Originally Posted by mavm86,Mar 30 2010, 08:02 AM

This page of Corey D's shock thread on SCCAForums has some interesting stuff in it (mainly the shock dyno sheets from his Penskes, Drew V's shock dyno sheets from his Penskes, and some interesting comments from Jason Collett, Steve W, and Andy Howe).

Interestingly, both Drew and Corey's Penskes had a TON more rebound than the above posted Konis, although Steve W commented that he thought this was more rebound than ideal.

Originally Posted by alvanderp,Mar 30 2010, 08:31 AM

65% critical is almost completely irrelevant on a stock class auto-x car. As others have noted, adding a bunch of low-speed rebound can compensate for the lack of spring rate in a stock car. Wynveen and I had this discussion back when I was still running in Stock, and we both ended up with rebound settings that were well north of 100% critical.

I think Nick said it in another thread. The only way you are going to figure this out is to set a baseline, make some runs, turn the knobs and make some more. I've found vastly different setups on different cars and it all has a lot to do with what a driver wants. The calculations can get you in the ballpark, but don't be surprised if you end up a ways away from what some people might say is "optimal"

And yes, as noted in the above thread, my shocks had way too much rebound, I never got anywhere near the upper end of the adjustment range. I'd like a bit more front compression though.

To the OP - did you get a dyno for all 4 shocks? I think you'll find that they vary a good bit, and you'll need to adjust them differently to get the same amount of force left to right. At least that's been my experience with Koni Yellows, even ones they have rebuilt to the "RACE" version.

Originally Posted by TheNick,Mar 30 2010, 08:38 AM

You are going to be overdamped. Thats what is necessary to make the cars work. Is it good? If it makes the car faster - then yes.

Overdamped rebound CAN lead to a loss of traction on the inside tire - will it? Thats a question you have to answer with your testing. Overdamped low speed compression CAN lead to skipping and a lack of grip from the outside tire....will it?

Originally Posted by alvanderp,Mar 30 2010, 09:31 AM

And yes, as noted in the above thread, my shocks had way too much rebound, I never got anywhere near the upper end of the adjustment range. I'd like a bit more front compression though.

Originally Posted by Forcednduckshn,Mar 30 2010, 06:38 AM

At the event, on 140 run Hoosiers and a bumpy Nick

Originally Posted by glagola1,Mar 30 2010, 07:36 AM

IMO:

Rebound and especially large amounts of low speed rebound is the devil. It essentially neuters the shock. When a wheel hits a bump or the body rolls, the shock compresses and before it extends it goes though a change from compression to extension. At the transition between the two, it stops and accelerates. Between being stopped and extending, the shaft is traveling at low speeds. If there is a lot of low speed dampening, the shock doesn't allow the wheel to extend back down to the road surface and the tire spends a brief time not contacting the ground. You know what happens when the tire doesn't make contact with the ground. On bumps, the car floats and braking is weakened as well as corner grip if the car is side loaded.

It's the convention that rebound is the way you control a spring and that springs and sway bars control compression and body roll. When this convention is applied to auto-x where transition is the name of the game, things get funny. Auto-xers start demanding large amounts of shock force so that the car transitions quickly. Here in lies the problem as explained above.

.... and really, does anybody think the weight of the unsprung mass on the inside of a car in a turn is going to hold down the mass of the chassis as it rolls to the outside tire? It's only going to be lifted off the ground and minimize traction.

In my experience, only enough rebound should be used so that extension of the shock and the resulting upward acceleration of the sprung mass is kept under control and doesn't disrupt traction and car dynamics. This is probably what is called 65% of critical.

Here's some anecdotal evidence from the Texas Tour this last weekend: The site is bumpy especially when traveling across the direction of paving. On day 1, I ran my shocks with a rebound damping setting of 4 which is what I used on the smooth asphalt of Dixie. The car had a hard time braking in the slalom across the bumps and was generally bouncy and difficult to keep on line. For a video reference of this see the day 1 video in the Texas Tour Results thread.

On day 2, I remembered this and decided to start with the rebound set on position 3. Sure enough, the car was a good deal less bouncy and the braking was much improved. I then noticed that the rear of the car took a little longer to transition so I added a click of compression which solved the problem with no ill effects.

Now, my thoughts on compression:

High speed bad, low speed good... especially when applied to transitional nature of auto-x. I run a ridiculous amount of low speed compression. I feel it loads the tire faster and does all of the chassis control we demand but doesn't lift the unsprung mass of the ground like rebound. It provides that feel of stability and instant tire loading that we're looking for in transition but with the right blow off force, it allows the sprung weight to still accelerate quickly when faced with a road irregularity. This ability to "blowoff" is crucial and is why I was still able to run high amounts of compression on the bumpy Texas site. It's only when a shock either has a hard time reaching that blow off force or if the high speed compression is too high that we run into problems maintaining traction.

The temporal nature of shocks also allows large amounts of low speed compression to act as a "disappearing sway bar". On entry, the shock is resisting roll as a sway bar or spring might do but a fraction of a second later during mid corner, the shock is finished with it's influence over body roll and allows the springs and sway bars to dictate a roll bias. This effect can be utilized to achieve a car that is stable in slaloms but balanced in sweepers.

Now, putting these concepts to use means you have to have a shock capable of allowing a smooth blow off of the low compression. That's the trick.

Originally Posted by zo7vette,Mar 30 2010, 09:28 AM

There is your problem, not the shocks, put some fresh tires on.

Jeff

Originally Posted by Forcednduckshn,Mar 30 2010, 09:34 AM

Definitely a factor. Sticker Hoosiers will be on for the next event. I just wanted to get a feel for the new shocks and beat up my Hoosier Rocks (boy, are they toast)

Originally Posted by mavm86,Mar 30 2010, 09:16 AM

Drew,

How close do you feel the re-valved Konis come to being "optimal" in terms of rebound force? Do you know how the forces on Steve W's shocks compare to the re-valved Konis?

Originally Posted by alvanderp,Mar 30 2010, 10:51 AM

I'm really not at liberty to discuss specifics of what Steve had/ran, that's something I'd rather he do himself.