Sunday, September 2, 2012

The knob

The rally cars I drove (and pretty much every rally car I have ever seen)  have systems with which you can manually adjust the brake bias (usually a knob which is within the driver's reach, in the cockpit). As a driver, one of the first things I did when sitting in a new car was check the brake bias before driving. By new car I mean a car I have not driven before, or somebody else has prior to me, or after a rebuild.

On gravel and snow I would drive some little bit and brake hard, making the car slide. What I looked for on loose surfaces was for the back to step out and stay out, steadily, under a steered braking slide. If the back would not step out I'd give it more bias to the rear. If the back stepped out too much and started to spin out I'd give it less bias to the rear. 

On gravel the bias ratio that I felt comfortable with usually hovered around 54:46 (Front:Rear). This is what I used on the '04 spec Focus and I believe the other drivers were on similar biases. As you can see this ratio shows only about 13% in difference between front and rear. Bear in mind the '04 Focus has a near 50:50 weight distribution front to rear and a 4 wheel drive transmission. 

When the bias is adjusted properly, you can brake from any speed, straight, and put the car sideways with help from a bit of steering (and speed). As you keep braking sideways the back stays stable and does not spin out. That is how, under full attack, you can brake very late and hard as you steer into corners, under full control, with no danger of spinning out.
Once I had figured out my favorite brake bias I did not need to re-test it every time I sat in a different chassis. There were about 5 different chassis, between the test car which I got to drive on a few occasions, and the rally cars which the team rotated around for me. The different '04 spec rally cars I drove were EO03XYG, EG53BDU, EF04VVB, ET53UJP. The way to check the bias was cool on the Focus: I'd turn onto a specific screen page on the cockpit computer which shows the front and rear brake cylinder pressures. I'd push on the brake pedal until 24 Bar showed on the front cylinder; at this point the rear had to show 21 Bar if it was correctly adjusted. If not, well......turn the knob...  

1 comment:

  1. Thank you for the explanation. I'm trying to currently decide if I put a pedal box in my rally car.