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Old 03-30-09, 05:22 PM   #1
CCrew
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Rotor size vs Front/Rear

Swapping one of my Gary Fisher's to Avid BB7's from Avid Single Digit V's. LBS gave me a great deal on the BB7's, but I realized that 1 is a 160mm and 1 is a 185mm. Bike will take either at either end from a clearance standpoint, but from a mechanical standpoint where am I going to benefit from the bigger rotor?

And yeah, I could just go back to the LBS and exchange the 185, but they said the 160's are on backorder so it would be a while..

Thanks.. I'm thinking bigger on the front due to the larger swept area, but threw this out for a sanity check since the arguement can be made that the rear sees heavier braking.

-R
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Old 03-30-09, 05:32 PM   #2
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I've always heard bigger rotor up front, but I run matched anyways.... looks more normal.


Also, what's wrong with exchanging the 160mm for 2x 185s? If there's no clearance problems and no major weight concerns (as the difference isn't all that much), what's stopping you from getting a pair of the larger rotor versions?
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Old 03-30-09, 05:34 PM   #3
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larger for front.
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Old 03-30-09, 05:36 PM   #4
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Also, what's wrong with exchanging the 160mm for 2x 185s? If there's no clearance problems and no major weight concerns (as the difference isn't all that much), what's stopping you from getting a pair of the larger rotor versions?
Impatience more than anything. They order once a week and if I return it it'll be a week to get another one.

At least I'm honest about it
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Old 03-31-09, 04:11 AM   #5
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I put my 185 in front, since that does more of the braking, and my 160 in back. Similarly, you'll notice that cars normally have bigger/more powerful disc brakes on the front wheels than on the back.
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Old 03-31-09, 06:01 AM   #6
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One has to ride very strangely if the heavier braking is on the rear. The front does probably 80% of the braking. I uses BB-7 with a 185 front and 160 rear. I went to the 185 as I do one finger braking and wanted faster response. I set my levers for maximum leverage which gives the maximum sensitivity (and the maximum lever travel). 185 on the rear is over-kill and will result in lock-ups which reduces control not to mention the trail damage.

Al
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Old 03-31-09, 07:05 AM   #7
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Thanks all. With the 185mm on the front and the 160mm in the back it works great.

-R
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Old 03-31-09, 07:24 AM   #8
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One has to ride very strangely if the heavier braking is on the rear.
Not really. During my most eager MTB days we rode a lot of really slippery trails, which meant that the rear brake certainly saw more action than the front brake.

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The front does probably 80% of the braking.
On surfaces with decent friction, sure.
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Old 03-31-09, 08:01 AM   #9
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What's the bike and what brand/model fork do you have on it?

On some XC forks the manufacturers recommend that only 160mm be used, I guess that these forks aren't made to handle the extra forces that a larger size rotor might put on them.
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Old 03-31-09, 10:53 AM   #10
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I'm thinking bigger on the front due to the larger swept area, but threw this out for a sanity check since the arguement can be made that the rear sees heavier braking.
Larger swept-area doesn't do anything for braking, just gives longer rotor and pad life. It's the diameter that generates the reverse-torque that slows the wheel down. For any given clamping-force from the caliper, a larger-diameter rotor will generate more reverse-torque. For example, if you've got a 100mm rotor on one wheel and a 200mm rotor on an identical wheel, the same finger-squeeze pressure on the 200mm rotor will generate twice as much braking force.

The smaller diameter of discs is why you must have much more mechanical-leverage of the caliper for much higher squeezing-force than rim-brakes, which has a much, much larger diameter. You have to squeeze the disc-brake X-times harder than a rim-brake where X = the ratio of diameters of the rim versus disc.

Similarly, since the front-brake generates easily 10x more braking force than the rear (even more at maximum braking), you need a larger rotor in front. In fact, a lot of racing motorcycles have such small single rear-discs, they're actually about the same 200mm size you see on bicycles! While the front have dual 320-350mm rotors; some even in the 400mm sizes.

Last edited by DannoXYZ; 03-31-09 at 11:01 AM.
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Old 03-31-09, 11:17 AM   #11
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What's the bike and what brand/model fork do you have on it?

On some XC forks the manufacturers recommend that only 160mm be used, I guess that these forks aren't made to handle the extra forces that a larger size rotor might put on them.
It's a Marzocchi Atom Z2 Race (air) . It's on an older Gary Fisher Mt Tam.
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Old 03-31-09, 12:02 PM   #12
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Not really. During my most eager MTB days we rode a lot of really slippery trails, which meant that the rear brake certainly saw more action than the front brake.

On surfaces with decent friction, sure.
I ride a lot on steep trail on wet leaves. The breaking is still the same: most of it on the front. It just takes more skill.

