Replacing Brake Discs
on Your BMW
Wayne R. Dempsey

Difficulty Level: 4
Difficulty scale: Adding air to your tires is level one
Rebuilding a BMW Motor is level ten

[click to enlarge]

     Brake discs (or “rotors” as they are often called) are a very important part of the braking system. The brake pads rub against the discs to create the frictional force responsible for slowing the car down. If the rotors become too thin or grooved, their ability to stop the car decreases.

     When replacing the brake pads, always measure the thickness of the brake discs. If they fall below the specified value for your car, replace them. Check for grooves in the rotor, and take several measurements of the disc in several different places to guarantee an accurate reading. If the brake disc has a groove in it, it should at least be removed and resurfaced by a machine shop—or, even better, replaced with a new one. Discs with grooves brake less efficiently and also heat up to higher temperatures, further reducing their overall braking ability.

     Take the micrometer measurements from the center of the discs. It is common for OEM rotors to have the minimum thickness stamped on the rotor hub. If you can’t find this information, use the following chart to determine if your rotors need to be replaced.

     If you need to replace the rotors, the process is relatively simple. The procedure for front and rear rotors is very similar, but for the sake of this project we’ll look at replacing the rear rotors, which are slightly more complicated due to the parking/emergency brake.

     First, jack up the car (see Project 1) and remove the road wheel. If you haven’t already done so, remove the brake pads from the caliper (see Project 51). The flexible rubber brake hose attaches to the trailing arm of the car with a large clip. This clip retains both the flexible line and the hard line that connects to the rear caliper. Remove this clip in order to remove the caliper without bending the hard metal brake line.

     Now, unbolt the caliper from the trailing arm where it is mounted. There should be two bolts that mount the caliper and hold it in place. After you remove these two bolts, you should be able to slightly move the caliper out of the way of the disc. Exercise caution when moving the caliper around, however—do not let the caliper hang from the rubber brake line, as this will most certainly damage the line.

     Once you have the caliper out of the way, remove the small screw that holds on the brake disc (once you’ve made sure the parking brake is off). You will need a 5-millimeter hex head tool for this task. Pull the disc off of the hub. If there is any resistance, use a rubber mallet to tap the brake disc loose.

     If you are having a difficult time getting the disc off, the parking brake shoes may be stuck on the back of the disc. Adjust the parking brake so it’s not gripping the disc (see Project 53).

     Installation of the new brake disc is a snap—simply push it onto the hub. Before you install the new disc, though, take a close look at the parking brake shoes to see if they warrant replacing. If you can see metal on the shoes, or if the previous owner had a hard time remembering to remove the emergency brake, then it might be a good time to replace them. After you install the new discs on both sides, test the parking brake and adjust it if necessary (see Projects 53 and 55).

     Once the new disc is installed, replace the retaining screw, reattach the caliper, and install the new brake pads. The new rotors should last a long time, and you should see improved braking after the wear-in period for your new brake pads.

     On some late E36 models (in particular, the convertibles), the rear brakes came equipped with rear vented rotors and slightly larger calipers. The theoretical reasoning behind the larger caliper in the rear is due to the heavier weight distribution on the rear wheels of the cabriolet. Although you might think the cars are lighter because they have a cloth roof instead of metal, almost all convertible cars are actually heavier than their coupe counterparts because the chassis is stiffened up underneath. It would appear there is a larger weight bias toward the rear on the convertibles because of this chassis stiffening.

     Are these vented rear rotors a worthwhile upgrade for the nonconvertible cars? If this is a dedicated track car, then the answer is probably yes. Only add the vented rotors if you’re approaching the heat dissipation limits of the rear brakes. Simply adding vented rotors over solid ones won’t buy you any more braking power under normal conditions where the rotors are adequately cooled. The larger piston diameter of the 328ic caliper might change front/rear brake bias as well, but in general, the stock braking system is designed for the weight distribution of the coupe and sedan. Putting too much braking power on your rear brakes may in fact cause them to lock up sooner, which hurts braking control. Many cars from previous generations are equipped with proportioning valves that limit the braking force applied to the rear just for this reason. Adding the rear vented rotors and calipers would only seem to benefit you if you’re on the track all the time, or if you somehow changed the weight bias of your car.

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