Pre-88 rear susp. geometry

[Atilla the Fun]

The '88 cradle is a lot better in regards to the andi-dive issue, but the supply is somewhat limited to what GM built and what a few enthusiasts are fabricating. For the rest of us, is there any chance that we could find benefit from maybe moving the lower control arms either an inch forward or an inch rearward? Or how about combining that with swapping the rear suspension side-for-side? Then the knuckles would be turned front-steer style, so make new brackets for the tie rods, which would also give opportunity for an alternate remedy for bumpsteer.

[eHoward]

Will's talked about that. I chose to buy an 88 cradle and needed suspension pieces five or six years ago and store them until I'm ready for them.

I think your step 1 would be taking measurements.

Step 2 would be inputting the measurements into a model and charting a range of movement.

Step 3 would be considering alternative suspension mounting points and running them through the model.

and the last would be trying them.

Good luck.

[cactus bastard]

Step 1 was to ask if anybody else has already gone through the other steps for you :thumbleft:

[Atilla the Fun]

It seems step one is to ask Will what keywords to search, since he has already gone into this but I have yet to find anything similar. how about it, Will? If no reply in a couple of weeks, I'll PM.

[The Dark Side of Will]

The early rear suspension is basically fubarred. It has a LOT of problems.

-Bump steer
-Pro Squat
-Roll center too low
-Poor camber gain

There is no *easy* way to fix any one of these, and to fix all of them would basically require a clean-sheet design.

[Atilla the Fun]

the camber gain is a simple matter of adjusting the spring rate, shock damping, and inboard mounting height for the lower control arm so that the arm is parallel to the ground at maximum body roll. So, if that's getting moved anyway, and the bumpsteer will be adressed, how about moving the arms fore or aft, or swaping them side for side, or both? Or am I on my own here?

[The Dark Side of Will]

To do any meaningful fix, the forward pivots of the control arms need to be raised. This can't happen to any significant degree because the left one is directly under the transmission.

Spring rates and damper settings don't actually address the camber gain. They simply restrict suspension motion to a range with acceptable camber characteristics.

[Atilla the Fun]

Then the forward pivots will be raised. This is the sort of info I was hoping for. Thank you Will! Now, can anyone give an educated opinion about moving them forward or rearwars? Back there, I'm inclined to have the struts truly vertical, to kill any remaining bump steer, unless anyone has a better idea.

[Fastback86]

To do any meaningful fix, the forward pivots of the control arms need to be raised. This can't happen to any significant degree because the left one is directly under the transmission.

Spring rates and damper settings don't actually address the camber gain. They simply restrict suspension motion to a range with acceptable camber characteristics.

Would lowering the rear pivots produce the same effect (ignoring ground clearance issues)? Is it any more workable?

[The Dark Side of Will]

Lowering the pivots would hurt the camber curve.

The Indy Fiero (the actual pace car) used wedge shaped rear cradle bushings to reduce pro-squat. The effect could be increased with eccentric front cradle bushings.

[Atilla the Fun]

In that case, I'm more inclined to try extra rith rear cradle-bushing spacers, to let me fit the truck LSX intake under the decklid easier. I learned something important yesterday. You can't run the camaro LS1 intake manifold turned backwards, with a Muncie, because theTB will interfere with the shift linkage/bracketry. But you can run the camaro intake feeding from the passenger side, the TB will clear the strut tower, and you can fit the truck intake reversed and have the TB clear the shift linkage/bracketry. So, any thoughts on this proposal? And I like the idea of dropping the rear of the control arms, thank you whoever thought of that.

[The Dark Side of Will]

Dropping the rear arms will hurt the camber curve, while helping to reduce pro-squat.

The strut can't go vertical because of the tower being offset to the rear of the axle centerline... the "kingpin angle" doesn't have jack to do with bumpsteer anyway... that's on the toe link.

[Atilla the Fun]

then raising the front would also hurt the camber curve. As to the bumpsteer, I see you're correct. I should've gave it more thought before typing. I just dropped my cradle this morning, and did notice the struts are not vertical.

[The Dark Side of Will]

The control arms are level at factory ride height. Raising the pivots would help the camber curve.

[Atilla the Fun]

once again, Will is correct. I did some mock-up work this morning. However, my findings lead me to believe that if the rear is not dropped more than the front is raised, the camber curve gets no worse than stock. I'll have to try mocking up the extra thick bushings between the rear of the cradle and the rear of the chassis.

[The Dark Side of Will]

I think I mentioned above the possibility of making offset front cradle bushings to raise the front of the cradle. Combined with thicker rear cradle bushings, these might help the pro-squat problem significantly without making the other aspects of the rear geometry any worse than the already are (although the change in caster curve might have a tiny effect on bumpsteer).

This is one of the things I was planning to try on The Mule. Since the topside clearances are so tight with a Northstar, I'd orient the new cradle bushings so that the cradle would be rotated around a line close to the Northstar throttle body. Without that space constraint, the more ideal orientation could be used in which the front of the cradle is raised straight up and the rear dropped straight down.

I still think that raising the front pivot of the left control arm without taking a chunk out of the transmission would require a clean sheet redesign of the suspension. What Toyota did with the Mk III MR2 was to have a trailing arm, a single lateral link directly underneath the axle, and a toe link about the same level as the axle.

The packaging compromises on a suspension that has to have a transverse powertrain in the middle of it are very interesting. The early FWD cars on which the early Fiero rear is based have MUCH more kingpin inclination than GM's later FWD cars. The later FWD cars have less kingpin inclination to make more room inside the engine compartment, both by spreading the strut towers and by reducing the "swing" that the tire goes through in the full range of steering angles, which allows the frame rails to be further apart. Consequently, A-bodies and Fieros have superior camber curves to modern W-bodies and the like.

[Atilla the Fun]

Okay, I am positioned to take the plunge and drill new holes in my aluminum cradle bushings, under the existing ones, but it occurrs to me; how do I keep the bushings from rotating? I'll try something and post pics.

[The Dark Side of Will]

Personally, I'd remove the bushings and do the deed in a mill.

[Atilla the Fun]

that would create a challenge with the "clocking", so I drilled, tapped, and bolted the bushings to the cradle. Monday morning I'll drill the 1/2" holes. It got too hot outside to do it this afternoon.

[Indy]

If your bushings are like mine, they don't "clock" in to any position upon install. You can backdrill through your cradle and into your bush to set the position once you're happy after you've already drilled a lower position in the bushing.

Anyway...What's the ID of the bush? Just had a dirty thought about an eccentric bushing for adjustment so you could move over a greater range and keep the cradle in the right position front-back.