Larry Herman

No, because you are forgetting about the body. It is the rebound that keeps the body from "floating away" from the suspension. If you didn't have any rebound, every bump would cause the body to float up, and then hang there for a while because of the compression damping. Remember that the shock has to control both the suspension and the body.

KaiB

Remember that the shock has to control both the suspension and the body.

So very easy to forget because of its simplicity...I'll keep this comment pasted on my forehead for a while.

the90

Most pro race teams, particularly in Europe, are using compression biased damper setups. The logic has been touched upon above; track surface irregularities cause variation in contact patch loading. Obviously, we want to minimize this variation, as it is a sort of first order measure of mechanical grip. Compression-biased damping will cause greater platform disturbance, but the affected wheel will not move upward as much as it would with a conventional rebound biased setup. Now, the wheel can return to its original state more quickly as it is not pushing against a strong rebound force. Also, damping 'style', (digressive, progressive, linear) is under some review as well. Most drivers will prefer a digressive setup, though a linear damper will give more grip. Few shocks can really achieve this (Multimatic DSSV, Ohlins TTX). Conventional shimmed dampers will cavitate.

the90

A couple more thoughts....
With minimizing contact patch load variation in mind, it is helpful to have a high ratio of spring stiffness to unsprung mass, and a high ratio of spring stiffness to damping force. So keeping unsprung mass and rebound damping force at a minimum are good things.
Additionally, a compression biased damping setup will use less damper stroke; less strokes means you may be able to reduce the average ride height, lowering the CG with obvious benefits....

The best way to set up springs and shocks? A multi-post shake rig. You will learn an enormous amount about suspension dynamics in general and about your car in particular. Certainly worth looking into....The best such facility that I know of is at Dynamic Suspension in Toronto.

Dan Shea

Am I wrong that in every comment it seems we're leaving out the word Damping behind Rebound/compression? If you tell me more rebound, I think of the force the shock wants to extend with is increased. If you say rebound damping, I think the forces holding the shock from extending is increased.. I may be a little over my head here, but that would also explain why the OP hears different advice from everyone.

PedroNole

I've read Jan's (JRZ founder) book and find it pretty difficult to follow between all of the bad grammar and the mostly anecdotal way he presents the information. I've probably read it 3 times and just come away not really feeling all that comfortable with the presentation/information.

I approach it like this:

1) Set the shocks at the mid-point for both bump/compression and rebound.
2) First start by driving the car and see how "bumpy" the car is. Start adding/removing compression until you've dialed out as much of the bumpiness as possible and then back the compression stiffness off 10-15% from that point.
3) Now focus on how the car handles in the corners from a chassis/platform stability standpoint. Does it roll? Does it lose grip in certain areas? Start adjusting rebound until you dial out a lot of the "roll" in the platform. You are really focusing here on weight transfer and what happens to the unloaded tire(s). The idea is to make sure you're getting as much out of all four of the tires as possible.

Unfortunately (for those of us who run in spec classes where you can't change certain things on the car), I think the greatest strides can be made with different spring rates. No matter how good your shocks are or you are at adjusting them, they can only do so much within the certain range that the springs operate in. Remember, in the corners, when the car is fully-loaded on one side or the other or is on one/two tires more than the others, the shocks really aren't doing a lot. No matter how much you want to adjust the car at that point, you really can't overcome the effect the forces on the car when it is on the spriings. You've just got what you've got. At that point, you either just have to drive it or modify your braking, turn-in, speed, line, etc to try to overcome some of the issues you still have.

TheOtherEric

I start by calculating the % critical damping (in Excel) for my setup and shock dyno plots. Then I find some initial settings that get close to what I want, e.g. 60% critical damping for low-speed bump, etc.

This got me good starting points on my 993 (and hopefully my e36) and helped me understand the shock settings of a fellow racer's e36.

But it's critical to follow that up with experimentation on track.

Larry Herman

Mark, the problem here is that you don't know what you have until you change it. From your videos, it "looks" like everything is working ok, but I don't think you can really evaluate it just based on that. One thing is apparent based on your car's dislke of the turtles; your spring rates are probably too soft and you are compensating with overly high compression damping. THAT is the nice thing about 3 way Motons. I ran 1400+ lb springs and my RSA was fine over the turtles. I could run higher low speed compression for body control, and a little lower high speed compression damping for more compliance over the rough stuff.

Larry Herman

I very much understand and agree with this.

jrgordonsenior

Larry can you elaborate a little on how this works?.....

the90

A damper has two half cycles; a compression biased approach simply means that more energy is being dissipated on the compression half. Rebound biased damping has been around since (and because of) the days of the boulevard cruisers, for reasons of ride comfort. In the interest of mechanical grip, compression biased damping will reduce load variation at the contact patch.

jrgordonsenior

OK so would you then set your bump at almost max and your bound close to zero at both ends? I've never tried it but I'm testing Friday and willing to give it a try....

the90

JR,
I have to caution you: damper setup is very dependent on damper architecture. A traditional shimmed damper will likely be happier in a rebound biased setup; a linear or compression biased setup is likely to produce a fair amount of hysteresis, which can be thought of as putting a spring in series in line with a pure (viscous) damper.

the90

JR,
I had to leave before completing my response to your question...sorry.
A good compromise might be to use digressive valving for compression, and progressive for rebound: this would give compression biased damping for low velocity inputs, and rebound biased damping for high(er) velocity stuff. This likely means revalving your current dampers, and if so, you might investigate the possibility of a linear setup if the builder feels he/she can do it. As always, dampers have to work with both corner springs and the spring rate of the tires.
Remember that what feels best is not always fast: the stopwatch is king.

Larry Herman

IMHO it is actually simplier than it sounds and "the90" explained it pretty well here:

Compression-biased damping will cause greater platform disturbance, but the affected wheel will not move upward as much as it would with a conventional rebound biased setup. Now, the wheel can return to its original state more quickly as it is not pushing against a strong rebound force.

If, rather than have a bump compress the suspension X amount, with more compression bias, it may only compress it by .7 X, the rest of the displacement being absorbed through greater tire distortion and body movement. Now the suspension (which would be controlled by shock rebound) only has .7 X distance to re-extend. This means that it will recover faster, even with an identical rebound setting as in our X example.

Cars with higher rebound bias will compress more, and then have to recover more. This really slows down suspension response and creates greater flucations in grip.

Does this help?