captain_pants

I finished off Jan Z's book last night. Of course I couldn't find any mention I made of incorrect definitions of critical damping in a few minutes of flipping through earlier chapters, so forget I said that.

I found it interesting that he never suggests that you could have too much bump damping. All suggested changes are always reducing rebound and increasing bump stiffness on the end of the car that's loose. If you hit the upper limit of bump on your shocks, get stiffer springs and start again. I assume that Jan's experience is that every single car in grid has too little low-speed bump damping. How to know when you have too much?

Also interesting was that he considered low-speed damping to be 0-2 in/sec, anything above that was high-speed. Most shock dynos have a such a large scale that you can barely see the 0-2 in/sec range! Obviously he feels that this lower range of shock speeds is more important than most of the other builders, and his JRZ design accounts for that difference. Very interesting! My Penskes don't even open the shimstacks (knee in the curve) until ~5 in/sec on rebound! They open at ~1 in/sec on the compression side.

I was hoping to find some calculation methods suggesting a baseline starting point for shock valving. In my case I have shocks that clearly have too much rebound stiffness for my driving style and track surfaces, but how much to soften them during a revalve? My searching so far says the way is to guess and change if necessary after testing. I have a hard time believing that any decent race engineer would do that and expect to still have a job after the first test day.

TheNick

In my experience - you know when the car won't stop like it should under braking and the front end skitters on a slippery/cold surface.

The stickier the surface - the more compression you can get away with.

When I do my revalve - I'd like to add more compression too - its only under limited circumstances that I have "too much" right now. I'd like to be able to have "too much" for Peru or Lincoln so that I can run them closer to the middle of their adjustment range for local sites and at least have some tuning options with various tires.

macr88

Page 43 at the bottom.
If the graph above that shows the explanation is correct which I'm guessing it is then he's incorrect or now I'm reading it wrong?
Is he wrong in his explanation on page 43?


His explanation of low speed being 0-2" is because that is in fact considered low speed or because that's where the low speed adjustment on his shock stops and the valving opens? I think each builder might have their own idea of low speed, I don't know? Is that in fact what builders consider low speed?


I'm on page 157 but should be done today.

angryfist

Your knee should be around 2-3".

macr88

The knee for both compression and rebound?

It seems like a good idea the way captain_pants has his rebound starting at 5". The later the knee starts the smoother it can be right?

mLeach

depends. you really want the adjustment knob to move the knee, not increase the amount of HS damping.

angryfist

I'm no shock guru but it seems to me if your knee started at 5" it would essentially act like linear valving. Not enough low speed or too much high speed depending on where it was set at. I personally like a digressive setup.

And yes, i have my knee set the same on both compression on rebound.

TheNick

Thats impossible. Changing bleed is going to effect HS damping no matter what you do unless you run the Regressive Piston from Penske. http://www.penskeshocks.com/files/racecar.pdf

HS Damping doesn't start until the bleed valve chokes off and the oil bypasses it to move through the shims/HS valves. The piston has to move through the low speed valve before it gets to the high speed - if you run less bleed/more force at low speed, then the high speed force is going to be additive on top of that.

macr88

It can still be digressive. Bleed or low speed won't change the type of piston you have but you can valve so that a digressive piston is more or less linear. The knee is only where the valving starts to open.

captain_pants

P.S. I was quoted around $1000 per Regressive piston from Stimola. That's the piston alone, then add the rest of the shock. Ouch.