Fletch

I asked Andy to take a look at my set up and tell me what he thinks.

The rear suspension and bump steer bracket is a problem. First, the suspension is in reverse. That is, what is mounted on the left side of the car should be on the Right side of the car. The brake calipers should face towards the front of the car, This may have been done on purpose so that the toe arm (what was used for steering in the Cavalier) would be up front instead of the rear as in the original Fiero. This would have made it a bit safer than the Fiero since as you would go around a turn, the wheels would "toe-in", providing more understeer. In the Fiero, the wheels would "toe-out", which would give you more oversteer. But this can be eliminated if we redesign the suspension a bit, while still using the lower control arm. But this will require that the wheels be finished and ready to bolt on.

The following photos are looking at the right rear.

This is a Fiero Rear Bump Steer Kit from Held Motorsports.

Endre (Andy) Bujtas

My tubular chassis uses the Ryane (now Held) engine cradle, which supports an independent rear suspension. I'm also using a sway bar, custom made for me by Sway-a-way (http://www.swayaway.com). The sway bar is a hollow tube (1.125" OD) with a spring rate equivalent to a 22mm solid bar. This suspension design will exhibit no bump steer since the upright is held firmly in place by the upper control arm system - two parallel arms. However, it could be susceptible to torque steer with a sufficiently powerful motor because the upper control arm might twist under heavy load. I compensated for this with a modification adding diagonal links or panhard rods.

The Ryane (Held) unit is designed to use the Wilwood 12 inch (Dynalite) brake system.

General Rear Suspension Information

Sway Bars
Sway bars, or more accurately anti-roll bars, are used to add some roll resistance to the car. But they are also used to control weight transfer and thus, control the car's handling characteristics. The more weight (force) you can put on the tires, the more friction with the ground, the more grip you will have. Of course, having a full tire contact patch is also desireable as well. When a car goes around a high-speed bend, the body/chassis will tend to "roll" because of centrifugal force. The anti-roll bar will add some resistance since as one side of the bar is getting compressed (due to roll) the other side of the bar is tending to follow the suspension on that side. The bar will twist, the amount of which will depend on the stiffness of the bar itself. But in addition, because the force going to the side of the car that is being compressed is greater than the other side, the bar will tend to lift the wheels on the side that is not being compressed. This causes an additional weight (force) transfer from the raised side to the compressed side, giving the car more grip. The stiffer the bar, the more weight is transferred because the bar can resist the downward force of the springs on the raised side.

The key to handling, especially with mid-mounted engines, is in the selection of the front-rear spring rates. Everything being equal, a larger anti-roll bar up front will cause a weight shift to the front, reducing any oversteer. Therefore, you do not want a particularly stiff or large roll bar in the rear, but up front. The opposite would be true for front-engine cars. Putting a large sway bar in the rear of a mid-engined car is asking for trouble.

Brakes
The point about brakes is generally in the location where the caliper is mounted. Understand that when the caliper "clamps" onto the rotor (stopping), a torque is being generated that is transferred to the chassis through the suspension. When looking at a chassis from the side and the car is moving (say from left-right) the rotor is rotating clockwise. If I clamp the rotor in front, the resulting torque will cause a tendency of the chassis to raise giving more grip to the rear wheels. If I clamp the rotor from the rear, the torque will cause the chassis to squat and to reduce traction.

Designers tend to place the brake calipers as so: towards the front on the rear suspension, and towards the rear for the front suspension. Or both ends facing the passenger compartment. With some anti-dive geometry up front, the car will tend to brake (sort of) level and not "nose" down in the front so much.

Bump Steer
Bump steer occurs when the rear (or front) wheels toe (in or out) while under bump. This is due because of the steering tie rod is moving up and down along with the steering knuckle. This is particularly true for the pre-88 Fieros since the rear suspension and cradle on those cars were actually the front end of a Chevy Cavalier and as such, were originally meant to steer. To prevent the rear wheels from turning, the Fiero engineers replaced the steering tie rod with a toe adjustment arm attached to the cradle. However, having this rod attached there and to the upright will cause the wheels to turn slightly when the car hits a bump. The 88 Fiero took care of that by redesigning the rear suspension from a MacPherson strut to a Chapman strut design. The Chapman strut uses 2 parallel lower control arms and thus, the wheels would not turn under bump conditions. The way bump steer is minimized is through the proper setting of the front wheel alignment (specifically the toe) and the location of the steering rack. The rack location variable [in the equation], however, is fixed so that there is not much you can do about it.

To correct the bump steer condition for those builders using the pre-88 rear suspension, you would either have to purchase a bump steer correction kit or make something yourself. There are 3 ways that this can be done: either by making a new lower control arm similar to the 88 Champan system, or add a link to the upper end of the strut clamp that is bolted to the upright. This link would be a diagonal rod that would be attached to the chassis at a point such that it will allow the wheels to move up and down as if it were not there, but would hold the wheel from turning. Or by moving the toe arm point on the chassis higher or lower, depending on how the car sits at full ride height. The object here is to have a very slight downward angle to the upright so that under small bump deflections, the toe rod would be almost level with the ground and not turn the wheels any noticeable amount.

Race cars like the open wheel F1 and Indy cars do not have this problem because the steer rack is located high and the tie rods are along the same rotating plane as the upper control arms. That means as the wheels go up and down, the steering tie rods move up and down on the same plane as the upper control arms. Therefore, the tie rod movement does not force the wheels to turn. This can't be realistically done for regular passenger cars because: a) the engine up front will be in the way, or b) for mid/rear-engine cars like the Diablo, the designers wanted a front trunk and a steering rack mounted to prevent bump steer would eliminate any useable trunk space up front.

