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  1. #1
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    Does Rotor Size Make A Difference?

    I am 220 and 5'10 and I have always wondered about rotor size. I am building my first bike and I would like to know if having a 203mm rotor as opposed to 160mm makes a difference. I mean, is having a larger rotor purely aesthetic or is their a science behind it. " Bigger Belly, Bigger Rotor"?

  2. #2
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    Rotor size in the front does matter, bigger than 160 in the rear is overkill. I'm 6'1 265lbs and run 180mm front and 160 rear, perfect lvl of braking balance.

    Be careful though if u try to go above 180mm front can cause issues and your forks may not support them.
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    Not purely aesthetic, bigger rotor = more stopping power. I'm no cylde, but I prefer 180 mm front and rear. That's just my preference though, most people have more rotor in front, as most of your stopping power is generated from your front brake.

    There can be compatibility issues so check with frame/fork manufacturer or LBS.

  4. #4
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    you really think there's a chance it's just aesthetic? Torque - force x radius. Bigger rotor = more stopping torque.

    I see no reason to go less than 200/180. 160 rotors are for little kid bikes.

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    Joules got it. Larger rotor is more stopping power when all other things are equal.
    Think using a cheater bar on a stuck nut. Longer wrench = more torque. It's the same with brakes. Just make sure the fork is rated for a large rotor...
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  6. #6
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    Before you get all crazy on rotors... I'd try composite vs. steel disc pads. Composites have more grab meaning you stop faster. Cheaper and less weight than more rotor.........They may wear out quicker. I got 900 miles on mine before changing to steel and I want to go back to composite.
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  7. #7
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    Larger rotors also mean lighter lever effort. Too light a brake lever reduces control, that's why some riders prefer NOT to use too big a rotor. They are also heavier and more prone to damage.

    The size rotor you use is also subject to wheel size. Smaller rotors make perfect sense on smaller wheels because smaller wheels spin faster for a given speed.

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    there's a heat capacity side to it as well

    the larger rotor has a larger heat sink as well(sort of depends on rotor design as well). Do a lot of fast techy stuff on the way down and you can get the brakes pretty hot.

    If you have 160s with good tires, brakes, pads and fluid now and you can't lockup your rear brake when you are ballanced within the bike, you probably should consider 180s.

    If your current Organic pads tend to smear onto the rotor, might want to look elsewhere.

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    That Shimano stuff looks good. handles the heat and keeps the water out too.
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    @ 235 lbs I like 180 front , 160 rear.

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    Quote Originally Posted by craigsj View Post
    The size rotor you use is also subject to wheel size. Smaller rotors make perfect sense on smaller wheels because smaller wheels spin faster for a given speed.
    Not exactly. The reason you can use smaller rotors on smaller wheels is torque, not rpm. The contact patch to the hub is the lever arm you're working against, and the rotor diameter is the lever arm working for you. For a given braking effort, you want to keep the ratio consistent.

  11. #11
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    Frankly I think one needs to look at the tires/speeds/terrain as well. Using big rotors on a cyclocross front end may allow the rider to brake harder than the tires just like using a big rotor on back may not be appropriate. I like a lot of brake, but I want it to take considerable pressure to lock-up a wheel so I don't crash with an inadvertant panic brake (like I did in 1988 on my FXR-SP).
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    Quote Originally Posted by berny2435 View Post
    If you have 160s with good tires, brakes, pads and fluid now and you can't lockup your rear brake when you are ballanced within the bike, you probably should consider 180s.
    If you can't lock up your rear wheel, something is wrong with your braking system... even if you are big and heavy. Usually it's harder NOT to skid.

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    Quote Originally Posted by gsmith11 View Post
    If you can't lock up your rear wheel, something is wrong with your braking system... even if you are big and heavy. Usually it's harder NOT to skid.
    and that's why I said what I said.

    Gotta pick a happy medium with a little extra brake for the trails you ride most.

    Here's some of my feelings braking
    If skidding into a corner to get rotation isn't your thing, you can probably get away with less brake.

    If you aren't very good at modulating, you probably don't want big brakes with metalic pads.

    IF you run skinny XC tires that can't brake worth crap, you probably don't need a ton of brake. Probably why many Cyclocross bikes come with mechanical brakes unless you spends tons of money.

    If you are big guy riding big tires that brake well and you do frightening fast decends that have some oh sheeeet areas where you have to slow down for features, you might want to consider some 205/180s with good pads and learn to modulate like a pro.
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    Quote Originally Posted by berny2435 View Post
    Probably why many Cyclocross bikes come with mechanical brakes unless you spends tons of money.
    Nope. disc brakes have only recently been made legal for CX and for road racing. Given that CX is far closer to road than mountain bikes for the tech (drop bars etc) then you'll find disc brakes are a new thing, so they cost more - exactly the same as when mountain bikes got them.

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    Depends what type of riding you are doing. I had 160's front and rear on my last bike. Fine for regular trail riding, but not so great for long DH runs.

