This thread prompted me to forward a few ideas about evaluating motorcycle or scooter disk brakes. A lot of people automatically equate brake disk diameter with performance, but there are a few other factors that are equally, if not more relevant.
Skip to the Cliff Notes at the end if tech stuff bores you.
First up I’ll start by saying that brake performance should be measured in two ways, brake torque and heat capacity. The first refers to how much negative torque the brake system can apply to the wheel, in other words how much rotational force the brake can counteract. The second refers to how long the brake can keep absorbing rotational force before overheating, since what a brake does is to convert kinetic energy into heat energy. These two aspects of performance do not always go hand in hand.
You’ll notice that I don’t use any terms like ‘stopping power’. Tires stop vehicles, not brakes. Without the friction between the road and the tire the best brakes in the world would not slow you even 0.1kph.
There are five main factors which determine maximum brake torque:
- The average radius of the brake disk.
- The surface area of the brake pads.
- The pressure applied to the brake pads.
- The coefficient of friction between the pad material and the brake disk.
- The percentage of the average brake disk diameter to the rolling radius of the tire.
Some of these parameters are fixed, some of them change with wear and temperature.
The average radius of the brake disk is the point at which the average friction force is applied to the wheel, measured from the axle center. In the same way that using a long breaker bar helps to loosen a stubborn nut, increasing this radius results in more brake torque without changing any other inputs. The downside is that for any given road speed the surface speed of the braking area past the pad will increase, and more heat will be generated.
The surface area of the brake pad is easy to understand, and it’s obvious that the larger the pad area the more friction would be generated, as long as the pressure stayed the same. In real life however it’s not easy to increase the pad area without increasing the size of the caliper along with it, and that brings other headaches. Of course one trick is to double up on the disks.
The pressure applied to the pads is (on a motorcycle without a brake booster anyway) limited by the ratio between the sizes of the pistons in the hydraulic cylinders at either end of the system and the levers used to apply pressure from the hand or foot to the master cylinder. It’s possible to increase pad pressure by fitting calipers with larger pistons or more pistons. In practice there are limits to line pressure that normal hydraulic seals and hoses can put up with, so ultimate pressure at the pad does have finite limits.
Motorcycle brake disks these days are usually made from stainless steel because owners don’t like looking at rusty bike parts. Cast iron disks are superior in many ways, but stainless is price of fashion. Of course race bikes may have plasma coated alloy disks or even ones made from carbon, but let’s stick to what’s streetable eh?
Most pads meant for street use are a compromise between friction, wear, heat range and dusting. The wear and dusting considerations are somewhat dominant in the compromise and most street pads have similar performance.
The relationship between the average disk radius and the rolling radius of the tire is very important and often overlooked. Applying the brake torque very close to the wheel rim is much more efficient than applying that force close to the hub, or center or rotation. For one, the higher surface speed makes it easier to develop friction without needing pads of extremely high friction coefficients or using excessively high line pressures.
So, when eyeing up the potential brake torque you should first consider how close in diameter the brake disk is to the wheel rim rather than it’s overall diameter. In other words, an 8” disk on a 10” wheel has better potential brake torque than a 9” disk on a 12” wheel, all other things being equal. The mitigating factor here is the tire contact patch. A 12” tire will have a much bigger contact patch through which it can apply all your brake torque to the road than a 10” tire.
The other side of brake performance is heat capacity. A brake is just a means of converting kinetic energy into heat energy via friction. More brake performance naturally means more heat. You may have seen tests on cars where they see how much distance it takes to stop from (say) 100kph once, and then on repeated stops afterwards. It’s a good measure of how much heat capacity the brakes have, how the stopping distance changes with repeated tests. On a motorcycle you usually only surpass the brake system’s heat capacity at the race track with repeated hard braking corner after corner, lap after lap. On a scooter however, having no engine braking to speak of, long mountain descents can be an issue. In this case you may want to buy the scooter with the biggest disk brake available so it staves off brake fade for as long as possible.