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.
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.
Along with removing the requirement to keep the wheel flange aligned with the axle, Moser's floating setup moves the rotor away from the wheel flange. Instead, the rotor attaches to a rotor adapter mounted inboard of the wheel flange and directly on top of the bearing housing. This arrangement reduces lateral rotor movement during maneuvers to effectively zero.
When we started to do research into floater kits, we found that Moser has a Grand National setup that can be had with 35-spline axles, which we need in order to hang on to 1,300 hp in drag racing mode. Unfortunately, Moser's GN kit only supports smaller rotors, since circle track racing is limited to 15-inch wheels. We also had a problem in that floater kits have no reluctor or wheel sensor capability other than using the rotor adapter bolts. This would only give us five signals per revolution, while the folks at RaceLogic told us that we really needed 30 signals per revolution for the best possible performance.
Given this "almost there" state of affairs, I contacted a local CNC machinist. Armed with Moser's original rotor adapters, a technical drawing from Wilwood for their big rotors, and a sketch of how a reluctor could be integrated into the design, we were able to make some rotor adapters that solved both the reluctor and large-diameter rotor adapter problems. During assembly, the custom parts bolted up with no issues, which is very important to help avoid balance issues at high speeds.
To complete our rear suspension setup, we contacted RideTech for a set of its new triple-adjustable shocks. Armed with such great shock adjustability, we should be able to switch modes from drag racing to road racing just by switching the shock adjustment. Even if we do have to change springs, the RideTech parts will make that a quick and easy swap, even at the track.
No story about Project Unfair is complete without some fabrication, and this one is no exception-we had to fashion custom brackets with which to bolt the calipers. I used an All Star Performance bracket to start, but had to modify it slightly so it would bolt to Wilwood's radial mount bracket.
The caliper bracket also provides a convenient spot to mount a tab for the wheel speed sensor.
Have a glance at the photos to get a close-up look at how a floating axle setup works and how we adapted Moser's GN setup for use with Wilwood's huge rear brake kit.
An astute reader will notice that there isn't an emergency or parking brake in our setup. Many enthusiasts will be OK without one, and I know I rarely use mine-I just put the car in gear to keep it from rolling away. However, that's not good enough. Not only is a parking brake useful, but many states require one to pass inspection in order to get tags to drive the car on the street. I've found a solution for the parking brake, but it is different than what one might think.
First, let's back up. Most of us use "emergency brake" and "parking brake" interchangeably, but they are different things. The U.S. Department of Transportation allows that using separate hydraulic circuits for front and rear brakes meets the definition of an emergency brake, since it is unlikely that two hydraulic failures will happen simultaneously. Since we took that one step further and used separate master cylinders for the front and rear brakes, we are covered for an emergency brake.
That leaves the concept of a parking brake. A parking brake is used to keep a car from rolling away while on an incline/decline, or to keep in place during jacking or trailering operations. Take a look at the photo above. It's a mechanical hydraulic line lock designed for holding hydraulic pressure until released. To use, one presses the brake pedal, then depresses the plunger. Doing so will hold the hydraulic pressure until it is released by simply pushing the brake pedal again. We're going to use this hydraulic lock as a classic parking brake by plumbing it into the rear brakes and using the rear calipers both for normal braking operations and for a parking brake. - John Parsons