Naka

[QUOTE=Kaolinte,Oct 25 2006, 07:00 PM]Stick with buying tires.

Naka

[QUOTE=Windscreen,Oct 24 2006, 06:50 PM]The down side for stock class autocross is that large shaft area leads to increased lifting force due to the shock gas pressure (lifting force per corner=pressure * shock shaft area).

Windscreen

Nope, Naka, you're confused, or at least not understanding how a monotube shock works. With a monotube design, when the shock moves, the pressure of the oil on one side of the piston will drop. To keep from dropping to less than atmospheric and having cavitation of the oil, the oil is put under pressure with a nitrogen gas charge. The nitrogen is kept separate from the oil with a floating pistion (but that is inconsequential for this discussion).

A knock on effect of the nitrogen pressure is that the pressure forces the shock shaft out of the shock body. The force pusing the shaft out of the body is the cross-sectional area of the shaft mulitplied by the nitrogen gas pressure. Therefore, for a given gas pressure, a larger diameter shaft will have more force pushing it out of the shock body.

Steve

Naka

Excellent write ups Steve. I'm learning so much. I appreciate your patience and willingness to teach.

I am no engineer, by no means. So I get a little confused trying to understand the dynamics of the damper. However I find it to be quite fascinating.

My original question remains, though. Based on your following statement:
Could I re-phrase it saying: "for a given force pushing it out of the shock body, a larger shaft diameter will have less gas pressure"?

Orthonormal

Yes, that's how the equations work. However, you don't really have a choice on the gas pressure. There is a certain minimum amount required to prevent cavitation. 100-150psi seems typical. So then you choose your shaft diameter, and you can calculate the resulting static force.

Naka

Are Moton shocks not able to give low enough static force (lifting force) to keep the car at the normal ride height, without dropping its internal pressure low enough to cause cavitation?

What is confusing to me, is how two different shocks, while providing the same lifting force, will give different ride heights (assuming the installation is "legal").

jguerdat

Sorry, but I've seen folks run on "old" tires quite competitively. You (and I) are not at the top level of competition. Your times are going to be be more affected by your driving than whether you have "obsolete" or "wrong" tires. It's nice to be able to afford the latest hot stuff but they likely won't make you faster.

If you do still want to always have the newest tires and still have your old ones, I'll be more than happy to buy your old ones. At a healthy discount, of course...

Windscreen

That's the crux - the two different shocks, in this case Moton and Penske, do NOT give the same lifting force. Assume you have to run 150 psi gas pressure in both shocks to prevent oil cavitation (a good assumption, BTW). The Moton has a 0.866" shaft diameter and Penske 0.620". Using F = p*a, you will see the Moton tries to raise ride height with a force of 88 lbs per shock and the Penske 45 lbs.

Does that clear things up? The strange part is that we stock class auto-xers are the only ones that care about this issue. Anyone else just compensates by lowering the ride height a touch.

Steve

Naka

Mmmmm.... Very interesting. Thanks a lot for clarifying Steve.

Now, yet another question: You brought Ohlins to the equation. I didn't even know they made ones for the S. What's your take on them?

Windscreen

I don't have any direct experience with Ohlins, other than my shock builder highly recommends them. A few years back he even told me that between Penske and Ohlins, he felt Ohlins was the more user friendly shock. Not necessarily better or worse, but easier to dial in.

That said, I don't know anyone at the National Solo level that is running Ohlins, so you'll be on your own to develop them.

Steve