Originally Posted by Servo on Sep/06/04
Holy crap, Dave! I wish I had something more intelligent to add, but the only thing most of us are capable of is to ask questions that you probably answered for yourself months ago.
Here's my question... I seem to remember a couple rwd projects in my local area that were demoted to "drag race only" duty because the suspension geometry couldn't handle the new weight balance, power-on characteristics, or whatever. I'm curious how well using the suspension geometry from the front of another car will work. How do the upper/lower control arm lengths/angles, wheel rate, etc. compare to, say, your MR2? Do you expect to have problems here or instability to dial out?
I had the same curiousity as you when I started thinking seriously about this project. The first thing I did was measure and draw to scale the original Prelude front suspension. (Actually, the very first thing I did was to spend many hours reading and studying "Race Car Vehicle Dynamics" by William F. and Douglas L. Milliken, and the terms I'll use here are from that book.) From these front view and side view drawings, I was able to determine the front view instant centers (IC) and roll center and side view instant center and caster. I also measured the bump steer, which was "not too good". If I used the Prelude front suspension geometry as originally designed, the stability would have been questionable and drivability challenging. The original Prelude front geometry had the side view IC behind and below the wheel center, which created pro-lift and rearward wheel path in bump. The roll center was 7.5 cm above ground. Static caster was around +2.5 degrees, however the upper control arm (UCA) pivot axis caused caster to increase with bump travel.
The changes I've made and their affects are:
1) Raised lower control arm (LCA) front mounting point to move side view IC above wheel center to increase anti-squat and to keep wheel path rearward in bump.
2) Moved UCA forward to zero static caster.
3) Shortened UCA to increase camber gain without affecting roll center.
4) Lowered front UCA mounting point to increase camber gain, bring UCA pivot axis near parallel with LCA pivot axis to minimize caster change with wheel travel, and set the side view IC ahead and above the wheel center to set the anti-squat and wheel path. (This change also raised the roll center by 2.5cm.)
5) Lowering the front UCA mounting point in effect rotated the UCA counter clockwise as viewed from the side, so I had to rotate the upper ball joint clockwise in the UCA to realign the ball joint vertically.
Ride and roll rates will be controlled with springs, shocks (adjustable coil-overs), and anti-roll bars. Corner weight will be set at 50/50 with ride height adjustments. I'm planning on 5" ground clearance. Wheel rates will have the same proportion to spring rates as the installation ratio has not been changed. Negative scrub radius will increase slightly with increase in static camber from 0 to -1 degree (increased kingpin inclination). Scrub should be about the same because even though the front view swing arms (affective swing arm pivoting at IC) have been shortened, the front view ICs have been lowered. Static toe will start at 1/16" total toe in and bump steer will be set to maintain that toe as closely as possible and such that no toe out occurs.
Your point about "instability to dial out" is well taken and I've made both upper and lower control arm mounting points adjustable, however I believe what I have is pretty close. I won't know for sure until I've driven it, taken tire temps, and looked at tire wear patterns.
The front suspension is another story. The Prelude uprights have the wheel center ahead of the kingpin axis which reduces mechanical trail. This isn't necessarily bad as it increases the proportion of pnumatic trail to the steering feel, however I may have to increase the caster for stability. The Integra has very little caster to begin with (about 1 degree). Also, the steering arms are slightly longer, which affects steering ratio and ackerman.
Not much I can do about the steering ratio without changing the rack ratio, but I may have to shim the rack for ackerman and bump steer.