car.  1908


For years, attempts have been made to combine these two points of view: comfort and road holding. For years, people have put up with a more or less satisfactory compromise.

© 2000 Julian Marsh


 

A SHORT HISTORY OF SUSPENSION  

Part Six


A PERPLEXING PHENOMENON: LEVEL VARIATION

We know that the softer the spring (and this is desirable), the more sensitive it is to load variations (this is undesirable). The softer the spring, the more it will give under the load, thus decreasing the space between the car floor and the ground level (road clearance). There will also be a corresponding decrease between the wheels and the body.

When a driver is alone in his car, the only load is the driver's weight, say 170 lbs. But if five people and their luggage are carried, the load will be about 900 lbs. The springs will give under this weight. Thus the clearance will vary according to whether the car is driven empty or with a full load. Now, for good road holding qualities, the car should always have the same position. Conventional suspensions are prevented from being made softer than they are by the excessive variations in position that would result. On the other hand, a suspension of the decreasing flexibility type restricts the clearance variations and guarantees the essential clearance the wheels require.

But it is possible to do better still, by adopting a suspension which corrects the car's position due to static and dynamic variation in height. 

Static variations are due to load increase which, of course, causes a spring deflection. Dynamic variations in height pose another problem for suspension engineers. Let us take an example: a car running on a flat road.  It comes to a steep hill. The front wheels will reach the hill first and start climbing up the slope. But the suspended part of the car, which was so far horizontal, will tend to remain in this position, owing to the force of inertia. The lay of the land has changed, causing a dynamic variation.  The faster the car is going and the softer the springs, the greater it will be. This phenomenon will occur whenever the lay of the land changes significantly: hills, hollows, humpbacks. Also, when going up a hill, the car's centre of gravity is displaced towards the rear, and when going down hill towards the front. These changes in the centre of gravity will also affect the ground clearance. If these ups and downs are close together - if, instead of being hills to drive up and down, they are holes and bumps in the road, or if the road surface is corrugated, constant dynamic variations will occur and give the car an unpleasant pitch. It will behave like a ship on a rough sea, heading into the crest of the waves to lurch down again afterwards. Like a ship, it will pitch.

If the ship sails along the crest of the waves, they will make it sway from side to side. It is said to roll. This occurs when a soft sprung car is driven over a road with holes and bumps staggered on both sides if and when sharp corners to the left and right are taken in rapid succession. Other influences must also be taken into account: side winds, centrifugal force on corners; a highly cambered road, making the car look lopsided; the rearing of the car in a quick getaway; or the way it puts its nose down when the brakes are jammed on. To counteract these phenomena, a certain number of devices have been designed, for example stabilizer bars, to prevent the car from rolling.

Several manufacturers, including Citroën, have also adopted independent wheels, with each wheel reacting individually to the state of the road. Watch a mule on a mountain track; its four legs are independent It can bend its left legs to walk along the hill-side or climb on a stone with a joint bent, without unbalancing the load it is carrying. In the same way, the independent wheels of a car respond separately to the state of the road, each wheel reacting on its own to the bumps of the surface.

A good suspension, then, requires four independent wheels. But, clearly, this would not be sufficient. It would also require an automatic clearance corrector which, as we have seen, would not only reestablish the car's position if the load were altered, but also react immediately to road surface conditions and to any change in the centre of gravity.

We shall see that this rare bird - the automatic height corrector - really does exist. But first let us continue our review of suspension problems. Unfortunately, we have not finished with them. How are shocks to be counteracted?

THE DAMPING PARADOX

The elasticity of a spring works both ways, as we have seen. Thus any stress on the spring will lead to a series of oscillations comparable in theory to those of the pendulum of a clock.

Take your favorite clock and stop the pendulum at a standstill. Then push it gently to the left. It will start moving in this direction, to return through its position of equilibrium and swing over to the right, and so on.  Your action has produced a series of oscillations on either side of the position of equilibrium. This is a pendulum motion.

Similar things happen in an automobile. At the standstill, the spring is in the equilibrium position. In motion, the variations of terrain create series of oscillations similar to the pendulum, i.e. series of movements going from one side to the other of the equilibrium position. This has two tiresome results: first, the repeated, upward oscillations are transmitted to the body and thus to the passengers; secondly, downwards, the same oscillations transmitted to the wheels make them bounce on the ground, thus losing adhesion -- the basic factor of road holding.

This is all the more marked in that the wheel has its own highly elastic suspension, the tyre, which reacts to road surface conditions immediately.  It is vital to dampen these oscillations, both in the suspension and in the wheels. This is all the more necessary when the road surface is corrugated in such a way that the frequency of the bumps coincides with the period of the suspension (time between two complete oscillations on either side of the position of equilibrium). This will produce the phenomenon of synchronous resonance, which will increase the oscillations beyond limits.

It is the same phenomenon that enables children to reach dizzy heights on a swing, merely by coordinating the impulsions of their legs with the period of oscillation of the swing.

