Set the dampers after you have got the wheel rates and ride height set. Wheel rates and ride height affect the setup more 'deeply' than dampers. (By the way - referring to dampers as 'shock absorbers' is just plain daft. The springs absorb shocks, the dampers damp the springs.)
Dampers do three things:
The next few paragraphs discuss the last of those in more detail.
The dampers control how fast each of the four wheel springs contracts and expands. So, tweaking the dampers lets you adjust the behaviour of the car in transient states - typically entering and leaving a corner. When you're in the middle of a long corner - and the amount of roll and pitch are constant - the damper settings do nothing for you. They only take effect when the roll or pitch is changing.
Specifically, increasing the bump setting of a damper would increase the 'drag' on the spring as it compresses, making the spring compress more slowly - and leave the rate at which it expands unchanged. Likewise, increasing the rebound setting of a damper will make the spring expand more slowly, and leave the rate at which it compresses unchanged.
The above seems intuitive. Nevertheless, damper tuning in GPL troubles me. Have you ever tried to saw the legs off a table to make it stop wobbling? Damper tuning seems a bit like that.
For example, imagine you are going into a corner at a constant speed, say a right hand turn. The springs on the outside wheels (the left ones) will compress, the bump dampers will control the rate at which those springs are compressed, and thus the rate at which the body rolls (to the left) will be controlled. Meanwhile, on the other side of the car, the springs on the inside (right) wheels are expanding and the rebound dampers are controlling the rate at which those springs expand and thus the rate at which the body rolls. So, on each axle there are two opposing dampers controlling the rate at which the car rolls. Err..?
Then it gets more complicated when you think of entering a corner and decelerating at the same time, because the car pitches and rolls at the same time. In this case, it will be the two diagonally opposing springs and dampers that affect the cars attitude, not those on the same axle. For example, when braking into a right hand corner, the front outside (left) spring will compress, and the rear inside (right) spring will expand.
During these two examples, is the bump or rebound damper dominant? Which setting, bump or rebound, has the most effect on the overall rate of pitch or roll? Which setting 'wins'?
It turns out that the dominant damper is the rebound one, for both body roll and pitching. Carroll Smith: “The compression stroke controls the motion of the unsprung mass and the extension stroke controls the motion of the sprung mass”. In effect, the bump setting controls the way the wheel moves (and therefore the amount of time it spends in contact with the bumpy road), and the rebound setting controls the way the chassis moves.
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How does changing the damper settings change the transient balance of a car? Here is an example. Say you have a car that has been tuned to have a neutral steady-state cornering balance. Say it has all four dampers set to critially damp the springs (3). You want the car to turn in better. You leave the front rebound dampers at (3) and set the rebound rears at full hard (5). The damper on the outside rear still has the same characteristic in compression. The damper on the inside rear has much more resistance to rebound, so it is going to attempt to 'pick up' the inside rear wheel. This takes weight off the inside rear tire. Since the total weight on all four tires must always remain constant on a flat smooth surface, the other tire-patch weights must rise. Hopefully - if the car is not twisting in the middle - the extra weight will be shared by all three of the remaining tires. The front tires will be doing a better job of sharing the weights than the rear. The front will stick better and thus allow the driver to turn in quicker.
Another example. Imagine you are turning into a fast right-hander; fast enough that you let off say, half the throttle as you do so. You find that as you go into this corner, the back of the car loses grip - maybe it is induced to break free because of bumps on the circuit. What might be happening is that the relatively stiff damping on the rear wheels is causing the inside rear wheel to be 'picked up', transferring more of the car's fore/aft grip to the front wheels, causing some turn-in oversteer. You fix the problem by softening the rebound damping on the rear wheels.
The bump resistance of a damper is (almost) always less than the rebound resistance, by a factor of about 1:3 on an average road car; a relationship that has been distilled over many years in the motor industry. The reason is this: when a wheel hits a bump, the wheel travels up and compresses the spring, storing the energy of the bump into the spring, stopping it being transmitted to the chassis. We want this to happen as quickly as possible, hence we want a fairly soft bump damper. (Also, if the damper was stiff in bump, then some of the energy would be transmitted through the damper and into the chassis, bypassing the spring. Not what we want.) Then, the time to apply the damping we need is when the spring rebounds, hence the damper being stiffer in rebound than bump.
