Editor's Note: This month's story about the fabrication design choices of our Project Unfair's rear axle and brakes takes place back at II Much Fabrication, in John Parsons' small shop. -Jim Campisano
This month, we're covering Project Unfair Camaro's floating rear axle setup. Traditional rear axles use a mounting flange for the wheels, which is the same piece of steel that is splined in the differential. A floating rear axle allows the wheel flange to be held in place by wheel bearings that aren't a part of the driven interface from the differential. The axles in such a setup "float" between splined interfaces on either end. As one might suspect, such an unorthodox setup has some other differences that must be addressed as well.
In order to understand why we are using a floating axle setup on Unfair, one first needs to understand the problem we are solving by using floating axles. The issue is known as "knock back," and it is caused when the brake caliper pistons are pushed back in their bores. This, in turn, is caused by excessive sideways movement of the rotor. Knock back ordinarily occurs when a significant lateral force is generated (like when making a high-g turn) and the rear axle flexes enough to push the rear wheel caliper pistons back. When that happens, it takes a couple of pumps of the brake pedal to remove the free play between the brake pads and the rotor to the point where the brakes work again. It's a scary feeling to push the brake pedal all the way to the floor and realize that nothing seems to be happening!
Most stock brakes use a spring-loaded floating caliper setup with pistons on only one side of the caliper that can move laterally with the rotor, but we aren't using anything like that. Instead, we're using Wilwood's awesome 14-inch SRP rotors and huge W4A calipers, for ultimate braking performance. While the huge calipers can stop a car immediately, the big rotors provide even more leverage to potentially cause knock back problems.
Many seasoned racers deal with knock back by pressing the brake pedal with their left foot until the brakes come back while running flat out down the straightaway, heading to the next turn. Others "double-clutch" the brake pedal while coming into a turn.
If these "driver solution" methods don't appeal to you (and they don't to Frank Serafine of Prodigy Customs and me either!), there are some equipment changes that can help. The idea behind these methods is to reduce wheel flange flex. One of the most common causes for this knock back problem is a deep offset wheel-when the mounting flange is relatively far away from the center of the wheel, there is more leverage from the tire to the wheel flange in a turn. This can result in greater lateral rotor movement.
Another typical cause of this problem is the C-clips GM uses for axle retention, which can allow the whole axle to slip sideways during a turn. Companies like Moser and Strange make C-clip eliminator kits, which, along with solving the axle movement issue, can also reduce knock back.
One possible solution is to use larger-diameter axles that have more deflection resistance. Moser Engineering even offers axles-they call them hobby-stock axles-with a bigger cross-section where the flange meets the axle itself.
1 Our Art Morrison Enterprises 9-inch housing gets prepped with ARP studs and hardware.
But with Unfair, as the name suggests, we went a couple of steps further. Moser Engineering also makes a circle track setup using floating axles that utilizes dual taper roller bearings to hold the wheel hub precisely in place, without deflection. In order to do that, the traditional axlehousing ends are cut off and a new snout is welded on. The wheel bearings and wheel flange ride on the snout, removing the requirement for the axle to hold everything in place. Now, the axle only supplies the rotational force needed to make the wheels turn, while the huge wheel bearings keep the flange in alignment.
2 Moser Engineering’s big 3.250-inch bearing third member, complete with a nodular iron c
3 We are using the new WaveTrac differential. It supports the use of 35-spline axles and
4 Wilwood’s 14-inch rotors and four-piston calipers are perfect for our application. They