rotor size for a 29er?
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I recommend the 185's. Great braking power with low fade, yet still retains a lot of modulation capability. I run a 185 up fron on my 9er
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1,503 Posts
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:
No clue about Gatorbrakes
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1,598 Posts
I think 203's start getting in the way, and are easy to tweak. And the last thing on a 29er wheel you need is more weight. Unless your a big dude and do a lot of fire road downhilling, the con's out weigh the pro's of giant rotors. Don't give in to "rotor envy." And google'n the gatorbrakes, they look a lot better than what I was expecting. But 30.00 sounds too good to be true. They mispriced something or they are some funky remanufactured oem bastard parts (not that the big e-tailers would sell anything like that).
).here's the link....
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:
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:
I'm running 185/160 BB7s on my Niner. I'm happy with the setup. My commuter Monkey has a mullet - 160mm BB7 front, with V-brake rear. Even though the Monkey only sees pavement, the 160mm brake seems a bit weak on that bike. Could be the over 30lb weight and big sluggish tires (Big Apple 2.35)...
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11,880 Posts
FWIW gatorbrake
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?
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
I have 160/160 on my SIR9 and 185/160 on my FS 26er. I am impressed by the 185 on the front and will be upgrading the SIR9 for sure... definitely a noticeable difference for me.
160 on the back is more than enough.
160 on the back is more than enough.
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3,284 Posts
I've always run 185/185 on my 29"ers, and for several years believed that it took more leverage at the axle to get the same braking force at the rim of a bigger wheel.
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 rotates more slowly at a given bike speed, negating the need for increased leverage at the axle and canceling the radius out of the question. I no longer believe it is necessary to go up a size when going 29".
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.
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 rotates more slowly at a given bike speed, negating the need for increased leverage at the axle and canceling the radius out of the question. I no longer believe it is necessary to go up a size when going 29".
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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I'm not sure the wheel size makes any difference either, on consideration, unless it just naturally makes you ride faster.
/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.
/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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2,298 Posts
Here's how I explain this result:
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.
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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I'm ignoring the rotational KE. my guess is it's less than 10% of the bike's overall KE.
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
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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NB if you want to add it in you have to start thinking about the relative masses of the bike, rider and wheels. Let's say you start with a 100kg bike and rider and a 4kg set of wheels/tires/tubes on both bikes. Assume the mass is concentrated at the rim. Finally, let's say your initial velocity is 5 m/s.
Give me K for the two bikes.
Give me K for the two bikes.
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2,298 Posts
sorry to keep chuntering on, but to get an idea of how little KE the rotation of the wheels contributes, imagine your bike upside down and spinning up the wheels as fast as you can, the equivalent of travelling at 30mph or so. Now yank the brakes. The wheels stop REALLY quickly, almost instantaneously with good disk brakes. Compare that with how long it takes to slow down a bike and rider travelling at 30mph. That's how little they contribute. And I suspect the big wheels have more KE if anything.
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3,525 Posts
LOL.....you misunderstand. i'm not arguing your results. too many people on this board read a formula in a physics textbook and assume they are correct.
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!!!
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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53 Posts
I use 160s with no problems.
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I agree that the 160s do fine... I have them on my Niner... but the difference with the 185 was enough to make me want one...
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