Rotor Size?? Does it matter?

[Giantrider]

Well lately i've been wondering does it really matter what size rotors you use?

There are so many different shapes and sizes but mainly i'm wondering,
whats the difference between running a 6" and a 8" rotor on a DH bike?
How does it affect performance?
Is there any difference in stopping power? slower/ faster?

I've seen a few people run 6" rotors on there DH bike, but nearly all the DH bikes i've seen run a 8"...

At the moment i'm running 8" G2 Cleansweep avid rotors and was wondering whether having a smaller rear rotor would have an affect on the braking power of my bike.

Cheers Hamish

P.S I did do a quick search, but didn't come up wiht much about whats the difference in rotors sizes.

[contagion]

I'm not an expert on the subject, but I've heard that the caliper and pads are what affect braking power and 8" rotors are used for DH because they don't heat up as much.

So to answer your question, no it won't affect braking power however it may cause a higher rotor temperature (I'm not sure of the effects this will cause). Also the front brake is responsible for approx 70% of braking power due to the rear locking if used too much, so running a 6" rear shouldn't be a worry.

[jasnos]

hey!
and 8 inch rotor doesn't rotate as fast. so its better for braking when your not skidding. which is why they run a 8inch on the front of dh bikes.
i think so any way.

[thecat]

The more metal (8") the better the heat dispersion. the better the heat dispersion the quicker it cools down. Keeping the brakes cooler keeps them operating at their peak.

DHer user 8" for this reason. XCer generally don't have the long continuous braking sections so go for 6" because there is less weight.

[xc_machine]

Taken from MBAction.com -

"The larger the diameter of the rotor, the more braking force is generated by the same caliper. If you ride fast and brake hard, eight-inch rotors shed heat and stop harder with less squeeze effort than six-inch rotors. It is the present fashion to use an eight-inch rotor on the rear for extra stopping power. That is stupid really, because only the front brake can develop enough traction to use the extra stopping power because the rear unweights when you hit the brakes--so use a six inch rotor on the rear and an eight inch up front and your Santa Cruz will perform as well as anything out there. My pick for brakes are Shimano Deore XT."

Iv'e also heard more and more DHillers are going to 6inch all round as they dont use their brakes enough for 8 inch rotors to make a signicant change!

Oh, and i think this is one of the best MTB questions in Farkin history..

[wombat]

Heat dissipation is definately a part of it, but the important bit is simple: torque.

A bigger rotor provides more leverage, and thus a greater braking moment for a given friction force applied on the rotor by the pads.

[tnankie]

hey!
and 8 inch rotor doesn't rotate as fast. so its better for braking when your not skidding. which is why they run a 8inch on the front of dh bikes.
i think so any way.

you are joking right?

an 8" rotor rotates faster than a 6"

[derelikt]

you are joking right?

an 8" rotor rotates faster than a 6"

Damn, You beat me to it.

[jasnos]

yeah. sorry.
just relized that the 8 is alot faster then the 6.
way to look like a huge tool

[matty_101]

Damn, You beat me to it.

This makes no sense to me, seeing as the disc is dependant on the wheel for its RPM, i don't see how the size of the disc alters its revolution speed.

[RaID]

yeah. sorry.
just relized that the 8 is alot faster then the 6.
way to look like a huge tool

I think you both appear to look like huge tools ;)

Both 6" and 8" rotors spin at the same angular speed, because they are directly attached to your wheel. So your wheel and your rotor spin at the same angular speed no matter what size rotor it is.

They have a different linear speed: think about it, if you were to roll a
10cm disc at 10rpm and a 100cm disc at 100 rpm on the ground. The 100cm would travel much faster along the ground.



This is the correct answer, regarding the different use different size rotors. The bigger the rotor the bigger the braking torque, the more "powerful" the brake.

Heat dissipation is definitely a part of it, but the important bit is simple: torque.

A bigger rotor provides more leverage, and thus a greater braking moment for a given friction force applied on the rotor by the pads.

Please DONT EVER quote anything from MBAction,

Yes they are right that you have enough torque using 6" rotor to break traction on the rear (you have enough torque to do that on the front using a 6" rotor as well). However 8" rotors are also used to allow better modulation of the brake as well as help in the heat dissipation which can definitely be a problem on the back on longer descents. I really dont see the how the small weight benefit of the 6" rear rotor outweighs the performance benefit of the 8" rotor.

Pro DH racers sometimes use smaller rotors eg 7" or 6" because they dont necessarily use brakes in the same way we do. They want the lightest bike possible hence every gram counts,they use their brakes less, check their speed rather then panic brake and loss a lot of speed at once.

Taken from MBAction.com -

"The larger the diameter of the rotor, the more braking force is generated by the same caliper. If you ride fast and brake hard, eight-inch rotors shed heat and stop harder with less squeeze effort than six-inch rotors. It is the present fashion to use an eight-inch rotor on the rear for extra stopping power. That is stupid really, because only the front brake can develop enough traction to use the extra stopping power because the rear unweights when you hit the brakes--so use a six inch rotor on the rear and an eight inch up front and your Santa Cruz will perform as well as anything out there. My pick for brakes are Shimano Deore XT."

Iv'e also heard more and more DHillers are going to 6inch all round as they dont use their brakes enough for 8 inch rotors to make a signicant change!

Oh, and i think this is one of the best MTB questions in Farkin history..