disk size

jaybocrazynips
jaybocrazynips Posts: 6
edited July 2008 in MTB workshop & tech
hi all

was just contemplating replacing my disks and was wondering if anyone knew the maximum size disk that can be fitted to a 04 - 05 santa cruz bullit

cheers

jay

Comments

  • Anonymous
    Anonymous Posts: 79,665
    I don't know specificalyl what's recommended for the frame, but most swingarms can handle a 180mm rotor.

    However, it's generally pointless to go over 160 on the rear.
  • rob cole
    rob cole Posts: 706
    However, it's generally pointless to go over 160 on the rear.

    perhaps for XC riding, but try doing serious downhill or freeride with 160mm and you will soon find your brake cooking...

    the increased leverage of the larger rotor means the brake is more powerful so you use it less often to decrease your speed (you will need to brake drag more with a smaller rotor), and it cools better because the larger rotor dispassiates heat better than a smaller rotor

    min. for downhill or freeride on the rear would be 185mm, preferably 203mm if doing long descents (i.e. Alpine)
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  • shin0r
    shin0r Posts: 555
    If it will fit and you can deal with the small increase in weight, go for rotors as large as possible.

    I don't know of any advantage (apart from weight) in going for smaller rotors, but I'm willing to be proved wrong!

    Shi
  • Anonymous
    Anonymous Posts: 79,665
    rob cole wrote:
    However, it's generally pointless to go over 160 on the rear.

    perhaps for XC riding, but try doing serious downhill or freeride with 160mm and you will soon find your brake cooking...

    the increased leverage of the larger rotor means the brake is more powerful so you use it less often to decrease your speed (you will need to brake drag more with a smaller rotor), and it cools better because the larger rotor dispassiates heat better than a smaller rotor

    min. for downhill or freeride on the rear would be 185mm, preferably 203mm if doing long descents (i.e. Alpine)
    On the front, that's all well and good, but you'll never need a 203mm on the rear unless your bike weighs as much as a motorcycle
  • shin0r
    shin0r Posts: 555
    yeehaamcgee wrote:
    On the front, that's all well and good, but you'll never need a 203mm on the rear unless your bike weighs as much as a motorcycle

    I don't see the logic in this: Surely bigger rotors mean less effort needed to brake and less chance of the pads (and discs) experiencing fade. In bad conditions, a bigger braking surface will still perform better than a smaller one. I can't really see why anyone would want small rotors if they can fit big ones, except maybe to save a few grams in weight.
  • supersonic
    supersonic Posts: 82,708
    Less modulation is a reason ie lock up too quick without much lever control. Larger rotors mean less lever effort to lock the wheel up - and locking the wheel up depends on tyre grip.
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  • Splasher
    Splasher Posts: 1,528
    I boiled a 180 rear in the Alps on a 32lb AM bike. A Bullit is a burly bike, I'd be amazed if it would not take anything up to and including a 203mm rotor.

    Bigger rotors mean less finger effort required for a given amount of braking, and less likelihood of brake boil due to larger radiating area. The downside is that they weigh more and that weight is both rotating and unsprung.
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  • jaybocrazynips
    jaybocrazynips Posts: 6
    im currently running an old hope c2 with a 160 mm disk and im finding myself suffering from a fair bit of fade, i have seen a bullit running a hayes 8" disk on the net but looking at my frame i was unsure as to how this was done (probably a tight fit) while weight is an issue im running shivers, mrp cranks and a heavy wheelset so i dont really think the extra weight of the disk will make much difference

    also anyone had experience with the hope 6 pots ?

    cheers

    jay
  • Anonymous
    Anonymous Posts: 79,665
    hopeC2 + 203mm rotors = crazy brakes :D
  • jaybocrazynips
    jaybocrazynips Posts: 6
    ha ha i was planning on changing the callipers as well but i suppose it could work ???

    jay
  • Anonymous
    Anonymous Posts: 79,665
    they really are mental when you stick BIG rotors on them.

    But I still don't think you need 203mm on the rear. Most World Cup DHers don't use 203mm on the rear.
    Rear brake hardly does anything, at the end of the day.
  • Splasher
    Splasher Posts: 1,528
    yeehaamcgee wrote:
    Most World Cup DHers don't use 203mm on the rear.
    Rear brake hardly does anything, at the end of the day.

    I'm not sure that's true. Most pro DHers alter their bike depending on the course and ride completely different set-ups from race to race. Also, they tend to weigh less and brake less than us mere mortals.

    I'm not going to go into too much detail because I got slagged off last time, but the heat produced by a brake is directly proportional to the rider and bike combined weight. So for example a 15 stone bloke on a 40lb bike, the heat produced is 1.3 times the amount for an 11 stone bloke doing the same thing.
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  • cee
    cee Posts: 4,553
    shin0r wrote:
    yeehaamcgee wrote:
    On the front, that's all well and good, but you'll never need a 203mm on the rear unless your bike weighs as much as a motorcycle

    I don't see the logic in this: Surely bigger rotors mean less effort needed to brake and less chance of the pads (and discs) experiencing fade. In bad conditions, a bigger braking surface will still perform better than a smaller one. I can't really see why anyone would want small rotors if they can fit big ones, except maybe to save a few grams in weight.

    Nope. the braking surface is the same....the pads are the same size and the tire/ground contact patch is also the same size. The bigger rotor gives you a bigger lever, and so it requires less input to acheive the same amount of braking force. As has been said before, on the front, this makes a huge difference, but on the back, if you lock up the wheel, you have already got past the point of max braking. (obviously slinging your weight back and down gives more pressure on the back wheel so traction brakes later.)
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  • Splasher
    Splasher Posts: 1,528
    A bigger rotor gives you a bigger radiator so less chance of brake boil. On long Alpine descents, it tends to be the rear brake that gets trailed to manage speed and leave all of the available front grip for cornering.

    I ride 180/160 in the UK, rode 203/180 last time in the Alps, will ride 203/203 next time - not for the brake force, just for the heat management.
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  • jaybocrazynips
    jaybocrazynips Posts: 6
    just weighed the bike it came in at 42.6 lbs which i was quite impressed at however iv rang leisure lakes and they are in turn ringing santa cruz so i will post the largest disk size an 04 - 05 bullit can handle once i find out

    jay

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