Another Ride Height Thread (Input Needed)

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Sep 19, 2007 | 05:30 PM

#46

ZzyzxM

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From: zzyzxmotorsports.com

Quote:

Originally Posted by Solo Evo

Stiffer springs will help up to a point. It's not like you can add another 100lbs and drop another inch, there is a limit.

Also, another thing to note, per Mitchell:

"When the chassis moves to place the instant centers near the ground the kinematic roll center moves rapidly laterally. When one IC is above ground and one below ground the KRC is outside the track. Before I understood the limited role of the KRC I thought this was bad. One way to avoid it is to keep the ICs either above or below ground."

What I take from this is that it's not crucial that your RC start above the ground, the real killer for roll stiffness is the RC transitioning through the ground plane under load, which causes one IC to be above and one below ground. If both were to start below the ground plane, and stay below the ground plane (read: stiff springs), it would be ok since approaching the ground plane causes the IC's to move rapidly.

Therefore, Chris, in your case, your springs weren't stiff enough for your ride height is what I'm interpreting from above.

Perhaps I am misreading, so again, anyone, feel free to correct.

Devin

You're correct in that the ICs are more "stable" either above OR below ground and not passing through the ground plane. I just did a scenario with a -3" ride height where the RC stays below ground. The change in FAP was only 3% for 4 deg. of roll - contrast that to the -2" model in the examples.

However, let's keep in mind that the COG is always above ground. So, by definition, any RC that remains below ground level produces a significant FAP-CG moment arm. Combine this with stiff springs and you have significant jacking forces working to overturn the chassis. If you've increased the spring rates to the point that it won't roll much, then you'll just end up picking the inside tires off the ground.

The point here is that keeping the COG/RC "FAP-CG" moment arm short is important. It's also important to keep the RC from significant lateral migration. This leads us to the conclusion that the RC needs to be reasonably above ground - or so it would seem.

Sep 19, 2007 | 07:49 PM

#47

Solo Evo

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Steve,

Thanks for the correction. Yes that would induce significant jacking forces. Now we only have to figure out how to place our CG below ground as well ...

Either way, just saying that the real killer is the ground plane transition.

Devin

Sep 19, 2007 | 08:47 PM

#48

Dave Mac

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From: Dillsburg

Quote:

Originally Posted by stimpy

Here are my ride height measurements from hub center to wheel arch:
Stock F=37.5cm R=35.5cm
Initial F=33.5cm R=32.5cm
Current F=35.5cm R=34.5cm

Honestly, I don't remember exactly how I came about my current height. I originally based my height on some suggestions in the STU thread. The car, just sitting, was dumped and I just had a hunch it was too low.

I later raised it up, corner-weighted it, and haven't looked back until I started thinking about roll centers, static camber, and rear grip.

I glanced under the car today and the control arms slope slightly downward from the chassis to the wheel. It is actually almost equal in inclination to the control arms of an e36 M3 on stock suspension.

-Jon

Jon-

I'm going to revise the lead post on the STU thread to reflect the current "better" data that we are finding here. I'll also link to this thread to help direct new STU class Evo builders. I'm higher in the front than I initially started with too.

Sep 21, 2007 | 11:27 AM

#49

DaWorstPlaya

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From: Denver, CO

Quote:

Originally Posted by ZzyzxM

The point here is that keeping the COG/RC "FAP-CG" moment arm short is important. It's also important to keep the RC from significant lateral migration. This leads us to the conclusion that the RC needs to be reasonably above ground - or so it would seem.

Steve wouldn't you say that verticle migration/increase is more of the issue than lateral migration because moment is always taken as a force acting perpendicular to an arm around a pivot point. If you are taking a force acting at an angle to the longer moment arm, to calculate the actual moment applied the vertical distance of the arm from the force which is acting on the car should be taken.

Acutual moment will be = FAP x moment arm x sine A (where A is the angle between the moment arm and a verticle line)

So in theory if FAP is constant and the RC doesn't move vertically but only laterally the acutal applied moment will stay the same ... regardless of where the roll center moves laterally, correct?

I do agree that the moment arm needs to be shortest possible but only vertically. When we move the the height lower the CG moves lower as well and so does the RC. Without being able to use the Whiteline RC kit to correct the RC, when is the verticle distance between them the shortest? Since this seems to be basic trig. could be be expressed in a linear model?

Jan 28, 2008 | 07:14 AM

#50

crystalhelix

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From: Pittsburgh

Sorry to bring this thread back from the dead but if anyone here would like to comment on this thread I am doing similar investigation with the STi (MY04-07 models).
http://forums.nasioc.com/forums/show....php?t=1419481

I worked on this chart over there as well and modeled the suspension in Solidworks.

For a MacStrut..