'84-'87 Rear Tie Rod Mounting Location to Reduce Bump Steer

[pmbrunelle]

I figure that the 84-87 Fiero makes for a better project car than the 88, by virtue of it being a bunch of cobbled-together hardware.

There's more room for improvement, and therefore project potential.

[CaptainHindsight]

So solid bushings for the cradle and a tri link all in the same plane for the '88.

[ericjon262]

So solid bushings for the cradle and a tri link all in the same plane for the '88.

Based on the drawings in Blooze's thread (linked below) I don't think that assessment is accurate.



the green line is drawn from approximately center of the forward trailing link, to approximately center of the rear lateral link, note the line passes over the top of the center of the middle of the forward lateral link.

https://www.fiero.com/forum/Forum3/HTML/000116-13.html

[The Dark Side of Will]

The rear 84-87 cradle and suspension is really bad. You should look into transplanting an '88 cradle and suspension. Especially if this is your daily.

he rear 84-87 cradle and suspension is really bad.

What is so really bad about it? The toe changes slightly near the end if its suspension travel with the factory bushings and tie rod mounting locations.

I can argue that whole car is an under powered and twisty and needs a proper frame, wheels, tires and drive train.

Everything. It's a FWD cradle flipped 180* and mounted backwards. It was never designed to be in the back of a car, much less a sports car. The difference between the older suspension and the newer is night and day. My little brother used to own an 86 GT that had a lot of stuff done to the cradle including solid bushings. When I would drive his car after driving mine it felt like a minivan.

You don't have to, entirely your decision. And I'm sure at this point in time finding an 88 for part out would be very difficult. But you can spend a ton of money and time on the older suspension and it won't be as good as the newer stuff right out of the box. There's a Fiero 3 feet from me that's got a cradle swap in it. I would have been mad if I didn't know any better and had tried upgrading the original cradle first. Just sayin'.

Supposedly the '88 suspension was what the Fiero engineering team had ready for production in 1984, but then were directed to make a last minute changes to use the components that ended up being the '84-'87 suspension. GM's stodgy culture claims another cool car.

The early cradle is pretty bad stock. The '88 is much better, but not wonderful until you install rod ends in the lateral links.
I'm very impressed by how my Formula handled with rod end lateral links, rod end (Rodney's) sway bar end links, Koni struts and an aggressive alignment. I haven't driven it in a while since I've been dorking with the front suspension, but I'll be returning it to service soon.

The early cradle is a WWD front end moved straight back. The Fiero team rotated the forward mounts to horizontal in order to enhance assembly precision.
The forward pivots of the control arms are further apart than the rear pivots. I call this "plan-view angle". This was done to move the wheel back as the suspension compresses, improving ride quality.
The forward pivots of the control arms are lower than the rear pivots. This was done to produce anti-dive in a front suspension.
Because the geometry of the control arm pivots ended up kind of weird, the geometry of the toe link ended up kind of weird.

Early suspension shortcomings:
The plan-view angle of the control arms doesn't really have much of an impact on handling, other than making the toe link geometry weird.
The front-end anti-dive geometry turns into PRO-squat geometry when the suspension is used in the rear. This does negatively affect performance, as it causes a high powered Fiero to lift its nose a lot when accelerating. In addition to "not feeling like a supercar", large changes in pitch attitude negatively affects aerodynamic stability at high speeds. The control arm pivot axis and kingpin axis not being perpendicular in side view probably has a bigger effect on toe link geometry than does the plan-view angle.

1988 suspension superiorities:
With the '88 strut towers closer together, the '88 has a slightly greater kingpin inclination, resulting in a slightly better camber curve than the early suspension. The '88's anti-squat also produces better pitch behavior at on vs. off-throttle. I have the stock engine in my Formula, so I can't comment on its behavior with higher power levels.

Parity:
Both systems can achieve similar alignment specs. Since alignment is very important for making the most of the tires, this is a big deal.
Both systems use the same struts.
Both can take 18x9.5-36 wheels with 285/30-18 tires. This is serious rubber. Maybe the '88's can fit 1/2" wider at the inner edge before hitting the strut, but they require mods to the trailing arm to do so.
Both systems have improvable roll center characteristics

Early suspension improvements:
Hard cradle mounts and hard control arm pivots will get the early suspension to close to the camber performance of an '88 with rod end lateral links. This does not do anything about the pro-squat geometry, but are not difficult modifications.

Early suspension superiority:
The early suspension can mount hub carriers from other GM cars, which allow use of either 27 spline 5x115 hub bearings or 33 spline hub bearings which are available in either 5x115 of 5x4.75. In addition to eliminating the Fiero's weak stock outer CV joints, the larger bearings are better able to handle the loads of wide sticky tires than the stock type 5x100 hub bearings. The 5x100 hub cartridges have severe supply chain problems, in that the supply chain can not source anything but hot garbage.

