84-87 vs 88 front suspension (Page 6/10)
LornesGT FEB 12, 12:35 AM

quote
Originally posted by Bloozberry:

The kick-back in the steering is due partly to the scrub radius and partly due to the amount of caster. Since the caster settings between the '88 and earlier cars is the same (+5 deg), then the difference in kickback must be due to the difference in scrub radius.

Scrub radius is the distance between the centerline of the tire contact patch and an imaginary line extended to the ground drawn between the upper and lower ball joints (king pin angle), as viewed from the rear of the car. The larger the distance between those two points on the ground, the greater the kick back. Here's a drawing of an '88 to illustrate what scrub radius is:



On the '84-'87's the scrub radius is 47 mm with the P185/80R/13 tires and is probably several mm's more with the later 14" Hi-Tech and 15" lace wheels. On the '88's, the scrub radius with the stock 15" lace wheels is only 40 mm's as shown above. That's a 15% reduction in scrub radius so it surely would be felt. It's also the most likely reason why GM left the steering damper off of the '88's.

The reason a larger scrub radius gives more kickback is because the tire pivots on the ground not about the center of the tire contact patch, but rather around the point on the ground drawn between the ball joints. When you hit a bump though, the force acts through the center of the tire contact patch, so the further the center of the patch is from the actual pivot point, the longer the lever arm trying to wrench the wheel around the pivot point.

To improve the kick back in earlier cars, the trick is to find wheels with a greater offset than stock. That's part of the way GM reduced the kickback on the '88's. Higher offset wheels move the center of the tire contact patch further inboard, closer to the ball joint line reducing the scrub radius. But everything is a compromise. Higher offset wheels will tuck under the fender further and most people want a wider looking stance, not a narrower one. By playing with wider rims and offsets though, you can achieve a reduced scrub radius and still maintain the sidewall at the outside edge.

(Edit for clarity)




So Rodney's Lowering ball joints raise the upper control arm angle decreasing the scrub radius correct?
Bloozberry FEB 12, 06:41 AM
Edit: see Will's explanation below.

[This message has been edited by Bloozberry (edited 02-12-2015).]

Will FEB 12, 07:01 PM

quote
Originally posted by LornesGT:


So Rodney's Lowering ball joints raise the upper control arm angle decreasing the scrub radius correct?



No.

AIUI, the actual ball, and therefore pivot, moves up relative to the control arm. As we discussed above, kingpin angle is fixed relative to camber. the locations of the pivots don't change relative to the knuckle. Have the car realigned to the same static camber as before the lowering ball joint installation and you won't change the scrub radius any.

[This message has been edited by Will (edited 02-12-2015).]

wftb FEB 19, 07:24 PM
Is it possible to use a front 88 spindle as a rear spindle ? Will an 84 to 87 rear hub and bearing fit in to an 88 front spindle ?
olejoedad FEB 19, 08:03 PM
No.
And no.

[This message has been edited by olejoedad (edited 02-19-2015).]

fieroguru FEB 19, 08:52 PM

quote
Originally posted by wftb:

Is it possible to use a front 88 spindle as a rear spindle ? Will an 84 to 87 rear hub and bearing fit in to an 88 front spindle ?



Sure its possible in theory, but the real question is if it is strong enough. There isn't much excess material in its design.

Nashco modified the 88 front uprights to accept a rear wheel bearing for his AWD/Hybrid fiero setup, but it isn't a bolt in install.

jediperk FEB 20, 09:32 PM
This was a highly educational thread. I'm going to have to re-read this one though. For some reason I have always had a hard time with wrapping my head around suspension geometry... Thanks for everyone's contributions and a special thanks to Will and Blooz!
Knight MAR 05, 08:30 AM
The Arraunt (formerly Held) drop spindles use a 84-88 rear bearing on a custom upright.
mender NOV 24, 10:20 PM

quote
Originally posted by Will:


Brake dive can be basically eliminated, but at the expense of reducing caster to almost nothing.


I'm going through my front suspension on my '87 GT to optimize the geometry for racing. My mod for anti-dive is a lot simpler, just involves slotting the lower control arm pivot points to move the front of the arm down and the rear of the arm up. No change in caster and the anti-dive went up to about 30%.

Big improvement under braking; before the tires would tuck up into the wheel wells enough to rub through the plastic fender liners. The camber change in the last bit of compression was excessive enough to cause the fronts to wash out. Now with the anti-dive the nose only drops about 1 1/2" despite the sticky tires and hard braking. Tire wear is much better as well.

I've moved the upper ball joint to get the static camber that I want. I'm going to move it back to get more caster, then weld in a plate to reinforce the control arm. Final step will be to move the rack to minimize the bumpsteer that will result from the geometry changes.

[This message has been edited by mender (edited 11-24-2017).]

pmbrunelle NOV 25, 02:14 PM
Did you do the spherical bearing mod to the front wishbones as well?

Actually, from the factory, the pivots didn't line up...