As any impulsion on the spring makes it store up energy which is gradually released by oscillation, part of this energy must be removed in order to decrease the oscillations.

This is a problem schoolchildren are familiar with. If a tap fills a bath in a given time, how long will the drain require to empty it before it overflows ? In the motor car, the drain is the shock absorber. The shock absorber has to eliminate part of the energy the spring has stored.  Friction, an excellent energy consumer, is used for this purpose.

There are various types of shock absorbers: the friction or solid type (friction damper) and the hydraulic type, in which the speed of the movement is reduced by the effect of lamination of a fluid. This is also called viscous dampening and is preferable to the solid type because it is more sensitive and responsive, its action is continuous and progressive in all circumstances.

On most cars, the shock absorbers are added to the suspension proper, as an extra. However, it would obviously be preferable for the shock absorbers to be built into the suspension. Anyway, the result is a paradox. We are trying to obtain a suspension as soft as possible and yet we are doing everything in our power to prevent it from being too soft.

If we exaggerate, we may say that a very powerful shock absorber would stop entirely the motion of the spring. This would amount to doing away with the suspension altogether. If the shock absorber is very weak it will have no effect at all. Here again, the difficulty consists in reaching a happy medium between a suspension that is sufficiently soft to absorb the bumps of the road and a shock absorber firm enough to brake the oscillations of the body and wheels quickly. According to the type of vehicle, manufacturers have to choose between two solutions:
 

 
 
     

  • for a sports car, where road holding is the vital factor, the shock absorbers will be powerful; thus, the springing will be hard. 

     

  • for a luxury Sedan, on the other hand, the shock absorbers will be weak: the suspension will therefore be very soft, but the road holding will not be so good. 

Lieutenant de la Besse's sledge.  1909
 

Westinghouse car.  1908


For years, attempts have been made to combine these two points of view: comfort and road holding. For years, people have put up with a more or less satisfactory compromise.

© 2000 Julian Marsh


 

A SHORT HISTORY OF SUSPENSION  

Part Six


A PERPLEXING PHENOMENON: LEVEL VARIATION

We know that the softer the spring (and this is desirable), the more sensitive it is to load variations (this is undesirable). The softer the spring, the more it will give under the load, thus decreasing the space between the car floor and the ground level (road clearance). There will also be a corresponding decrease between the wheels and the body.

When a driver is alone in his car, the only load is the driver's weight, say 170 lbs. But if five people and their luggage are carried, the load will be about 900 lbs. The springs will give under this weight. Thus the clearance will vary according to whether the car is driven empty or with a full load. Now, for good road holding qualities, the car should always have the same position. Conventional suspensions are prevented from being made softer than they are by the excessive variations in position that would result. On the other hand, a suspension of the decreasing flexibility type restricts the clearance variations and guarantees the essential clearance the wheels require.

But it is possible to do better still, by adopting a suspension which corrects the car's position due to static and dynamic variation in height. 

Static variations are due to load increase which, of course, causes a spring deflection. Dynamic variations in height pose another problem for suspension engineers. Let us take an example: a car running on a flat road.  It comes to a steep hill. The front wheels will reach the hill first and start climbing up the slope. But the suspended part of the car, which was so far horizontal, will tend to remain in this position, owing to the force of inertia. The lay of the land has changed, causing a dynamic variation.  The faster the car is going and the softer the springs, the greater it will be. This phenomenon will occur whenever the lay of the land changes significantly: hills, hollows, humpbacks. Also, when going up a hill, the car's centre of gravity is displaced towards the rear, and when going down hill towards the front. These changes in the centre of gravity will also affect the ground clearance. If these ups and downs are close together - if, instead of being hills to drive up and down, they are holes and bumps in the road, or if the road surface is corrugated, constant dynamic variations will occur and give the car an unpleasant pitch. It will behave like a ship on a rough sea, heading into the crest of the waves to lurch down again afterwards. Like a ship, it will pitch.

If the ship sails along the crest of the waves, they will make it sway from side to side. It is said to roll. This occurs when a soft sprung car is driven over a road with holes and bumps staggered on both sides if and when sharp corners to the left and right are taken in rapid succession. Other influences must also be taken into account: side winds, centrifugal force on corners; a highly cambered road, making the car look lopsided; the rearing of the car in a quick getaway; or the way it puts its nose down when the brakes are jammed on. To counteract these phenomena, a certain number of devices have been designed, for example stabilizer bars, to prevent the car from rolling.

Several manufacturers, including Citroën, have also adopted independent wheels, with each wheel reacting individually to the state of the road. Watch a mule on a mountain track; its four legs are independent It can bend its left legs to walk along the hill-side or climb on a stone with a joint bent, without unbalancing the load it is carrying. In the same way, the independent wheels of a car respond separately to the state of the road, each wheel reacting on its own to the bumps of the surface.

