Over the last couple of weeks I've been thinking about the bump steer conditions in the '84-'87 model's rear suspension. We had made a spacer for the race car similar to the RCC Specialties unit that we have on the Copper Finale but it has some of the shortcomings of the RCC Kit.

What are those shortcomings? I believe I can speak to this because I have and have tested throughout the travel range what it does. The biggest problem is deflection. There is an additional problem of using medium quality Heim joint ends, but that is a problem of component selection and not design. The RCC kit basically uses a piece of square bar stock that bolts in the same location as the stock inner tie rod point and moves the inner tie rod attach point out a few inches, but in the same plane and height as the original. Under loading, this bar exerts a considerable amount of force under compression, pushing it UPWARDS unless changes are made, such as we did by laying a piece of steel plate between the bar stock and the frame and welding it solidly. An elegant solution this is not.

According to the RCC website:

quote Bump Steer in : The pre 88 Fieros have 1.8 degrees of toe inbump steer at full compression of the strut and 0.6 degrees of bump steer at full extension of the rear strut. This means that each wheel will toe in about 0.44 inches at full compression of the strut. Compared to the 88 Fiero which has minimal bump steer in the rear, this is certainly one of the causes of the handling instability in the pre 88 Fiero's. Our Bump Steer Kit moves the inboard pivot point of the rear drag link out to eliminate this compression bump steer. The kit has zero degrees of bump steer (+/- 0.3 degree depending on alignment and Pontiac manufacturers tolerance) from full compression of the strut through the first 6 inches of travel. In the last 1 to 1.5 inches of extension travel, the kit will exhibit 1.2 degrees of toe out bump steer, which we deem immaterial since this amount of extension travel will usually be seen only when the car is jacked up for service.

Not taking anyone at their word, I ran my own tests this weekend and today. I put a spare strut in place, without a spring, and had 100% pure stock tie rod attached at the stock pick up point. The results were interesting.

Stock, at full extension equating to 0 toe change, first toed out as the strut was compressed to a maximum value of .859°, then coming back to zero at approximately 4.5" of compression, then toeing in to a maximum of 3.43° at full compression which is about 7" of travel as measured at the wheel hub (Studs). Notice that I was getting a considerable amount MORE toe change during compression than RCC claims the stock suspension has.

I then roughly measured the toe change on the Finale with their kit installed. I don't put a lot of stock in these numbers because I didn't remove the springs so I couldn't get full compression and it was not the same, identical car so values will be different due to the differences in specific cars, but I was curious as to what it showed. Their claim of .3° through the first 6" of travel was not correct. We showed a slight toe out of about .15° (again, starting with "0" at full extension) at about 1.5" of compression. By 3" of compression, it was back to 0 and beginning slight toe in where we showed about .7° at 4.5 inches compression (or a bit more) which, at that point, the stationary jack began lifting that corner of the car off the hoist. Because of the different car, and the inablity to completely compress the strut, I did not plot the data, but offer it only for informational purposes.

I have been playing with this on a CAD program and had what I thought was a superior pickup point. We made a temporary mount, welded a heim joint to a shortened stock tie rod, and tested the results. The first location chosen was a benefit over stock but not much over the RCC kit. It showed a toe out deflection of about .716° almost immediately, holding there, then coming back to zero at about 3 1/2" of compression, then moving toe in at a steadily increasing rate to a maximum of 1.909° at full deflection. Remember we don't know what the RCC kit will actually do at full deflection but given the results of my tests I'm certain it can't be much different. We did show an improvement of over 1 full degree at full compression over stock.

We gradually moved the pick up point towards the rear of the car until we were actually against the rear cradle, plotting points as we went, and settled on a location that gave us results that we believe are as good as we're going to get with the relatively stock suspension. Our tests showed that, again with full extension at "0", that the rear toed out no more than .239°, then back in to 0 at 3" compression, then toe in to .239° at 4" compression. At 5" of compression it moves to less than 1° toe in, only slightly more than that of 1.193° toe in at 6" deflection, and 2.386° toe in deflection at full compression.

