Does the rear suspension have to be a tie rod end? (Page 1/1)
Additivewalnut JUL 19, 11:45 AM
I was thinking.... Which normally is never good... Does the rear suspension on an 86 require that tie rod? Obviously it's for toe adjustment but couldn't it just be converted into a trailing arm similar to the 88s? You could get spherical bearing on the cradle side and figure out some solution for the spindle side and it would stiffen up the rear a fair amount, right? Or would that introduce a comical amount of bump steer?
cvxjet JUL 19, 12:20 PM
The biggest mistake by any engineer on the Fiero was the design of that Tie rod; On a STRUT-style suspension, the length of the tie rod should be based on a Triangle, formed by the inner and outer ends of the lateral arm, and the top of the strut. Since the Fiero's tie rod is mounted ABOVE the arm, it should be SHORTER than the A-arm, but it is 3 inches LONGER...That is where most of the Bump-Steer comes from. There are some kits out there to shorten the Tie rod, but A) I don't know where they are located, and B) they may not be manufactured anymore. You may be able to make your own....

I swapped in an 88 rear subframe/suspension- MUCH better, both in handling and ride...and the frame is >>Hard-mounted<<, so it acts as a reinforcement to the chassis

[This message has been edited by cvxjet (edited 07-19-2025).]

Brian A JUL 19, 12:53 PM
Some thoughts on the pre-88 rear suspension ...

There is lots of information on this website regarding how to shorten the rear tie-rods and relocate the mounting points to eliminate any bump steer.

My track car is a 1987 GT and can't say I feel any bump steer. I am a-ok as is. I really don't know what the fuss is about. Granted, I have stiffer than stock springs. Having said that, I once pirated the rear tie-rods from a 1987 in a junk yard to maybe use them someday relocate the mounting points.

A cute factoid is that, apparently, there are also McPherson struts in the back suspension of some Lotuses. When McPherson struts are in the back, some people (Lotus people!) call them Chapman struts.
cartercarbaficionado JUL 19, 05:35 PM

quote
Originally posted by Additivewalnut:

I was thinking.... Which normally is never good... Does the rear suspension on an 86 require that tie rod? Obviously it's for toe adjustment but couldn't it just be converted into a trailing arm similar to the 88s? You could get spherical bearing on the cradle side and figure out some solution for the spindle side and it would stiffen up the rear a fair amount, right? Or would that introduce a comical amount of bump steer?


it not have to be a tie rod end. ive been trying to come up with a way to use the 1998+ grand prix gtp rear trailing arms (aftermarket tubular ones) on the pre 88 rears. needless to say its a real pain and my fab skills suck.
as for the tire rod if you did the relocation mod and then made that a solid non adjustable rod with solid joints it should help handling alot
1985 Fiero GT JUL 19, 06:50 PM
What's wrong with tie rods? I understand the design is sub optimal, although I've never experienced "bump steer" I've had similar symptoms from bad bushings in the cradle and control arms, but what's wrong with tie rods in this use, if the geometry is fixed, are tie rods a known weak point? If all you want is the least toe change across the board, then wouldn't a "link" probably with rubber or poly bushings have more play than the ball joints that make up a tie rod?
Brian A JUL 19, 07:27 PM

quote
Originally posted by 1985 Fiero GT:

What's wrong with tie rods? ... are tie rods a known weak point?



They are not a weak point.

The only problem is Pontiac's configuration with tie rods. They must have grabbed "parts bin" tie rods and located their anchor points in the Fiero to utilize them without modification.

Because the tire rods are longer than the lower A arm, the tie rod pushes and pulls the wheel inward and outward as the wheel moves up and down over bumps. If it were the same length as the other pivot points, it would not push and pull. Its sounds horrible but, at least to me, the effect is trivial.


quote
Originally posted by 1985 Fiero GT:If all you want is the least toe change across the board, then wouldn't a "link" probably with rubber or poly bushings have more play than the ball joints that make up a tie rod?



There are no rubber or poly bushings in the tie rod; there are ball joints on each end.
1985 Fiero GT JUL 20, 01:01 AM

quote
Originally posted by Brian A:

They are not a weak point.

The only problem is Pontiac's configuration with tie rods. They must have grabbed "parts bin" tie rods and located their anchor points in the Fiero to utilize them without modification.

