The M Files

Changing The Brake Pads On Your E36 M3 Coupe

Changing the brake pads on your E36 M3 is a great first project if you are looking to foray into the world of home auto-repair. This procedure is accessible, requires no special tools and is almost foolproof. If you've been shying away from attempting this procedure because you're afraid to bleed the brake system you're in luck - you don't have to bleed the brakes to change the pads. For those that are unfamiliar with the phrase "bleeding the brakes", this is the procedure by which trapped air bubbles (which reduce braking efficiency) are removed from the hydraulic system.

The brake system on your car is a closed, dual-circuit hydraulic system with 4-channel ABS (for an excellent schematic see Bentley, Figure 3, p. 300-3). I was unable to confirm this in Bentley (please e-mail me if you can confirm this hunch), but I imagine that each of the two dual circuits is responsible for one front and one rear brake, thus maximizing fail-over capability in the event of a failure in one circuit or the other. The front brakes in any car are responsible for 70% of total stopping power (due to weight transfer in the vehicle under stopping conditions ... think about how your chest is pressed into the shoulder belt in an emergency stopping condition). As such, you would rather lose one front and one rear brake in the event of a failure than loose both fronts.

The brake pedal in your M3 is connected directly to the master cylinder. Hydraulic lines run from the master cylinder to the ABS unit and then to each of the wheels. The calipers in your M3 are a single-piston arrangement. Rotors in the M3 are beefier than in other 3-series models and are directional (which means that there is a right and a left, which is apparently not true for the stock sedan).

Before proceeding with this procedure it is important to understand a little bit about how the brakes in a car work. I'll spare my audience a lengthy discussion about how hydraulic systems work (fundamental to the operation of your brakes) but suffice to say that a small force (your foot) applied over a large area (the master cylinder) translates to a tremendous force produced over a piston of smaller diameter (at the caliper). If memory serves from my days as an automotive engineer at Cornell University, a very simple formula governs this relationship (see margin).

Looking at the second arrangement of this formula it is easy to see that if the area of the master cylinder is, say, four times the area of the associated caliper cylinder, the force applied by your foot will be four times as great at the caliper piston. Anywho, I'll have to look this up when I have a moment.

Outer pad. See larger image later in procedure.

Inner pad. See larger image later in procedure.

More important than the science is a basic understanding of some of the more counter-intuitive elements of this system, visible to the eye of the layman. The two brake pads (one outer, one inner) on each wheel form a sandwich with the associated rotor. The outer pad floats freely (it is "connected" rigidly to the car) and the inner pad clips to the inside of the caliper cylinder. The three components of this sandwich are constantly in contact with one another. Even when you are not applying pressure to the brakes, the pads rub on the rotor. This arrangement gives you two things. First, it allows for instantaneous braking reaction when you step on the pedal. Second it permits pad and rotor wear to be compensated for automatically. As these parts wear, the piston simply extends slightly further at its neutral point which keeps the sandwich in tact regardless of degree of wear. This means that no matter how worn your pads and rotors (except, perhaps, in extreme conditions) you will always have the same brake feel and stopping capability. Do not expect that your car will "brake like new" when you are done replacing your pads. If your car has mushy brakes or is experiencing some other braking related problem, you should look into it right away - it's probably not your pads.

Before you get started you should think about what type of new pads you'd like to put on your car. Different pads are made of different compounds. Some wear more quickly (and/or produce more brake dust), but provide greater stopping power. If you are not a serious weekend racer I recommend using the OEM pad or a suitable replacement. I choose a set of OEM-style (I'm not sure if these are the actual OEM pads or not) Pagid pads which I purchased from All OEM BMW Parts for about $45. Bavarian Motorsports has a nice little write-up, that you may find interesting, if you want to learn more. You should also take a look at my brake pad FAQ.

For this procedure there isn't even any prep to do the night before you begin. If you are planning to do front rotors at the same time you should soak the caliper carrier bolts (two on each side) with penetrating oil the night before you begin. These bolts were a b-tch to remove. Make sure you've thoroughly read the procedures you are about to perform before you begin, so you know what you are getting into. I've provided a full tool and part list which lists the items you will need. I've also performed a cost analysis for this procedure.

Ok, enough chalk-talk. Let's get started.