Our pickup points and shortened tie rod end resulted in a total change from full extension to full compression of 2.625° as compared to the stock setup that results in a total toe change throught full strut range of 4.293°. We were able to reduce toe change by almost 40% and, more important, almost all of that toe change is at the very end of the stroke of the strut compression.

I uploaded the graphs which should be self explanatory if you'd like to take a look at them. The graph for toe change in inches (based 12" in front of the wheel hub) is HERE and the graph (which is the same thing, only with different units) in degrees is HERE. They aren't that large in file size but I didn't want to shrink them as images as they lose a lot of resolution.

We are now in the process of making the permanent brackets for the new pickup points and will probably have them done tomorrow. When they are finished, I will share the pickup point location, which is also a compromise (just not as much of one as the RCC kit) so that if you're daring, you can make your own. I'm not making any kits and won't sell brackets, so don't ask. This was for our own use in our race car and I'm just sharing the information. We played with at least 20 different locations in working with this using a laser level shooting against a target board so our readings are, I believe, quite accurate.

This change is not a bolt on. Welding of the new pickup points is required and you will also have to shorten the stock tie rods, turn them down to .500", then thread them for female heim joints. Only use the best quality heim joints and bolts, a failure here could be catastrophic.

I don't believe it's possible to completely eliminate the toe change with a McPerson strut style suspension designed as it is in the early Fieros. I even simulated the Held setup by locking the hub to the lower A-arm and as the suspension compressed, a toe-in change was evident. I did not measure it, but I looked at enough measurements today to estimate that at about 1.5 - 2.0°. The pickup points we found, and tie rod length, gives a much improved result over stock and, I believe, will work fine for 95% or more of the use the car is going to see.

John Stricker

Nice work! You've obviously made some real improvements.

I am curious as to where, on the graph, the car would sit in a resting state.

Good advice about using quality heim joints.
Mike (mrfiero) had one of the Held units to break, going down the road. He was quite lucky that no damage was done (other than, perhaps, nicely "squaring up" one tire. ).

The obvious answer is.........it varies.

The WS6 springs are quite a bit heavier. One of these days I'm going to fix up a rig to test all the springs I have laying around. We did measure where the race car sits and that is at about 2.2 inches of compression. If you look at the graphs and where that is with the stock setup youll see that's at about the most extreme toe out position. Now if the alignment is set at that point to 0 toe in or 1/8" toe in, whenever the car moves up and down it's going to toe in even more, and a lot more.

Our new pickup point location is at nearly 0 toe. We are going to lower the car at least an inch, though, so that's going to affect that a bit. If you want to know where yours sits, take a tape and measure how much exposed shaft there is on the strut. It's almost, but not quite, 1:1 ratio with the outer hub so if you have, say, 5" of shaft exposed at rest, you can look on the chart and see where you're at because most all the struts have about 6.5" of travel or so.

John Stricker

quote Originally posted by Raydar: Nice work! You've obviously made some real improvements. I am curious as to where, on the graph, the car would sit in a resting state. Good advice about using quality heim joints. Mike (mrfiero) had one of the Held units to break, going down the road. He was quite lucky that no damage was done (other than, perhaps, nicely "squaring up" one tire. ).

Good info!

Thank you John!

I'm very curious to see where you ended up. I played with it on paper, making a scaled drawing and found that lengthening the toe link and moving it up was the best....

Bob

Very nice work. I have spare stock tie rods and my son can turn and thread them. Will be interested to try this....

Can't wait to see the attach points.

It's going to be a few days before we have any real update on this. I have some 1/2" Heim joints here that are all Right Hand Thread. Those would work, but when you went to set your toe (something that gets changed from time to time on a race car) you would have to unbolt the heim joint and turn the rod in and out. If we used left hand thread heim joints, we could simply loosen the jam nuts and turn the tie rod to move the toe. The right hand thread would work, but the left hand thread is better.

I ordered a pair of left hand thread heim joints from Speedway, along with some LH Jam nuts and cone spacers. We're not going to weld this up until we have these joints in our hands to get the correct dimensions on them so it's going to be a day or so. When we get them done, I'll post pics and drawings of our brackets. We're using 3/16" steel for the brackets.

