Mountain Biking - disc size

disc of:
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

Larger discs are typically used for downhill and freeriding. The reason is they have more surface area to dissipate heat.

As for the smaller discs being rougher I've never heard of that. I have 6" Magura Marta's and 6" Magura Marta SL's and they seem pretty smooth to me.

On a flat surface you probably won't notice much if any difference between a 6" rotor and an 8" rotor. The odds of you heating the brake up to the point of failure is pretty low.

disc of:
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 ;-)

What he said ^^. My 8 inch discs keep overheating and warping which causes a horrible high pitched squeak. If you're just doing XC stick to 6 inch rotors, it will save weight if you're a weight weenie anyway :D

What he said ^^. My 8 inch discs keep overheating and warping which causes a horrible high pitched squeak. If you're just doing XC stick to 6 inch rotors, it will save weight if you're a weight weenie anyway :D

Dude, seriously, learn to brake. 8in discs in ontario overheating and warping?....you ride your brakes to much.

unless hes a blue hoe.

unless hes a blue hoe.

No offence. It isn't long enough to be burning up 8in rotors. Unless he is riding them

if u use standard QR on the front, it is not recommanded that you use any rotor size larger the 7" says on my my avid juicy 7

so, no matter what she tells you, disc size does matter.

if u use standard QR on the front, it is not recommanded that you use any rotor size larger the 7" says on my my avid juicy 7

Thats pretty old info and limited to xc forks. There are many forks made with qr's that can accept 8in rotors.

Stupid question, but what does QR stand for?

Stupid question, but what does QR stand for?

Quick release.

Thanks alot!