Because the tire rods are longer than the lower A arm, the tie rod pushes and pulls the wheel inward and outward as the wheel moves up and down over bumps. If it were the same length as the other pivot points, it would not push and pull. Its sounds horrible but, at least to me, the effect is trivial.

There are no rubber or poly bushings in the tie rod; there are ball joints on each end.



Ok, yes I understand the theoretical bump steer problem, whether it is noticeable or not, I was more asking why people are talking about reinventing the wheel and modifying other parts to this, like a grand prix trailing arm/link mentioned above. If it's a link with rubber or poly, it will be worse than the tie rod, depending on the design a "solid bearing" could have wear issues from what I've read, the tie rods work, and getting them shorter would seem easier than adapting other arms/Links to work.
I know that, I was saying the rubber or poly would have play in a "link/arm" adapted instead of the tie rods.
fieroguru JUL 20, 07:17 AM
Over the years, several people have done a complete rear suspension replacement, so it is possible, but most will not have the skills or patience to do it with enough precision to come up with something that is actually better. It all comes down to properly locating the suspension pivots and finding the opltimal placement to minimize unwanted effects through testing, modeling, and trial and error.

Bump steer happens basically in 2 manners:
1. Geometry issue - toe changes significantly with suspension travel.
2. Non-uniform deflection during lateral loads.

The 84-87 cars have both of the above.

It has a geometry issue with the tie rod length and the tire rod has sperical bearings paired with rubber bushings in the lower control arm so under lateral loads, toe will change as the rubber bushings compress (and the tire rod doesn's). Additionally the rubber bushings in the cradle allows the two lower pvots to move relative to the upper strut tower pivot.

1. Stiffer bushings in all areas will help reduce the deflection cause bump steer.
2. Stiffer springs and shocks will reduce suspension travel, which greatly reduces bump steer. Bump steer gets progressively worse the more the suspension travels. You might get 0.1 degree for the first 1/2" of travel, then get 0.5 degrees over the 2nd 1/2". So limiting suspension travel to 1/2" limits bump steer to 0.1 degree vs. 0.6 degrees.
3. You can also limit geometry induced bump steer by adjusting the pivot points, but you might cause other unwanted side effects in one of the other suspension geometry parameters.
The first two will have negative impact on road harshness and ride quality, and the 3rd mght help bump steer, but hurt another more critical susension behavior.

The 88 rear suspension has toe change (bump steer) designed in. With the front and rear lateral links being different lengths, as the suspension cycles, toe changes (normally toe in) a small amount. However, both links use the same bushing material, they are also on the same plane, and the cradle is solid mounted, so there is much less deflection induced bump steer.
Brian A JUL 20, 07:00 PM

quote
Originally posted by fieroguru: ... It has a geometry issue with the tie rod length and the tire rod has sperical bearings paired with rubber bushings in the lower control arm so under lateral loads, toe will change as the rubber bushings compress (and the tire rod doesn't).



Very interesting. I never thought of this. Thank you.

I have polyurethane a-arm bushings which I have never installed. I will now consider it. (Although, again, I have never had a bump steer problem.)


EDIT:
Thinking about this a little more, lateral loads may change toe-in but -- to my thinking -- wouldn't cause bump steer. Once a car takes a set in a corner, the load on the outside tire stays relatively constant so the driver wouldn't notice anything at the steering wheel. Wouldn't the car just hold its track?



quote
Originally posted by fieroguru:... 3. You can also limit geometry induced bump steer by adjusting the pivot points, but you might cause other unwanted side effects in one of the other suspension geometry parameters...



What performance aspect might I damage by shortening tie rod length?

[This message has been edited by Brian A (edited 07-20-2025).]

fieroguru JUL 20, 07:23 PM

quote
Originally posted by Brian A:
EDIT:
Thinking about this a little more, lateral loads may change toe-in but -- to my thinking -- wouldn't cause bump steer. Once a car takes a set in a corner, the load on the outside tire stays relatively constant so the driver wouldn't notice anything at the steering wheel. Wouldn't the car just hold its track?



This would assume a constant radius to the turn, the majority are not.
Lateral loads can and do increase/decrease mid-turn as well as during the transitions into and out of a corner which will change the compression of the rubber bushings. With this happening at the rear of the car, the driver will likely need to make steering angle adjustments to counteract.


quote
Originally posted by Brian A:
What performance aspect might I damage by shortening tie rod length?



If all you did was change the location of the inner tie rod, only thing altered is bump steer.