How to Choose Compression and Rebound

[sbiggi]

This isnt a DSM specific question as it pertains to my VW, but I assume the process would be the same.

I'm going to get my Bilstien shocks/struts revalved and rebuilt.

Right now my I'm running 350f/350r lbs/in springs and will be switching to 450f/250r. The Weight disrtibution is about 70/30 and the car weights ~2800 lbs.

I really have no idea what compression and rebound I need. I know what ever it is right now there deffinantly isnt enough rebound control, as you can feel the springs unload and then kind of bounce over large bumps/weight transfers.


How do you know what Compression and Rebound are needed?
The cataloge I'm looking at list Newton Meters at .52 m/s velocity.


-Seth

 

[ACM]

Unfortunately this is a little like asking 'How long is a piece of string ?'

As a general rule of thumb, one would decrease the compression and increase the rebound when increasing the spring rate. For a given vehicle, the actual damping levels depend greatly on the car's intended use - autocross damping rates are very different from road course rates, which are very different from drag rates.

Find a rebuilder who is familiar with your car and your intended usage. Detail the spring rates you intend to use long term, and how much bump travel you will have (less bump travel requires more compression damping to stay off the bumpstops). If there are any significant changes from the stock weight/distribution, detail that as well - corner weights will be very helpful.

Charles

 

[bwhughey]

Sorry, man, but you're going to have to do what everyone else does:

#1 Worry about calculating too much.
#2 Resolve to take a wild guess.
#3 Start fiddling with it.

I've had adjustable dampers be off by 20% when they were at the same setting.

You're going to have to play around to get them calibrated.

Regards,

Brad

 

[bjones18]

...I'm going to get my Bilstien shocks/struts revalved and rebuilt....
... I'm running 350f/350r ....to 450f/250r...Weight disrtibution ..70/30 ... car ~2800 lbs. I ... have no idea what compression and rebound I need......
How do you know what (s)..needed?
The cataloge I'm looking at list Newton Meters at .52 m/s velocity.
-Seth

I recommend one of two options:

1. purchase adjustable dampers.
2. make a set of take apart dampers, obtain a tuning kit (pistons, valves, washers, springs, oil, seals, & gas charge equipment.

A. Even the best development engineers "tune" the shock valving for different tires, wheel rates, and ride/handling preferences. This involves a guess at a valve code, building the dampers, testing the car, decide on compression/rebound low/med/high speed damping changes, take dampers apart, re-build/re-valve, test... and so on.

B. I was planning on taking option #2. Time constraints and the need for a set of shocks forced me to option 1 (Tokico Illuminas). I am very happy.... why? I cranked them to 4 in front and 5 in rear (stiff). I liked the handling... after a week or two I reduced the damping for a more enjoyable daily drive. It only takes two minutes to "tighten" the handling back down.

C. As for the calculations... go ahead if you want to learn more... but don't expect to calculate the answer. Full vehicle CAE models with pacejka tire coefficients, non-linear bushings, springs/bars... etc cannot even replace the development engineer from part A.
info: http://www.visteon.com/utils/whitepapers/2002_01_1567.pdf#search='tire%20coefficient%20modeling%20pa%2A'