spa-zz

Steve, I like your explanation. Thanks. I've had a hell of a time explaining these concepts to people in the past. Now it'll be easier.

silvershadow

OK understood now - chemical engineer here - too much time spent in the chem lab and not enough statics and dynamics.

So, basically we are talking about transferring weight to the road surface.

In the case of auto-xing, if there are parts of the course where you need to tap the brakes to set the front (get the front tires to bite), you will get a quicker response from the tires if the shocks are somewhat stiff rather than soft.

Ok I think I have caught up so far.

But then extending that explanation, how is it that rebound has such a dramatic effect on the car handling when compression doesn't seem to.

As an aside, my car was understeering badly. With Dave McCombs advice, I lowered the front shock pressure from 200 psi down to 125 (essentially a compression adjustment) and it made a huge difference in the understeer, to the point where you could begin to feel rebound adjustments make a difference in the cars handling.

Orthonormal

As for why compression seems to have less of an effect, go back up to my table. In half the cases, changing compression damping has two effects that work against one another. With rebound, half the cases have two effects that work in the same direction.

Also, Andy and Steve both mentioned that the compression adjustment on the Penske 8100 cannisters doesn't have a very wide range. Mike Maier has told me the same thing. That may be part of why turning that knob doesn't seem to do much.

Gas pressure isn't really a damping adjustment. As I understand it, a certain amount of pressure is required to prevent bubbles from forming in the oil, but above that threshold the gas pressure has two effects:

It generates a constant force to extend the shock. The force is equal to the gas pressure times the cross sectional area of the shock rod. When you lowered the pressure you probably lowered the front end of the car and maybe got a bit more camber as a result.

It also generates a very weak spring rate. Compared with the static force, the spring rate is very small. To visualize how small, think of both the spring rate and the static force as the result of taking a mile-long spring and compressing it to the length of the shock so that you have a huge amount of pre-load force compared to the spring's linear rate.

I think Andy has a quick-and-dirty estimate for how much spring rate you gain for a given change in pressure.

shaggy

I'd have to look at my notes again regarding gas pressure specifics, but gas pressure is NOT a damping adjustment. It is relevant to compression damping in the Penske shocks, but only because too little pressure causes a problem.

Gas pressure acts like a spring under the ideal gas law... gas pressure times piston area gets you force. So an increase in gas pressure is an increase in force required to compress the damper. If I recall, its something like 50 psi = 15 pounds of equivalent spring rate. If anyone has a loose canister floating piston, measure it and give us the surface area.

Andy

silvershadow

I haven't measured the canister but it is about 2" in diameter. Therefore the crossectional area of the cannister = pi*(d^2)/4 or 3.14*2^2/4, yielding an area of about 3.14 sq. in.

It appears that 15 pounds of pressure would = ~50 lbs of equivalent force (3.14 x 15). That doesn't sound right. I don't remember what the spring rate is for the car (and search isn't working) but I seem to remember it being somewhere around 300 lb/in. If that were the case, raising the canister pressure from 125 psi to 200 psi, would change the force by ~250 lbs, which would raise the car by about 0.8 inches.

The result of this change in load with pressure is the shocks would be way too sensitive to heating and cooling, plus as the piston moves the gas is compressed or decompressed, depending on whether the piston is moving up or down. This change in volume would raise or lower the nitrogen pressure, changing the force. Something must be wrong with some of the assumptions here, as I can't believe that the shocks are that sensitive to environmental influence (i.e., hot day vs cold day)

Gas is one thing I understand quite well (as most people tell me I am full of the hot variety of it).

Orthonormal

Silvershadow, maybe tonight I will scan in and send you my calculations for the spring rate.

For the static force, the relevant cross sectional area is the shaft coming out of the top of the shock. The pressure throughout the inside of the shock should be the same as the gas pressure in the reservoir, shouldn't it? Anyway, the volume change inside the shock due to the shaft moving a distance dx is A * dx where A is the shaft area. So the force is P * A.

The spring force comes from how much the pressure changes when the shaft moves. The easy way out would be to assert that the process is isothermal and use the ideal gas equation to determine dP/dV. But I think that's cheating because I don't think that the gas in the reservoir has time to keep its temperature in equilibrium with its surroundings. In my calculations I assumed an adiabatic process (no heat is exchanged with the surroundings). I think that's closer to being correct. Over the course of long operation, heat may be gradually transferred to the oil and the casing, but not on the scale of individual piston movements.

Unfortunately, my result requires knowledge of the volume of the gas reservoir to determine the spring rate...if anyone knows that, it would be interesting to plug it in to my equations and see just how unreasonable the result is

Andy, did your estimate come from dyno testing at different pressures? That is of course the practical and empirical way to determine the correct relationship.

silvershadow

I agree adiabatic is the right assumption to make. I think we need the volume also.

I think I have a drawing of the inside of the canister somewhere - I need to dig it out, because I think that the volume change in the canister is dx (as defined above) * the piston area (not the shaft area) - just not sure - need to see how the inside of the canister is designed.

We can shelve that part of the discussion until we get confirmation of the design of the inside of the 8100 canister.

silvershadow

Look what I found

Penske manual

shaggy

My numbers are *very* fuzzy. They come from my recollection of a conversation with Andy McKee who was relating a conversation with Guy Ankeny. And you know what happens when people spread rumors...

I think you guys are on the right track. Gas is not my gig, but what you're saying rings a bell. Adiabatic... I haven't heard that word since my junior year in college. I hated Thermo! Give me statics and strength of materials problems any day!

Andy

shaggy

John,

If I were going to rebuild your shocks today, I'd add one shim to the stack. I'd add VW-2NX.008 as the base shim on the rebound side. I say this because all the funky hysteresis came about after removing VW-5NX.008 because I couldn't get enough low speed rebound to suit my fancy. At the time, I didn't know the two slotted bleed shim existed. If I did, I'd have swapped to it.

Andy