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The Iron Duke Resource Site



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

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