Al
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Old 03-31-09, 01:32 PM   #13
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I ride a lot on steep trail on wet leaves. The breaking is still the same: most of it on the front. It just takes more skill.
So you're a more skilled rider than me, good for you.

The point I was trying to make was that all that is needed to encourage heavy use of the rear brake are slippery conditions (and maybe limited rider skills then). Doesn't qualify as strange IMO.
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Old 03-31-09, 04:07 PM   #14
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I thought that the only difference was in the mounting bracket? The calipers are the same, you just fit on different mounts to get the 160 or 185 (and isn't there even a 210)? oh yeah, and you need the corresponding rotor too, but those are available aftermarket...

anyway, oh yeah, go with the bigger in front.
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Old 03-31-09, 06:01 PM   #15
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So you're a more skilled rider than me, good for you.

The point I was trying to make was that all that is needed to encourage heavy use of the rear brake are slippery conditions (and maybe limited rider skills then). Doesn't qualify as strange IMO.
As long as it does not lock up the rear wheel and you don't extend the stopping distance by not applying the front brake to the limit. I don't know what "encourage heavy use" means. Most folks just ride and don't think about it. That comes with experience. One really needs to focus on the trail and anticipate braking conditions to know where to apply the brakes most effectively.

Ned Overend's book and video are excellent primers for all aspects of mountain biking including braking.

Al
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Old 03-31-09, 07:34 PM   #16
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Originally Posted by DannoXYZ View Post
Larger swept-area doesn't do anything for braking, just gives longer rotor and pad life. It's the diameter that generates the reverse-torque that slows the wheel down. For any given clamping-force from the caliper, a larger-diameter rotor will generate more reverse-torque. For example, if you've got a 100mm rotor on one wheel and a 200mm rotor on an identical wheel, the same finger-squeeze pressure on the 200mm rotor will generate twice as much braking force.

The smaller diameter of discs is why you must have much more mechanical-leverage of the caliper for much higher squeezing-force than rim-brakes, which has a much, much larger diameter. You have to squeeze the disc-brake X-times harder than a rim-brake where X = the ratio of diameters of the rim versus disc.

Similarly, since the front-brake generates easily 10x more braking force than the rear (even more at maximum braking), you need a larger rotor in front. In fact, a lot of racing motorcycles have such small single rear-discs, they're actually about the same 200mm size you see on bicycles! While the front have dual 320-350mm rotors; some even in the 400mm sizes.
+1

As always a great answer from Danno.

The other benefit of a larger rotor is more surface area for cooling. Those holes in the rotor are not for brake dust, they're to increase the surface area and lighten the disc.

Also, if you are using the rear brake more heavily than the front, please don't ride our trails.
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Old 03-31-09, 08:23 PM   #17
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I find that even a 160 rotor in the rear is more than plenty. You see some light XC rigs running 160 front, 140 rear.
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Old 03-31-09, 08:28 PM   #18
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I find that even a 160 rotor in the rear is more than plenty. You see some light XC rigs running 160 front, 140 rear.
That depends entirely on riding style, terrain and rider weight.

Living in the Rockies, weighing in at 230, I like 160/185 for XC, 205/205 for DH
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Old 03-31-09, 08:35 PM   #19
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It's a Marzocchi Atom Z2 Race (air) .
80mm XC fork?

Probably not recommended to have a rotor larger than 160mm fitted to it and probably not necessary either.
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Old 03-31-09, 09:42 PM   #20
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80mm XC fork?

Probably not recommended to have a rotor larger than 160mm fitted to it and probably not necessary either.
Yup. 80mm XC. I'm probably going to switch the 185 rotor mounts and rotor to another GF I have that's running a Surly Instigator rigid front fork. I know that one will have no issue with the load.
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Old 04-01-09, 04:23 AM   #21
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.... I don't know what "encourage heavy use" means.
It's easier to recover from a locked-up rear wheel skid than it is to recover from a front wheel skid, so it's my desire to avoid a faceplant that encourages me to use the rear brake, if conditions are slippery and the available braking distance allows it.

The front brake gets used too, but with more care and a bigger margin against lock-up than the rear during slippery conditions. It's a tradeoff between longer braking distance and a reduced risk of front wheel skid when riding at the limit of my ability. Seems like a good deal when circumstances allows it.
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Old 04-01-09, 05:05 AM   #22
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It's easier to recover from a locked-up rear wheel skid than it is to recover from a front wheel skid, so it's my desire to avoid a faceplant that encourages me to use the rear brake, if conditions are slippery and the available braking distance allows it.

The front brake gets used too, but with more care and a bigger margin against lock-up than the rear during slippery conditions. It's a tradeoff between longer braking distance and a reduced risk of front wheel skid when riding at the limit of my ability. Seems like a good deal when circumstances allows it.
Check out Overnd's book. You can probably get it for a few $'s used on Amazon. There is no reason to skid at all, period!

Al
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