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The Iron Duke Resource Site |
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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.
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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
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