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What I have been told is go one size up from your existing 26" set up in the front. The rear is less dependent. I run 160's on my 26er, so I would get a 160 for the rear and 185 for the front, for instance. I weigh 160 or so and ride mainly xc-ish stuff. Hope that helps. Also check in with the peeps on the 29er forum.rivrmutt said:What size rotor is best for a 29er? I found 185's and 203's on ebay, just not sure which way to go.

By the way anyone ever here of Gatorbrakes? Pricepoint has them for 30 buckz

No clue about Gatorbrakes

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1,598 Posts

http://www.pricepoint.com/detail.htm?stylepkey=15796&offer=IN070MTD

They're 160's. how bad can they be?:eekster:

These are my first discs, so I;m alittle leary. I kinda drag my feet when it come to new things. lol:skep:

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made in taiwan.

A google search turned up many foreign (as in non american) websites with those links. The 8 piston looks really works. Nothing about the mechanical ones though. Still 30 clams for a disc brake. What's the worst that can happen?

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2,298 Posts

160 on the back is more than enough.

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After reading endless discussions of this on the 29" board, I'm finally now convinced that I was wrong, and that wheelsize is irrelevant. I hadn't considered the bigger wheel

That said, I love my 185mm rotors, especially in front. Tons of stopping power, still plenty of modulation. May be overkill for all but my FS bike, but I like being able to swap wheels between all 3 of my bikes, so they're staying 185 in front. I might consider going 160 in back someday, but it hardly seems worth the trouble.

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/EDIT -- I proved myself wrong. I always like that. See below.

For any given speed, you have pretty much the same kinetic energy to scrub off on a 29er or a 26er. Let's call that K.

Let's say that you apply a constant braking force F, measured at the tyre contact point, tangent to the wheel, to bring the bike to a stop, ie reduce its kinetic energy to zero. It takes distance D to stop the bike.

Then FxD = K, for both bikes.

Suppose the bike has a rotor of circumference C and the calipers result in a force F' measured along the tangent of the rotor. Since the work done here must be the same as the work done scrubbing off the kinetic energy of the bike...

F'xCxN = K

Where N is the number of revolutions of the wheel it take to stop.

For a 29er, N=D/29

For a 26er, N=D/26

(this is measured in some weird unit I just made up which involves pi, but it makes no difference in the end as the units cancel)

Okay, so let's plug it all in. The 26er has a rotor of circumference C, and the 29er has a rotor of circumference C'

So, to stop the same bike in the same distance...

F'xCxD/26 = F'xC'xD/29

Dividing by F'xD we get

C/26 = C'/29

in other words

C' = 29/26xC = 1.11 x C

The circumference C is obviously just 2xpiXR where R is the radius of the rotor.

So to stop the same bike in the same distance you need an 11% bigger rotor on the 29er wheel.

Which would be a 178mm rotor. Add in the extra weight of the 29er and the 185mm rotor looks right to me.

QED I think.

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The Kinetic Energy has to be scrubbed off by the calipers, which apply the same force to the rotor on the 29er and the 26er.

The 26er wheel makes more rotations to cover a given distance, so the force is applied to the rotor over a greater number of rotations, ie through a greater distance,

On a 29er, the wheel rotates fewer times to cover the same distance, so unless the rotor is scaled up, the force is applied by the calipers through a smaller distance, and the requisite KE is not used up.

That is, after the bike has travelled the same distance as it took to stop the 26er, with the same size rotors and same force applied to the brakes, and no slipping, the 29er still has some KE, and is still moving.

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3,525 Posts

your math is incorrect. the kinetic energy of the two wheel IS NOT THE SAME.QED I think.

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you can add it in if you like... it won't change the result very much, and quite possibly just proving my point even more

the math isn't made up

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Give me K for the two bikes.

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you have the right idea....but in my job we don't use QED unless it will stand up to rigorous mathematical proof, which yours would not if we accounted for everything properly.

just being an anal turd, that's all!!!

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I use 160s with no problems.

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