Thursday, May 28, 2009

on brakes & bekker moments

The physics of how brakes work, in plain English
by Pierre Martins

You may think that the job of your brakes is to slow you down, but have you ever given any thought as to the physics of how brakes actually work? The common misconception is that brakes just slow you down by the pads squeezing against the discs, but there is much more to it than that…

brakes%203 You’ve got speed and mass, something the learned guys call the energy of motion, better known as Kinetic Energy.

One of the rules of physics states that energy cannot be lost. Energy cannot be created or destroyed – it can only be converted into another form of energy. Your brakes work by converting kinetic energy into heat – Thermal Energy. This is where the interaction between your pads and discs actually comes into play. The friction created by the pads squeezing against the discs is a mechanical moment that converts the kinetic energy of your car into thermal energy.

That’s easy enough to understand, but it’s only one aspect of braking. There are many other useful things to learn, like brake fade and Bekker Moments…

What are Bekker Moments?

ian2It’s not something you’ve heard of before. It’s my own little saying that comes from an unfortunate experience I’ve had with brake fade in someone else’s track car.

Picture this scenario – I’m in the passenger seat, coaching a rookie track driver by the name of.., you guessed it, Ian Bekker. We’re in his Porsche 944 Turbo Cup going full taps down the 800metre long back straight of Midvaal Raceway.

At the 100m mark Ian threw anchors for the hairpin coming up and what happened? The brake pedal suddenly dropped to the floor. Hello Brake Fade! Ian panicked and instead of using the runoff he suddenly pitched the car into the corner. Time slowed down, even at that speed. We went sideways off track with the tyre wall coming up at a rate of knots.

I braced for the impact. Next thing a bump in the boondocks threw the car up in the air. Spectators say it was airborne by about 10 feet. We came down hard on the tyre wall, smacked it sideways at about 160km/h and pushed it along a couple of metres.

P1040263 This was a left-hand drive jobby, driver’s side took the impact, but I slid out of my seat and clouted the driver’s seat pretty hard. The passenger seat was a generic Porsche OEM unit with stock 3-point belt. Driver’s seat was a proper job, 6-point belts and whatnot. I’ve done many a track day in similar OEM seat & belt setups in 911’s and never thought anything of it. I guess when the crap hits the fan you kinda change your thinking a bit when it comes to safety…

Anyway, I was lucky to walk away from that one with some pretty colourful bruises and a buggered up shoulder. Ian wasn’t badly hurt either, but his car was toast. Sad story, but I learned a lot of lessons that day, the most important one that safety must come first.

So now you know what a Bekker Moment is. It’s that all of a sudden Oh Crap! moment when you realize you have no brakes, He-he…

About Brake fade

brakes%202We all know that brake discs get super-heated on a race track. You’ve seen them glow red-hot during night races.

There’s nothing inherently wrong with that, it’s supposed to happen. It’s a visual real-life display of physics at work, of kinetic energy being converted into heat.

Let’s assume you’re having some track fun in your bone stock daily driver, a car that wasn’t designed for the track. On your out lap you take things easy, warming up the tyres and brakes. By lap two you have everything up to operating temperatures and you start giving it stick. By lap six your brakes are not working so nice anymore. You’re developing a long pedal and the brakes are losing stopping power, but you keep on driving. On the next lap you’re tonking it through the stadium section and as you line yourself up for the final corner you ht brakes and the pedal suddenly drops to the floor - a Bekker moment!

Why did that happen? According to the laws of thermodynamics, your brakes were heating up during your first few laps and they produced stopping power, just like they were supposed to. But you kept going, not giving the brakes sufficient time to cool down. Remember, the brakes on your daily were not designed for track work. By lap six you were starting to cook them nicely and they were telling you they could not absorb much more heat.

Let’s take a closer look at what was happening to your brake pads at that stage. Most modern brake pads are designed to grip harder when hot, but only up to a point. Brake pads contain friction material that is held together by a concoction of bonding agents and resins. Once this lot starts to overheat, they start to form gasses. The gasses form a boundary layer as they get trapped between the contact areas of the pads and discs whilst trying to escape. Your pads started drifting on ball bearings of gasses in a similar fashion to ‘aquaplaning’ on a wet road when water gets trapped between the contact patches of your tyres and the road surface.

At that point you should have slowed down for a lap to give your brakes time to cool down to operating temperatures, or come in to pit. But you were hooked on driving and enjoying yourself, so you kept tonking it and eventually the pedal refused to slow the car and you lost all braking.

Why did the pedal stop working?

It has to do with heat-soak. The brakes were overheating big-time, and that heat transferred from the pads, through the callipers to the brake fluid. The brake fluid overheated and started to boil, and what did they teach you in primary school about a liquid that reaches boiling point? – It starts turning into a gas. The function of brake fluid is to transmit the pressure of your foot on the brake pedal to the pads by means of hydrostatics.

When the brake fluid in your callipers started to boil the gasses from the fluid were not able to escape like steam from a boiling kettle. They had nowhere to go, so they formed tiny little bubbles in the brake lines. Gas is compressible, brake fluid isn’t. When you pressed down on the brake pedal you were compressing the little gas bubbles in the brake line and got full travel from the pedal with no effect on the other end, and Voila – a Bekker moment!

Good luck and look after your brakes.


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