In a circuit racing car, the bump:rebound ratio tends to be a little more like 1:1.3 (i.e. the bump setting is often about 75% of the bump setting) as we are interested more in handling than ride.
The two tasks performed by the dampers, i.e. dissipating the energy of the bumpy track and managing the rate of lateral and longitudinal load transfer, are handling by 'fast' and 'slow' settings, respectively. ('Fast' and 'slow' refer to the damper piston speeds, not the velocity of the car.) GPL only models one setting, which is generally assumed to be the 'slow' one. Real-world bumps in the track are not modelled fully by GPL.
How much damping should be applied to a wheel? The following graph shows the effect of non, under-, optimal and over- damping:
(a) No damping - the wheel oscillates indefinitely about the zero reference. In reality, there is always some energy dissipation so that, in time, the wheel will settle back to the zero reference.
(b) Underdamped - the wheel oscillates about the zero reference but with decreasing amplitude and eventually reaches a steady state.
(c) Critical damping - the wheel returns to its starting point smoothly and quickly, without under/overshoot.
(d) Overdamped - the wheel returns smoothly to its starting point, but more slowly than when critically damped.
As originally recommended by KONI, the 'normal' way to set dampers in the 'real world' is as follows:
You start out by making the adjustments on all four dampers at once, which will give you a feel for the overall level of damping your car needs (which will vary with wheel rate). You may then go on to fine-tune the dampers separately at each end of the car.
Set the dampers in concert with the differential, as these are the things that determine how the car enters and exists a corner.
You can observe the effect of damping in GPL by the following method. Set the dampers to fully soft and look (using the F10 view) how the car reacts to being 'dropped' onto its pit lane stall. You should be able to see it oscillate in the second or so before it comes to rest. Underdamped! Increase the stiffness of the dampers and try again. With a setting of 2-3, the car looks critically damped; with a setting of 5, the car looks overdamped because it settles more slowly than with 2-3.
Conclusion: in GPL, the damper settings have a useful range of 2-4.
A second observation: while experimenting with both very high and very low wheel rates, I noticed that I didn't need to alter the dampers to match the springs. In the real world, as you stiffen the springs on a car you generally need to stiffen the dampers too. It seems you don't need to in GPL, because the damper settings are not absolute values, but rather they are relative to the wheel rate. Furthermore, after a bit more experimentation and intuition, I reckon that a damper setting of '1' means the spring is underdamped, '3' means critical damping and '5' means overdamped. (Thinking as an ex-games programmer, that's how I'd implement it.)
If true, that gives you a great starting point for tuning the dampers; just set them to 3 bump and 3 rebound on each wheel.
Since GPL doesn't model real world bumps, we can pretty much now ignore the 'tweaking the dampers to make the car ride the bumps' stuff you have to do in the real world. So that means leaving the bump settings at their critically-damped values. What we're left with is to tweak the rebound settings to affect the handling of the car at corner entry and corner exit.
| Corner entry | |
|---|---|
| Problem | Solution |
| Oversteering | Soften the rear rebound dampers from 3 to 2 |
| Understeering | Stiffen the rear rebound dampers from 3 to 4 |
(However, if you're having trouble tucking the nose of the car into the corners, then I suggest you attend to the differential before fiddling with the dampers. With a low coast side ramp angle and a high number of clutches (e.g. 85/30+4), the car will have a lot of trouble rotating into the corner, no matter what you do with the dampers.)
| Corner exit | |
|---|---|
| Problem | Solution |
| Oversteering | Soften the front rebound dampers from 3 to 2 |
| (Stiffen the rear bump damper from 3 to 4) | |
| Understeering | Stiffen the front rebound dampers from 3 to 4 |
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