Upper Control Arm Location Question

[gimp]

Background

1981 Chevy Camaro being prepped for C-Prepared

• I will be running a 12" wide wheel with 5.5" backspacing. Tires will be R-compound radials.
• Tubular lower control arms, stock length, with screw in ball joints to adjust roll center.
• Going to a coilover setup, so spring pocket will be cut off flush and frame boxed back in and the factory upper control arm mount will be removed. Coilover mount on the lower control arm is right next to the ball joint, upper mount location will be to the front cage members.
• Upper control arm will have a screw in ball joint, and adjustable rod ends (for camber/caster adjustment) instead of the traditional mounting system. The new mounts on the rotational axis of the control arm will be set back a little to improve caster.

My question relates to the location of the upper control arm pivot axis when viewed from the front of the car.

Is there an advantage to moving the control arm pivot axis inboard or outboard on the frame? As an illustration, let's just say that the stock pivot point is in the center of the frame member. Is there an advantage to moving it towards either edge of the frame rail? Assume that the upper control arm length will be adjusted to maintain the same static negative camber setting through all possible configurations.

I don't have any suspension software, so for now I'm drawing on graph paper and playing with arbitrary numbers. Let say that the base configuration is a lower control arm that is 100 units (made up number) long, and an upper that is 65 units long (this is using a upper control arm that is 50-80% of the lower length rule, and splitting it down the middle). The static camber will never change in the drawing, only the mounting point on the frame (either 10 units in or 10 units out) and the upper control arm length to compensate for the relocation of the mounting point. The mounting point will not change vertically.

Doing a quick drawing, it looks like moving the point of rotation towards the outside of the frame rail will shorten my swing arm length by 18% and shorten my control arm 14% (changing the length from 65 units to 55 units). I don't think this is desirable, even though you get a shorter control arm because the swing arm length is made longer.



Drawing again, but this time placing the axis of rotation inboard (closer to the engine). The swing arm length now increases by 18% and the upper control arm gets 18% longer (changing the length from 65 units to 75 units). Swing arm length is pretty good now, but the upper control arm is longer. I know a longer swing arm length has its benefits, but is it worth the additional upper control arm length?

Just looking for some opinions. Should I move it, or leave it stock?

Thanks!

[Norm Peterson]

The shorter swing arm length will give you faster camber gain, which among other things gives you an idea where you'll end up setting your static camber. Roll center height is affected slightly (normally it's higher with a shorter swing arm as long as the lower arm inclination is held constant). But keep in mind that it's entirely possible to have too much of a (sort of) good thing. Excessive camber change under braking, extra bumpsteer and lateral scrubbing are likely consequences.

Static position only tells you part of the story. You need to see how everything changes once the suspension starts moving in response to bump (during braking) and roll (cornering).

Without suspension software, you can either draw your suspension in various positions or make a pin pivot cardboard model to see how the various things change as a result. A halfway step past that might be a 2-D Excel solution for roll center, FVIC, and camber if you're comfortable with creating spreadsheets and have a bit of time on your hands.


Norm