Bicycle Brake Theory

[Adabiviak]

I'm after opinions about mountain bike disk brake setup. I do three types of braking: one where I need to come to a stop fast (emergency, for example), shedding speed over a long mellow downhill (ten+ miles for a long one), and shedding speed over a steep downhill.

For emergency stopping and steep downhill rides, both brakes are used as needed.
For mellow downhills (or casual stops for that matter), I ride the rear brake. This leaves the front brake (which has better stopping capability) cool and ready in case I need it for an emergency stop/drop in speed or an upcoming steep section.

I have two disk brakes: one with a 160mm disk, and the other with a 185mm disk.

Two 160mm disks will handle the bicycle just fine. A larger disk gets you a bit more stopping power, and better heat radiation. When comparing the front wheel to the rear for braking, the front already has inherently better stopping power because it's in the front, and since I ride the rear brake at length (for reasons mentioned above), it needs to shed heat better. For these reasons, I put the larger disk on the rear wheel.

Ultimately, I think the performance difference between these two disk sizes won't be incredibly noticeable, but I'm open to suggestions. Should I put the larger disk on the front brake and use that for shedding light speeds? Everything I read online says that the larger disk should be in the front, but they don't say why. If the only reason is better stopping power, the smaller disk already has plenty of stopping power.

 

[Stigmata]

Stopping power is basically the ability to keep a wheel from moving while under force, and since far more of your weight ends up on the front wheel during a stop, it's harder to keep the front wheel locked than the back one. Extra surface area would help keep it from slipping, but you don't seem to have any real problems anyway so I think it's just a question of which wheel you want to be able to brake more reliably.

 

[Higlac]

it's just a question of which wheel you want to be able to brake more reliably.

This. Emergency stops and their reliability always take precedence over convenience.

 

[Dan]

For dumping heat on long down hills I find the best method is to alternate squeezing the front and back until you think it is as hot as you are comfortable with. The hotter the brake is, the more heat it will shed to convection, and you always have one cool brake for if you need to slow even more.