Muscle Mustangs & Fast Fords
1993 Ford SVT Cobra Rear Suspension And Rear Axle - Straightening the Slithering Part 2
Project Stolen Goods gets rearended and suspended.
Welcome back to the buildup of our '93 Cobra project car, appropriately named "Stolen Goods." No, it wasn't stolen, though judging from the condition and the price we paid, we did get a steal of a deal, picking it up for a mere pittance.
This month, we plan to finish up the Maximum Motorsports Road & Track suspension we began installing in the February issue. We also need to sling the newly built 8.8 rear axle beneath the Cobra. We've already received mail on this project-some suggesting we should return the car to stock, but other than using the story as a restoration guide, it wouldn't be much fun. Of course, we realize the potential value of the Cobra, so we won't do anything to it that we can't restore back to stock in the future.
In keeping the suspension setup relatively simple, we installed Maximum Motorsports' Road & Track Box, which utilizes the factory-style coil spring arrangement coupled with matched Bilstein struts and shocks, along with a host of other components that tighten up the suspension and make it work more efficiently.
Up front, we installed Maximum's coil spring and Bilstein strut setup along with caster/camber plates, aluminum steering rack bushings, a solid steering shaft, and urethane antiroll bar bushings and end links.
Bridging the gap between the front and rear components are Maximum's full-length subframe connectors that we welded in for superior chassis support. Yeah, yeah, we know the car has 1,331 miles on the chassis, and we want to keep it riding that way. Once you start working a car hard and throwing excessive amounts of power and torque load at it, the chassis can flex, and we wanted to practice some preventative maintenance in order to keep the snake straight.
The rear suspension of the car starts off with Maximum Motorsports' ride-height-adjustable, heavy-duty lower control arms and coil springs matched with Bilstein shocks. "These new Road & Track springs are made to our specifications," says MM's Chuck Schwynoch. "They are similar to H&R's Race springs. They are a nice com-promise, suitable for a daily driven performance car, and for open-track use."
MM's heavy-duty rear lower control arms utilize specially designed, three-piece urethane bushings at the chassis end and high-quality Teflon-lined spherical bearings at the axle end.
According to Maximum Motorsports, the rubber bushings of the stock control arms prevent the rear axle from maintaining its correct position under the chassis, leading to instability from rear steer. Most aftermarket rear control arms use hard, two-piece urethane bushings, Delrin bushings, or steel bushings, but these do not allow the angularity needed for the Mustang's suspension to articulate freely. This results in suspension bind that causes the rear tires to easily break loose. Poor traction and handling ensue, but this binding also causes damage to the torque boxes. Control-arm deflection, as a result of flexible bushings, is also the primary cause of wheelhop.
Maximum's front control-arm bushings have a hard centersection to prevent fore and aft deflection, and softer outer sections to allow the angular motion necessary to prevent bind. These three-piece urethane bushings are employed at the chassis end of the arm, rather than at the axle end to reduce noise, vibration, and harshness. Placing them there also keeps the control arm and chassis from getting too far out of line with each other.
The MM heavy-duty adjustable lower control arms we're using allow us to raise the rear ride height up to 2 inches or lower it as much as 1 inch, thanks to its weight-jack bolt, a piece similar to what NASCAR race cars use.
As for the upper control arms, Maximum does not recommend using urethane in the upper arms to restrict the side-to-side motion of the axle. "Because of the three-dimensional movement of the upper arms, severe binding will be induced as the suspension moves-with or without a Panhard bar," Schwynoch says. "This effect may not be evident in a drag-only car, but if you plan to drive the car on the street at all, we do not recommend urethane in the upper arms."
That being the case, we'll reuse the stock upper control arms for the time being and ditch them later on down the road when we install the torque arm. Until then, and after, we'll use MM's Panhard bar to locate the rear axle assembly. In relying on the four control arms to locate the axle, Mustangs can experience unstable and unpredict-able handling behavior while cornering near the limit. This is caused by the rear of the car moving sideways and attempting to steer itself without any input from the driver.
There are a couple of companies that offer large aftermarket sway bars to reduce body roll. When we queried Schwynoch regarding what to use on Stolen Goods, he replied, "The '93 Cobra sway bars are the best." Unfortunately, they weren't in our pile of parts that we grabbed with the car. We then asked why he liked the Cobra bars so much.
"The smaller-diameter Cobra front sway bar will result in a car with less understeer and better braking traction, especially on rough surfaces," Schwynoch says. "The stiffer the sway bar, the less independent the suspension acts. Bumps hit by the left front wheel result in suspension motion in the right front suspension. Very stiff sway bars, like the stock GT bar, are Band-Aids for springs that are too soft."
According to Al Kirschenbaum's The Official Ford Mustang 5.0: Technical Reference & Performance Handbook, '93 Cobra's stabilizer bars measure 1.125 inches in diameter in the front versus 1.30 inches for the stock GT, while the rear stabilizer bars are the same 0.83-inch diameter from Cobra to Mustang. The Cobra also received stiffer bushings in its stamped rear upper control arms.
Luckily, Stolen Goods' previous owner had the '93 Cobra sway bars stashed in his garage, but before we could mount the rear sway bar and Panhard-bar setup, we needed to get the 8.8 axle fixed up for extreme duty. We called the folks at Reider Racing and told them we had a housing and that was it-we needed everything from axle seal to axle seal.
