I have a 2000 GT XCR4000 I would like to put disc brakes on. What size of disc brakes would I need to do this upgrade the brake system for downhill and dirt trail ride.
1I don't know the answer to your question, I'm afraid, but the callipers of disc brakes attach differently to the frame/forks than with rim brakes. Does this affect your plans?– PeteHJul 22, 2016 at 17:59
2Unless your bike is already equipped with disc brakes, it's very unlikely that it's possible to put disk brakes on it.– KibbeeJul 22, 2016 at 21:41
1That bike does not appear to be disk-ready, and even if it were you'd probably find it cheaper to buy a new bike than install after-market disks.– Daniel R HicksJul 22, 2016 at 22:15
The picture above that's been edited in by a 3rd party shows "ears" on the frame and fork to accept disks. However, the picture of a 2000 model I found online did not have these ears. But even with the ears the cost is not worth it, vs buying a new (or used) bike that is disk-equipped.– Daniel R HicksJul 23, 2016 at 11:33
Probably 200mm for down hill. 180mm will work if you are not too heavy and don't ride hard.
Disadvantages of bigger rotor is heavier and more prone warping and getting damaged. Advantages are more stopping power and better heat dissipation. However, that's presuming same quality brakes. Better brakes trump a bigger disc.
Does you bike have mounts for discs?