A good suspension, then, requires four independent wheels. But, clearly, this would not be sufficient. It would also require an automatic clearance corrector which, as we have seen, would not only reestablish the car's position if the load were altered, but also react immediately to road surface conditions and to any change in the centre of gravity.

We shall see that this rare bird - the automatic height corrector - really does exist. But first let us continue our review of suspension problems. Unfortunately, we have not finished with them. How are shocks to be counteracted?

THE DAMPING PARADOX

The elasticity of a spring works both ways, as we have seen. Thus any stress on the spring will lead to a series of oscillations comparable in theory to those of the pendulum of a clock.

Take your favorite clock and stop the pendulum at a standstill. Then push it gently to the left. It will start moving in this direction, to return through its position of equilibrium and swing over to the right, and so on.  Your action has produced a series of oscillations on either side of the position of equilibrium. This is a pendulum motion.

Similar things happen in an automobile. At the standstill, the spring is in the equilibrium position. In motion, the variations of terrain create series of oscillations similar to the pendulum, i.e. series of movements going from one side to the other of the equilibrium position. This has two tiresome results: first, the repeated, upward oscillations are transmitted to the body and thus to the passengers; secondly, downwards, the same oscillations transmitted to the wheels make them bounce on the ground, thus losing adhesion -- the basic factor of road holding.

This is all the more marked in that the wheel has its own highly elastic suspension, the tyre, which reacts to road surface conditions immediately.  It is vital to dampen these oscillations, both in the suspension and in the wheels. This is all the more necessary when the road surface is corrugated in such a way that the frequency of the bumps coincides with the period of the suspension (time between two complete oscillations on either side of the position of equilibrium). This will produce the phenomenon of synchronous resonance, which will increase the oscillations beyond limits.

It is the same phenomenon that enables children to reach dizzy heights on a swing, merely by coordinating the impulsions of their legs with the period of oscillation of the swing.

As any impulsion on the spring makes it store up energy which is gradually released by oscillation, part of this energy must be removed in order to decrease the oscillations.

This is a problem schoolchildren are familiar with. If a tap fills a bath in a given time, how long will the drain require to empty it before it overflows ? In the motor car, the drain is the shock absorber. The shock absorber has to eliminate part of the energy the spring has stored.  Friction, an excellent energy consumer, is used for this purpose.

There are various types of shock absorbers: the friction or solid type (friction damper) and the hydraulic type, in which the speed of the movement is reduced by the effect of lamination of a fluid. This is also called viscous dampening and is preferable to the solid type because it is more sensitive and responsive, its action is continuous and progressive in all circumstances.

On most cars, the shock absorbers are added to the suspension proper, as an extra. However, it would obviously be preferable for the shock absorbers to be built into the suspension. Anyway, the result is a paradox. We are trying to obtain a suspension as soft as possible and yet we are doing everything in our power to prevent it from being too soft.

If we exaggerate, we may say that a very powerful shock absorber would stop entirely the motion of the spring. This would amount to doing away with the suspension altogether. If the shock absorber is very weak it will have no effect at all. Here again, the difficulty consists in reaching a happy medium between a suspension that is sufficiently soft to absorb the bumps of the road and a shock absorber firm enough to brake the oscillations of the body and wheels quickly. According to the type of vehicle, manufacturers have to choose between two solutions:
 

 
 
     

  • for a sports car, where road holding is the vital factor, the shock absorbers will be powerful; thus, the springing will be hard. 

     

  • for a luxury Sedan, on the other hand, the shock absorbers will be weak: the suspension will therefore be very soft, but the road holding will not be so good. 

Lieutenant de la Besse's sledge.  1909
 

Westinghouse car.  1908


For years, attempts have been made to combine these two points of view: comfort and road holding. For years, people have put up with a more or less satisfactory compromise.

© 2000 Julian Marsh


 

A SHORT HISTORY OF SUSPENSION  

Part Six


A PERPLEXING PHENOMENON: LEVEL VARIATION

We know that the softer the spring (and this is desirable), the more sensitive it is to load variations (this is undesirable). The softer the spring, the more it will give under the load, thus decreasing the space between the car floor and the ground level (road clearance). There will also be a corresponding decrease between the wheels and the body.

When a driver is alone in his car, the only load is the driver's weight, say 170 lbs. But if five people and their luggage are carried, the load will be about 900 lbs. The springs will give under this weight. Thus the clearance will vary according to whether the car is driven empty or with a full load. Now, for good road holding qualities, the car should always have the same position. Conventional suspensions are prevented from being made softer than they are by the excessive variations in position that would result. On the other hand, a suspension of the decreasing flexibility type restricts the clearance variations and guarantees the essential clearance the wheels require.

But it is possible to do better still, by adopting a suspension which corrects the car's position due to static and dynamic variation in height. 