To give you a rough position idea, though, the ends are about in line height wise (just a touch lower) than the stock pickup points and almost to the inside of each frame rail, and slightly ahead of where the original tie rod would run. You will have to increase the size of the notch on the space frame that is already there for clearance for the tie rods about as wide as the existing notch, but that's a minor adjustment. The tie rod is just moved forward that much and there isn't sufficient clearance for full travel.

John Stricker

quote Originally posted by Dodgerunner: Very nice work. I have spare stock tie rods and my son can turn and thread them. Will be interested to try this.... Can't wait to see the attach points.

A picture, drawing or measurements would be nice when you get to that point.

The nice thing is Speedway is right in town. Going there Thurs night for a party in the museum.

I'm having a hard time seeing how that would decrease your bump steer.

Ride height is normally at or close to the lower arm parallel to the ground. There are three pivot points (vertically) on the rear suspension. The lower ball joint, the upper strut mount, and the control arm bushings. The control arm is fixed at the bushings allowing the outer end to ONLY move up and down. The strut and knuckle, once the strut bolts are tight, are effectively one unit, pivoting on the lower ball joint and the upper strut mount.

If we assume a starting point (at rest) of roughly parallel with the ground, as we move up in the stroke the attach point for the tie rod is going to move in towards the cradle. Visualize that in your mind. The higher you make the attach point, and the farther away, the more the tie rod will pull or push the knuckle one way or the other. Ideally, my drawings showed the attach point to be directly above the control arm pivots, about 5" or so above (I don't have my notes in front of me so this is from memory) the pivots, and it should be at, or close to, the height difference between the pivot point of the ball joint and the pivot point of the tie rod end. This is important as the ball joint pivot is a significant amount below the knuckle and the tie rod pivot is a significant amount above the tie rod attach point on the knuckle.

I'd have to sketch it out but I don't think a long, high mounted tie rod is going to help in fact, I think it's going to make things significantly worse at full deflection.

John Stricker

quote Originally posted by RCR: I'm very curious to see where you ended up. I played with it on paper, making a scaled drawing and found that lengthening the toe link and moving it up was the best.... Bob

I think I see what you're saying...I think I didn't account for the knuckle to strut joint, and that it does not change angle as the strut compresses. I'll have to relook at my figures tonite.

Thanx for the heads-up.

Bob

Sigh...more work than I would want to do...never have enough time to do really nice things to the car...or the money. Think you could manufacture said brackets, mounts, etc.? Perhaps partner with some place like the Fiero Store or Rodney dickman? (or heck, make em yourself. I'll begrudge you a decent markup). If it's as good as you say, I think you'd have a decent market.

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1986 SE V6, stock, auto, fastback. GT Trim and Body panels.

quote Originally posted by PineyCreek: Sigh...more work than I would want to do...never have enough time to do really nice things to the car...or the money. Think you could manufacture said brackets, mounts, etc.? Perhaps partner with some place like the Fiero Store or Rodney dickman? (or heck, make em yourself. I'll begrudge you a decent markup). If it's as good as you say, I think you'd have a decent market.

From my first post:

quote Originally posted by jstricker: We are now in the process of making the permanent brackets for the new pickup points and will probably have them done tomorrow. When they are finished, I will share the pickup point location, which is also a compromise (just not as much of one as the RCC kit) so that if you're daring, you can make your own. I'm not making any kits and won't sell brackets, so don't ask. This was for our own use in our race car and I'm just sharing the information. We played with at least 20 different locations in working with this using a laser level shooting against a target board so our readings are, I believe, quite accurate.

The reality of this is that I could have the brackets laser or waterjet cut locally for not much money. The reality is also that someone, at some point, will probably bubble gum these on with an $89 harbor freight welder, they'll break, and then they'll want to sue me.

I started the thread for the purposes of sharing information with the Fiero family. What people do with this information is entirely up to them. We're cutting the brackets out right now and I hope to have the dimensions of the pickup point and pictures of the new brackets uploaded sometime this evening or so. Beyond that, I'm just not going to get that deeply involved. You also have to remember that this is going on our race car and since it's currently engineless, there is no way I'm going to have an opportunity to test these on the track before early next spring. There's no way I'd sell something that's untested.

