stimpy

iTrader: (3)

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

ZzyzxM

iTrader: (1)

Ok guys... turns out I was wrong and reversed the relationship.

That said, I figured I'd better put something together for you and get things straightened out.

Here's a document I put together that has 8 figures to peruse. Let me know what Qs come up and certainly let me know if you think there's an error in the document.

http://zzyzxmotorsports.com/library/...h-20070918.pdf

dannykao

iTrader: (6)

Thanks Steve for your informative write-up.

I will try to increase my front ride height some to test it out.

This thread brings out the geek in all of us, and it's great!

Solo Evo

iTrader: (7)

Steve,

Thank you so much taking the time to write that up. It's always nice to turn conjecture into hard facts. Much appreciated.


Jon,

You may want to measure your LCA inclination as at an ~36.5cm ride height in the front, my LCA is now parallel. Granted, my measurements on the LCA may be incorrect, but just thought I'd let you know. I did a couple quick glances as well for mine and I thought it looked parallel, but when measured, it was not.




Devin

Silencer

iTrader: (4)

Thanks Steve!

Looking at your data, it shows an RCH and IC movement reduction utilizing the camber plates to their full inward position at a specific LCA angle with 4 degrees of roll. However, figure 8 is very interesting with the drop of 2" which positions the LCA at a downward angle towards the center of the car. Can I speculate that moving the RC to the inside of the corner is a benefit while the significant drop in RCH is not? Can I get your opinion on the offset of one over the other?

Dave

DaWorstPlaya

iTrader: (31)

Solo Evo: Can you tell us the ride height (from wheel center hub to fender) after you set the LCH to be parallel to the ground? Stock ride height I believe is 15", I'm guessing the drop would be 0.5" - 0.75"

ZzyzxM

iTrader: (1)

To address your question, I think the following reading material is required:

http://zzyzxmotorsports.com/library/...nd-reality.pdf

I'll quote one particularly relevant section:

This introduces a new concept - the Force Application Point (FAP). You'll note the left and right FAP values in each diagram in the document I put together.

From the paragraph above, he states that we want to minimize lateral movement of the "KRC" = kinematic roll center = roll center and that change in FAP heights is a more direct way of measuring the FAP-CG moment arm, and should be minimized.

Sum up the pre and roll FAP values and take the difference. Now, compare that to RC movement. You'll notice a direct relationship between the difference of total FAP static (no roll) / during roll, and the movement of the RC ("KRC").

The following are % diff between static and roll total FAP for each scenario:

Figure 2: 2.1%
Figure 4: 17.7%
Figure 6: 1.5%
Figure 8: 33.9%

Which states:

Figure 6 > Figure 2 > Figure 4 > Figure 8

My personal observations tend to support this conclusion.

chrisw

iTrader: (2)

that's great stuff steve! thanks for sharing.

Solo Evo

iTrader: (7)

The heights are in the first post of this thread. I will again check the heights this weekend and report back my findings to this thread. All of the measurements this weekend will be taken on a completely flat surface with multiple measurement instruments on the control arms and mounting points.

You are correct, though, as I found the drop to be roughly more than half an inch (14.7mm - 17.8mm to be more exact, again, I give a range because fender measurements aren't that exact).


Note: My original measurements were taken on a flat surface by my measurements, but this weekend, the surface I am using I know is flat.

Devin

ZzyzxM

iTrader: (1)

Note that I changed the location of the files, so I can better organize them. I added up a Tech section to the website so you can grab them from there:

http://zzyzxmotorsports.com/tech/

... should have done that a long time ago, I'll keep adding articles there that are informative.

Jeff_Jeske

iTrader: (66)

So a rule of thumb is max lowering should make front lower control arm parallel to the ground?

Solo Evo

iTrader: (7)

Not necessarily. Parallel to the ground is a good starting point.

Depending on your track width, camber, spring rates, etc. your LCA's can be sloped downward from the wheel to the chassis and still accomplish an acceptable roll center. Because some of us STU guys aren't running really stiff rates, we don't have much leeway under parallel LCAs. The stiffer your rate, the lower you can go since the amount of roll on the same turn will be less than with softer springs. Therefore, since your suspension is moving less, the roll center won't migrate as much, allowing you to run lower on the car an not have the roll center cross the plane of the ground (considering you started above the ground to begin with).

Takes alot of calculation or a 3D suspension program to really zone in on what your height needs to be for your own personal setup. Other than that, it's alot of test and tune.

Please correct me if I am wrong.

Devin

DaWorstPlaya

iTrader: (31)

Well said Solo Evo ... A lot of the open track guys who don't follow any classing are quick to point out the use of Whiteline Roll Center Correction kit to help move the LCA to point in the correct orientation after lowering the car. Those of us who follow the SCCA Solo 2 rules are limited to allowances of the class. As Devin said with stiffer springs you can go lower as your car won't roll as much ...

chrisw

iTrader: (2)

stiffer springs won't help (been there done that) because you are not making any adjustments to the basica suspension geometry. But depending on the class rules the whiteline roll center kit does help, even at the stock suspension height.

Solo Evo

iTrader: (7)

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