For the front end, the Ryane/Held Slalom front suspension system compensates for bump steer in their design. Their unit is bump-steer corrected, though it does not totally eliminate it.

It has been suggested that bump steer could be corrected in the rear Fiero suspension by relocating the toe rod from the chassis (or engine cradle) to the lower control arm. But this will not work either and in reality, will make it even worse. Why? Because the rear knuckle pivots (rotates) in and out relative to the lower control arm due to the lower ball joint. Worse how? Because the toe rod will have to be shorter than the original rod, which will cause the rear wheels to toe faster and much more (s = r * theta). The theoretical way to minimize bump steer in the pre-88 rear suspension design (as it is) would be to have an infinitely long toe rod:) But that is obviously impossible. Or make a longer toe rod such that the left/right rods criss-cross so that the end point for the left side is on the right and vice verse - if this is possible and the rods don't interfere with anything else. Or they (the rods) can be joined at the car's center line - if that is possible and still keep the down-angle of the tie rod small enough. You want the angle of the tie rod going down to the knuckle slightly (at ride height) so that when the wheel hits a bump, the rod movement will cause the wheels to toe-in a small bit (NEVER TOE-OUT) and reach level (zero angle) under normal bump conditions.

The Held bump steer correction kit (basic design idea) is the only way to eliminate bump steer with essentially the same Fiero suspension setup. It replaces the lower ball joint with 2 Heim joints and the upright is held in place with the Heim joint pivot bar - preventing the knuckle from turning or steering. But I'm not too keen on its overall design and have some concerns if used with engines of significant power and torque. I can easily see a couple of areas in this design that will bend out of shape if someone dumped the clutch with a torquey V8 - causing permanent misalignment or worse.

Torque Steer
Torque steer occurs when the rear suspension responds to the effects of hard (or maybe moderate) acceleration. This probably will be especially noticeable on Diablo builds using the Fiero rear suspension and having torquey V8s powering them with those wide 335mm tires. What happens is that under acceleration when the wheels turn one way, the upright will go the other way. This is a result of Newton's 2nd law: a force or action in one direction will cause an equal and opposite reaction. So that when the upright shifts back it can cause the suspension to twist and the wheels will tend to toe-out. Having the rear wheels toe-out under acceleration will tend to make the car act "squirrelly" and is even more dangerous in a high speed turn (e.g. severe oversteer). Torque steer is generally not a problem with Hotchkiss setups (i.e. rear carrier-tube differentials) since the wheels can't turn.

How do you handle this with a Fiero setup? With some difficulty. You would either have to completely design a suspension for it or add some longitudinal links like the Corvette rear suspension. The link that was mentioned in the previous section for Bump Steer could aid in handling torque steer. It would keep the upper end of the upright from rotating back.

Squat
Squat occurs in almost every passenger car. Squat is when the chassis dips or "squats" in the rear under acceleration. This has the effect of losing traction and causing wheel hop. You see this occur in the early Camaros where they only had a single leaf in their leaf springs. Under acceleration, the car would squat, causing the spring to twist (called wrap-around) making it ineffectual. The result was wheel hop and loss of traction. Thus, the traction bar was born. The traction bar, being attached to the differential, would rotate under acceleration, hit the frame and transmit the torque to lift the chassis and increase the down force on the rear wheels. Race cars (especially dragsters) have anti-squat built into them. As a matter of fact, dragsters have so much anti-squat that the entire rear of the car lifts up under acceleration. I've personally seen a T-bodied Fuel Altered (AA/FA) actually lift completely OFF the ground when the Christmas tree went green because it had so much anti-squat.

But it is difficult to incorporate anti-squat into independent rear suspensions. The only thing you can really do is to increase the rear spring rate - if you don't mind a stiff ride. But unless you plan on drag-racing your build, squat should not be a big concern.

Andy

Kelly Hays

Here is a pic of the Held bumpsteer.

Bump steer is what occurs on the back of the fiero suspension on the 87 back cars. The reason this happens is because the the back of a fiero is the front of a citation. So they used the tie rod end to tie adjust the toe in and toe out. The problem lies with they secured the tie rod end to the frame instead of the lower control arm. So you have two separate pieces moving up an down with the suspension and at different rates of travel. This causes the rear wheel to move in and out of adjustment as the suspension travels. When you are driving this you notice because it feels like the car hops to one side or the other when going over bumps. (This has been my personal experience.) It causes ill handling and excessive tire wear. The 88 fieros corrected this problem with a different suspension set up. Held Motorsports has a very nice solution with their bump steer kit. This replaces the lower control arm and makes the toe in adjustment on the lower control arm. Held also offers this kit with custom lengths to assist with the correct offset on the wheels. Lee is great to work with and also helped with our build by custom manufacturing parts to move the coilover unit back inboard. This allowed for correct geometry of the strut and we were able to use a 13” wide wheel.

David Melbourne

The rear suspension was going to turn as it moved up and down. This is a design fault with the 1984 - 1987 Fieros. a condition called " Bump Steer " A Fiero is basically a front wheel drive car running backwards. It uses a front suspension in the back. The steering arms are still there, the knuckles, everything. Here then,
was the problem. Since the steering arms are attached to the engine cradle, they turn the wheels when the rear suspension goes through it's motion. Bump Steer. Inside the car, it feels like a twitch as you get on, or let off, the power in a turn. The car tries to swap ends. Not too much fun. A company called Held Motorsports makes a kit that is a total CURE of this problem.
This kit replaces the silly original rear suspension with totally new parts and no weird steering arm. So this is what I am doing now. Putting this on, you can see my progress below.