Static variations are due to load increase which, of course, causes a spring deflection. Dynamic variations in height pose another problem for suspension engineers. Let us take an example: a car running on a flat road.  It comes to a steep hill. The front wheels will reach the hill first and start climbing up the slope. But the suspended part of the car, which was so far horizontal, will tend to remain in this position, owing to the force of inertia. The lay of the land has changed, causing a dynamic variation.  The faster the car is going and the softer the springs, the greater it will be. This phenomenon will occur whenever the lay of the land changes significantly: hills, hollows, humpbacks. Also, when going up a hill, the car's centre of gravity is displaced towards the rear, and when going down hill towards the front. These changes in the centre of gravity will also affect the ground clearance. If these ups and downs are close together - if, instead of being hills to drive up and down, they are holes and bumps in the road, or if the road surface is corrugated, constant dynamic variations will occur and give the car an unpleasant pitch. It will behave like a ship on a rough sea, heading into the crest of the waves to lurch down again afterwards. Like a ship, it will pitch.

If the ship sails along the crest of the waves, they will make it sway from side to side. It is said to roll. This occurs when a soft sprung car is driven over a road with holes and bumps staggered on both sides if and when sharp corners to the left and right are taken in rapid succession. Other influences must also be taken into account: side winds, centrifugal force on corners; a highly cambered road, making the car look lopsided; the rearing of the car in a quick getaway; or the way it puts its nose down when the brakes are jammed on. To counteract these phenomena, a certain number of devices have been designed, for example stabilizer bars, to prevent the car from rolling.

Several manufacturers, including Citroën, have also adopted independent wheels, with each wheel reacting individually to the state of the road. Watch a mule on a mountain track; its four legs are independent It can bend its left legs to walk along the hill-side or climb on a stone with a joint bent, without unbalancing the load it is carrying. In the same way, the independent wheels of a car respond separately to the state of the road, each wheel reacting on its own to the bumps of the surface.

A good suspension, then, requires four independent wheels. But, clearly, this would not be sufficient. It would also require an automatic clearance corrector which, as we have seen, would not only reestablish the car's position if the load were altered, but also react immediately to road surface conditions and to any change in the centre of gravity.

We shall see that this rare bird - the automatic height corrector - really does exist. But first let us continue our review of suspension problems. Unfortunately, we have not finished with them. How are shocks to be counteracted?

THE DAMPING PARADOX

The elasticity of a spring works both ways, as we have seen. Thus any stress on the spring will lead to a series of oscillations comparable in theory to those of the pendulum of a clock.

Take your favorite clock and stop the pendulum at a standstill. Then push it gently to the left. It will start moving in this direction, to return through its position of equilibrium and swing over to the right, and so on.  Your action has produced a series of oscillations on either side of the position of equilibrium. This is a pendulum motion.

Similar things happen in an automobile. At the standstill, the spring is in the equilibrium position. In motion, the variations of terrain create series of oscillations similar to the pendulum, i.e. series of movements going from one side to the other of the equilibrium position. This has two tiresome results: first, the repeated, upward oscillations are transmitted to the body and thus to the passengers; secondly, downwards, the same oscillations transmitted to the wheels make them bounce on the ground, thus losing adhesion -- the basic factor of road holding.

This is all the more marked in that the wheel has its own highly elastic suspension, the tyre, which reacts to road surface conditions immediately.  It is vital to dampen these oscillations, both in the suspension and in the wheels. This is all the more necessary when the road surface is corrugated in such a way that the frequency of the bumps coincides with the period of the suspension (time between two complete oscillations on either side of the position of equilibrium). This will produce the phenomenon of synchronous resonance, which will increase the oscillations beyond limits.

It is the same phenomenon that enables children to reach dizzy heights on a swing, merely by coordinating the impulsions of their legs with the period of oscillation of the swing.

As any impulsion on the spring makes it store up energy which is gradually released by oscillation, part of this energy must be removed in order to decrease the oscillations.

This is a problem schoolchildren are familiar with. If a tap fills a bath in a given time, how long will the drain require to empty it before it overflows ? In the motor car, the drain is the shock absorber. The shock absorber has to eliminate part of the energy the spring has stored.  Friction, an excellent energy consumer, is used for this purpose.

There are various types of shock absorbers: the friction or solid type (friction damper) and the hydraulic type, in which the speed of the movement is reduced by the effect of lamination of a fluid. This is also called viscous dampening and is preferable to the solid type because it is more sensitive and responsive, its action is continuous and progressive in all circumstances.

On most cars, the shock absorbers are added to the suspension proper, as an extra. However, it would obviously be preferable for the shock absorbers to be built into the suspension. Anyway, the result is a paradox. We are trying to obtain a suspension as soft as possible and yet we are doing everything in our power to prevent it from being too soft.

If we exaggerate, we may say that a very powerful shock absorber would stop entirely the motion of the spring. This would amount to doing away with the suspension altogether. If the shock absorber is very weak it will have no effect at all. Here again, the difficulty consists in reaching a happy medium between a suspension that is sufficiently soft to absorb the bumps of the road and a shock absorber firm enough to brake the oscillations of the body and wheels quickly. According to the type of vehicle, manufacturers have to choose between two solutions:
 

 
 
     

  • for a sports car, where road holding is the vital factor, the shock absorbers will be powerful; thus, the springing will be hard. 

