by Phil Ethier

A stiffer rebound adjustment WILL change the transient balance of a car.

Springs and sway bars help define the steady-state balance. Shock absorbers (let's call 'em "dampers", because this Briticism actually describes them better) are said to affect this balance in transitional moves. The effect of dampers diminishes to zero as the vehicle stabilizes in a steady state of cornering on a perfectly flat and round skid pad. But the effect is profound while the car is beginning to lean, before it reaches its steady-state body roll.

To understand how a change in rebound adjustment changes transient balance, it helps to compare how dampers work in transients to how sway bars work in both transient balance and steady-state balance.

Simply put, the rebound action of the damper does (in transients) HALF of what a sway bar does (in all cornering).

Here's the short explanation of how a sway bar (anti-roll bar is a more accurate description) works: "The sway bar pulls up on the inside wheel." That's it.

As the outside wheel is pushed up into the body by cornering forces, the sway bar rotates up. The inside end of the sway bar then tries to pull up the inside wheel. The stiffer the sway bar, the harder it tries to lift the inside wheel. This works AGAINST the inside spring which was HELPING the car to roll. The force of the sway bar is split evenly between opposing the inside spring and helping the outside spring.

Thus, in a corner, the sway bar adds weight to the outside tire patch and subtracts weight from the inside tire patch. The degree to which it does this for a given amount of body lean depends on the stiffness of the sway bar. Manipulating the relationships among these corner weights is how cornering balance is tuned. The end of the car that shares the weight more evenly between the right and left tires is the end that gets more stick.

The characteristics of dampers work (in transients) like a sway bar, but the compression and rebound effects can be different on each side, because they are not connected across the car like a sway bar. You could say they split up the job of dynamic roll resistance.

In a transient (like the turn-in to a corner), body roll is increasing in one direction. The compression characteristic of the outside damper adds weight to the outside tire patch. The rebound characteristic of the inside damper subtracts weight from the inside tire patch.

Now we finally come to how a stiffer rebound adjustment will change the transient balance of a car. Say you have a car that has been tuned to have a neutral steady-state cornering balance. Say it has all for dampers set full soft. You want it to turn in better. The dampers are only adjustable for rebound stiffness. You leave the front dampers full soft and set the rears at full hard. 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 patch. This slows the rate of roll down, so the inside front damper (whose rebound characteristics have not been changed) will not be subtracting weight as quickly as before the damper adjustment was made. Since the total weight on all four tires must always remain constant on a flat smooth surface, the other tire-patch weights must rise. Since the car is not twisting in the middle, the extra weight will be shared by all three of the remaining tire patches. 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.

Regarding "rebound adjustment would mainly be useful to adjust for the bumpiness of a particular course." Bumps are a separate issue. Problem is, you can't separate your damper settings. Ideally, the car should be made to handle by geometry and spring rate setups, then just enough damping added to prevent bouncing around after a bump. Stock autocross rules do not allow this approach, so the art of attempting to set cornering balance with dampers was born. It calls for a lot of compromises.

This has been an explanation of one effect, taken in isolation. Autocrossing is the art of handling a car while it is under the influence of many effects simultaneously, most of them changing rapidly. When testing a car, don't change too many things at once, or you will not be able to tell which changes caused which effects.

Regarding "is the rebound stiffness the same from full compression out to full expansion?" I think it is on most dampers.

Regarding "is the compression stiffness the same from full # rebound back to full compression?" I think it is on most dampers.

Regarding, "does it only matter that the stroke is increasing for rebound stiffness and decreasing for compression stiffness?" I think that is true for most dampers. I know that it seemed that way when I have operated dampers by hand, but my arms are hardly calibrated instruments. I do know there have been some street dampers that were intentionally manufactured to have different responses at different extensions, but I don't have any experience with them nor know how common they are.

Now we come to the tricky part. The "damping rate" may indeed be the same all though the range of the damper, but that is not important from the point of view of the car. The damping rate at the tire patch is what counts. Because of the suspension geometry and the angle at which the damper is attached, the rate at the tire patch may vary. In the Europa front end, for example, the damper (and it's concentric spring) is mounted from the frame to a point on the lower control arm inboard of the ball joint. The angles of force all change depending on the deflection. The relationship between the damping rate at the damper and at the tire patch can be expressed as the ratio of

vertical movement of the tire patch relative to the chassis
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coaxial movement of the damper piston relative to the damper cylinder

How constant this ratio remains is a function of suspension design and body roll. At the front of the Europa it varies around 1.56/1 or so. At the rear, it runs about 1.16/1 and stays much more constant through the travel. I wouldn't worry about this too much though.

Copyright 2002, Temple of VTEC