//

I'm working on eccentric forward/wedge rear cradle mounts to try to correct some of the pro-squat geometry. I don't think I'll be able to make enough of a change to eliminate it or do as well as I did with front anti-dive blocks.

[The Dark Side of Will]

So solid bushings for the cradle and a tri link all in the same plane for the '88.

There's nothing wrong with the '88 rear end which would be fixed by that.

GM deliberately built a smidge of roll steer, which is really just controlled bump steer, into the '88 rear end. The toe link inner pivot is slightly outboard of the lateral link inner pivot. This makes for a smidge of toe in on jounce, which translates to roll understeer. I'd try ditching that roll steer along with raising the rear roll center. That's probably my next major tweak to the Storm Trooper rear.

[CaptainHindsight]

Going with stiff bushings, larger dia wheels and lower profile tires, coil overs/shocks initially. Next spring i can place sensors on all the chassis and suspension components and monitor their positions and behaviors in real time to make adjustments based personal preferences and real time data.

[CaptainHindsight]

This also easy enough to fab while using the stock lower control arm that has been upgraded for more strength.



i have it all apart right now.

[CaptainHindsight]

Another question is, are there hubs off the shelf that bolt into the stock knuckles that have the same spline size and count as stock but also have a 114.3mm x 5 bolt pattern?

Which leads to the next question, are there rotors that match the stock caliper locations if you swap to 5 x 114.3?

Yes I know about completely changing over the hubs, rotors and calipers to other much larger and better combos but I'm just wonder about taking intermediate steps.

[ericjon262]

Another question is, are there hubs off the shelf that bolt into the stock knuckles that have the same spline size and count as stock but also have a 114.3mm x 5 bolt pattern?

Which leads to the next question, are there rotors that match the stock caliper locations if you swap to 5 x 114.3?

Yes I know about completely changing over the hubs, rotors and calipers to other much larger and better combos but I'm just wonder about taking intermediate steps.

IIRC, 90's "dustbuster" minivans have an upright that has the same geometry as the Fiero knuckle, and uses the 5x114.3 bolt circle,

[The Dark Side of Will]

Another question is, are there hubs off the shelf that bolt into the stock knuckles that have the same spline size and count as stock but also have a 114.3mm x 5 bolt pattern?

Which leads to the next question, are there rotors that match the stock caliper locations if you swap to 5 x 114.3?

Yes I know about completely changing over the hubs, rotors and calipers to other much larger and better combos but I'm just wonder about taking intermediate steps.

IIRC, 90's "dustbuster" minivans have an upright that has the same geometry as the Fiero knuckle, and uses the 5x114.3 bolt circle,

GM never used 114.3. GM used 5x115.
The FWD A-Body (Chevy Celebrity, Pontiac 6000, Olds Ciera, Buick Century) with heavy duty brakes (RPO JA2, IIRC) has 27 spline outer CVs and 5x115 hubs. These outer CVs will plug onto the Fiero manual transmission axle bars.

The U-Body minivans (Chevy Lumina APV, Pontiac TransSport, Olds Silhouette) went through a couple of iterations of knuckle hardware, but the '92-'96 version has 33 spline outer CVs, heavier duty wheel bearings and the same front brakes as an LT1 F-Body. The Mule currently has A-body hub carriers in the rear. I'll be swapping to U-Body hub carriers shortly after I get it on the road.

As long as you get centering rings, you can probably just bolt up 5x4.5 wheels to 5x115 hubs and never have a problem. People bolt 5x120.65 Corvette wheels onto 5x120 BMWs and go AutoXing.

[CaptainHindsight]

Thanks. Yeah it's weird

114.3mm . = 4.5 in
115mm ... = 4.5275 in
120mm ... = 4.7244 in
120.65mm = 4.75 in

[The Dark Side of Will]

Also, the cast iron A-Body and U-Body knuckles have clearance holes in the hub mounting flanges and threads in the knuckles. Later platforms with aluminum knuckles have clearance holes in the knuckles and threads in the hub mounting flanges. Drilling out the threads and spot-facing the inboard surfaces of the iron knuckles allows use of the later hubs intended for aluminum knuckles.
With 33 spline outer CVs, hub options are then: 2nd Gen W-body (5x115), C7 Corvette (5x4.75) and Chrysler 300M (5x4.5). Series 8217 recently discovered the Chrysler 300M hubs. All the spec dimensions are the same as the GM bearings noted.

ALSO, since the U-Body and F-Body share brakes, those rotors have an intermediate bolt pattern drilled with larger holes, so they fit both 5x115 and 5x4.75 wheel bolt circles.