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  1. #1
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    why do bigger disc's equal better stopping??

    Sorry for the stupid question but I am realitively new to the disc brake world.

    Why, all things equal (best I can tell is the same model with 160mm vs 185mm still uses the same pads, caliper etc) does a bigger disc diameter equal better stopping. The pads still only bare on the outer1/4 of the disc.

    Is it that there is more baring surface due to the larger diameter or better heat discipation, both (neither lol).

    Thinking about putting a 185mm (up from 160mm) upfront for next year (with the correct bracket ofcourse) so I want to make sure I know why it works better, lol

    J-

  2. #2
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    The same pad and caliper grip the same amount of disk with the same pressure....

    Thing is the bigger disk can dissipate heat better...

    and it has a bigger lever arm to work with.

    The heat dissipation is the big thing.

  3. #3
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    The unscientific answer is it takes less energy to stop a spinning wheel closer to the outside of its diameter. A larger disc effectively moves the braking area closer to the outside of the wheel.

    Someone will chime in on the science....

    Cheers!
    And don't do the burrito jump... Francois

  4. #4
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    Quote Originally Posted by cmdrpiffle View Post
    The unscientific answer is it takes less energy to stop a spinning wheel closer to the outside of its diameter. A larger disc effectively moves the braking area closer to the outside of the wheel.

    Someone will chime in on the science....

    Cheers!
    The energy required to stop the bike remains the same regardless of the disk diameter.

  5. #5
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    Quote Originally Posted by jeffscott View Post
    The energy required to stop the bike remains the same regardless of the disk diameter.
    Yes but you have a better mechanical advantage with a bigger rotor. It's not solely a matter of heat although it also plays a role. If we take heat out of the equation, the bigger rotor will still have more power.
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  6. #6
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    One word: torque

  7. #7
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    Quote Originally Posted by PissedOffCil View Post
    Yes but you have a better mechanical advantage with a bigger rotor.Yes a longer (bigger) lever arm It's not solely a matter of heat although it also plays a role. If we take heat out of the equation, the bigger rotor will still have more power.
    You are mixing your terms.....Power refers to energy per unit time. The brake converts kinetic power, to heat...and it must dissipate or store this heat. The bigger rotor will be better for both dissipation and storage of heat. So heat dissipation and storage provide more braking power.

    The lever arm provides greater braking torque because of the longer lever arm.

    End result bigger rotor equals more braking torque and more braking power.

    For me a 160 mm rotor provided ample one finger braking torque...(I could do a really good stopee)....but on long descents I would smoke the brake and lose braking torque due to overheating.

    So for me the bigger rotor 203mm improved the braking power by increasing heat dissipation/storage.

  8. #8
    Master of the Face Plant
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    One word: Leverage (which basically means torque)
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  9. #9
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    Quote Originally Posted by sandmangts View Post
    One word: Leverage (which basically means torque)
    +1 think of it as a lever on a fulcrum and you are trying to lift a large weight if you have 10ft of lever on your side of the fulcrum it will be easier to lift the weight then if you have 5ft on your side

    Did i make any sense?

    so if you have a larger rotor you then have a larger lever so to speak which makes it that much easier to stop the bike.
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  10. #10
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    More surface area to compress upon...

    "bigger is better"....oh, baby!
    ...and proud member of the anti-sock puppet desolation

  11. #11
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    cool thanks guys all makes sense and was kinda what I was thinking.

    Second quick question. To go to a bigger front rotor all I need is the rotor itself and the appropriate caliper adaptor?

    Same cailper/pad would work, correct? I have BB5's, if that matters.

    Thanks
    J-

  12. #12
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    Yes, make sure to get the proper FRONT adaptor.
    ...and proud member of the anti-sock puppet desolation

  13. #13
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    Quote Originally Posted by ambassadorhawg View Post
    Yes, make sure to get the proper FRONT adaptor.
    copy thanks all for ur input.

    J-

  14. #14
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    Check if your front fork will support the bigger rotor. I wanted to upgrade but RockShox said that my fork wouldn't support the bigger rotor. I assume the fork didn't have enough support for the extra stopping power or something.

