Million Dollar Skunksworks Kirkham

[David Kirkham]

This is a really interesting part. This is the pedal box. On the original 427 Cobra, the master cylinders for the brakes and clutch is under the floor. My customer told me one day he wanted to drive this car down the PCH from San Francisco to LA. One of the problems with the original car is you can't slide your foot under the brake pedals because they are floor mounted. We thought a lot about this and decided to hang the pedals from the top.

The problem is; if you hang the pedals from the top, they are right above the exhaust and they will "pre-heat" your brake fluid for you! Bad, very bad--especially if you are facing turn 1 at the end of the straight and you already have your brakes hot. So, we flipped the master cylinders around so they face the driver and the brake pedal turns into a class 1 lever (just like a teeter totter). It made things tight, but it was the only answer that worked.

[David Kirkham]

Notice in the above box we made 2 internal supports for the pivot pin. The stiffness of the pedal box is critical to brake feel. You do NOT want your brake pedal or pedal box flexing! You do not want anything in the brake department flexing as it gives the driver a vague feel of what the brakes are doing. Flex makes modulation of the brakes impossible (or at least very difficult). So, we placed intermediate braces in the box--between the brake and clutch pedal levers--to mitigate any flex that could creep into the system. Also, the front of the foot box is made from 1/2 inch aluminum plate. The top of the foot box is made from 1 inch plate! (All hogged out and lightened up, of course.)

[David Kirkham]

Here are the brake and clutch pedals.

You can see we designed them so there is clearance for your toe as you work the pedal. We designed the pedals straight so there would be no chance of the pedal flexing under hard braking. The original Cobra brake and clutch pedals are bent to put them in the right place for the driver to use them. The bend in the pedals is weak spot that causes the pedals to flex. That was a problem we had to solve in this car.

So, we placed the pedals optimally in the foot box for the driver, but that caused problems lining up the pedals with the master cylinders as the pedals are nowhere near directly under the master cylinders. So, we machined the offset into the top of the pedal--along the axis of the pivot pin--thereby eliminating any side loading of the pedals or the master cylinders.

The down side is the billet to make the pedals was 4 x 2 x 20! That is 16 pounds of billet! The finished machined pedals are just over 1 pound each.

We have been asked many times how much of the billet goes into chips. I would guess on average 90-95% of the aluminum billet goes into the chip barrel. Aluminum does recycle, however!

[David Kirkham]

Oh yes, in the above picture, the brake pedal is on the right; the clutch pedal is on the left.

[David Kirkham]

This is a close up of the top of the brake pedal. This is where the brake bias bar attaches to the brake pedal. The hollow cylinder holds the bias bar bearing. When we put the bias bar in the car, I will take a pic. As you can see in the picture, the bias bar itself is also supported on the pivot pin to absolutely minimize any flex in the system.

[David Kirkham]

This leads me to a discussion of the brake bias bar. It is very common in hot rods and even in our own Kit Car and Replica industry to see people place chokes on the rear brake line to control the brake bias. This is a very bad idea--especially in our cars that generally lack power brakes.

You see, you only have so much brake line pressure that is available in the system. If you limit the pressure to the rear brakes to get them to not lock up before the front brakes, then you are eliminating some brake line pressure that could effectively be used on the front brakes.

It is far better to mechanically bias the brakes toward the front of the car so you are then mechanically applying more pressure to the fronts than to the rears. Unfortunately, the original 427 and 289 Cobras did not have a way to adjust the brake bias mechanically. They had to do it by changing the master cylinder sizes front to rear.

In our dropped foot box cars, (where we designed the pedal box ourselves) we do run this type of a bias bar. When we do that, we actually put the largest size piston calipers available on the rear calipers so we can get the highest pressure possible on the rear caliper pistons. We then mechanically bias the brakes to the front so we can get even higher line pressure to them. That way, for a given pedal pressure, we get a higher line pressure--and hence, easier stopping of the car for the driver.

This will all become a lot clearer when I post a pic of the bias bar assembly.

Any questions are most welcome.

David