Hoosier_Daddy

I want to play with things and would like some thoughts on what to expect… I am no engineer and hope the collective here has some input… I have the adjustability of rebound and figure I should understand it a little more than I do.

Set Up: 700# ft and 900# rear springs with JRZ singles on a 2800 pound 964. Compression is set at 160 psi and for the most part the car feels great other than it might be just a tad bit ahead of me in terms of reaction. The rebound is set at 5 clicks from full soft front and 7 clicks from full soft in the rear. (This was suggested from previous owner who has the same car and weight as I do and had set up the car for his liking in the race series.)

My questions are: What can I expect to feel if I soften the rebound? If I increase the rebound? Any suggestions on how to approach this at the track? Should this be fiddled with only if I intend on messing with the compression as well? (Compression is not adjustable with ***** and I won’t have nitrogen with me at the track)

All suggestions/instruction appreciated.

Seth Thomas

Softening the rebound will cause the suspension to travel faster. For the rear this would mean that under braking with softer rebound the rear will rise up at a faster rate transfering the weight to the front of the car quicker. Same with teh front when you go to the throttle. The front with a softer rebound setting will transfer the weight to teh rear of the car at a quicker rate. This is a similar situation over the bumps too.

Basically the rebound adjustment affects the rate of the suspensions upward travel. Softening rebound will speed the rate up and stiffening the rebound will slow it down.

SundayDriver

Esses are a key place to check rebound. If it is too stiff, then when you, as the driver, feel like it is time the change direction, the car will not be ready. I would always stiffen until you feel that, then back off until you feel like the car is with you. What Seth talks about with braking is also critical so you balance those.

Any more than that is pretty advanced and involves a lot of feel and potential compromise so I would start with those two aspects.

HIGHBOOST

I thought the stiffer the setting of a shock, the faster weight is transferred?

Lemming

The questions concerns rebound. So going soft speeds things up.

Professor Helmüt Tester

You have to remember what's going on at the opposite end of the car. Low rebound at the rear allows quicker transfer to the front, etc.

Having separate diddles on bump and rebound is really enlightening, once you get brave enough to run setting from one end to the other. And that's why they put ***** on them, right ?

Hoosier_Daddy

Great input.

In simplistic terms…

Rebound too stiff = will (may) begin to induce understeer
Rebound too Soft = will (may) begin to induce oversteer

So, in the event of say, a 90 MPH sweeper that I feel I have to wait a little for the car before going hard to the gas I would in theory add a tad of rebound stiffness? (For conversation sake let’s assume all other parameters are acceptable, such as entry speed, sway bar settings, etc)

Now, what might play a more important role in the above scenario, sway bar setting or the rebound setting?

M758

This brings up some interesting questions that I have.


How about you rebound experts help out with what may happen in certain situations.

1) Feel on turn in - medium speed corner
- Corner is medium speed with a light reduction of speed taken at lowish rpm in a high gear. Assume it starts our neutral

- If you soften the rear rebound will the back end feel lighter or more planted?

- If you soften the front rebound will the back end feel lighter or more planted?


2) Feel on turn in - slower/medium speed corner with heavy braking (trail braking required due to a curved entry
- Again - softer rear rebound
- Softer front rebound


I remember adjusting the front rebound on my 944 spec car and it seemed to help me get front end grip in a full throttle corner exit in gentle bend (see the sig picture). When I hard the fround rebound full soft the car would understeer a bit and I could not hold my line. By stiffening the front shock rebound the front seem to grip better as I could go flat and easily point the car where I needed.

Veloce Raptor

This is the sort of thing that Seth and I help with routinely when we do private coaching. Think about what Seth said above: if you soften front rebound, under hard throttle, the front will be more prone to lifting up and unweighting more quickly (all other things being equal). This will generally lead to understeer. By firming up front rebound, the front will not rise as fast under hard throttle, and thus you will have more front grip for a longer time.

Larry Herman

Rebound is a simple concept. Increasing the setting makes your shocks extend more slowly. The net result of that is what is complicated, because it can affect the movement of the body as well as the response of the suspension.

With respect to the body, the softer the rebound, the more easily the body will extend the shocks making the car feel floaty. In turns, it will allow the inside of the car to roll up more easily. Except for very short transitions, it will not affect how far the car will ultimately lean, just how quickly it happens. So tightening up the rebound will help control the body movement......but.......

Tightening up the rebound will slow down the "re-loading" of the tire when you go over bumps or undulations. That can make your car pull and wander over the track if your rebound is set too stiffly with respect to your spring rate. It is all a compromise.

Remember that shocks have the greatest effect on initial movement, and then the car will settle-in based on the actual balance as determined by spring rates and roll-bar sizes. So you could induce a little turn-in oversteer by running a higher rear rebound rate, and then the car would settle back down mid-corner. Keep in mind that this will also affect your stability under initial braking too!

Larry Herman

And the extra diddles on the 3 way and 4 ways makes it a real challenge. Lots of ways to screw them up.

onefastviking

It is very important to determine what the car is doing in a turn and WHEN in the turn. You will use different adjustments dependent on if it is at corner entry, mid corner, or corner exit. Obviously this has to do with the weight transfer as Seth and VR have eluded to.

onefastviking

I need a cool title like you VR, what do I have to do to earn a Professional Driving coach and driver title ?

Veloce Raptor

Lawyers, guns, and money. Or a bad attitude!

Professor Helmüt Tester

Also known as "A nearly infinite combination of WRONG settings".