Autocrossing the Fox: A Primer
Filed Under Fox Mustang
At this point, my autocross experience is pretty limited and my knowledge of Mustangs is only a little better than that. What follows is basically just compiled information I’ve picked up as the internet’s biggest Mustang Bench Racer. It’s meant only as a primer, folks!
What is autocross?
Basically, its a miniature road course set up in a parking lot or maybe some blocked off roads and marked by cones. The competition is divided into classes depending on car and modifications, the course being run one vehicle at a time, with each run timed. Every cone hit adds two seconds to your time and you usually will have three or four runs, with quickest time winning. 5.0 Mustangs will find themselves in one of the following classes; F Stock (FS), E Street Prepared (ESP), Street Modified (SM), C Prepared (CP), X Prepared (XP) or E Modified (EM).
The Fox Mustang
When someone refers to the Fox platform Mustang, they’re generally referring to the 79-93 model years. The 94-04 Mustangs are commonly referred to as the sn95, but are still Fox based. People who try to autocross either of these must be either sick, or hopeless Mustang nuts (or both?), as these are poor handling cars, plain and simple. The suspension design comes from the 78 Fairmont for goodness sakes! In 2005, the Mustang got a complete redesign with the s197 chassis and is much better handling, but the s197 isn’t the focus here.
All Mustangs from 1979 to 2004 use a strut front suspension. Strut suspensions are simple designs that use only one control arm, a spindle, a strut and a spring on each side of the car. This design can be manufactured inexpensively and is light weight, but it also limits the performance potential of the suspension. Dual A-arm suspensions can be designed to perform far better than a strut style suspension, but this doesn’t mean that strut suspensions all perform poorly; both the M3 and 911 use strut designs. And just because its a dual A-arm suspension doesn’t mean it handles well – check out the Mustang II’s Pinto sourced front suspension. The problem is, the strut design in the 79-93 Mustang plain sucks. From 79-93, every few years the k-member was tweaked for a little better geometry, so basically, the newer the k-member, the better off you are. In 94 the lower control arm (LCA) length is a little longer which widens the track which makes them a little better. The 96+ (when Ford switched to the modular motors) has even better geometry.
The rear suspension in all of these cars, except the 99-04 Cobra, is a non-parallel 4-link solid axle design. Here the upper control arms (UCAs) are at dramatically different angles to the LCAs so that using only these 4 arms the rear end is held in place during both torsional/twisting forces (acceleration and braking) and lateral forces (left and right turning). The problem with this is that when cornering, the UCAs bind as the car leans over. This is because the angles of the UCAs and LCAs conflict and need bushing material for compliance. When the compliance is gone, the suspension binds and the rear end suddenly loses traction, resulting in snap-oversteer. Many think (myself included) that aftermarket UCAs or harder bushings in both ends of the stock UCAs make this problem worse as they offer even less compliance, creating even more snap-oversteer.
How do you fix these problems? The most common way is to bandage them with higher spring rates and after market control arms. The real solution is to redesign the suspension. Companies such as Griggs, Maximum Motorsports, and Evolution Motorsports are taking care of this upper-end market. Most people who are more into the autocrossing V8 pony cars than Mustang brand loyalty choose a 3rd or 4th generation GM f-body (Camaro or Firebird). These are far better handling cars with the 3rd generation cars having a strut front suspension far superior in design to the Mustang’s. The 4th generation cars get a short-long arm (SLA) dual a-arm front suspension and both 3rd and 4th have a torque arm rear suspension. For what its worth, Griggs’ and Maximum Motorsports’ popular rear suspension redesign for the Mustang also utilizes a torque-arm.
That’s as deep as I’m going to go into the specifics of the Fox Mustang’s suspension geometry. If you’re interested in more specifics on the Mustang’s suspension, Miracerros.com has a really nice write-up on Late Model Mustang Suspension Basics.
FS
Just because this is a stock class, doesn’t mean you can’t modify anything. Air filters, struts, shocks, tires, brake pads, cat-back exhaust and the front sway bar can all be changed. The Fox is completely outclassed by the sn95 cars, and the newer s197 chassis is even better.
ESP
Street Prepared and Prepared work with cars classified along “lines.” The 79-93 Mustang and 79-86 Capri are all listed on the same line, so parts can be swapped between them. Here intake and exhaust are open, struts, springs, wheels, tires, swaybars, panhards, torque arms, suspension bushings, camber/caster (CC) plates can all be changed as well as updating/backdating between designated years. You cannot add non-stock subframe connectors (currently), non-stock rear gears, aftermarket control arms, do much internal motor work or add forced induction. The Fox chassis hasn’t proven to be competitive due to its poor suspension geometry, flexible chassis, and low power (comparatively). The sn95s do far better here because of the stiffer chassis, better front grip, and better power of the modular Cobra or 351 Windsor from the 95 Cobra R.
SM
At first glance, SM sounds like a good fit for a Fox as it allows several of the common upgrades; subframe connectors, gears, cowl induction hoods and radical engine modifications. However, you can’t change any of the control arm mounting points, and you can’t replace the k-member, so you can’t fix the poor front suspension’s geometry. In SM, a rear wheel drive car has to weigh 1800 pounds, plus 200 pounds per liter. This puts a stock 5.0 Fox Mustang at 2800 pounds. Stroking the motor or adding forced induction will cause more weight to be added. However, the most the car will have to weigh is 3100 pounds. In general, SM is a faster class than CP and doesn’t allow you to truly “fix” the suspension geometry, which is far more important than horsepower in autocross. In a nutshell, you should be able to build a more competitive dual purpose street-autocross car for CP than for SM.
CP
C Prepared has to be the coolest class in Solo2, period. CP cars tend to be crude, gutted, stiff V8 sedans with loud exhausts and very fat tires. The high end prepared cars have completely redesigned front and rear suspensions that are usually adjustable, high rpm, high horsepower 310 CID V8s (cars with engines over 310 have to weigh 3000 lbs, 300 more than 310 engines) and radically flared bodies to cover the massive slicks. Having a tubular k-member adds 10% to your base weight as it causes your car to be considered “in excess” as a tube framed car. You can modify the heck out of your stock k-member though. And forced induction is a no-no.
XP
XP in an interesting class, as it’ll allow you to run any engine you want, and figure out your car’s minimum weight from a formula. You could run a LS Chevy in your Fox if you wanted. However, “in excess” cars are not allowed, so you couldn’t do a k-member swap. But you could still modify your k-member. XP will end up a hella-fast class though.
EM
So, where do you autocross your twin-turbo modular powered, tubular k-membered, IRS equipped FOX? EM, that’s where. And you’ll be uncompetitive on anything other than a local level. The typical cars here weigh half as much as a Mustang, have as much power as any Mustang out there, and are usually as, or more, purpose built than a CP car.