     

  • for a luxury Sedan, on the other hand, the shock absorbers will be weak: the suspension will therefore be very soft, but the road holding will not be so good. 

Lieutenant de la Besse's sledge.  1909
 

Westinghouse car.  1908


For years, attempts have been made to combine these two points of view: comfort and road holding. For years, people have put up with a more or less satisfactory compromise.

© 2000 Julian Marsh


 

A SHORT HISTORY OF SUSPENSION  

Part Six


A PERPLEXING PHENOMENON: LEVEL VARIATION

We know that the softer the spring (and this is desirable), the more sensitive it is to load variations (this is undesirable). The softer the spring, the more it will give under the load, thus decreasing the space between the car floor and the ground level (road clearance). There will also be a corresponding decrease between the wheels and the body.

When a driver is alone in his car, the only load is the driver's weight, say 170 lbs. But if five people and their luggage are carried, the load will be about 900 lbs. The springs will give under this weight. Thus the clearance will vary according to whether the car is driven empty or with a full load. Now, for good road holding qualities, the car should always have the same position. Conventional suspensions are prevented from being made softer than they are by the excessive variations in position that would result. On the other hand, a suspension of the decreasing flexibility type restricts the clearance variations and guarantees the essential clearance the wheels require.

But it is possible to do better still, by adopting a suspension which corrects the car's position due to static and dynamic variation in height. 

Static variations are due to load increase which, of course, causes a spring deflection. Dynamic variations in height pose another problem for suspension engineers. Let us take an example: a car running on a flat road.  It comes to a steep hill. The front wheels will reach the hill first and start climbing up the slope. But the suspended part of the car, which was so far horizontal, will tend to remain in this position, owing to the force of inertia. The lay of the land has changed, causing a dynamic variation.  The faster the car is going and the softer the springs, the greater it will be. This phenomenon will occur whenever the lay of the land changes significantly: hills, hollows, humpbacks. Also, when going up a hill, the car's centre of gravity is displaced towards the rear, and when going down hill towards the front. These changes in the centre of gravity will also affect the ground clearance. If these ups and downs are close together - if, instead of being hills to drive up and down, they are holes and bumps in the road, or if the road surface is corrugated, constant dynamic variations will occur and give the car an unpleasant pitch. It will behave like a ship on a rough sea, heading into the crest of the waves to lurch down again afterwards. Like a ship, it will pitch.

If the ship sails along the crest of the waves, they will make it sway from side to side. It is said to roll. This occurs when a soft sprung car is driven over a road with holes and bumps staggered on both sides if and when sharp corners to the left and right are taken in rapid succession. Other influences must also be taken into account: side winds, centrifugal force on corners; a highly cambered road, making the car look lopsided; the rearing of the car in a quick getaway; or the way it puts its nose down when the brakes are jammed on. To counteract these phenomena, a certain number of devices have been designed, for example stabilizer bars, to prevent the car from rolling.

Several manufacturers, including Citroën, have also adopted independent wheels, with each wheel reacting individually to the state of the road. Watch a mule on a mountain track; its four legs are independent It can bend its left legs to walk along the hill-side or climb on a stone with a joint bent, without unbalancing the load it is carrying. In the same way, the independent wheels of a car respond separately to the state of the road, each wheel reacting on its own to the bumps of the surface.

A good suspension, then, requires four independent wheels. But, clearly, this would not be sufficient. It would also require an automatic clearance corrector which, as we have seen, would not only reestablish the car's position if the load were altered, but also react immediately to road surface conditions and to any change in the centre of gravity.

We shall see that this rare bird - the automatic height corrector - really does exist. But first let us continue our review of suspension problems. Unfortunately, we have not finished with them. How are shocks to be counteracted?

THE DAMPING PARADOX

The elasticity of a spring works both ways, as we have seen. Thus any stress on the spring will lead to a series of oscillations comparable in theory to those of the pendulum of a clock.

Take your favorite clock and stop the pendulum at a standstill. Then push it gently to the left. It will start moving in this direction, to return through its position of equilibrium and swing over to the right, and so on.  Your action has produced a series of oscillations on either side of the position of equilibrium. This is a pendulum motion.

Similar things happen in an automobile. At the standstill, the spring is in the equilibrium position. In motion, the variations of terrain create series of oscillations similar to the pendulum, i.e. series of movements going from one side to the other of the equilibrium position. This has two tiresome results: first, the repeated, upward oscillations are transmitted to the body and thus to the passengers; secondly, downwards, the same oscillations transmitted to the wheels make them bounce on the ground, thus losing adhesion -- the basic factor of road holding.

