Should you use a Full Floater Rear End?
A Grand National Full Floating Axle Setup Solves Several Issues
With updated suspension systems, high horsepower and large sticky tires, car owners are pushing the limits of their street car further than ever before. From auto crosses to drag-and-drive events, there are a lot of high powered cars on the street today that are using race car level parts. With the higher stresses of a heavy street car, sticky tires, and lots of power, these parts commonly used on the racetrack are making their way into the muscle car market. One of those items is a full floater rear end housing.
What Is a Full Floater Rear End?
A full floating axle mounts the axle hub directly to the axle housing with large bearings and uses a separate, splined axle shaft between the differential and the hub. There are many benefits to this style of axle housing for high horsepower street use. But before we delve into that we must first explain how your standard semi floating axle works so you can understand why a full floating rear axle is so beneficial.
Your standard GM 10-bolt and 12-bolt muscle car housings, as well as the popular Ford 8.8-inch axle housing are considered semi-floating axles and are designed to retain the axles at the differential end of the axle shaft using C-clip retainers that slide into a groove machined into the end of the axle shaft.
In a standard flanged Ford 9-inch housing, the axles are held in at the outer end using a bolt-on bearing retainer. If a C-clip fails or an axle breaks, you could lose a wheel off your vehicle. We’ve seen countless dragstrip launches where the rear wheel and a portion of the axle exit stage left (or right!). The 9-inch upgrade is popular due to retaining the axle right at the axle bearing, but an axle failure right at the bearing is still possible, allowing the loss of the wheel.
With a semi floating style of rear-end housing the axle must support the weight of the vehicle, cornering loads, and torque loads. Another issue with a semi floating rear end is disc brake pad “knock back.” Most applications mount the caliper directly to the rear end housing. When cornering hard on track, the axles will deflect. Your rotor, which is seated on the axle flange, moves, and pushes the caliper pistons back into the caliper. The result is you will not have rear brakes until the brakes are pumped back up, pushing the pistons back out until the pads touch the rotor surface. That lump in your throat feeling of “no brakes” until you give the pedal a quick pump or two after hard corning on track is the result of knock back.
What Are the Advantages of Full Floating Vs Semi Floating?
Upgrading to a Grand National style full floating rear end has many benefits. Think of the design like your front spindle. The rear hub rides on two tapered roller bearings directly on the full floater axle tube itself. The axle “floats” between the drive flange plate mounted to the hub and the differential itself. The full floating axle drive flange is sandwiched between the floating axle hub and the wheel, which is driven by the wheel studs. The vehicle’s weight and cornering loads are now on the full floater axle housing instead of the axle itself. The design of the full floater rear end means the axle assembly can handle more vehicle weight and more g-forces in cornering and braking.
The only force now being applied to the axle itself is the torque load to drive the car. This makes a full floating axle the ideal setup for a high horsepower car. Also, now you do not have any of the normal axle deflection from cornering loads. This eliminates the pad knock back issue for those running autocross, road courses, or just some fun-spirited driving! Another benefit of the full floater rear end is the ease of removing the third member. You can often leave your wheels on and just remove the end caps of the hubs (depends upon wheel design). Then pull the floating axle out of each end of the full floating rear end to remove the third member. This allows quick and easy gear ratio changes for different track conditions or types of road course tracks. No more pulling a standard axle, tearing up bearings or grease seals, etc. just to change gear ratios or to switch to your spare 9-inch third member.
So, after reading the above why wouldn't you choose a full floater rear end? One issue you may run into is wheel fitment with the drive flange used with a full floating axle. Typically, this not an issue on race cars, but for street wheels you must be cognizant of the wheel’s center bore size to allow clearance for the drive flange used on the full floating vs semi floating that simply uses a relatively flat axle flange to mount the wheel. Below is a drawing with the full floater axle drive flange dimensions to check clearances or provide these dimensions to your custom wheel manufacturer.
The brake caliper bracket is typically welded to the rear-end housing, though there are some clamp-on versions available. If you ever decide to change the size of your brake rotor diameter you would have to cut off your caliper bracket and weld on a new bracket. We encourage most people to run at least a 13-inch rotor in the rear unless you are limited to a 15- or 16-inch wheel size, then a 12-inch rotor will work best. This way you can minimize having to cut off your caliper bracket and perform any welding in the future.
What Is the Price of a Full Floating Axle?
Lastly, some people will tell you that a full floating rear end setup is expensive. Perhaps people are confusing “exotic” or “race car parts” with “expensive” here, but in all reality, when you compare the cost of an aftermarket 9-inch axle housing to a full floating rear axle housing, you will see that the difference in cost is minimal for the increased strength and increased safety you are obtaining for your high horsepower street car. Additionally, the rotor bolts directly to the hub on a full floater rear end, which further saves you money down the line by not having to buy brake rotor hats.
For those looking to run a full floating rear axle on the street you can use our G-Comp Grand National Hub Kit that features a 5x4.75-inch bolt pattern using street friendly 1/2-20 wheel studs or upgrade your current 5x4.75 bolt pattern hub with our custom replacement 1/2-20 screw-in wheel studs. This opens your wheel choices substantially, as traditional full floating axle hubs are fitted with larger 5/8-11 wheel studs designed for circle track wheels.
Updated by Mark Houlahan
