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
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
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
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
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
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