Pedals and brakes

We all know the importance of reliable brakes. When you push the stop pedal, the car better slow down. No exceptions, ever. Ensuring that that is the case involves more than having physically strong components. The components have to be appropriately sized for the application, must be able to dissipate a sufficient amount of heat, and the components also have to be appropriate for the temperatures that they will reach(i.e. a track car requires pads that can handle hotter temperateness than a street car). How much heat do brakes generate under hard stopping? Well, brakes work by using friction to resist movement. Friction is what warms your hands when you rub them together. Now imagine that your hands were rougher, you pushed them together with 4,000 pounds of force, and you rubbed them together at 112 feet per second. The cited force is an approximation of what will happen when I start to brake at the end of a 150mph straight–112 f/s is the speed of the outsides of the rotors relative to the calipers at 150mph. This is true for each corner in the front, and the numbers for the rears are about 20% of the numbers cited for the fronts. Your hands will not hold up to that and a lot of brake systems will not hold up to that either. Lots of heat is created, and the amount of energy that the brakes must absorb is exponentially proportional to the velocity of the vehicle. The heat is dealt with by using large vented rotors, using pads that retain their coefficient of friction under extreme conditions, and in some cases, by ducting air to the rotors for additional cooling. I’m doing the first two, and may have to do the last as well–time will tell.

Under heavy braking, a larger portion of the mass of the vehicle is over the front wheels. On a Miata with significant grip, the dynamic front weight bias could be as high as ~80% under heavy braking. This means that if you want to utilize all available grip to slow the car down, the brakes should have the same front bias as the dynamic front weight bias. That being said, it is common to give brakes additional front bias for the sake of safety. The car would have a tendency to spin out if the rear wheels locked up before the fronts, so brake systems are engineered to have the fronts lock up first. My brakes are no exception to this, however I am using a balance bar for fine control over front/rear bias. The balance bar will allow me to make adjustments to the bias, while strapped into my seat, to optimize the settings for the conditions that I am driving under. Other ways to adjust the bias are to adjust the caliper piston sizes, the number of pistons in the calipers, the diameter of the rotors, the diameter of the master cylinders, and the friction coefficient of the pads. These same things can be adjusted, as well as the pedal ratio(think mechanical leverage), to alter the amount of force the driver must exert to achieve the same outcome at the wheels. One thing to be mindful of is that nothing comes for free–if you have a pedal that requires little effort, that pedal will also have to be pushed further.

For the rotors and calipers, I chose to go with a proven setup. I got the Trackspeed Engineering front big brake kit and M-Tuned caliper spacers for the rear. The front setup includes 11.75″ Wilwood rotors and 4-piston Wilwood calipers. The pistons are 1.38″ in diameter. The kit also includes brackets and stainless steel lines that make these brakes plug and play. Trackspeed Engineering’s attention to detail manifests itself in things like safety wire to keep the bolts that connect the rotors to the top-hats from backing out. They also provide a good set of installation instructions. The stock rear calipers will be retained and spaced out further via brackets to allow clearance for 10.9″ sport rotors, which came on the Mazdaspeed Miata. Wilwood master cylinders were used for the brakes and clutch. I am not going to share the specifics there because my master cylinder choices, as well as the pedal ratios, have yet to be tested and proven. That said, my math indicates that there should be no issues. The pedals themselves are from a 2004 Pontiac GTO. They came with my drivetrain and have been modified to suit my application. The modifications included cutting and welding to alter the overall lengths and pivot points. The gas pedal was the simplest part, and minimal modification was necessary. I welded mounts into the car for the gas pedal to bolt onto, and added a point to the pedal assembly to provide the throttle cable with a place to attach in the new configuration.

In addition to greater capacity, these brakes weigh less than the stock components, and consumable items(pads and rotors) will last longer and are cheaper to replace.

Early stages of the pedal box. The balance bar is clearly visible here. The pivot point is adjustable so the forces applied to the front and rear master cylinders can be varied.

This is the finished pedal box.

The switch will operate the brake lights.

All three pedals can be operated as needed. The gas pedal was placed with heel-toe shifting in mind.

The plumbing still has to be done. Steel hard lines will be run through the interior to the front and back, and tee off at the ends to provide pressure to the calipers on both sides. The stock rear subframe comes with a tee fitting bolted to the passenger side. I relocated that to the drivers side to better accomodate my intended line routing.

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