6" is all you need (she said)
Originally Posted by NuclearParanoid
Bigger diameter - smooth breaking, longer breaking distance, heavy weight
Smaller diameter - rough breaking, shorter breaking distance, sudden breaking causes bike to drag, light weight
Are there any other differences? Are big discs more appropriate for downhill riding?
What if we would comapare ~6' disc with ~10' disc performance on flat surface (road/cross country style)? Especially on the front wheel
Unless you run downhill or are 350#, you probably don't need 8" discs. I'm 280# and on XC my 6" discs do just fine. If you can lock up your front wheel, that's more stopping power than you actually need.
Like the other guys said, the larger discs are for heat dissapation. The brakes don't work so well if you glaze the pads or make the brake fluid boil.
Regarding power, yes the force on the rotor is expressed as a function of it's distance from the hub. Basically the larger surface area per revolution results in greater friction. This basic priciple is what allows rim brakes with their rubber pads to even work. This translates to your needing to apply less pressure to the lever in order to affect the same stop power.
However the issue of power is something that scales. On flat terrain it rarely matters. On a prolonged downhill however, your hand will get tired. This is the other reason dowhillers (and free-riders to some extent) need bigger rotors while XCers probably do not.
Finally, I will point out that larger discs get knocked out of true easier. If you're a downhiller this doesn't matter much. But if you ride cross country, it's critical that you're breaks don't rub as this will rob energy from you.
Simply put, it's not the size of the rotor, it's the modulation and weight distribution ;-)
Last edited by willtsmith_nwi; 04-20-06 at 02:05 PM.
Reason: Adding power comments ...