Dale Amy
July 1, 2003

Horse Sense:
If you've seen the Reider Racing name and can't quite place where it was, it was on the side of Joe Silva's Pro 5.0 racer. The company's logo has been on the side of the turbo racer for the last five years.

Despite an appellation that does nothing to glorify its hard work, your Mustang's rearend plays a number of essential roles, perhaps the most obvious of which is changing crankshaft/driveshaft rotation into rear-wheel rotation. Additional tasks assigned to the rearend assembly's various components include reducing input driveshaft speed to output axleshaft speed, while simultaneously increasing output torque, and dividing that torque between the drive axles. Production rearend design must also permit a difference in speed between these two axleshafts, primarily to facilitate cornering.

Major players in this whirring ballet include the ring-and-pinion gearset, the differential assembly, a pair of axleshafts, and a housing to keep everyone playing well together. Here we're looking at the specific roles of these various components, and why it may become necessary to upgrade them for reasons of performance or durability.

To obtain subject material for our photos, and for valuable expert help on the subject, we turned to Thomas L. Reider, whose self-described vocational specialties are "gears and rears." One of Tom's companies, Precision Gear, is a manufacturer of high-performance ring-and-pinion gearsets for street, off-road, and race vehicles, while his other corporate enterprise, Reider Racing, is a major distributor of driveline components from a variety of manufacturers. If it's downstream of your Mustang's transmission, Tom not only sells it, but he also knows all about it. In addition, he's a Ford enthusiast-and a drag racer when his hectic schedule permits-so trust us, if anyone can help save your rearend, Tom can.

The ring-and-pinion gearset's primary function is to "bend" the torque of the rotating driveshaft through 90 degrees to an orientation suitable for driving the rear wheels. But equally important is its task of multiplying torque available at the axleshafts by gearing down their speed relative to driveshaft speed. These latter two functions are a direct result of the input gear-the pinion-having fewer teeth than the output gear, known as the ring. Dividing the number of teeth on the ring gear by the number on the pinion determines the axle ratio (not to be confused with the final drive ratio, which is the product of the transmission's top-gear ratio times the axle ratio).

As an example, the factory 2.73:1 axle ratio found in most 5.0 Mustangs has 2.73 times as many teeth on its ring gear as on its pinion. This particular ratio means that the ring gear (and, subsequently, the axleshafts) will turn only once for every 2.73 turns of the pinion (i.e., the driveshaft). But at the same time-and this is critically important-torque input by the pinion will be multiplied 2.73 times by the ring gear on its way to the axles. So then, input torque times the gear ratio equals output torque, while input rpm divided by the gear ratio equals output rpm. Why? Because for a given horsepower level, the product of torque times rpm is a constant-reducing the rpm causes torque to rise, and vice versa. Thus, increasing this ratio between ring-and-pinion teeth results in even lower axle speeds and proportionately higher torque multiplication.

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With that little caveat out of the way, one of the most cost-effective ways to improve the seat-of-the-pants feel of a Mustang is to install a numerically higher gearset. The most commonly seen upgrade ratios for the 8.8-inch or 7.5-inch axle are 3.08:1, 3.27:1, 3.55:1, 3.73:1, and 4.10:1 (steeper ratios are also widely available, but really only have application on a pure race car). Street-driven pushrod Mustangs are often teamed with 3.55 or 3.73 gears for all-around use, while the less torquey but more rev-happy 4.6 modulars respond well to a bit more gear in the 3.73-4.10 range.

These are generalizations, of course. Supercharged or turbo'd cars, for example, can get by with much less gear-due to their prodigious power output, they simply don't need as much torque multiplication. In the end, vehicle use should be the ultimate deciding factor. A 5.0 facing a daily 100-mile highway commute, for instance, wouldn't be the best candidate for a steep 4.10 gearset.

The internals of the pictured conventional, or "open," differential reveal the basics of differential operation. The top and bottom spider gears are pinned to the diff case, and this crosspin and spider assembly thus rotates at the speed of the case as the bolted-on ring gear drives it. The spiders mesh with side gears, which are splined to the individual axleshafts (not installed here). But these spiders are also free to rotate around the crosspin itself. Under ideal traction conditions in a perfectly straight line, both drive axles are driven at identical speed by the spider/side gear interface. In a turn, however, the outside wheel can turn faster than the inside, since that faster-moving side gear simply causes the spiders to rotate around the crosspin, in turn causing a proportional decrease in the relative rotational speed of the other side gear and axle. The open diff's problem is that it has no mechanism to stop one wheel from spinning at full speed while the other sits uselessly stationary. It's important to understand one additional attribute of gears in mesh. As input load, or torque, increases, so does a separation force between the spider and the side gears. In other words, the side gears are forced laterally outward, away from the spider gears, under the loads applied during acceleration. This gear separation force is the key to how limited-slip diffs operate. However, in the open diff pictured here, this lateral force is not utilized and acts upon simple thrust washers on the outer ends of the side gears.

