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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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Weight
distribution
and
anti-roll bars.
All
other factors being equal (which they seldom are) it is weight distribution
which has the greatest influence on cornering attitude. Yes this is an over
generalization but will serve to illustrate how handling is effected by
changes in roll stiffness. Newton's Laws of Motion tell us that the more mass
an object has, the more of a tendency it has to continue in a straight line
and the greater the force required to change its direction. This
translates into this automotive statement, the heavier end of the car will try
harder to go straight, or:
1)
Forward weight bias, understeer
2)
Rearward weight bias, oversteer
3)
50/50 balance, neutral.
I
have oversimplified up to this point so let's continue. An anti-roll bar
resists chassis roll by transferring some of the energy from the outside wheel
to the inside wheel on the end of the car it is mounted. This makes that
end of the car work harder during cornering. The end which works the
hardest is the first to loose lateral grip and increase slip angles, but the
cornering speed has increased.
If
I have lost anyone lets look at some examples. If a very front heavy FWD sedan
has no anti-roll bars it understeers (example VW Rabbit) if we add a front
anti-roll bar the understeer is worse, BUT the cornering speeds are still
higher despite the greater slip angles, if we then add a rear bar we get a
more balanced car with less understeer (on the early A1 chassis Jetta, VW
actually used just a rear bar, there was lots of body roll but a fairly
neutral car for a FWD). In a car with a serious rear weight bias such as
early Porsche 911s the tendency was to oversteer, as a result a front
anti-roll bar is used to produce understeer by making the lightly laden front
wheels do more of the work.
The
point to all of this is that the end of the car with the greatest mass is
already working hard, so putting an anti-roll bar on the lighter end will
compensate and improve overall balance. Pontiac did this with the 84-87
Fiero but may have gone a little too far in the interests of safety and ride.
Adding a rear bar will decrease the under steer by putting more of the load
back on the rear tires, BUT remember the rear weight bias, too much roll
stiffness at the rear can get us back to oversteer again (we do not want to go
there with a rear weight bias). This is of particular importance to the
people who have done V8 swaps and further increased the rear bias. So
how do you reach that balance?
Note:
All
of the following is in reference to the 1984-87 Fiero, I have no experience
with the 1988 at all.
Before
we begin keep in mind that none of these recommendations will be of any use if
the shocks, ball joints, tie rod ends and bearings are not in good condition.
Fix whatever is broken or worn before you modify anything.
1) Your
first concern should be to get rid of the unwanted motion in the rear
suspension which causes much of the twitchyness. Rear control arm
bushings should be replaced with polyurethane (along with the cradle bushings
if you don't mind the added noise and harshness) be warned, urethane bushings
can bind, squeak and moan, stiffer rubber bushings would be more ideal.
Herb
Adams
recommends using steel bushings at the rear, but this is rather extreme for
street driving. Now that the rear is somewhat tamed (the bump steer is still
present but there are companies who have developed kits to relocate the toe
links and reduce that or redesign the lower control arm and eliminate it
altogether) we can set out to balance things.
2) Second
on your list, leave the front suspension pretty much alone, it is much better
than it is given credit for. The only change here is to replace the anti-roll
bar mounts and end links with polyurethane pieces (leave the control arms
alone, the ride is rough enough and the deflection in these bushings do not
cause the havoc the rear ones do). If you have a V8 conversion or any
other which increases the rear weight I would go with the larger 1"
anti-roll bar, but for stock powertrains the stock bar is adequate, after all
it understeers already.
3) Third
step, add a rear anti-roll bar. The most readily available bar is the
7/8" available through several sources. This bar should work well with
the existing front bar on all stock models. This will make the car more
neutral, if there is now a tendency toward oversteer, go with the larger front
bar as well, but try the rear bar with the stock front bar first, this will
reduce understeer the most. Again I refer you to Herb
Adams
who uses a 1" front bar with a 1.25" rear, I don't think I'd try
this with an engine swap however.
If
you have that V8 sitting back there then go with the larger front bar, most of
the handling problems with V8 swaps is due to bushing deflection at the rear
so be sure to fix that first. With the swap you may actually want more
understeer then we would want with a stock powertrain since the much stronger
torque reactions of a V8 will cause throttle induced oversteer much more
easily than the smaller engines will. Early pony car Trans Am racers
actually used a Z-bar on the rear which worked exactly opposite to an
anti-roll bar (I guess you could call it a pro-roll bar:-) just to keep the
rear tires on the track for traction with the torque of the V8 engines. The
result was tremendous understeer which could be counteracted easily by gobs of
power on oversteer, it wasn't pretty but it worked. I'm not suggesting
this, but remember, get rid of the rubber bushings so your rear suspension can
do its job properly, then try the smallest available rear bar with a larger
front bar.
Ira
Crummey
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