This is all the more marked in that the wheel has its own highly elastic suspension, the tyre, which reacts to road surface conditions immediately.  It is vital to dampen these oscillations, both in the suspension and in the wheels. This is all the more necessary when the road surface is corrugated in such a way that the frequency of the bumps coincides with the period of the suspension (time between two complete oscillations on either side of the position of equilibrium). This will produce the phenomenon of synchronous resonance, which will increase the oscillations beyond limits.

It is the same phenomenon that enables children to reach dizzy heights on a swing, merely by coordinating the impulsions of their legs with the period of oscillation of the swing.

As any impulsion on the spring makes it store up energy which is gradually released by oscillation, part of this energy must be removed in order to decrease the oscillations.

This is a problem schoolchildren are familiar with. If a tap fills a bath in a given time, how long will the drain require to empty it before it overflows ? In the motor car, the drain is the shock absorber. The shock absorber has to eliminate part of the energy the spring has stored.  Friction, an excellent energy consumer, is used for this purpose.

There are various types of shock absorbers: the friction or solid type (friction damper) and the hydraulic type, in which the speed of the movement is reduced by the effect of lamination of a fluid. This is also called viscous dampening and is preferable to the solid type because it is more sensitive and responsive, its action is continuous and progressive in all circumstances.

On most cars, the shock absorbers are added to the suspension proper, as an extra. However, it would obviously be preferable for the shock absorbers to be built into the suspension. Anyway, the result is a paradox. We are trying to obtain a suspension as soft as possible and yet we are doing everything in our power to prevent it from being too soft.

If we exaggerate, we may say that a very powerful shock absorber would stop entirely the motion of the spring. This would amount to doing away with the suspension altogether. If the shock absorber is very weak it will have no effect at all. Here again, the difficulty consists in reaching a happy medium between a suspension that is sufficiently soft to absorb the bumps of the road and a shock absorber firm enough to brake the oscillations of the body and wheels quickly. According to the type of vehicle, manufacturers have to choose between two solutions:
 

 
 
     

  • for a sports car, where road holding is the vital factor, the shock absorbers will be powerful; thus, the springing will be hard. 

     

  • for a luxury Sedan, on the other hand, the shock absorbers will be weak: the suspension will therefore be very soft, but the road holding will not be so good. 

Lieutenant de la Besse's sledge.  1909
 

Westinghouse car.  1908


For years, attempts have been made to combine these two points of view: comfort and road holding. For years, people have put up with a more or less satisfactory compromise.

© 2000 Julian Marsh


 

A SHORT HISTORY OF SUSPENSION  

Part Six


A PERPLEXING PHENOMENON: LEVEL VARIATION

We know that the softer the spring (and this is desirable), the more sensitive it is to load variations (this is undesirable). The softer the spring, the more it will give under the load, thus decreasing the space between the car floor and the ground level (road clearance). There will also be a corresponding decrease between the wheels and the body.

When a driver is alone in his car, the only load is the driver's weight, say 170 lbs. But if five people and their luggage are carried, the load will be about 900 lbs. The springs will give under this weight. Thus the clearance will vary according to whether the car is driven empty or with a full load. Now, for good road holding qualities, the car should always have the same position. Conventional suspensions are prevented from being made softer than they are by the excessive variations in position that would result. On the other hand, a suspension of the decreasing flexibility type restricts the clearance variations and guarantees the essential clearance the wheels require.

But it is possible to do better still, by adopting a suspension which corrects the car's position due to static and dynamic variation in height. 

Static variations are due to load increase which, of course, causes a spring deflection. Dynamic variations in height pose another problem for suspension engineers. Let us take an example: a car running on a flat road.  It comes to a steep hill. The front wheels will reach the hill first and start climbing up the slope. But the suspended part of the car, which was so far horizontal, will tend to remain in this position, owing to the force of inertia. The lay of the land has changed, causing a dynamic variation.  The faster the car is going and the softer the springs, the greater it will be. This phenomenon will occur whenever the lay of the land changes significantly: hills, hollows, humpbacks. Also, when going up a hill, the car's centre of gravity is displaced towards the rear, and when going down hill towards the front. These changes in the centre of gravity will also affect the ground clearance. If these ups and downs are close together - if, instead of being hills to drive up and down, they are holes and bumps in the road, or if the road surface is corrugated, constant dynamic variations will occur and give the car an unpleasant pitch. It will behave like a ship on a rough sea, heading into the crest of the waves to lurch down again afterwards. Like a ship, it will pitch.

If the ship sails along the crest of the waves, they will make it sway from side to side. It is said to roll. This occurs when a soft sprung car is driven over a road with holes and bumps staggered on both sides if and when sharp corners to the left and right are taken in rapid succession. Other influences must also be taken into account: side winds, centrifugal force on corners; a highly cambered road, making the car look lopsided; the rearing of the car in a quick getaway; or the way it puts its nose down when the brakes are jammed on. To counteract these phenomena, a certain number of devices have been designed, for example stabilizer bars, to prevent the car from rolling.

