The (abridged) story of the Texas Speedwerks BDM brake kit

As some of you know, Texas Speedwerks (TSW) is serious about creating and testing solid, track-tested products. I'd like to take a little time to tell you the story of the TSW big brake kit: where the idea came from to create it, how we developed it, and how we tested it.

To begin, let's get in the way-back machine and return to 2004...



Figure 1: My MINI Cooper S back in 2004

Ah, my 2003 MCS. In 2004, I allowed myself to enter the MINI in autocross competition, after holding off for a year or so. I just didn't want to spoil such a new car. To my delight, the MINI turned out to be a sturdy beast indeed. Notice here that the brakes are completely stock, except for some performance brake pads.




Figure 2: Stock brake setup

I autocrossed the MINI a lot, and I also went to some track events. I had previously driven racecars off and on for about fifteen years at that point, so I was already up to speed on the track.



Figure 3: Rut Roh

At the track, I began to find the limits of the stock brakes. I tried progressively more aggressive brake pads, but the pads would quickly wear down, and I would experience brake fade after about twenty minutes on track.



Figure 4: Facepalm

Finally, something bad happened. Heading into a tight left-hand corner after a really fast, on-camber sweeper at Texas World Speedway, I applied the brakes, and the brake compound on one of the front pads sheared off of the backing plate. I managed to keep the car on the track through the left-hand turn, but the car was out of shape for the next right-hander just after. I managed to minimize the damage to the car by keeping the car from spinning into the wall, but I clouted a tire barrier with the left front.



Figure 5: Wierdest brake duct ever

After the car was repaired, I looked into adding brake ducts to the car. This led me to do several simple experiments to determine how the ducts would be routed, and from where they could get some airflow. I fabricated several inlet ducts, and also tried the usual hole for the driving light. At the time, I decided that a proper duct would require the driving light hole to be cut and opened up. Otherwise, the airflow (as measured by the tube running into the cabin here) would be too low to make enough of a difference. And at the time, the idea of cutting up the MINI's face was just too much to bear!



Figure 6: Fresh tires, same old brakes

So, for the next autocross season, I decided to step up to R-compound tires, in this case the Kumho V710. I thought that the slower speeds of autocross would allow me to stay with the stock brakes. If I did go back to the track, I would just take it easy. Riiiiiight.



Figure 7: Oh snap

One autocross day, and the front rotors were toast. In Figure 7, you can see big stress cracks along the surface of the rotors. That was (finally!) the last straw. So I went and designed the TSW kit, right?



Figure 8: Wilwood 11.75" brake kit

Wrong. Wilwood sells a brake kit that fits under 15" wheels. It includes 0.81" wide disks. I bought that and used it to good effect at the autocross in 2006. The disks were still getting hot, but at least they lasted for one season.



Figure 9: Back to brake ducts

I continued to upgrade the suspension to the point that the brakes were starting to need new pads and disks every two or three events, which was getting pretty expensive. And then I got a new job and moved to the Mojave desert in California.

As you can imagine, it's hot out here. The nearest track facility is Willow Springs, and the first track I went to was the Streets of Willow. "The Streets" appears everywhere, from Autoweek to Top Gear, mainly because it is fairly close to Los Angeles, it is devilishly difficult to master, and it is relatively cheap to rent.

The Streets of Willow has it all. Off camber, on camber, elevation changes, sweepers, tight turns, evil pavement, good pavement, transitions, curbing, sandy corners, and HEAT. I signed up for a two day event, and at the end of the first day, I had trashed my rotors and eaten through a new set of racing pads. The brakes faded after ten minutes on the track, and when the pedal touched the floor, I said "uncle" and backed off.

That night, in desperation, I removed the driving lights, opened up the holes, and fabbed up brake ducts.



Figure 10: Finished brake duct

The brake ducts helped: the brakes faded after 16 minutes, and the pedal never quite touched the floor. But by Sunday night, the new rotors were cracked and gouged, and I had eaten through all of my brake pads.



Figure 11: Heat-soaked brakes

Worse, the brake hat had turned a funny color from the heat, and so had the caliper. I think I've heard this song before, and it ended with a bang - a sort of MINI-meets-wall bang.

