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Welcome to my newly redesigned Fiero 2M4 resource site.  The original site recorded over 12000 visits,  hopefully this one will also prove as useful.
Ira Crummey

The Bump Steer Story

 

Fiero owners, especially those who do not have a technical background, are often left with the impression that the 1984 to 1987 Fiero has a unique case of bump steer.  Bump steer has in fact been an issue with many vehicles over the years, for several different reasons.  Let us get the terminology straight first.

Bump steer is in fact a change in toe angle as a wheel moves up or down in its suspension travel.  When a bump is encountered in a turn, the resulting toe change will cause a slight steering effect which is felt as anything from a slight "twitch" to a major direction change.  Bump steer can exist in front or rear suspension designs, although most front bump steer has nearly disappeared from factory suspensions.  The early Fiero chassis used an Opel derived front suspension as also used in the Chevette. Although it suffers from limited travel and "kickback", due to the large scrub radius, it does not have significant bump steer, in fact the only real problem with the front end is the pro-dive geometry which unbalances the car under braking.  It is bump steer in the rear suspension we are concerned with.

The early Fiero's rear suspension was another front end design, this time from the X-body series of GM small sedans, such as the Pontiac Phoenix, and in light duty versions of the A-body series, such as the Pontiac 6000.  This MacPherson strut, lower A-arm suspension was mounted to a cradle similar to those used on the front wheel drive sedans, but since there was no steering the tie rods were bolted to the cradle to act as toe links.  Herein lies the problem, if toe links are not perfectly parallel to the control arms at all extremes of suspension travel, there will be a change in toe angle, thus bump steer.  The Fiero suffers from this problem which is then compounded by the larger diameter, soft rubber bushings used in the control arms.  These bushings allowed deflection of the control arm, forward and aft under acceleration and braking, and in and out under side loads during cornering.  Since these motions were independent of the toe link the results were more toe changes, and more bump steer.  This is not a unique condition.

There have been many different rear suspension designs over the years, many of which gained popularity and wide spread use despite a significant bump steer problem.  Among solid axle (non-independent) designs we have used are a number of coil sprung types which include a Panhard rod or track rod which locates the axle, some of these have used a rod which was too short.  The result of this short Panhard rod is not really bump steer but it involves the body moving left or right relative to the axle.  This is an uncomfortable sensation which results in the same off-balance feel as the Fiero's bump steer.  The Chevrolet Monza, one of the better handling cars of the 1970's suffered from this problem.

The most universally used independent rear suspension design in rear wheel drive cars  has probably been the semi-trailing arm suspension.  When swing axles began to disappear from beneath many cars, including all of the great German marques, they were usually replaced with semi-trailing arms.  Everything from the last VW Beetles of the 1970's to the famous Datsun 510, to every BMW, Mercedes and Porsche of the same era were equipped with this compact efficient design, and all suffered some degree of bump steer.  This system is called semi-trailing arm because the wheels move in an arc determined by a triangular suspension arm with pivot points at the differential and forward of the rear wheels.  Motion around this pivot axis causes big camber changes and a toe out condition as you move the wheel up or down from the normal at rest position.  Multi-link designs, most originating in the 1980s,  have replaced this design in most modern cars, although the BMW Z3 still uses a variation of this design to good effect.  These companies took great pains to reduce the effect but in some cases, such as very powerful rear engined designs, like the 911 Turbo, it could not be truly tamed and added to this cars reputation for punishing the inexperienced or careless driver with a snap spin.  Many cars which used semi-trailing arms avoided the worst of the bump steer problem, and the related camber change problem, by having the suspension set so low that there was already considerable negative camber, and the toe was then set for this height. Since all of the suspension travel was now really above the "level" position the toe change was minimized, and thus the bump steer as well.  Larger rear tires also helped tame the rear end's antics, along with limiting suspension travel.  These cars used detail improvements to make the best of a "less than ideal" design.  The Fiero's bump steer problem is the opposite, the design does not have inherent bump steer, but the details cause it.
 

The bump steer problem can be solved, or at least minimized by several methods.  Manufacturers of Fiero suspension systems have come up with low cost fixes which reduce the problem significantly, and at least one more costly solution which should solve the problem, however, even with some low cost changes the average owner can reduce the problem to reasonable levels.   If all of the rubber bushings in the rear suspension are replaced with something with less deflection, such as polyurethane, the problem would not go away but it would no longer be increased by the additional unwanted motion caused by the bushings (Herb Adams actually recommends steel bushing replacements).   Stiffer rear springs and shocks will reduce vertical deflection and therefore the bump steer.  A rear anti-roll bar will keep the rear flatter and therefore again reduce vertical motion and bump steer.   None of these suggestions will completely eliminate the problem, but if you are on a budget, it will reduce the bump steer from the factory levels.

The Fiero's bump steer problem is more related to handling feel than actual handling.  It will not upset the chassis enough to be really dangerous at most normal speeds.  What it will do is put a real scare into the driver who first encounters it, and due to its effect on overall feel it will result in most drivers being a little nervous of their cars.

AND No one wants to be afraid to drive their sports car.

Ira Crummey