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Originally Posted by sperry
I always thought double adjustables were low speed bump and low speed rebound, since they determin the car's handling. Fast bound/rebound are more for absorbing "shock", which isn't as critical.
My problems at RFR aren't really the harsh bumps/curbs/washboard... it's the longer stuff that loads up the rear of the car then launches it into the air... most notable at turn 1, where my current suspension basically tosses the rear of the car into the air making me balistic just after turn-in till the apex when the car comes back down and I have enough traction to turn right towards the apex of 2. Ask Eric, it's scary as hell in my car.  |
Here's my $.02:
The particulars of what the adjustments on your shocks are actually doing in terms of Resistance Force vs. Velocity depends on how the manufacturer designed the adjustment mechanism and the shock itself. Even if they say it is only supposed to adjust the "low-speed" part of the curve, there will probably be some cross-talk and the "high-speed" part of the curve will be affected a bit too.
I wouldn't really agree with the statement that high speed isn't as critical as low speed, because the real purpose of a shock is to keep its tire planted on the pavement as firmly as possible at all times. It's best to approach shock tuning with this mindset instead of setting out to use them to fiddle with handling balance. There are plenty of other tools in the toolbox you can use to adjust handling balance. From my experience the mid to high-speed part of the curve is very critical to get right, since that's the region you're in during most disturbances to a tire (pretty much any bump, dip, curb, etc.) at speed. These sharp input situations are also usually the hardest to recover from if the car is really upset as a result. The low-speed part of the curve is mostly only encountered during body roll movement like you get at intial turn-in, or very long wavelength elevation rolling in the track.
As far as how to set up a track car, you have to be careful with the idea of using a lot of bump and not much rebound like Dean was saying. Track cars usually have pretty stiff wheel rates, and that will require more rebound stiffness to avoid having the car feel "floaty" and uncontrolled (underdamped). You can usually feel a lot of upward travel or "hang time" after a bump input when this is happening. When you've cranked in too much rebound, you'll really feel the car being pulled down after a bump input, and it will start feeling harsh (overdamped). In extreme cases of this the car will actually jack itself down onto the bumpstops if there is a series of rapid inputs, because the suspension doesn't have time to fully extend back to its normal ride height position before the next input.
You don't want to neglect bump stiffness either though, because too little can lead to extra travel during the compression stroke, and then you feel a lot of travel on the upward extension stroke which mimics the symptoms of not having enough rebound stiffness. So, you can see shock tuning is a bit tricky.
It's hard to speak in generalities because to really get a car right you need to know a LOT of specifics, but hopefully that helps. I would highly recommend looking through the Penske adjustable tech manual (
http://www.penskeshocks.com/technica.htm ), because it helps understand how these things physically work, which helps understand how tuning them works, and they have a good basic start-up tuning procedure in there too. Good luck man!