  15. #15
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    Quote Originally Posted by adrianmoisey View Post
    Check if your front fork will support the bigger rotor. I wanted to upgrade but RockShox said that my fork wouldn't support the bigger rotor. I assume the fork didn't have enough support for the extra stopping power or something.
    copy i'll def check that before I lay out the cash.

    thanks

    J-

  16. #16
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    It's torque and surface area. The torque has been explained. But on the larger diameter, more rotor passes through the caliper with one rotation of the wheel. More circumference = more brake length for the same distance traveled. A 160mm has 502.654mm of braking length and a 185mm has 581.946mm, that's an extra 3" of brake per revolution.
    Btw, circumference = pi x diameter.

    Oh yeah, and better heat dissipation.
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  17. #17
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    Quote Originally Posted by jeffscott View Post
    You are mixing your terms.....Power refers to energy per unit time. The brake converts kinetic power, to heat...and it must dissipate or store this heat. The bigger rotor will be better for both dissipation and storage of heat. So heat dissipation and storage provide more braking power.
    I'm not mixing terms at all. Mechanical advantage is the same as leverage and this is what gives you better stopping power with a bigger rotor.

    From Sheldon :
    "'Mechanical advantage', or 'leverage' is the ratio between how much you get out of a linkage and how much you put in."
    "Installing smaller wheels [... reduces] the mechanical advantage accordingly. For instance, substituting 622 mm (700C) wheels on a bike built for 630 mm (27 inch) wheels will degrade the braking."

    The same applies to rotors.
    Ref : The Geometry of Cantilever Brakes
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  18. #18
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    Quote Originally Posted by PissedOffCil View Post
    I'm not mixing terms at all. Mechanical advantage is the same as leverage and this is what gives you better stopping power with a bigger rotor.

    From Sheldon :
    "'Mechanical advantage', or 'leverage' is the ratio between how much you get out of a linkage and how much you put in."
    "Installing smaller wheels [... reduces] the mechanical advantage accordingly. For instance, substituting 622 mm (700C) wheels on a bike built for 630 mm (27 inch) wheels will degrade the braking."

    The same applies to rotors.
    Ref : The Geometry of Cantilever Brakes
    He's trying to point out that you are using the word "power" in the wrong context. Notice how neither of the statements you just listed reference "power"?

  19. #19
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    Quote Originally Posted by kan3 View Post
    He's trying to point out that you are using the word "power" in the wrong context. Notice how neither of the statements you just listed reference "power"?
    No his edited quote mentions that a longer lever gives better mechanical advantage, which it does but a bigger rotor also gives a better mechanical advantage.

    Yes I used "power" as in "stopping power". Given the context I think it goes without saying and without getting into semantics.
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  20. #20
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    nice ^ ! way to sum it all up.
    Keep trying to do the awesomest thing you've ever done.

  21. #21
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    Quote Originally Posted by PissedOffCil View Post
    No his edited quote mentions that a longer leverlever arm as in the radius between the brake pad and the axle gives better mechanical advantage, which it does but a bigger rotor also gives a better mechanical advantage.

    Yes I used "power" as in "stopping power". Given the context I think it goes without saying and without getting into semantics.
    Nope still got it wrong but what the hey....you at least can understand yourself.

  22. #22
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    Have you ever extended the handle of a tire iron to make it easier remove lug nuts?

    I believe a bigger rotor works on the same principle to provide more stopping power.

  23. #23
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    Quote Originally Posted by JanBoothius View Post
    Have you ever extended the handle of a tire iron to make it easier remove lug nuts?

    I believe a bigger rotor works on the same principle to provide more stopping power.
    Yup the extended handle, gives you a longer lever arm....hence more torque....not more power.

    Anyway we call that a snipe around here.

  24. #24
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    Quote Originally Posted by jeffscott View Post
    Nope still got it wrong but what the hey....you at least can understand yourself.


    What exactly are you talking about? Are you saying that a bigger rotor doesn't have a bigger mechanical advantage over a smaller rotor?
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  25. #25
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    Quote Originally Posted by jeffscott View Post
    Yup the extended handle, gives you a longer lever arm....hence more torque....not more power.

    Anyway we call that a snipe around here.
    You're really just playing with words. When people talk about stopping power, they mean the ability to stop in a shorter amount of time/distance from a given speed. You can go around correcting people with your torque vs. power all you want, it really makes you look like a dork... Everybody here understood what we are talking about, we couldn't care less about your semantics and uptight comments.
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