Work, Horse

Project Mustang has finally seen some autocross action, so the official process of evaluating and sorting has, at long last, begun. Our first autoX outing, with the Steeda G-Trac pieces and Goodyear GS-CS tires in place was a remarkable success considering the car has not really been tweaked much, if at all, yet. We finished a close third in Street Modified (behind two AWD cars in the rain) and we were faster than E Street Prepared.

Let’s first address the subject of Fox Mustang rear suspensions which are notorious for doing extremely odious things in the heat of battle. The problem with the Mustang’s rear end, unfortunately, is its inherent design. The rear suspension of the Mustang is a four link. It has two nearly parallel links out near each end of the axle to locate the assembly, then two links that attach from the diff housing to the chassis that are at about an 80º angle to each other. The reason for these non-parallel arms is to control both lateral and front-rear motion of the axle with as few pieces as possible.

In theory, and in limited practice, it works. But problems soon crop up when the car is pressed a little harder. Since the various control arms move in non-parallel arcs to one another, they eventually bind as the arcs begin to conflict. This bind effectively stiffens the suspension and send the wheel rate skyrocketing, resulting in a loss of traction and snap oversteer.

Basically, there is no way to “salve” this problem using the stock suspension geometry.

The Steeda pieces that replace the factory pieces will bind, as well. Any control arms that simply replace the factory arms will also bind, there’s just no way around it. What the Steeda pieces do, however, is use specific bushing densities to keep the bind as controlled and linear as possible when it does occur. It’s a band-aid, certainly, but if you must run stock-type suspension due to rules/packaging/budget requirements, it’s a pretty effective band-aid.

The only way to truly “solve” the bind problem is by running a completely different type of rear suspension alltogether. This means either a Panhard rod/Torque arm setup, or Steeda’s five-link setup. The PR/TA setup uses a Panhard rod to control side-to-side motion of the axle, and a torque arm that attaches from the differential to a poin near the transmission to control axle twist and front/rear motion. It’s a good solution, but it has it’s drawbacks, namely lack of ground clearence, axle hop under braking, and legality issues in certain racing arenas.

The Steeda five link also uses a Panhard rod to control side to side motion, but uses a pair of parallel upper arms to control axle windup and front/rear motion. It doesn’t have the ground clearence issues of the torque arm, but the extra hardware on top of the axle necessitates the use of side exit exhaust.

Great volumes have been written by fans of both systems on the internet, but we hope to do an actual test in the near future to uncover the good, bad, and ugly of both of these types of systems.

In the meantime, now that our Mustang handles a lot better, we needed to do something to help keep us in place better than the SN95 spec seats we were using. Some Corbeau Corsa II seats fit the bill, being that they had the holy trinity of light weight, comfort and reasonable price sewn up.

Now we get to start tweaking. We’re playing with some different brake compounds on our Baer Sport kit (KFP Blues seem to be a good place to start), and Steeda says that if we use adjustable upper control arms we can get a little more bite out of the rear end by setting the pinion angle more precisely. They also say that their five-link is THE hot setup for the Mustang rear. We’re also going out west to visit Griggs Racing and Maximum Motorsports, two heavy hitters in the Mustang performance wars, to see what kind of good they have cooked up for these cars.

Look for the first full installment of Project Mustang in the December issue of GRM

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