Adjusting Rear Wheel Bearing Axial Clearance
In our quest to update VW drum brake technology to a touch more modern disc set up, we find a few “bumps” in the project that may need attention.
Before we get into the fixes, it is important to understand the function of the components.
The original rear wheel bearing system and drum brakes function with a very small amount of axial “float” in the bearings. This float is needed as the bearing retainer also bolts/compresses the drum brake backing plate to the rear axle housing and acts to accept brake torque. With the use of drum brakes, the amount of float is less critical and will not affect braking capacity as the drum moves in/out under cornering loads. The Germans are pretty picky about holding nice close fits/tolerances, so the original float is very small and rarely noticeable.
Now, we come along and bolt up our newer disc brake technology onto something it was never designed to have and a few troubles can show up. The common issue is that the manufactures of these kits build in plenty of bearing axial clearance to ensure the bearing retainer fully compresses down on the caliper bracket. The problem for us is that as cornering loads are applied, the increased axial bearing float allows the rotor to push the brake pads back. The next time the brakes are applied, the pedal has that much farther to be depressed to compensate for the increase in brake pad travel. It took a while to find this out, as it is not typically seen with the car stationary in the paddock.
The simple way to check if this needs adjustment can be done anywhere in this fashion. Pump the brake pedal to get maximum pedal height. Next, push very strongly side-to-side on the roll bar to simulate cornering load. Depress the brake pedal again and note any increase in travel. If the rear bearing float is too much, the applied pedal height change will be dramatic. By design, the bearing float must be present and some variation in pedal travel (1/4” or so) can be measured, but typically not noticed by the driver. The point of un-acceptability is typically when pedal travel is described as “it goes to the floor on the first pump, but then it’s OK”.
Now that we understand our problem, the solution is relatively easy. The rear axle bearing float needs to be minimized while maintaining sufficient compression on the caliper bracket. There are two ways to accomplish this. One method is to shim the bearing to the correct/desired float. The other method is the way we have done all of our cars to date, by machining the bearing retainer cap to size. Some of the disc kits come with various thickness bearing shims and will function just fine. I have preferred to machine the caps in order to get the correct float.
The best way to measure float with the components un-assembled is with a depth micrometer (see photo #1). One of the dimensions you will need is the height of the installed bearing in the axle housing from the edge of the caliper bracket (bolted down) (see photo #2). The other is the depth of bearing retainer (see photo #3). The objective is to establish bearing float (bearing height smaller than retainer depth) of .005” minimum to.010” maximum. This will minimize rotor float and still maintain a good compression of the caliper bracket. For machining, we use a lathe to trim the flange end of the retainer (see photo #4).
Once correct bearing float has been established, a few helpful ideas when re-assembling. Make sure the bearing retainer threads in the rear axle housing are chased (10 x 1.5). Most axles are used and the last few threads are filled with years of rust and could give false torque indications. Use Loctite on the retainer bolts and torque them to 65 ft lbs. Also, do not use the thin spacer washer that comes with the rear axle seal kit. They were a replacement for the original oil seal failure cup that would keep gear oil off the brake shoes if the seal started to leak. We don’t need it and the quality of the current ones will only give you trouble by collapsing under the rear axle nut torque forces as they are not hardened.
Once the bearing float is correctly set, the pad push back issue becomes a non-issue and you can fully enjoy the benefits of maintenance free disc brakes in your FST.