Reider sent us every bearing, crush sleeve, and shim along with a 3.55:1 ring-and-pinion and a Torsen T-2R differential, which we chose on Schwynoch's recommendation. We brought the housing and box full of Reider parts to HP Performance in Jacksonville, Florida, where the highly capable Sean Story was assigned the task of rebuilding the empty housing.
After a three-hour journey across the Sunshine State, we were made aware of the fact that the axlehousing differential caps were missing. The caps are cast with the housing so you have to use the ones that came with it. Even switching them from side to side can ruin perfectly good bearings and the ring-and-pinion set.
A call was made to Stolen Goods' previous owner, but the caps were MIA. Luckily, we had scheduled a couple of other stories at HP, so the day was not entirely lost.
We returned, however, the following week, as HP Performance's proprietor Tony Gonyon had called up Bob Eubanks at Rusty Acres Auto in Jacksonville to procure a suitable replacement. Eubanks, who happens to race a few Mustangs himself, went into the auto recycling business in 1973, and by 1984 he specialized in Ford/ Lincoln/Mercury products.
Shortly after Gonyon's call, the Rusty Acres delivery truck dropped off a properly aged and bone-stock 8.8 axle assembly replete with Traction-Lok and 2.73:1 ring-and-pinion ratio. Everything short of the all-important differential caps, though, was replaced with the various items from Reider Racing.
In choosing a rear differential for Stolen Goods, there were a lot of avenues to check out. With Schwynoch's bountiful road-racing knowledge and experience, we asked him what he recommended and, based on his choice, we opted for the Torsen T-2R torque-biasing differential.
"A conventional limited-slip differential uses some sort of preloaded clutch to limit the speed difference between the two axles," Schwynoch says. "A Torsen differential delivers torque from the driveshaft to the rear tires based upon how much torque each tire can take before spinning. It uses worm gears to achieve this instead of clutches, so there are no clutches to replace or preload to adjust. The behavior of the differential does not change as it ages, and the Torsen does not heat the gear oil much as there is little friction (no clutches to slip)."
Another benefit is a constant axle endplay, which results in less kickback and a more consistent brake pedal height, at least with C-clip axles. Each rear tire gets 100 percent of the maximum torque that it can deliver to the ground, and thanks to the low break-away torque, the vehicle is easy to drive in the rain.
Some drawbacks to the Torsen include its higher price point relative to conventional limited slips, and its cast-iron case, which can crack under high shock loads that may occur during drag racing. Schwynoch pointed out something else about the Torsen, which only appears as the result of a poor suspension setup.
"If one drive wheel becomes completely unloaded (in the air), it will spin freely," he says. "The Torsen can't bias the axle torque in this situation. This should never be a problem with a vehicle that is designed to be driven on asphalt. If your Mustang is pulling the rear inside tire off the ground, you have something wrong with your suspension setup."
The T-2 Torsen was originally designed for C-clip axle applications, and Reider Racing now carries the T-2R or Racemaster version. The difference between these units is in the maximum TBR (torque bias ratio) they can deliver. TBR is the maximum ratio of axle torque 1 to axle torque 2 that the differential can deliver.
Schwynoch explains, "If the vehicle accel-erates in a straight line, both tires can accept the same amount of torque before either one spins. If the vehicle is turning left with a lateral acceleration of 1g, there may be 400 pounds of weight transfer from the left rear tire to the right rear tire. If the vehicle started with a static weight of 750 pounds on each rear tire, the left rear now has 350 pounds on it and the right rear has 1,150 pounds on it.
This means the left rear tire can take approximately 30 percent of the drive torque that the right rear tire can take before spinning (350/1,150=30 percent). The TBR that you need the differential to deliver is the ratio of the weights on each tire. In this case, we need a differential that has a maximum TBR of 1,150/350=3.3. If the differential in the vehicle has a TBR of 2.5, then under these conditions too much torque will go to the left tire, and it will spin.
In general, the higher the lateral weight transfer of the vehicle the Torsen is used in, the higher the TBR needs to be to keep from spinning the inside tire. Street driving has the lowest cornering forces, thus the lowest lateral weight transfer. Autocross has the highest cornering forces, with the highest lateral weight transfer, while road racing falls somewhere in the middle. The T-2 has a TBR of 2.5, while the T-2R has a TBR of 4.5. In most multipurpose vehicles we recommend a T-2R. For pure street driving, a T-2 will work fine.
The higher TBR of the Torsen T-2R will prevent the inside rear tire from spinning in high lateral weight transfer situations. We have not seen any appreciable downside to using the T-2R instead of the standard T-2, while there certainly is a downside to using the T-2 in some situations."
Having now given you a short dissertation on the Torsen T-2R differential, we must say installation was a snap, at least for HP's Sean Story who had our 8.8 assembly ready for action in about an hour. The Reider Racing installation parts certainly made it easier, and it was nice to call up one place and be able to order everything we needed.
With the rusty 8.8 now suitably fortified, it was back to the garage for sanding and painting prior to installation. A Scotch-Brite sanding pad and some brake-parts cleaner left us with a good surface to break out some self-etching primer in a spray can. We then coated the 8.8 with three cans of semi-gloss black enamel from the local auto parts store. While powdercoating the axle housing was our first choice, time constraints did not allow for it.
With the rear axle assembly now ready, we swung it under the back of the snake and attached it to the chassis using Maximum Motorsports heavy-duty lower control arms and the factory upper arms. The rear half of the Road & Track coil spring package was installed along with the Bilstein shocks and Panhard bar.
Next month, we'll turn our attention toward the braking system and wheel/tire combination, so stay tuned.