Muscle Mustangs & Fast FordsHow To Chassis Suspension
1989 Mustang LX Rear Suspension Upgrade - Bringing Back The Bite
Overhaulin' An Outdated Rear Suspension To Handle Today's Horsepower.
Drag racing differs from other forms of motorsports in that it is a sprint to the finish line, unlike other segments where the cars drive around endlessly looking for the finish line. The short sprint down the track leaves little, if any, room for a mistake, and there are countless variables that have to be just right for your Mustang to perform its best.
It all starts with a hard-launch that requires your tires to grip the track surface. Thankfully, '79-'04 Mustangs feature a four-link-style stock suspension, which is quite effective when modified for drag-racing applications. The rear control arms aren't parallel like a racing four-link, but the factory suspension carries similar principles.
This month, we upgrade a '89 Mustang LX with UPR Products' Pro-Series rear suspension and adjustable rear-spring kit, adjustable Strange shocks, and some new guts inside the 8.8 rearend housing. Each modification was selected to provide grip at the track without sacrificing street performance.
DMC Racing of Halifax, Massachusetts, handled the work, which included more than just bolt-in parts. The hardest segment of the installment was the adjustable spring mounts. It was easy for the DMC crew, but it might present some challenges for someone installing the parts in their driveway due to the welding aspect.
"The stock suspensions in Mustangs are great. With a few modifications you can make adjustments to help plant the rear tires and get the nose in the air. Just look at Outlaw Drag Radial cars. The only two 6-second cars on drag-radial tires are Mustangs, and both run on stock-style suspension. Most of the low 7-second stock-suspension-type cars are Mustangs," commented Dennis MacPherson of DMC Racing. "Sure, the components are different in those cars than the setup we installed in this '89 coupe, but the same basic design is shared by both."
Running fast on stock suspension goes back to the early '90s when going mid-9s was a huge accomplishment. Most racers utilized nothing more than a set of Southside bars, non-adjustable square upper control arms, and a pair of 50/50 shocks. It made for some wild rides, as the power under the hood wasn't effectively channeled to the rear tires. Fast Mustangs across the country would leave the line in a corkscrew fashion as the body rolled excessively. The Southside bars would lift the body and plant the tires rather violently due to the lowered attaching point at the rear housing--and without any adjustable components, there was little the racers could do except try to limit the front-end travel and move weight around. Some of the early heroes of stock-suspension madness include Jimmy LaRocca, Ronnie Crawford, and Tim Lynch. All three, along with many other notable racers, seemed to hit a wall (no pun intended) in the mid-to-low 9s. That is, until the aftermarket responded with adjustable components and a better understanding of how things work under the unibody.
The reason the cars acted violently was because of the dynamics of a rear suspension setup. The rear suspension's action during a drag launch is simply described as controlling the rear axle's rotation due to the driveshaft turning. The control arms limit the rear end's ability to rotate (opposite of the direction of the tires), and it transfers that energy into leverage to lift the front end and plant the rear tires. In addition to the control arms controlling the axle rotation, the rear shocks control the rate at which the rear separates from the body. The UPR Pro-Series control arms and Strange shocks will allow us to control the action of the rear and help our car plant the tires more effectively. UPR also provided us with a drag-race-style antiroll bar that will help keep the body level as it leaves the starting line.
Another benefit of the Pro-Series control arms is the solid mounting points, which eliminates the rubber bushings that come from the factory. The rubber bushings collapse and distort under stress, causing inconsistent and unpredictable control arm movement. "Bushings flex and deflection occurs, which causes the control arms to lose adjustments. You then cannot tune for the track conditions. The UPR Pro-Series control arms have Heim joints, or spherical bearings, that are locked in and never change adjustment," commented Jeremy Martorella of UPR.
The Pro-Series upper controls also use a spherical bearing on the rearend housing to prevent binding. The upper control arms control side-to-side movement of the rear end, so the spherical bearing provides stability in straight-line performance but won't bind up on the street when the housing is moving up and down. A downside to the solid bushings is that the ride is rougher.
For those who don't want the solid mounts, there are control arms that have urethane bushings, which are much tougher and better than rubber, but not as harsh as the solid mounts. UPR does offer more street-friendly and less racy control arms with urethane bushings. For those who demand uncompromised drag-racing performance, there is the XD Pro-Series lineup that feature chrome-moly Heim joints, grade 8 bolts, and tool-steel sleeves.
One item we didn't install--but will in the future--is a UPR antiroll bar, which will be welded between the frame rails and has two arms that attach to the rearend housing. "The antiroll bar limits all body roll. If you watch cars at the track, a lot of the time they are twisting. Installing our antiroll bar eliminates the body twist by keeping the rear parallel to the body, which will lower reaction time and also help the car hook better," added Martorella.
