Suspension geometry calculation - Chevy Impala SS Forum
 
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post #1 of 5 (permalink) Old 03-02-2004, 01:17 PM
shiggins
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I am working on figuring out the geometric and force rolls centers for the front suspension in an effort the nail down the a-arm geometry I am considering building. I can and will do some measuring (after I determine ride height with new springs), but does anyone have numbers from there stock suspension and numbers from their desired (optimum) setup? I will share what I come up with ASAP.

Some of the information I need is as follows:

Rolling radius of tire (known)
static camber 0deg
centre of wheel (known)
contact patch (known)
Lower control arm to body (need)
Lower control arm to spindle (need)
Upper control arm to body (need)
Upper control arm to spindle (need)

thanks,
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post #2 of 5 (permalink) Old 03-02-2004, 01:36 PM
bengston
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Not exactly what you ask for, but... From a link in a recent post;
Quote:
JK,
Use the recommended specs from the IGBH website in conjunction with the
1.5in lower ride height. You could go to 1/2 deg. negative camber (that's
what I currently have), but there will be a slight bit of inside tire wear:
rotate the tires often. The -1/4 degree camber is a good compromise for
maximum tire life. Positive caster should be maximized, and there are no
negative effects, (unless you had manual steering). Five degrees positive
is commonly used on heavy, front engined racing cars, NASCAR,Trans Am, etc.
for road racing applications. A good compromise for the caprice (while
staying within service specs.)is 4.5 degrees positive with no cross caster.

I have taken measurements on my 95 9C1, inputted them into the Performance
Trends Roll Center analysis program, and made the following observations.
On a stock ride ht. 9C1 the roll center appears to be about .75in below
ground level. The camber gain appears to be a positive 1/2 degree per inch
of dive. I have verified the positive camber gain on the car with a camber
gauge. These numbers are by no means optimum. In a heavy front engined car
like the Caprice the roll center should be about 3-4 inches above ground
level, and the camber gain should be at least negative 1 degree gain per
inch of dive. For comparison to a street car with a well designed
suspension: a 63-83 Corvette has a roll center ht. of +3.25in. and a camber
gain of about -3/4 degree per inch of dive. The 84-97 Corvette(C-4)has
about a +2.9in roll center, I don't know the camber gain, but it is
probably similar.

Lowering the car will lower the roll center, but as it also lowers the
center of gravity: the roll moment will be about the same, so it will be an
improvement. Lowering the car will also decrease the angle at which the
outer ends of the upper control arms angle(down), and this will decrease the
positive camber gain in bump and roll, another good thing.

What is really needed is to raise the roll center, which would decrease the
roll moment (distance between cg height and roll center)decreasing roll and
increasing responsiveness, and this would simultaneously improve the camber
gain situation. Can you accomplish this without totally redesigning the
front suspension with new components and/or new chassis mounting points?
Yes: Stock Car Products sells x-tra long upper ball joints that have the
effect of raising the outer upper pivot point of the front suspension up an
additional inch. Along with lowering the front end 1.5 in, this
modification would point the outside ends of the upper a-arms up rather than
down, expediting a roll center height of around 2-2.5in along with a mild
negative camber gain curve. In addition, if the two upper a-arm inner mount
holes could be redrilled about 1/2in lower than stock than the roll center
would be raised to about 3.5in, and the camber gain curve would be around
-1deg. gain per inch of dive. Is there enough room on the a-arm mounting
tower to drill the holes 1/2in lower? I am not sure, but the extended ball
joint mod would be a change in the right direction. Of the two
characteristics: roll center height and camber gain curve, the camber gain
curve is probably more critical to ultimate cornering, especially for a
heavy front-engined car. Even with a low roll center the roll of the car
can be controlled with large sway bars. There is no easy fix to the camber
situation, although large amounts of static negative camber may help.

One last thought. If you stick with the stock design and take the car to a
road course, it would be beneficial, just for the day or week of the race,
to crank in allot of negative camber; probably around 2 to 3 degrees. This
would help compensate somewhat for the positive camber gain curve. Straight
line braking performance would be hurt to some extent, but you would gain
much more in the turns than you would lose in the straights. This much
camber would quickly wear the inside of your tires out on the street. When
you have the car aligned, you can have the technician align the camber to
two different specs. and he could hand you the extra shims to put on at the
track to increase the negative camber. A tire pyrometer would help at the
track to make the best compromise.

Owen Saucyn
post #3 of 5 (permalink) Old 03-02-2004, 02:01 PM
shiggins
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Good information Wayne. I don't like the thought of using the upper Spindle Spacer . I saw the 1" longer ball joints in your response in another thread as well. While I am working on the a-arm design, I will see if I can accomocate the mounting point change that would give the desired ~2" upper ball joint change that is desirable.

Keep it coming!

thanks,
 
post #4 of 5 (permalink) Old 03-02-2004, 02:25 PM
bengston
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Keep in mind that the additional height needs to be on the spindle, and not on the control arm.

A longer spindle, longer ball joint, and a spindle spacer, all accomplish the same thing, which it to move the pivot point of the ball joint up. Adding a spacer to the control arm will not change the geometry of the system at all.
post #5 of 5 (permalink) Old 03-02-2004, 02:37 PM
shiggins
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I understand and agree. I also saw someone with a missunderstanding of this issue in another thread.

thanks again,
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