But what could I do? There were no other brake kits available to address this problem that would allow me to use the 15 inch wheels I loved so much. Ah, now we get to the point of all of this.



Figure 12: Wider is better

With the 15 inch wheels, I could not go to a larger diameter rotor. But to be honest, a slightly larger rotor diameter wouldn't net enough of a performance gain. What I needed was a wider rotor.

When a car decelerates into a turn, the kinetic energy of the car is converted into heat energy by the brakes. The brake rotors then transfer this heat to the air around the rotors. There are two things going on here. First, when the brakes are applied, the rate at which heat is generated in the rotors is much higher than the rate at which the heat is carried away by the air. This is true for any vehicle brake system at maximum braking force, from a bicycle up to a Formula 1 car. This means that the brake rotors will heat up. For a given energy input, the increase in the average temperature of the rotor depends directly on the mass of the rotor. Therefore, a more massive rotor will store more heat per unit temperature (that is, it has a higher heat capacity) than a ligher weight rotor.

Conversely, a lightweight rotor will run hotter. A higher difference in temperature between the rotor and the air will increases the heat transfer rate, but the brake system has a maximum tolerable time-temperature curve (which I exceeded when I stripped off a pad face and hit the wall on stock brakes). That is, the life of a brake system is determined in large part by the area under the time-temperature curve. In other words, really hot brakes die really quickly.

The other half of the picture is the rate at which the rotor transfers heat to the air. As the car leaves the corner, the brakes are relaxed, and the rotors continue to transfer heat to the air. The amount of air flowing past (and through) the rotor makes a big difference, of course, but it still takes time for the rotors to cool. This works well as long as there is enough time for the brakes to cool before the next turn. If not, the brakes will get progressively hotter with each lap, until the rotor temperature reaches a point where the heat transfer to the air equals the heat input per lap - or the brakes fade away and the driver backs off.

I said before that I needed a wider rotor. While a larger diameter rotor (say, 12.2" instead of 11.75") does give slightly more area for heat transfer, and slightly more mass for heat storage, the difference is minimal. However, moving from a .81" wide rotor to a 1.25" rotor gives a HUGE difference in heat transfer area within the cooling passages of the rotor, AND it greatly increases the heat capacity of the rotor, all without greatly increasing the moment of inertia of the rotor.

In other words, the brakes stay much cooler, the components have a much longer life, and the car accelerates the same as it did before (minus an immeasurably small amount).

But, will it all fit together?



Figure 13: Calipers out, fingers crossed

At this point I needed to mock up the new brake kit to see if it would clear both the wheel and the suspension. The "hub" is little more than some metal scraps I pulled out of my toolbox, and some spacers I whipped up for the occation.



Figure 14: Air brackets

The caliper is then put in place, and air pressure is applied to clamp it (and the pads) to the disk. A little cajoling of the caliper is needed, and a lot of judgement is also needed so that the resulting "air bracket" can be converted into a real mounting bracket.



Figure 15: Eureka!

After a lot of furrowed brows, I managed to fit everything together. All I needed was actual hardware.



Figure 16: Prototyping

I made a pair of these brackets on the milling machine, and they worked perfectly the first time. I cleaned up the design a bit for the production run, but the geometry worked out the first time (that's always nice).



Figure 17: Done (whew!)

All of this effort finally resulted in a true prototype, which I promptly took back to the Streets of Willow, and also to Buttonwillow a few times. After many track days, I'm still on the original rotors AND pads! The difference in pad life is astonishing.

So, here ends the story of the TSW BDM brake kit. It was a fun trip down memory lane for me, as I had forgotten just how long it can take to follow through on a project when pesky things like day jobs and cross-country household moves get in the way!

 

The duct is hidden in the last picture just by chance. Ducts can only help the brake cooling. I don't think these brakes need ducts, but why take them off, you know?

Some may point out that the ducts could funnel road rocks to your brakes, and this is true. If that is a worry, then some coarse expanded metal will screen out the big stuff.

 

Mmmmmm... :arf: gunmetal.