Control Arm Alignment (No Pun Intended)

[TheJet-09]

For arguments sake (and possibly fact, either way), let's say the motor plate is square (90* or "perpendicular") to the chassis centerline. If you draw a line through the lower control arm mount (heim that bolts to the crossmember) and the front strut mount (heim that bolts to the front of the chassis), shouldn't that line be perpendicular to the motor plate (as viewed from above)? And assuming so, shouldn't a line drawn through the upper control arm mounts (or directly through the center of the upper control arm shaft, front to back) also be perpendicular to the motor plate (but not in line vertically with the lower control arm mounts due to obvious differences in mounting locations/control arm lengths)? As my car sits now (and it has had repairs) the distance between the "front" upper control arm mounts is about 1/4" less than the rear mounts (toed in). Wouldn't this introduce caster gain (or at least change) through suspension travel?

[Matt49]

Absolutely. Take an extreme example. Say your lower control arm is "square" with the chassis as viewed from above. And say your upper control arm is pointed inward 45 degrees when viewed from above. As the lower control arm moves in compression the ball joint will move straight up and down. As the upper arm moves in compression the ball joint will move up and back. This will give you caster gain.
Of course my example assumes that the arms are also horizontal and parallel when viewed from the side. If they are not, you are introducing additional potential for caster gain AND anti-dive. Anti-dive is an entire different discussion but at least worth noting.
I've gotten into measuring caster gain and became increasingly frustrated with the method used to measure caster. It's very hard to reproduce and that's largely due to its inaccuracy. So I made a custom little gadget that thread between the the stems of the ball joint to form an edge perfectly parallel with the center lines of the ball joints. I've learned a lot using this little gadget.

[TheJet-09]

Thanks Matt. I've only read where caster gain is bad but haven't heard much else on it. I think it would be hard to reproduce in a static setting (at the shop) in order to measure, other than simply moving the suspension through a variety of positions and checking caster. Increasing caster through compression on the RF would also seem to add at least some wheel rate (jacking effect), almost like a progressive spring. But undoubtedly hard to measure and tune.

[Matt49]

I think most people would agree that minimizing dynamic caster change is a good thing. It could lead to some real inconsistencies that are hard to track down. But if you ever go messing with anti-dive, caster gain is an inherent side effect.

[Kromulous]

Not sure about caster gain, but I always preferred more caster.

It has a jacking effect naturally.

[Matt49]

Not sure about caster gain, but I always preferred more caster.

It has a jacking effect naturally.

I think this depends greatly on track configuration and drive preference. We experimented extensively last season with higher than normal caster and found some unwanted side effects related to the loading you are referring to. Most notably, an inability to plant the RR tire when back steering. The effect basically was the more you turned the wheel to the right, the more the RR unloaded which made a loose condition just get looser when trying to correct...if that makes any sense.

[save the racers]

What is alot of caster?5 to 8 degrees?

[95shaw]

Although this article is written about autocross and road racing, interesting thoughts about KPI and caster/camber, and how to arrive at caster to run and why.

http://www.lateral-g.net/forums/showthread.php4?t=42467