Bump Steer Bum Rush

What is Bump Steer?

Very briefly, true “Bump steer” is the result of steering parts that don’t move thru the same Arc as the wheel moves up and down. If you have a control arm moving thru a short radius and a tie rod moving thru a long radius then as the wheel moves up/down they turn the wheel just like you would turn it with the steering wheel. It is present on many cars and trucks. (It is fairly rare that a vehicle doesn’t have this problem.) If you want more detail, try your favorite search engine. Bump Steer is well documented and there is no need to rehash all that here.

That said things other than steering geometry could cause what people too often call Bump Steer. Defective parts in the suspension or frame can cause a car to become extremely unstable over even small bumps. It is critical that you don’t confuse wear problems with design issues. (Many people also make the gross mistake of confusing alignment problems with bump steer…)

Does Fiero have Bump Steer “problem”?

If you listen to certain vendors and car rags… Fiero has a horrible problem with bump steer. If you actually sit down and look at the car, you quickly find that the problem isn’t nearly as bad as many make out. More importantly you’ll find out there are very cost effective and simple things that will greatly reduce the effects of Bump Steer on Fiero.

Geometry

84-87 Fiero does have suspension geometry in the rear that can cause bump steering. I’ve seen numbers of 1-2 degrees thru the full range of wheel travel. Notice that I said thru full range of travel. This is a critical factor that is left out in many discussions of bump steer. “True” Bump Steer can only happen as the wheel moves up and down. The more it moves the more it will be steered. (Some efforts to “solve” bump steer actually increase the amount of steering at the far ends of wheel travel.)

Under normal driving the wheels NEVER move over the full range. On rough roads and rail crossings the wheels may move much more but even then will usually not move thru full range UNLESS the shocks/struts are bad! The fact is that GM used very poor struts on Fiero and 15-20 years later the OE struts aren’t doing any better. Worse, the OE struts have damping only in one direction. This means a bump can throw the wheel to the stops with relatively little effort.

Cradle Mounts

The 84-87 Fiero has an engine cradle mounted in rubber with uneven loading caused by the placement of the torque reaction strut on the right side of the engine bay. This cradle setup can and does twist the entire rear suspension as engine load changes. This is painfully obvious on many manual transmission cars but automatics will also suffer this problem. While this problem can and often does steer the rear wheels, it is NOT defined as Bump Steer and must not be confused with it.

How do I get rid of Bump steer?

To start… The plan here isn’t to get rid of the “problem.” It is to fix basic chassis aging and control wheel travel.

Cradle Bushings

Replace them! Even if they look good replace them! Use Polyurethane or solid metal. (Polyurethane will never squeak in this use. There isn’t enough motion to cause that problem.) The cradle should never have been rubber mounted to start with and after 15-20 years the OE mounts are getting still softer even if they look good. This will stop the cradle from twisting due to engine load and potholes.

I went with polyurethane from Suspension Restoration Parts Co. This one change alone really tightened up the back of the car. I have one turn in my route to work that was a real pain. It is a stop sign that drops you into the middle of a banked turn. (Left onto Rt 9 from Hamburg Rd south of New Castle DE if anyone wants to try it.) Before locking down the cradle you always felt like you were going to lose the car. Now it just stays planted. (This was done before new struts. New struts made it even better.)

Struts

Replace Them! Especially if you still have the originals and many people do. The original struts were junk to start with. Use any brand of Premium Gas charged strut. These all have road sensitive valves and other features that will greatly improve control of wheel travel. This will automatically reduce the effect of bump steer. If the wheel moves one inch instead of three over a bump there will be that much less steering effect.

I used Gabriel myself. Some people prefer Monroe. Some tell you that you need KYB or Koni. Koni maybe if you are racing but it is way overkill for street use. Various people claim KYB is supposed to be better than Gabriel or Monroe but I’d like to see some actual tests. I’ve used Gabriel and Monroe for years and their top end products are very good performers at reasonable prices. (As I understand it… Monroe makes some of the shocks used in NASCAR and other racing. Koni isn’t the only one.)

Control Arm Bushings

Replace Them! Again these things are 15-20 years old. The OE Rubber design isn’t bad here, but aging bushings will be torn up and soft. Don’t use polyurethane unless you have no other choice. Poly may be stiffer than rubber but unless you want maintenance problems you really don’t want this stuff in control arms of street driven cars. (Read the bushing articles for more about poly in this use.)

Even in high horsepower applications, I recommend most people stay with rubber in the control arms. Remember that any increase in control arm deflection due to engine load will only occur when you romp the pedal. Under most other conditions the arms won’t move any more than they would with even the “weak” 2.5l L4.

But Blabblah says I need expensive part X to fix this…

One of two things here… Blahblah is frequently full of crap and/or trying to sell you parts. This includes some PFF members that constantly say you need whatever part. These folks all too often have no basis for comparison because they threw 50 parts at the car at one time and as a result have no clue as to what really improved their handling. (This sort of problem is very common with cars. The shotgun approach is nearly always bad in one way or another. Usually bad for your wallet at the very least.)

If you do the three things I just said, you will find the bump steer “problem” greatly diminished. Often to the point it’s unnoticeable in all but the hardest driving on the worst roads. I can drive over cobblestone with no tracking problems. It will shake your teeth loose but the car goes where pointed with little effort. Diagonal rail grades no longer make the car dance sideways even on the worst crossings.

Digression…

For those that have never seen it, Cobblestone is a stone pavement method that is very uneven especially after decades or even centuries of weathering. It can still be found on some streets of the old towns and cities like Philadelphia PA, New Castle and Wilmington DE. These streets are still cobbles because of historic value or a desire to make people drive really slowly. Philly has many narrow historic streets you can hardly get a car down. Wilmington has it on “Monkey Hill” to keep people from flying past the Brandywine Zoo’s Monkey House where many children are frequently present. Cobbles will heavily test any car and most trucks for that matter.

But anyway…

These changes will not eliminate the suspension geometry that causes bump steer. They will help confine wheel travel and reduce the ability of the geometry to steer the rear wheels.

Odds are you need the parts anyway so why not start with that and a good alignment. Don’t rush out spending money on expensive aftermarket tie rods or control arms until you are sure you really need them. To the dismay of some parts vendors the vast majority of Fiero owners simply don’t need their expensive products.

But I race…

My object with this article is to help the daily drivers. If you are racing and looking for fractions of a second then things change. Racers should look at what 8Shark has done with offset bushings and so on. (They cover Bump Steer in more detail.) 8Shark’s bushings potentially allow far greater alignment tuning to match your track than you can get with normal parts. Alignment may even be more of a factor than bump steer elimination.

SCCA racers could to have “bump steer” induced by high amounts of body roll… You folks may be better served with a better set of anti sway bars than just trying to kill the bump steer. Controlling body roll will affect traction and other things beyond whatever bump steer there is.

Bump steer is pretty much a non-issue for drag racing… You guys need to look at control arm bushings and rear toe setting. You may want to carry a “trammel bar” for setting toe at the track and then resetting it for the drive home.



Share

43 comments to Bump Steer Bum Rush

You must be logged in to post a comment.

Categories

Share