The suspension system is where the car comes into contact with the road and tries to a) keep it there and b) pretend as if the road doesn’t exist. That’s a lot to ask…
…and then get precious little thanks in return from customers or indeed motoring journalists. The former probably don’t know what suspension is. The latter want all suspension to do the same thing, namely to keep fast cars stuck on the tarmac at 145 mph. This conflict is as big as the one facing the suspension itself, which must mediate between the undulating road and the dynamic system that is the car in motion. The other circle to be squared is that of ride comfort versus handling.
Taking all this into account, the suspension system is the least well-understood part of the car. You can see the body, inside and out. That’s easy to comprehend. The engine is a controlled mass of moving parts, all tightly interlocked and with the degrees of movement and rates of motion firmly constrained within narrowly defined ranges. All that has to do is move the car as quickly as possible. You can see it work and judge action and reaction. The suspension can’t be so easily assessed nor its behaviour second-guessed even by those rare souls interested in its activities. In a way its role is to negate.
Life is easier for the orchestrators of motive forces than for the suspension wizards. To a large extent, the engine specialist has total control over the inputs: that’s the accelerator pedal. The oxygen content and temperature of the air can be allowed for and the vehicle’s weight and payload are predictable. The engine is essentially a closed system, as simple as a railway network. Not so the suspension.
The suspension engineer is not able to constrain the inputs, only define limits of operation with variables of potentially infinite values. The suspension engineer must define requirements of handling and comfort in relation to that mass of unknowns known as a road surface and even whatever tyres the car is eventually fitted with. The required output is that under normal use the car must be comfortable and secure. At the same time, when the car reaches the limits of grip, it must fail in a progressive and secure fashion. Failure must be predictable and gradual so that the driver can learn the limits of the car’s ability. And the manner of failure must be accounted for. There is a major difference between progressive understeer and snap oversteer.
Differentiating the chassis engineer from the others is that the science of suspension is largely empirical and related very firmly to subjective perceptions. Whereas an engine can be computer-modelled to a very high degree, the chassis can’t. The differences between the predictions made by a mathematical simulation and the reality of an engine’s performance are two orders of magnitude less than those between a mathemetical model of a suspension system and its actual behaviour.
The chassis and suspension engineer must select a set-up which, at best, can only approximate to the one required. Then they need to test it on a track and public roads to determine which elements need adjustment. To do this she or he must understand in principle how the elements work and then, in practice, judge how to vary them to get a result which satisfies the perceptions of an unknown group of human users. I refuse to call this an art but it is certainly an unusual blend of engineering and imaginative empathy.
The other imaginative part is to conceive of how an unknown group of users will want their car to feel. Companies such as Rover, Jaguar, Lancia, Citroen and Peugeot have been very good at producing cars with a good ride and handling compromise. Personally, I feel that ride is more important than handling. One can always drive more slowly but one can’t make a rumbly, harsh ride less so once the suspension engineer has decided the specification. I notice that most of the cars on my roll-call are either out of business or have abandoned any pretence at good taste in the ride department. Only Jaguar keeps the flame alive and I am quite confident the Leamington Spa chapter of the Jaguar XJ-6 club will tell me the new cars are no match for the ill-shaped rust-traps they champion so vocally.
And we have not even got as far as discussing sprung mass, friction, damping and roll velocities.