roadster front suspension loook-see

i may, or may not, tweak the front suspension geometry. with a lot of RTFM and trial and error and experimentation with the amazing free software at i worked out that the notorious rambler understeer built into the front can be vastly lessened. in stock form, with all new components and adjusted to as close as i could get to zero camber, the static Roll Center (RC) heigh is about half an inch below ground, and for each degree of body roll RC shifts 14" to the side. since the upper and lower arms are nearly parallel, the instant centers approach infinity. obviously the center of gravity for these cars is high off the ground (assuming it's around camshaft height, or about 18"), and so in a turn the car's mass pivots on the RC, causing a lot of lean. the rambler suspension's spring directly over the knuckle is weird, but builds in a lot of desireable anti-roll (unlike GM types with the spring on the lower arm, where in a roll the spring knuckles under).

the good news is that by simply lowering the upper A-arm's inner mount the roll center raises well above ground level, and more importantly, the lateral shift plummets. it will still be an understeer car, but vastly reduced. raising front RC lowers longitudinal chassis (unibody) stress by making the roll axis (chassis lengthwise) more level (the rambler's in no danger of having too high a roll center here...)

dropping the upper A-arm inner mount by 2" raises RC to 3.6" above ground but most importantly the RC shifts to the side only 2.4" per degree of body roll -- instead of 14". lowered 3" RC rises to 5.7" and shifts only 1.25" per degree roll.

that's easy to do in software, how about in metal? lucky me -- the convenient peculiarity of the pocket design of the front suspension A-arm pod makes this easy. first, the entire Nash suspension is modular, the hard parts bolt to a stamped pocket that is welded into the unibody inner chassis. the pocket is curved for the upper arm's motion, downward then outboard. simply re-drilling two 0.5" holes further down the pocket both moves the arm down and outward -- exactly what is needed. it's the exact same steel assembly. there's no change to stress or margins.

the amount of lowering/outboarding that can be tolerated will be limited by arm length plus camber adjustment that leaves camber at 0 to -1 degree. all of my measurements are a combinatino of on-car and of complete suspension units on the iron pile. since this looks feasible the next step will be to get precise chassis measurements, drill a 1/8" test hole to check for clearances and strength (stamping) issues and measure from there to make sure i can get camber back in range without having to modify the arm itself. if that's necessary i'll probably just make an upper arm from race car parts.

excellent discussion of "race car" suspension details here