John Stricker

As I understand your geometry, you're more or less making the tierod length as equal as possible to the balljoint pivot/A-arm bushing length on the lower suspension arm. This will get rid of a BUNCH of toe-in variation. But what would happen if you also lowered the outer and inner tierod ends so that they were in the same plane as the balljoint pivot/A-arm bushing centerlines? It might not be practical to lower the outer tierod end that far on a street car, ( might interfere with some wheel rims or something), but for something that gets constant attention like a racer it might be worth doing. Would it work??

I would LOVE to lower it. As you know if you've had it apart, the tie rod end goes into a tapered opening so there is no way to flip it from the bottom up. That would be a GREAT help, IMHO. But since it wasn't practical, I didn't plot that. The only way would be to go to a larger tie rod end that had a larger shank, then ream the opening from the bottom up, to flip the ball joint, then re-analyze the pick-up points.

Even doing THAT, I don't think it would go away completely. The reason I say that is that even assuming we could make the links perfectly parallel, as the strut and knuckle moves up, it WILL MOVE IN. When it moves in, if that tie rod mount is at all above the pivot of the lower ball joint, there MUST be toe change. If you picture the mechanics, you'll see what I mean. All that said, I think you are 100% correct that it would make an even larger difference than what I've done and would be a help, but machining of the knuckle is going to be required to make it work.

Another option from a ball joint out there would be to machine the tie rod mount on the knuckle to a straight hole. Then usins a straight stud, use a heim joint on that end as well. I considered doing just that but that leaves the heim joint flexing in the wrong axis. IOW, the Heim needs to be mounted vertically (as much as possible) so that it rotates in the ball as intended. Mounting it on a stud at the knuckle will leave it mounted horizontally, not a good arrangement. I also considered machining a block that bolts into the mounting stud, then mounting the heim to that. That has it's own set of problems as it will move it either back, or in, and would require the inner pickup point to also move either back, or in. As you can see, we're already in as far as practical without going past the inside of the frame rail and going further back gets us against the cradle upright. Both of these options would take some creative pickup point mounting with a lot more fabrication than has been required so far.

Your thinking is right along the lines of mine, though. There's just a balance there on what would be PERFECT and what is actually practically possible.

John Stricker

quote Originally posted by Jim Gregory: As I understand your geometry, you're more or less making the tierod length as equal as possible to the balljoint pivot/A-arm bushing length on the lower suspension arm. This will get rid of a BUNCH of toe-in variation. But what would happen if you also lowered the outer and inner tierod ends so that they were in the same plane as the balljoint pivot/A-arm bushing centerlines? It might not be practical to lower the outer tierod end that far on a street car, ( might interfere with some wheel rims or something), but for something that gets constant attention like a racer it might be worth doing. Would it work??

That looks just about where I thought they would end up when I sighted on mine.
When I first thought about it wondered if it would get in the way of pulling a transmission out the side tipping the cradle back but does not look like it really would be in the way much.

It shouldn't bother dropping the cradle but it most likely WILL be in the way on the other side of pulling the transmission out the side so if that's a concern for you then you probably don't want to move these. There is not a lot of room over there behind and around the rear transmission mount. It doesn't concern me because I always just drop the cradle completely.

John Stricker

quote Originally posted by Dodgerunner: That looks just about where I thought they would end up when I sighted on mine. When I first thought about it wondered if it would get in the way of pulling a transmission out the side tipping the cradle back but does not look like it really would be in the way much.

how's about eliminating the balljoint, and making it a hinge instead? a very strong hinge, of course, but one that does NOT allow the twisting. leave it adjustable, so you can adjust the toe. no more twisting/bumpsteer.

reminds me of an article from Joe Wynnman in one of the old Fiero magazines about welding the ball joints in place to remove any wiggle... My memory is bad but I may be able to dig out the magazine some day...

Be my guest.

Be sure to let us know how that works out.