The ring gear is bolted to the differential case, or ring-gear carrier, inside which is another set of gears whose job it is to distribute torque between the right and left axleshafts (some of the accompanying photos illustrate the basics of how it's done). The differential takes its name from the fact that it must do this in a way that allows each axleshaft to rotate at different speeds, as necessary when navigating a corner where the outside wheel must travel farther than the inside in the same amount of time.

If you only drove in a straight line on perfect road surfaces, you wouldn't need a differential at all. Welding the ring gear solidly to both axleshafts would suffice in a crude kind of way. A drag racing spool is basically just that, locking both axles permanently together for maximum forward traction, making it ideal for single-purpose quarter-mile cars, but also making it a device that must be avoided like the plague on the street, because a permanently locked rear axle is an absolute handling nightmare that does everything possible to stop a car from turning.

Still, for best forward traction-whether on the street, strip, autocross, or road course-we need power going to both drive wheels as often as possible. Two contact patches are better than one. This means there is no real performance use for an open, or conventional, dif-ferential. Its design certainly allows each axle to turn at its own rate, but the open diff has no mechanical means of limiting this speed differential, and will always route power to the wheel with the least traction-great for one-wheel burnouts, but little else.

Luckily, current V-8 Mustangs are factory equipped with a limited-slip differential called the Traction-Lok that, as its name suggests, does rein in this speed difference between axles, pro-viding grip for both rear wheels while still permitting basic differential func-tion when needed. It does this using clutch packs, which are quite adequate for routine street use but don't hold up well to the abuses of a dragstrip.

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In addition to the race-only spool, the aftermarket is chock full of high-performance differential designs, the full details of which are beyond the scope of this guide. Suffice it to say the choices come down to (A) a locker-type diff, where the axles are automatically or manually either 100 percent locked together, or completely unlocked, at any given time, or (B) a variety of limited-slip designs in which each drive axle is incrementally bound tighter to dif-ferential case speed as torque load increases, but which are never literally locked together.

The right and left axleshafts form the link between differential and wheels. We've already seen how the axles are splined to, and driven by, the diff side gears, and retained by C-clips inside the diff. The wheel ends of the axles have a machined surface area where they are supported by bearings pressed into the outer ends of the axle tubes.

Axles are obviously rather long and narrow, and are subject to heavy torsional (twisting) loads. Breakage is not unheard of. Tom Reider explains it best. "The capability of an axle to carry a load or to resist breakage in torsion, is a function of the diameter of the axle," he says. "The larger the diameter of an axle, the greater its load-carrying capacity. The strength goes up as a square function of the radius, so that small changes in the diameter of the axle make large changes in the capacity of the axle to carry torsional load."

Factory axles have 28 splines and are 1.2 inches in diameter (at the splines). Though failures on the street are exceedingly rare, these have limited life expectancy under the brutal shock loads of a sticky-tire drag racing environment. The most common upgrade is a 31-spline axle, with a diameter of 1.31 inches. Though this seems to be an insignificant difference in cross section, remember that a big increase in strength results from just a small change in diameter. Naturally, with greater spline count and diameter, you can't just slide new axles into your existing differential-the side gears must be splined accordingly. But if you are at the point where you need bigger axles, you'd also better be looking for an upgraded differential anyway.

Potential axle failure raises another issue. When an axle breaks, it's almost always at its point of smallest diameter, which is either at the spline or sometimes just outboard of the spline. Since the axle is retained by a C-clip at its innermost end, such breakage would allow the lengthy outboard portion of the axle and its attached wheel to say adios to the housing and become an unguided missile. The cure for this is something called a C-clip eliminator kit, which changes the retention point of the axle to the outer ends of the axle tubes. That way, when breakage occurs inboard at the spline area, the axle and wheel stay in the housing.

That's about all the room we have for this subject right now, other than to say that NHRA rules require C-clip eliminators for any car running 10.99 seconds or quicker.

The Mustang's "solid" axlehousing consists of a pair of 3-inch-diameter steel axle tubes pressed and welded into a cast-iron centersection. Both the pinion and the differential/ring gear assembly are installed from the rear of the housing before insertion of the axleshafts. A pair of stout bearing caps retains the differential. In factory form, these internals are concealed behind a stamped-steel cover secured by 10 bolts.

Other than the shortening of the axle tubes necessary if you go to C-clip eliminators, there's only one way to upgrade the axlehousing itself, and that's through the addition of a cast-aluminum rear cover, or girdle. The need for this goes back to the separation forces between meshing gears that we mentioned earlier. Interaction of the pinion and ring gears tries to push the ring gear/differential assembly both sideward (to the driver's side) and rearward, under forces sufficient to distort the housing under extreme shock loads such as those endured at the dragstrip. Substitution of a chunky, cast-aluminum girdle for the flimsy factory stamped-steel cover helps initially by stiffening the aft section of the housing. But many aftermarket girdles, such as the one from Ford Racing Performance Parts, also have additional bolts located so as to preload the differential bearing caps. The loads exerted on the caps are distributed to, and partially borne by, the heavily ribbed structure of the girdle.

That's it. We're tired. We're gonna rest on our rearends for a while.

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