ARTICLE: The Black Art of Suspension Set-up

By Guy Evans of Nitron and Steve Guglielmi

"In this first article, we will look at why we have dampers and how they contribute to grip levels. We will discuss how dampers work in detail in another article, but at this stage, we must appreciate that a damper is an energy converter and turns the energy stored in a spring and the energy contained in a vehicle's motion into heat energy (in the same way a brake disc does). A damper is not a 'shock absorber', in reality, that job is taken on by the spring."

When a tyre is in contact with the road, it forms something called a contact patch, which is rougly rectangular in shape. A tyre generates force (in any direction) proportional to the amount of vertical load on it. The more vertical the load, the more the tyre will grip. One of the main aims of a good suspension set-up is to minimise something known as Contact Patch Load Variation. We basically want the tyre to press itself onto the road with a maximum equal force at all times. The greater the load we can apply to the contact patch, the more cornering / tractive force is available to us. However, this is up to a point; it has diminishing returns as the load goes up, so the amount of extra cornering force is slightly less than the last increment for a given increase in vertical load. As the tyre generates traction based on vertical load, it will seem obvious that for a well balanced and optimally set-up car that is predictable to drive, we will need to maintain this load with the minimum vertical variation.

So how do we achieve this? First let's look at the dynamics of one of the sprung corners of our car.

If we imagine a spring with a mass dangling from it, and we extend the spring and let the mass bounce, it will take a fair amount of time to come back to rest. The vibrations will be "damped" but only lightly, the damping in this case is due to internal friction in the spring wire and the drag of the air around the mass. If we now imagine this virtually un-damped spring / mass system to be the corner of our car reacting to an input like hitting a bump, we can see that the Contact Patch Load Variation would be very high. The tyre would alternate between high levels of vertical load (high cornering force / tractive force) and low levels of vertical force (low cornering force / tractive force). The result is obviously a reduced amount of cornering / tractive force for the period that the corner is oscillating and the car will be un-drivable. By increasing the damping levels we are able to reduce the amount of oscillation resulting from the original disturbance.

The role of a damper is therefore to control the oscillations of the suspension to maintain the lowest possible Contact Patch Load Variation.

The tuning of a suspension system involves not only dampers, but springs, anti-roll bars etc. The best handling car has the best compromise of all these components; the results are totally dependent on the compromise of the total system. Dampers can play a significant part of this system, you can't, though, take them totally in isolation.

If for a moment we assume we have a perfectly damped car, then we change the springs, we may end up with a better handling car, but unless we re-tune the dampers to match the new springs, we will miss out on some of the potential of the new set-up.

The message here is that the springs and dampers go together and we must take account of this if we are to tune the car to its full potential.

The advantage dampers have is that adjustable versions are usually easily and quickly adjusted. This can have very dramatic results on the car's handling. So knowing what to adjust, and when, can be a big advantage.

Dampers are not like springs; the force created by a damper is totally dependent on the velocity of the dampers shaft, not its position. A spring's force is dependent only on the amount the spring is displaced, there is no velocity component. Therefore for a competitor it's important to realise that when a car is being tuned they think about what area of the system is likely to make the biggest improvement to their existing set-up.

For example, if you are in the middle of a long high-speed corner on a smooth circuit, then the dampers will not contribute very much to the contact patch load as the car is settled and the dampers are not really moving. On the other hand the springs will be compressed on the outside of the car, and the roll bar will be twisted, and thus contributing a large proportion of the contact patch load.

If our car understeers or overtsteers in long smooth high-speed corners, we consequently will not find very much improvement by changing the damper settings, we should instead be looking to make a change to the roll bar or springs to cure the problem. On the other hand, if we are experiencing the problems on the entrance to a corner just after turn in when the car is still transient, then damper adjustments might help us. The car's body is rolling, the outside wheels are rapidly moving up into the body and the inside wheels are rapidly moving dropping down and thus the dampers will be accounting for a fair proportion of the contact patch load. Changing the damping values should alter its reaction to the turn in.

So we should be thinking all the time: what are likely to be the biggest contributing factors to the contact patch load at the point we are interested in tuning? This way we will be able to identify whether the dampers need adjusting or whether the roll-bar / springs need changing.

It is usual for the driver to drive the car to the limit of the stiffest end. For example if the car understeers then it is because the front end of the car is too stiff in relation to the rear. Oversteer is the reverse of this, where the rear is too stiff in relation to the front. As we have just discussed, where this understeer or oversteer occurs will indicate where the solution may lie. What we are ultimately trying to achieve is a situation where we achieve a "balance" in the apparent stiffness at each end and the car becomes stable and hence more predictable and more drivable.

So in conclusion, we have seen how the springs and roll bars are really the components that provide the force on the tyres to generate grip during a steady state corner and we have also seen how the dampers provide additional force during periods when the chassis is moving around, as well as damping out the oscillations caused when the springs are compressed and then allowed to uncompress.

In the next article we will look at exactly how changing damper settings will affect the handling of your car.