John Stricker

quote Originally posted by Pyrthian: how's about eliminating the balljoint, and making it a hinge instead? a very strong hinge, of course, but one that does NOT allow the twisting. leave it adjustable, so you can adjust the toe. no more twisting/bumpsteer.

Any updates??

Bob

quote Originally posted by Pyrthian: how's about eliminating the balljoint, and making it a hinge instead? a very strong hinge, of course, but one that does NOT allow the twisting. leave it adjustable, so you can adjust the toe. no more twisting/bumpsteer.

The pivot axis of the control arm is not perpendicular to the kingpin axis in the side view. A single axis hinge would tear itself apart unless you relocated the control arm inner pivots.

quote Originally posted by Will: The pivot axis of the control arm is not perpendicular to the kingpin axis in the side view. A single axis hinge would tear itself apart unless you relocated the control arm inner pivots.

the way the struts are mounted should absorb that small amount deflection
I'm more worried rotational forces ripping the hinge apart.

Think about what I said. You're not worried about the right problem. The strut upper mount has nothing to do with it.

[This message has been edited by Will (edited 10-03-2007).]

I guess I need a pic....cant picture what you mean.
eliminate the tie rod, and use a hinge instead of a balljoint.
put the hinge on a plate, so you can make toe adjustments.
tho, instead of bumpsteer, you may end up with "bump camber"

Look at the rear suspension from the side. In order for the single axis hinge to work, the control arm inner pivots must be perpendicular to the kingpin axis (strut). They are not.

ok, now I see what you are saying
the inner a-arm points must be ____ while the strut must be | |
if the strut is / or \ yes, it will not work. I thought the strut was only tipped inwards, not forwards or backwards.

It's exactly the same as it is in the X/A body. In those car's the strut is tilted back for caster, the control arm is pitched forward for anti-dive and rotated back around a vertical axis to keep caster change through the suspension travel to a minimum. It works ok as a front suspension (in my AWD 6000) but it's a disaster as a rear suspension.

Any progress? Bumpity Bump

Well... I might be going out on a limb here... but here goes. It seems to me tha the best way to avoid bump steer while retaining stock rear control arms is to eliminate the tie rod connection to the subframe althgether. Wouldn't it be possible to attach an alternate arm from the steering arm attachment point on the spindle assembly to a fixed point on the control arm. It seems to me that by doing so, the "tie rod" would always be the same position relative to the control arm throughout the range of motion. Does this make any sense at all?



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FierOmar

[This message has been edited by FierOmar (edited 10-19-2007).]

yes, but - it would need to have multiple joints & stay out of the way of the axle

quote Originally posted by FierOmar: Well... I might be going out on a limb here... but here goes. It seems to me tha the best way to avoid bump steer while retaining stock rear control arms is to eliminate the tie rod connection to the subframe althgether. Wouldn't it be possible to attach an alternate arm from the steering arm attachment point on the spindle assembly to a fixed point on the control arm. It seems to me that by doing so, the "tie rod" would always be the same position relative to the control arm throughout the range of motion. Does this make any sense at all?

There is no point on the control arm that's fixed relative to the spindle.

Part of the reason I'm so in love with this car is the engineering that continues to go on in improving it. I can't wait to try this mod out on my car.

*anti archive bump*

bump again...

any developments? testing?

quote Originally posted by Pyrthian: how's about eliminating the balljoint, and making it a hinge instead? a very strong hinge, of course, but one that does NOT allow the twisting. leave it adjustable, so you can adjust the toe. no more twisting/bumpsteer.

This would work if the kingpin axis were perpendicular to the inner pivot axis of the lower control arm... but it's not. Blame GM for transplanting the intact geometry from an X-body into the back of a Fiero. The anti-dive at the front of the car turns into pro-squat at the rear. Since they had to develop some new stampings for the Fiero cradle ANYWAY, this could easily have been fixed, but they were too short sighted for that.

So does the '88 rear help reduce pro-squat?

I think so but I haven't seriously investigated the geometry.

The big potential for the '88 rear is that it could be made with adjustable anti-squat, like a drag racing 4 link, by adding additional optional pivots for the forward end of the trailing arm.