He went on to inform us that in a typical 11-second car, the 60-foot times are consistently reduced by 0.06-0.07 of a second with the UPR antiroll bar. The reason we didn't install the antiroll bar was due to our cheap exhaust system that blocked the mounting of it. Most exhaust systems clear the UPR bar; ours was old, nearly 10 years ago. Martorella suggested the removal of the antisway-bar links when driving on the street. The links will break under the stress of street driving. It's meant for track use only. Our goal is to install the system down the road, when the tailpipes are replaced with a set of turndowns.
Strange Engineering was tapped for a pair of its adjustable rear shocks, built specifically for '79-'04 Mustangs. The shocks feature rebound adjustment to allow the user to compensate for a fast or slow rate of rebound.
In drag racing, setting the shocks up loosely (setting of 1) will allow the body to move quickly and help plant the tires. The loose setting works great when traction is poor as the tire will need to be planted harder. The downside to the loose setting is tire shake and/or wheelhop. As the tires plant and distort under load, it hooks and thrusts the car forward. But as the torque of the engine takes over, the tires want to eventually unwind and return to their natural state. If the body/chassis is moving too quickly, the tires will react accordingly and cause shake or hop. To counter this, the shocks can be stiffened to slow the body movement and lessen the hit to the tires. On tracks that offer excellent traction, it's good to stiffen the rear shocks to help ease the hit. At the track, we recommend starting at a shock setting of 2 for a serious street/strip ride. Move up a few clicks for slower rides, and a setting of 5 is ideal for the street. Those settings are, of course, starting points and can be adjusted up or down to properly suit your combination.
Adding a good suspension system is useless without the inclusion of sticky rear tires. The suspension can work perfectly, but if the tires don't grip the ground, then performance will suffer greatly. Adding a set of sticky meats can and will exploit the weakest link inside the rear. We have a pair of 275/60-15 Mickey Thompson drag radials--a.k.a. rear breakers--under the car right now. Our inventory also includes a pair of M/T ET Street 28x12.50 (measuring 10.5 inches wide) DOT tires mounted on another set of wheels for track use. Just bolting on the radial tires made us think twice about the differential and axles. Our old combo used a set of M/T ET Drag 26x10 slicks--far smaller than our new tire line up. The coupe always spun a little out of the gate, sparing the puny 28-spline axles and traction-lock differential.
Our subject LX has survived some 10-second runs and countless 11-second performances over several years with its stock axles. The differential was replaced once, but with a regular Traction-Lok, nothing special. We attribute its lifespan to the fact that the car is equipped with an AOD transmission, sans transbrake. The very nature of the numerically low First gear of the transmission (2.40:1), coupled with a mild 3,200-rpm stall speed, meant this coupe left lightly compared to its stick-shift brethren. That was about to change due to the forthcoming powerplant upgrade.
Unlike the late-model GM F-body cars, the Mustang was built with a stout rearend housing. A few simple mods are all that is required to get the Ford 8.8-inch rear strengthened. Larger axles and a tough differential go a long way in the Mustang world. We tapped Latemodel Restoration for its five-lug conversion kit. Since we wanted to add 31-spline axles, it was a good time to increase our lug count from four to five. Adding five lugs opens up the choice of wheels on the market for both track and street use. Plus, the lug strength is increased due to distributing the load to five lugs, as opposed to four. We have seen the lugs sheer off on quite a few 10-second cars, so we wanted to avoid that unpleasant experience.
A quick phone call to Summit Racing netted us an Eaton Detroit Locker differential to fit the 31-spline axles. The Detroit Locker diff is a nasty unit that is more than capable of handling 9- and 10-second runs, but it still has limited-slip capabilities for subtle street manners. The same 3.73 gears were re-installed in the rear, but only after a thorough inspection. Miraculously, after 15 years of use, the gears looked great without any hint of wear. We saved money by reusing the gears, but still added new shims and bearings that we bought from the local NAPA store.
The modifications took about a day with two DMC techs working on our coupe. The process required a little cutting and welding in order to install the UPR adjustable spring perches on the body. Your author found an old set of five-lug Weld Draglite wheels in the basement for the car. The wheels added that classic and timeless 5.0L look to our coupe.
The new rear-suspension setup complements our recent front-suspension overhaul that included a tubular K-member and A-arm setup, along with coilover struts. The front-end mods removed 67 pounds, including the skinnies. The reduced weight will help weight transfer and let the rear suspension work even better in planting the variety of DOT tires we have for this car. Thanks to UPR, Strange Engineering, Latemodel Restoration, Royal Purple, Mickey Thompson, and Summit Racing, we have brought back the bite to go along with the supercharged bark under the hood.