Why do motorcycles usually have two brake rotors at the front wheel, whereas cars do fine with one per wheel?
There have been some good points made, although when drilled discs first came out, it was to dissipate water. The when the discs were wet on early disc systems, it's a darn good thing they had a drum brake in back because the front was practically useless. My Honda 750 super sport had a single front and back disc and in the rain, you kept a VERY LONG distance from the car ahead of you. Apply the brakes and at first nothing happened until finally the front brake especially would go to moaning, howling and groaning and FINALLY start slowing down.just a bit though. When it would finally stop, it came as an eyeball shaking shudder. The Kaw I traded that thing off for had drilled discs with metallic pads (also much better when wet) and the brakes worked great, wet or dry. In addition, bike discs have a lot of stainless steel in them, which is really a pretty poor friction material and being wet makes it that much worse. About the only way to make them any slicker would be to chrome the disc. Car discs are cast iron which is an excellent friction material except that it rusts. Old Moto Guzzis had Brembo cast iron discs.great brakes but just setting overnight had them rusted and sometimes that rusty water would drip on the right side of the front wheel, discoloring the chrome. They gave up on iron sometime in the late 70's or early 80's I think. A final reason for dual discs up front is that some bikes have an intergrated braking system. The foot pedal activates the rear disc as well as partially applying one of the front discs. The lever operates the remaining disc only. I think the older Gold Wings used this as well as the Suzuki Cavalcades and a few others. It was actually a way to overcome the common problem of too much rear brake and not enough front brake.
The gyroscopic action of the wheel takes care of any steering effect from braking, not just every action has an equal and opposite reaction although the opposite reaction to braking is that the forks dip and heat builds up in the brake assembly including the disc. The quantity of braking is improved by increasing the swept area – the further from the centre of the wheel and the more contact with the disc that a set of pads (which is why the number and size of the pads have also increased) has improves the amount of force that can be applied, being further from the centre the more material there is meaning that it takes longer to heat up and less time to cool down. Using two discs doubles the swept area and almost doubles the braking forces that can be applied (suspension and tyre condition are also in the equation). Evo brakes compensate for their smaller area by having, relatively, larger pads and spread the braking load over all four wheels – which doesn't happen with motorcycles nearly all the braking forces on a bike are on the front. The discs are a lot heavier and are made up of two separated discs – all of which, including the material, make for a quick cooling brake disc.
More is better! Actually due to the HIGH CENTER OF GRAVITY of a motorcycle (i.e. high seating position of the rider, and the rider is much of the weight) the amount of weight transfer forward to the front wheel is greater than the weight transfer that occurs for most cars, so this justifies big front brakes or multiple front brakes. Note: some bikes have single front brakes, but it is better to have 2 and most have 2, as it distributes the heat for better cooling so better sustained braking feel. For cars also it isn't very handy to put on 2 rotors because we don't place the wheel between suspension forks like a motorcycle, so if bigger brakes are needed, a larger single rotor is installed just on the inside of the wheel. Otherwise how would you propose to connect the second rotor to the outside of the wheel?
