Friday, May 02, 2008

Brake upgrades for the E36 M3

Open-track events are really hard on brakes, much harder than any street driving. It is common for rotor temps to exceed 1000 degrees F. Temps this high can shatter rotors, melt brake pads and boil brake fluid. Losing your brakes is bad, losing your brakes on the track is infinitely worse.

The first thing to consider is the capability of the stock brake system. Things like rotor size (both diameter and thickness), number of pistons, and system design should be considered.

After much research it appears the the stock E36 M3 brake system is robust enough for open-track use. Especially considering that here in Denver, the car is down on power due to altitude. Less power = less speed = less work for the brakes compared to sea-level. If I were building a car to compete nationally I would not consider altitude in the selection process.

Now that I have decided to stick with the stock system we can look at each part of the system and evaluate it's performance and explore alternatives.

The first and most important choice is the brake pad. This is by far the most important choice regarding track braking systems.

Things to consider in selecting a track brake pad

1. Cost -- you will be buying lots of these
2. Heat range -- Each pad has a preferred operating temp range, this needs to match your predicted use.
3. Friction -- Some pads work only with very sticky tires and will quickly destroy other tires.
4. Wheel damage -- At least one pad (Hawk Blue) creates a dust that when mixed with water forms a black/grey coating that is impossible to get off your wheels -- ask me how I know.

After discussions with a number of vendors and racers, I chose the highly recommended Performance Friction Carbon Metallic pads in their "01" compound. Here is what PF says about this compound:

01 is one of the most advanced race compounds in the PFC's arsenal, and has become the standard by which all brake pads are judged. It has good initial bite, with very little torque rise with temperature. At the end of the stop, 01 Compound has less torque scatter for improved modulation with excellent release. 01 Compound has good disc conditioning properties with low wear. 01 Compound is one of PFC’s most popular race compounds and wins more World and National Championships annually than any other brake pad on the market.

Now that we have our pads we can look at the rotors, the stock front M3 rotors are 315mm x 28mm, cast iron and vented. These are fairly large for a 3000lb car and reflect the performance oriented nature of the M3. While the OEM rotors are good, there are a couple of upgrades available including floating rotors from the European M3 or aftermarket floating rotors.

Floating rotors are the sime size and shape as the stock units but have one significant difference -- the disk (where the pads clamp) and the hub (the attaches to the suspension) are separate and connected in a way that lets the disk move laterally as it heats and cools.

Floating rotors provide a number of advantages:

1. It allows the disk to center between the pads as they clamp down
2. It reduces the amount of heat that is transfered to the wheel hub and wheel bearings.
3. The hub of a floating rotor can be made from aluminum reducing the weight when compared with a solid iron rotor.
4. Some aftermarket rotors allow the actual disk to be replaced while keeping the hub, reducing the cost of replacement.

A step up from the Euro Floating rotors are aftermarket floating rotors -- these are generally considered racing parts because the use different hardware to attach the disk to the hub. The hardware allows the disk to be removed and can require more maintenance that solid or Euro rotors. Imagine having to torque the rotor bolts every 5k miles on a street car. Not a good thing.

Both Racing Brake and Performance Friction offer stock-sized full floating disks for the E36 M3.

I made my choice based on primarily on long-term cost. I ordered Performance Friction front rotors -- PF doesn't make a rear set up so I chose to go with stock rotors out back. Since the front brakes do 70% of the work (and generate 70% of the heat) this shouldn't be a problem.

The PFs rotors are more expensive than the Euro rotors ($530 vs $330) but PF replacement disks are about the same price ($350) as the Euro rotors so long term the costs are comparable.

Now we can optimize the rest of the braking system. This will entail completely rebuilding all four calipers with new seals and dust boots. I will also be replacing the "guide rods" and rubber bushings with new brass rods with brass bushings. This will keep the two halves of the caliper in better alignment as they clamp down on the rotor.

I am also replacing all 6 brake lines with braided stainless. This is as much about safety as performance. the new lines will have little performance advantage but will make the driver more confident knoing the lines are new and not 13 years old.

When complete I will flush the entire system (including the ABS sytem) with Ate Super Blue (or alternately Type 200 Gold) fluid.

With shipping this is about $1400 for everything front and rear from Bimmerworld. I originally planned on replacing all four corners with the Bimmerworld Wilwood Stage III kit with costs about $3300 + 200 extra for the PF 01 Pads.

Initial testing will show if the investment in the OEM system was worth it or if I have to upgrade to the Wilwood kit anyway. Time will tell.

1 comment:

Anonymous said...

I am doing a similar upgrade to my car and this looks like a good plan. In addition to improved performance, I like the idea of the lighter unsprung weight with the floating rotors. I have been told its about 4 lbs per rotor, but I have not data to support that. -- Matt Bader, Delaware