Several manufacturers, including Citroën, have also adopted independent wheels, with each wheel reacting individually to the state of the road. Watch a mule on a mountain track; its four legs are independent It can bend its left legs to walk along the hill-side or climb on a stone with a joint bent, without unbalancing the load it is carrying. In the same way, the independent wheels of a car respond separately to the state of the road, each wheel reacting on its own to the bumps of the surface.

A good suspension, then, requires four independent wheels. But, clearly, this would not be sufficient. It would also require an automatic clearance corrector which, as we have seen, would not only reestablish the car's position if the load were altered, but also react immediately to road surface conditions and to any change in the centre of gravity.

We shall see that this rare bird - the automatic height corrector - really does exist. But first let us continue our review of suspension problems. Unfortunately, we have not finished with them. How are shocks to be counteracted?

THE DAMPING PARADOX

The elasticity of a spring works both ways, as we have seen. Thus any stress on the spring will lead to a series of oscillations comparable in theory to those of the pendulum of a clock.

Take your favorite clock and stop the pendulum at a standstill. Then push it gently to the left. It will start moving in this direction, to return through its position of equilibrium and swing over to the right, and so on.  Your action has produced a series of oscillations on either side of the position of equilibrium. This is a pendulum motion.

Similar things happen in an automobile. At the standstill, the spring is in the equilibrium position. In motion, the variations of terrain create series of oscillations similar to the pendulum, i.e. series of movements going from one side to the other of the equilibrium position. This has two tiresome results: first, the repeated, upward oscillations are transmitted to the body and thus to the passengers; secondly, downwards, the same oscillations transmitted to the wheels make them bounce on the ground, thus losing adhesion -- the basic factor of road holding.

This is all the more marked in that the wheel has its own highly elastic suspension, the tyre, which reacts to road surface conditions immediately.  It is vital to dampen these oscillations, both in the suspension and in the wheels. This is all the more necessary when the road surface is corrugated in such a way that the frequency of the bumps coincides with the period of the suspension (time between two complete oscillations on either side of the position of equilibrium). This will produce the phenomenon of synchronous resonance, which will increase the oscillations beyond limits.

It is the same phenomenon that enables children to reach dizzy heights on a swing, merely by coordinating the impulsions of their legs with the period of oscillation of the swing.

As any impulsion on the spring makes it store up energy which is gradually released by oscillation, part of this energy must be removed in order to decrease the oscillations.

This is a problem schoolchildren are familiar with. If a tap fills a bath in a given time, how long will the drain require to empty it before it overflows ? In the motor car, the drain is the shock absorber. The shock absorber has to eliminate part of the energy the spring has stored.  Friction, an excellent energy consumer, is used for this purpose.

There are various types of shock absorbers: the friction or solid type (friction damper) and the hydraulic type, in which the speed of the movement is reduced by the effect of lamination of a fluid. This is also called viscous dampening and is preferable to the solid type because it is more sensitive and responsive, its action is continuous and progressive in all circumstances.

On most cars, the shock absorbers are added to the suspension proper, as an extra. However, it would obviously be preferable for the shock absorbers to be built into the suspension. Anyway, the result is a paradox. We are trying to obtain a suspension as soft as possible and yet we are doing everything in our power to prevent it from being too soft.

If we exaggerate, we may say that a very powerful shock absorber would stop entirely the motion of the spring. This would amount to doing away with the suspension altogether. If the shock absorber is very weak it will have no effect at all. Here again, the difficulty consists in reaching a happy medium between a suspension that is sufficiently soft to absorb the bumps of the road and a shock absorber firm enough to brake the oscillations of the body and wheels quickly. According to the type of vehicle, manufacturers have to choose between two solutions:
 

 
 
     

  • for a sports car, where road holding is the vital factor, the shock absorbers will be powerful; thus, the springing will be hard. 

     

  • for a luxury Sedan, on the other hand, the shock absorbers will be weak: the suspension will therefore be very soft, but the road holding will not be so good. 

Lieutenant de la Besse's sledge.  1909
 

Westinghouse car.  1908


For years, attempts have been made to combine these two points of view: comfort and road holding. For years, people have put up with a more or less satisfactory compromise.

© 2000 Julian Marsh



 

A SHORT HISTORY OF SUSPENSION  

Part Seven


A car remaining perfectly horizontal and parallel to the road, whatever the load, the state of the road and the speed, yet having a very flexible suspension and very efficient shock absorbers, would have the ideal
suspension. To obtain this, it would need to have the following qualities:

  • very great flexibility, varying on the basis of the load, and self-adjusting. 

  • automatic height corrector. 

  • independent wheels. 

  • built-in shock absorbers. 

Is there such a suspension ? Yes, there is. It is the Citroën hydropneumatic suspension, known as  Air-Oil Suspension, and it is absolutely unique and exclusive.

It is the only type to offer the following possibilities combined:

  • the greatest riding comfort known to-day. 

  • optimum road holding at the highest speeds. 

