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  1. #1
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    7" vs 8" rotors - how much difference is there in braking power?

    I'm looking at buying a new set of Avid BB7's for my Giant AC. The BB7's I'm looking at are the same price, whether you get 6", 7", or 8" rotors. I'm thinking that having the 7" rotors front and back would be ideal. My current setup is 8" fr / 6" rr.

    Will I notice a big difference in braking power on the steeps with a slightly smaller front rotor? Will the back wheel be much more likely to lock up?

    Thanks

  2. #2
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    I thought I read somewhere the only difference is since the rotor is larger it spins slower thus doesn't build up heat as fast.

    I have a BB7 which has sintered pads and 6" rotors which are more "powerful" than my Juicy's with 7" rotors.

  3. #3
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    If you have 8" up front, stick with that. I would go 7" in the rear in this case. Having them the same size in the front and rear may cause you overheating issues on steep downhill blasts...
    fee-fy-fo-fum...

  4. #4
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    I prefer to stagger the sizes. Smaller on the rear to help minimize rear wheel lock up.

    Run a 180/160 f/r combo. I found that I was locking up the rear too much when it was equipped with a 180.

    As far as going to the 203mm, only you can decide that. If your a larger rider or ride lots of steep downhills, then a 203 would be best. If not, a 180 or 185 (Avids rotors) would work just fine on the front.

  5. #5
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    Quote Originally Posted by MaukaRunner
    I thought I read somewhere the only difference is since the rotor is larger it spins slower thus doesn't build up heat as fast.

    I have a BB7 which has sintered pads and 6" rotors which are more "powerful" than my Juicy's with 7" rotors.
    actually the larger rotor spins faster, but for the same deceleration, i.e. you lose the same amount of energy, since the larger rotor has more metal it will absorb less energy (heat) into any 1 unit of area as compared to the smaller rotor

    also the larger rotor provides a greater amount of torque on the wheel per given clamp force.

    \end physics

  6. #6
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    Quote Originally Posted by tyler243
    actually the larger rotor spins faster...
    actually, both spin at the same speed...that being the speed of the wheel. The larger rotor moves faster though the pads but not because it spins faster.

  7. #7
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    Quote Originally Posted by craigsj
    actually, both spin at the same speed...that being the speed of the wheel. The larger rotor moves faster though the pads but not because it spins faster.
    whoops my bad with the terminology, thats why I do numbers not english

  8. #8
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    Quote Originally Posted by tyler243
    ...also the larger rotor provides a greater amount of torque on the wheel per given clamp force.
    I think this is the key - more torque, or leverage. Similar to using a longer wrench to tighten or loosen a bolt.

    Anyways, I think the main selling feature of 7" front and back would be being able to swap rotors, if one should become damaged. On the steeps, I can very easily lock up the back wheel with a 6" rotor, so I'm guessing a 7" would be a bad idea. So, the back stays as a 6".

    So, now for the front rotor. Is there a distinct braking power difference between the 7" and 8" rotor? Someone mentioned weight - I'm about 220 lbs, plus I carry a lot of gear with me. Would that be considered heavyish?

  9. #9
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    Quote Originally Posted by bikedreamer
    Someone mentioned weight - I'm about 220 lbs, plus I carry a lot of gear with me. Would that be considered heavyish?
    That was me. You are what's classified as a clydesdale. Over 200 lbs. Not saying your fat, just a larger guy. If your 220 and carry a lot of gear - I'm guessing 15-20lbs, more? - then that puts you around the 250 lb mark. Just guessing here.

    You probably want to go with the 8" rotor for the front for piece of mind. But definitely nothing smaller than the 7".

  10. #10
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    Quote Originally Posted by bikedreamer
    I'm thinking that having the 7" rotors front and back would be ideal. My current setup is 8" fr / 6" rr.

    Will I notice a big difference in braking power on the steeps with a slightly smaller front rotor? Will the back wheel be much more likely to lock up?
    Yes and yes.

    I went from 7F/6R to 8F/6R and got a significantly stronger front brake as a result. That's what I wanted, so I was happy.

  11. #11
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    Quote Originally Posted by frdfandc
    That was me. You are what's classified as a clydesdale. Over 200 lbs. Not saying your fat, just a larger guy. If your 220 and carry a lot of gear - I'm guessing 15-20lbs, more? - then that puts you around the 250 lb mark. Just guessing here.

    You probably want to go with the 8" rotor for the front for piece of mind. But definitely nothing smaller than the 7".
    I thank you for your political correctness.

  12. #12
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    Quote Originally Posted by MaukaRunner
    I thought I read somewhere the only difference is since the rotor is larger it spins slower thus doesn't build up heat as fast.

    I have a BB7 which has sintered pads and 6" rotors which are more "powerful" than my Juicy's with 7" rotors.

    Ahhhh, they both would spin @ the same RPM, but the larger rotor would be going more IPS (inches per second) than the smaller rotor @ the swept area.

  13. #13
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    Quote Originally Posted by bikedreamer
    I thank you for your political correctness.
    No political correctness from me. I don't know you. Larger guy fits both fat and muscular or very tall. For all I know your 6'7". hehe

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    Quote Originally Posted by frdfandc View Post
    No political correctness from me. I don't know you. Larger guy fits both fat and muscular or very tall. For all I know your 6'7". hehe
    I'm that exact same weight but am 4'9". So I asked my wife since I'm over 200lbs if I would fit into this Clydesdale category too. She said, sure honey you can be a Clydesdale too, your just one of those short fat versions of a Clydesdale... :/ and to think I was awesome enough to buy her an electric can opener for valentines day... pfft.

  15. #15
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    Quote Originally Posted by frdfandc View Post
    That was me. You are what's classified as a clydesdale. Over 200 lbs. Not saying your fat, just a larger guy. If your 220 and carry a lot of gear - I'm guessing 15-20lbs, more? - then that puts you around the 250 lb mark. Just guessing here.

    You probably want to go with the 8" rotor for the front for piece of mind. But definitely nothing smaller than the 7".
    225 lbs here trying the 203mm (8") just to see how it works compared to the (180mm) 7"--my wife says Ill like it

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