    203/185 set up now. Overkill for many situations, but sure nice for a long 5500 ft descent like I did last night!
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    Quote Originally Posted by TooTallUK View Post
    Nope. disc brakes have only recently been made legal for CX and for road racing. Given that CX is far closer to road than mountain bikes for the tech (drop bars etc) then you'll find disc brakes are a new thing, so they cost more - exactly the same as when mountain bikes got them.
    I probably should have said Mechanic DISC like BB5.
    even the lower end cyclocross bikes are getting BB5s now. Some of the expensive ones are still mech. disc. Might have something to do with wheel strength as well.
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  17. #17
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    Like a lot have said.....
    The bigger the rotor the more control you have on the steep stuff.
    Just want to add that one more variable in your issue with disc size.
    It also depends on what type of brakes you have also........
    2 piston compared to the 4 piston. 4 Piston has more power but are also heavier and $$.
    Just thought I would throw that out there.......

    I myself use Shimano Saints on 203 discs (Front and back) and they are amazing!
    But not light in weight, however on steep rock faces they stop me on a dime and are quiet.
    Best brakes I ever bought but they are not cheap.....

  18. #18
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    My Stumpjumper has 203 front and 180 rear. Last February, when I finished my fatbike, my thinking was that the bike was going to be a crawler and 160 front and rear would be fine. I couldn't have been more wrong.
    When going downhill, they slowed me a little. There was no way I was stopping. I switched the front to a 180. Slightly better but still not enough and noisy as hell. I was about to give up on Shimano brakes when I switched to 203, 180. The MTB gods smiled upon me. I can one finger brake like my other bike.
    BTW, I weigh about 255.
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  19. #19
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    I'm 220-240lbs and I run 203mm Zees front and rear. The way I see it, there's no reason not to. If you need less brake.......don't squeeze as hard. Modulation is excellent, much better than any 160 or 180mm brake I've used. Instead of handicapping the brake system to make it hard to get maximum power, work on modulating the brakes. Again, I have all the modulation I could ever want and I can send myself over the bars with one finger.

    You gain two things. Power, not having to squeeze the lever as hard for a given decel rate. Heat rejection, the larger rotors run cooler so there's less chance of fade.

    Also, 4pot brakes are not automatically stronger than 2 pot. It's total piston area that matters and there are some 2 piston brakes with more piston area than some 4 pistons. The 4 pistons can use a pad with more pad area. This gives a little better fade resistance and longer pad life. Larger pads do not affect power.

    At my weight I automatically use 200-203mm rotors on any bike I buy. I feel like modulation is better and I have a lot of power in reserve. Even if it doesn't see downhill I like knowing the tires determine my stopping distances and not the brakes.
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  20. #20
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    240lbs here and running 203 front/180 rear. XT Ice Tech rotors. Metallic pads up front, resin in the rear.

    I notice much more difference with metallic vs resin and tires for sheer power. Larger rotors dissipate heat better, but that's it in my experience. Although that may be because I'm not needing to use them as much. But, again, tires and pads make more of a difference.

  21. #21
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    Quote Originally Posted by Alias530 View Post
    240lbs here and running 203 front/180 rear. XT Ice Tech rotors. Metallic pads up front, resin in the rear.

    I notice much more difference with metallic vs resin and tires for sheer power. Larger rotors dissipate heat better, but that's it in my experience. Although that may be because I'm not needing to use them as much. But, again, tires and pads make more of a difference.
    Larger rotors have a little more surface area so technically they dissipate heat better, but the real story about heat is that they don't generate as much in the first place because of their superior leverage.

    Because of that same increase in leverage, you will get more braking power on tap for a given setup regardless of whether the brakes were running hot or cold before.

  22. #22
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    Quote Originally Posted by kestrel242 View Post
    Larger rotors have a little more surface area so technically they dissipate heat better, but the real story about heat is that they don't generate as much in the first place because of their superior leverage.
    No. Braking is the process of converting kinetic energy into heat. The amount of kinetic energy is the same so the heat generated is the same. The larger rotor spins faster AND has greater surface area so it transfers heat to the air better.

    Leverage is a silly way of looking at it as well. The larger rotor has greater speed past the brake pad. Regardless, the math is the same.

  23. #23
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    I switched from 180 to 203 on the front and I feel like I get a bit more stopping power. Things like this are hard to quantify, but maybe 15-20%. Biggest improvement was the reduction in brake fade on long downhills. On one trail (4 miles long) with the steepest part at the end, I'd have a lot of trouble stopping with the 180. With the 203, I now have enough reserve to stop in an emergency.

  24. #24
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    At my weight (super clyde) I love my Hope floating rotors 203/185 with Shimano XT Ice Tech brakes. Had a 160 ice tech rear rotor and it did not have near the feel or power of the 185 Hope.
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  25. #25
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    Also depends on how / where you ride and design of rotor. A big rotor is heavier and takes a bit more to get to speed, so on XC bikes/trails, you typically have smaller rotors. On my FL trails, I can easily get away with 160's.

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