The Science of Brakes

300mm of steel, a rubber hose and some fluid is enough to bring 200 kilos of bike to a standstill from 200mph in 10 seconds. Here's how

Posted: 22 November 2010
by Kevin Ash
ABS - How it works: A sensor generates electrical pulses from serrations on a ring fixed to the wheel. These are counted by a computer interested in changes in the wheels' speed, which it measures by taking a sample of, say, 10 pulses, then measuring how long they took to arrive compared with the previous 10 pulses. Levels of wheel deceleration deemed unacceptably rapid (i.e. a wheel locking up) are programmed in. If it detects this by comparing the incoming pulse rate changes with those preset in its memory, the computer triggers a valve that releases the brake pressure. It then restores the pressure by opening another valve from a second hydraulic circuit, constantly pressurised by a powerful pump. If the wheel still looks like locking up, the process happens again, at 10 to 20 times a second - the pulsing you feel.

Brakes have been accelerating, at least in terms of their technology, in the past decade. Growing pistons, sprouting new mounting systems, gaining power assistance, and even some cast-in-steel certainties you could always depend on, like discs being round, have started to look wobbly. Or wavy.

The impressive reductions in mass of most manufacturers' bikes in the past few years might have taken some pressure off what stoppers have to cope with, if only riders weren't busy getting fatter and spoiling it.

But the main forces driving the advance are improvements in tyre technology and increasing top speeds. More grip from road rubber, pioneered by more grip being found in race tyres, means brakes have to be stronger to be able to exploit it. And they need to do so in as easy-to-control a manner as possible if they're to be of any use.

Higher top speeds (and on MotoGP bikes we're now talking in terms of 210mph-plus regularly, while many road bikes are capable of at least 180mph) carry a double whammy, or more precisely, a whammy squared. A brake's job, at its most fundamental level, is to convert your kinetic energy - the energy you have due to your speed - into heat. The faster it does that, the more rapidly you slow down.

What makes its job so demanding is that your kinetic energy increases in proportion to the square of your speed, so a bike at 120mph has accrued four times the kinetic energy it had at 60mph. It's not quite true to say the brake therefore has to shed exactly four times as much heat at twice the speed because wind resistance is on your side for once. This also increases with the square of your speed, so it's trying four times harder to slow you down at 120mph as at 60mph and the brakes get a bit of help.

But not enough to make a big difference - the square of your speed is the dominant factor, and you can be sure that the kinetic energy of a fully fuelled Hayabusa with a 15-stone rider at 200mph has to be converted into an awful lot of heat to lose through the brakes when it's slowing to a standstill. We wouldn't recommend sticking your tongue on the discs straight afterwards to hear how loud it hisses! In the dark, they'll be a glowing, red-hot 800°C. They can get up to 400°C even in normal road use, so if you value your taste buds, disc-licking is probably best restricted to before you set off. If you really must.

You don't have to use the brakes of course, you could coast to a stop from 200mph eventually. But still the same amount of energy would have to be lost, which happens as heat in the tyres and bearings, a small
fraction as noise, and kinetic energy (plus a tiny amount of heat) given away to the air as turbulence. It just takes longer. What brakes do is shortcut the process, grabbing big buckets of your kinetic energy and gobbing it out as heat as fast as they can, depending on how hard you're pulling the lever.

And that's the bit that matters, what happens when you pull the lever. Years ago, this would usually operate two shoes inside a drum via a cable. But drum brakes are limited because the heat is generated inside the wheel, away from the cooling airflow, and the length and curve of brake shoes means very high pressures can't be applied without distorting them. So disc brakes were invented.

The friction surface where the heat is generated is out in the open, the disc and caliper are simpler lumps of metal that suffer less from heat distortion and because brake pads are small, chunky, square-ish things, much higher pressures can be applied to them than shoes, resulting in an increase in friction and braking force.

In all but a handful of cable-operated oddities, those forces are applied by hydraulics, a very useful way of translating and magnifying a force from somewhere convenient to somewhere awkward and mobile, like from a handlebar to a caliper on a bouncing fork stanchion.

The advantage starts when you squeeze the lever, as this gives you immediate, er... leverage, over the brake's master cylinder of about four to one. Move the lever 20mm where you're squeezing it and it pushes a piston into the master cylinder by 5mm, but with a four times larger force than you're applying. If that piston has a 10mm diameter, it displaces about 0.4cc of brake fluid into the brake hoses. The fluid doesn't compress and decent hoses won't expand significantly, so at the other end 0.4cc is displaced into the only place it can go - behind the piston in the caliper that pushes the brake pad against the disc. Let's say this piston has a 40mm diameter, in which case it only needs to slide forward 0.31mm to accommodate this extra 0.4cc.

So, a 5mm movement of the piston at one end produces just 0.31mm of movement at the other, 16 times less movement but with 16 times more force. If you now factor in your brake lever gains, you've turned 20mm finger movement into 0.31mm piston movement and multiplied the force a further four times, a total of 64 times. You have the power...

The small amount of movement of the caliper piston is not a problem, as brake pads are brushing against a disc's surface anyway. The consequence is the pads can be squeezed against the disc with colossal force, the friction rises dramatically, loads of heat is generated and the bike slows down.

This is the underlying principle, what comes next is refining it to make it work more effectively. And this is where compromises come into play, as with all engineering. The first and most obvious compromise is calipers and discs add weight to exactly the area it's least wanted: the unsprung mass of a wheel assembly and suspension. More mass here adversely affects ride quality and grip and, worse still, brake discs add gyroscopic effects that slow steering speed (although they do improve stability), as well as increasing the rotational inertia of a wheel. And ironically, that impairs braking as well as acceleration.

Continue the technical focus on brakes page 2/2

HOW TO BED IN NEW DISCS AND PADS

Brakes perform best and last longest if properly treated - run in - when new. New pads bed in best on old discs, and likewise new discs are best bedded in with old pads, so try not to change both at once. On a brand new bike, the brake bedding in routine should be extended by around 50%.

With new pads, the aim is to boil off the resin's gases and avoid green fade or overheating the pads and causing them to glaze over, which ruins their performance. This means using them gently with no long periods of braking for 100 miles.

It also helps when you fit new pads to cross-hatch your brake discs lightly with a rotary 220 grit sander on an electric drill, to bust the discs' glaze.

Discs need to be tempered, a heating and cooling cycle that stabilises the metal structurally, making the discs more resistant to warping and wear. Do this by braking with medium pressure from 60mph to 30mph, releasing the brakes then accelerating back up to 60mph. Don't stop as it's important to maintain a flow of cooling air. After two minutes, repeat the process 10 times. Then stop to let the discs cool completely before running through another 10 cycles, using harder braking. Three or four of these 10-cycle sessions will temper your discs, but avoid very hard use for 500 miles.



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Brakes cont.


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Discuss this story

Good article, but I'd like to correct a factual mistake you've made.

 "Another solution is to make the disc narrower when viewed from the side, so its inner diameter is closer to its outer diameter. The disc becomes usefully lighter, but the problem is you would have to make the brake pads smaller, reducing friction and negating any other gains."

 Reducing brake pad size does not reduce friction. Brake pads will typically have a coefficient of friction of about 0.3 to 0.4, this doesn't change with size as its a property of the material.

The problem with a smaller brake pads is a reduction in thermal mass, so for an equal amount of kinetic energy that is turned to heat, the smaller pad will end up at a higher temperature. So the smaller pad will fade quicker and wear faster.


Posted: 23/11/2010 at 07:16

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