The Zen of Brake Bias

With all the talk about giving the drivers the ability to change aspects of the setup from within the car, I thought some comments on what types of changes they can make would be appropriate.  Since the only control they have right now is brake bias, let’s start there.

The first thing you need to know is that the braking system on a race car is different from the one on most passenger cars.  On a  standard passenger car, the brake pedal is connected to a master cylinder – a hydraulic-filled container that transfers force from your foot on the brake pedal to the brakes.

When you press on the brake pedal, the master cylinder exerts force on the fluid that fills your brake lines. That fluid pushes on the brake calipers and slows down the car. In everyday driving, most people don’t push the boundaries of traction or of their braking system, so this works pretty well.

In a race car, however, you’re always on the hairy edge of losing traction. Your goal is to go as fast as possible, so you have to run on the edge. That means the balance of the car is absolutely critical.

I’ll remind you of what I’ve decided to call the zeroth law of racing (we do that in science. When something is so fundamental, we make not the first rule, but the zeroth rule.) That rule would be: You can only go as fast as your least grippy tire. The corollary to this is that you can also only slow down as good as your least grippy tire.  For that reason, racers like to have a little more control over how the brakes are applied.  A NASCAR race car has separate master cylinders for the front and the rear tires.

 

Note the little dial where the brake lines split into front and rear. That’s the brake bias, a device that allows the driver to change the proportion of the force that’s going to the front versus the rear brakes.

You’ve heard drivers talk about cars being loose and tight.  Loose is when the front wheels have more grip than the rear wheels. When you’re going around a corner on an icy road and the rear wheels swing out, that’s loose.  Tight is when the rear wheels have more grip than the front wheels. That’s when you have the steering wheel all the way to the left and the car is still going straight.

Look at the extreme case of doing donuts.  You do donuts  (in a rear-wheel drive car, at least) by holding the front tires still, turning the wheel and powering the rear tires. The forces on the front tires essentially pin the front of the car to the ground, while the rear is free to move. This would be the extreme case of an unbalanced car. Because the front wheels are pinned and the rear wheels aren’t, the car goes in circles.

Back that off a little. Anytime you have unbalanced forces, the car is going to want to rotate. On a dirt track, drivers talk about steering with the throttle – you can turn the car by giving it a burst of power to the rear wheels. The need to steer with the throttle is primarily because of the lack of grip on dirt – the front wheels have less effect on steering. It’s a much smaller effect on asphalt. But the principle is the same. When you have better braking in the front wheels than the real wheels – or vice-versa – the car is going to want to turn. You can use that to your benefit to some extent, but remember the goal is always balance.

Hence the need for brake bias.  Let’s say, for example, that we have a front-heavy car. If we apply the same braking force to the front and the rear, we’re going to lock up the rear brakes before we’ve gotten everything out of the front brakes we need.  We need to bias the brakes so that the front gets more braking force.

Remember that the distribution of the car’s weight on the four tires is always changing, depending on whether the car is speeding up, slowing down or turning a corner. So no one brake bias setting is perfect through an entire corner. You’re generally optimizing for corner entry, so you an be the last car braking into the turn.  You want to build up as much speed as possible down the straightaway. If you can wait a fraction of a second longer than your competitors to put on the brakes, you’ve got an advantage.

Traditionally, set-up changes can only be made during pit stops. The brake bias is the one thing the driver can change from inside the car. (At the moment, at least.) The driver can’t change brake bias through a corner, but think about what changes over the course of a green flag run. There are two biggies.  Tires change in two ways. For the first few laps, they’re underpressured and heating up. After that, they’re wearing. Both things change their grip levels.

Second, you’re going from a full to an empty fuel tank. The fuel tank holds somewhere around 18 gallons.  Gasoline is roughly 7 lb per gallon, so that’s a rear weight change of about 120 lbs. Brake bias allows the driver to compensate for those rather significant changes in weight distribution – and thus need for braking – inbetween pit stops.  There’s a knob in the car that’s allows the driver to dial in how much brake goes to the front and how much goes to the rear.  I’ll get into the details of how that works in the next post.