What is its secret? Replace the elasticity of steel springs, by the far greater elasticity of a gas.

The body of a Citroën DS19 or ID19 rests on four air mattresses, one on each of the independent wheels.

In the 17th century, the French scientist, Mariotte, defined the elasticity of gases as follows: at constant temperature, the volume of a mass of gas varies in inverse ratio to its pressure.

Three centuries later, the principle of the pneumatic spring was to be discovered, thanks to this observation. It has properties of flexibility a metal spring cannot provide. To approach the possibilities of one pneumatic spring as regards variation in flexibility, by the so-called conventional methods, at least three helical metal springs, each with different characteristics, would be required.

Now, the type of air spring to be adopted must be determined, as there are two possibilities: a constant volume and a variable mass of gas, or a constant mass and a variable volume. The difference is important. Citroën's hydropneumatic suspension has sometimes been confused with pneumatic suspensions of the former type. This is a mistake; Citroën's hydropneumatic suspension is unique in its kind.

The principle of the current so-called pneumatic suspensions is as follows: if a certain quantity of air is introduced into, or withdrawn from a given volume, then the pressure of the gas will be decreased or increased in proportion. Generally, this type of suspension is combined with metal springs. It usually needs an air tank and often a compressor to handle the air pumped from the atmosphere and also an exhaust device to allow for the variations of the quantity of air in the constant volume.

Citroën's hydropneumatic suspension is based on the opposite principle: the mass of gas remains constant and the volume is increased or decreased to obtain the corresponding pressure changes. With the Citroën suspension, air is not pumped in from outside, to be routed under pressure into a volume already containing compressed gas. Thus, it cannot be affected by the ambient temperature or the humidity of the atmosphere. It is sufficient unto itself and does without any auxiliary metal spring with the greatest success. Its reliability and the speed and precision with which its springs respond, make it by far the most satisfactory design, in the present state of technical knowledge. This is because it utilizes a hydraulic control device. The Citroën suspension combines the action of a gas (compressible) with that of a liquid (non-compressible). This is one of the secrets of its superiority in all circumstances.

AN OLD FRIEND: HYDRAULICS

It might almost be said that Blaise Pascal invented hydraulics.

From the principle that liquids are non-compressible, he deduced the theory which seems obvious to us today: if a certain quantity of liquid is poured into one end of a tube, the same quantity will be recovered at the other end. This means that, provided the tube is long enough, a motion can be transmitted to any distance desired, following any path whatsoever, so long as the tube is flexible. This is no mean advantage, and has far-reaching consequences. Furthermore, the motion can be multiplied or demultiplied by varying the cross section of the piston in the receiving cylinder, in proportion to the one in the emitting cylinder. It is also possible to vary the speed of the movements by varying the cross section of the tubes. This speed can be adjusted, following a law of continual variation with an accuracy that could never be obtained by mechanical means.

Owing to their flexibility, hydraulics are used nowadays for many applications in very different fields: from old-fashioned hydraulic lifts, to the controls of a supersonic plane, without mentioning all the cranes, steel presses, machine tools, radar aerial controls, battle ship or tank gun turrets, jacks, rams, dump trucks, scrapers, agricultural machinery, lubricating ramps in service stations, etc., and even the base of the Eiffel Tower in Paris.

Hydraulics appeared in motor cars some 30 years ago, with the first brake systems. The advantages of hydraulic brakes are so well known nowadays that they are in general use and it is hard to imagine how they could be replaced. In the present state of technical knowledge, this solution may be considered the final one.

At about the same time, hydraulic shock absorbers appeared, as the auxiliaries of conventional suspensions, together with Diesel engine injection equipment, hydraulic starters, etc., to culminate finally in Citroën's Air-Oil suspension, which has carried the use of hydraulics to the most advanced and integrated form of the art. Is Air-Oil suspension complicated? No, not when one considers the sophisticated machine performance that is required and demanded in all fields today. Technical trends are evolving towards solutions which appear more complicated from a functional standpoint, as they are expected to meet more and more requirements and as they are tending towards an ever higher degree of perfection.

This is so true that every driver would complain if he had to do without the benefits of a certain degree of complication, as this complication in the machine makes its incredibly simple on the driver. Thus, this essential complication finally results in simplicity and in safety. More often than not, the complication is only apparent. To realize this, it is sufficient to consider the number of devices that would have to be installed to fulfill all the functions Citroën's hydropneumatic suspension performs on its own. In practice, the Air-Oil system proves far simpler than any conventional process which could never succeed in playing so many parts on its own, since all these controls require both precision and power in addition to an almost immediate response.

It was thus right to choose hydraulics. This technique has been perfected; it is reliable and has proved its worth. Citroën's engineers deemed that a suspension combining the compressibility of gas with geometrical precision, the speed of transmission and the operating flexibility of a hydraulic control would be preferable to any other type, which, in their opinion, would be less dependable and sure.
 


"Going away"


1922 model 5 HP Citroën

© 2000 Julian Marsh