Mustang MonthlyHow To Drivetrain
Inside the Full-Floating Rearend
Full Revolution: Technology drives the full-floating rear axle into the mainstream
Some technology that seems new is usually derived from older technology. The new Ford Coyote engine screams of the vintage 427 SOHC engines of the ’60s. Sometimes, technology or design is reused because other technologies have improved enough that their use becomes a requirement. The most recent application for this is the full-floating rear axle. What used to be a luxury item for high-end race cars has now become a staple for many performance street cars.
Full-floating rear axles have their history in large trucks and race cars. The primary reason for using a full-floating rear axle is simple—it takes the weight of the vehicle off of the drive axle and places it on the axlehousing itself. The result is less stress on the bearing, and if the axle breaks, the wheel doesn't come loose from the car.
All Mustangs came from the factory with a semi-floating axle. This was because of the costs and the fact that the need for a full-floating design isn’t necessary on most street cars. But improvements in tire technology and horsepower gains in street cars has pushed the bar for the need for a full-floating rear axle down into the street car arena, and may be a requirement for Pro Street and other high-performance street Mustangs.
We took a trip to Michigan to meet with Shaun Burgess of Street or Track to get the lowdown on full-floating rear axles and how they work, and why you might consider one for your high-performance build. Street or Track equipment is riding on many of the top SCCA and vintage road race Mustangs, and they market a full-floating axle specifically designed for the Mustang.
Semi-floating: The axle in your Mustang is a semi-floating design, which means that the axle is directly connected to the wheel, and it rides on a single sealed bearing. The bearing allows the axle to spin but makes the connection to the axle. The single bearing is taking all the side load and weight of the vehicle and transferring the weight to the axlehousing through the bearing. When in a hard corner, the bearing is taking both the load of the tires pushing against the bearing and the weight of the vehicle. Even though the bearing is pressed on to the axle, the axle can move in and out laterally and rotate up and down slightly, causing the brakes to move in relationship to the braking surface, which can cause poor brake performance. If the axle itself breaks, the wheel can become dislodged from the axle (we have all seen the photos and videos of the race car losing the rear wheel in the corner or on the starting line at the dragstrip).
Shaun reports that with the improvement in tire technology, the forces on the single bearing are amplified with more tire grip and less sidewall flexing. The energy is going right into the bearing, and with today’s horsepower numbers you are putting even more stress on the axle and the braking is greatly reduced. Shaun sees one symptom of this braking issue is pad knockback due to axle movement, which then requires the driver to pump the brakes a couple of times to square everything back in place right before going into a corner.
Full-floating: Shaun said the best way to think about the full-floating design is to think about the front spindle on your Mustang. The brake hub rides on two tapered bearings, which spreads the load of the wheel out over a larger area. In a full-floating axle, the axle is not attached to the axlehousing at all; it is allowed to “float” inside the axlehousing. A tubular shaft is mounted to the end of the housing and acts like your front spindle, only it is hollow to allow the axle to meet up with the wheel assembly. A hub rides on two bearings and is allowed to spin like your front rotor (think 4WD), and a splined drive plate is mounted to the end that the axle is actually driving. The weight of the vehicle is transferred through the wheel to the spinning hub and direct to the axlehousing through the two bearings, not the axleshaft. With the brake system attached to the axle and the load moving together, the brake surfaces stay in alignment with each other.
One of the biggest advantages is that the dual tapered bearings are used in the full-floating design. Shaun explains that “the tapered bearings are designed to take high loads in radial and axial rotation. The straight bearing on the semi-floating axle is not really designed to take the lateral loads of modern tires and big horsepower.”
Finally, a big safety reason for using a full-floating axle comes from the fact that the wheel is attached to the housing and not hard-mounted to the axle. If the axle should break, the wheel is contained and held to the axle and doesn't go rolling off Herbie-style.
Braking Now that the axle and wheel are in place, one of the big advantages to having things not move is the ability to take advantage of fixed brake calipers. With most full-floating or semi-floating calipers, the calipers themselves are moving on pins, which can become misaligned. With the semi-float rear axle, the caliper is attached to the axlehousing and the rotor is attached to the axle. With hard lateral movement in a corner, the axle will flex and take the rotor with it, and combined with caliper movement the two braking components can become misaligned and the brake would become less efficient.
With the fixed caliper and full-floating rear axle, both the caliper and the rotor are attached to the axlehousing, so they won’t move; or they will move in relation to each other. With no moving calipers, the only thing moving is the pistons in the calipers, so the brakes stay in alignment in the corners.
Street or Track assemblies
Street or Track has developed their axle assemblies with the vintage Mustang in mind. They are available in stock width for all first-generation Mustangs, and can be shortened for tubbed cars or cars requiring a different width. The all-new 9-inch housing boasts 3-inch axletubes, AN-style vent fitting, and separate fill and drain plugs. The spindle snouts are CNC-machined and welded to the housing. The drive spindle and drive plate are heat-treated forged units. Although they have yet to report an axle break, the axleshafts are forged 31-spline units to mate to a wide choice of differential gears. Custom center section offsets are also available. The axle can be adapted to a variety of rear suspensions, from standard leaf springs to Street or Track’s own 3-link systems. Street or Track takes advantage of the fixed calipers and offers a variety of forged brake calipers to work with your wheel size. They can also adapt drum brakes to the floater if your class requires it.
Our vintage performance/restomod Mustangs are now old combined with new, and with today’s low-profile tires and even bigger power gains from the engines, a full-floating rear axle that was once relegated to the likes of a heavy truck and all-out race cars is now a real consideration for some high-end street, race, and drag cars. They provide the safety of retaining the wheel should the axle break, and keep the brakes aligned and operating in hard corners. With the addition of modern suspension designs, Street or Track has our vintage Mustang competing on equal terms with late-model cars on the track and on the street.