I like disc brakes because:
1. work better in wet conditions. if your rim gets wet or muddy, rim brakes loose most of their power.
2. work regardless of how badly out-of-true your rim is. you should keep your rims true, but if yours gets wrecked during a ride, it's not going to rub unless the tire is actually rubbing on the frame.
3. I find that I get better modulation out of disc brakes and they lock up more consistently than any v-brakes I have used.
Their pros are that they're cheap, simple, and usually work more-or-less okay as long as the rims aren't too wet or muddy. They're usually lighter than disc brake systems. They do respond to good maintenance, tuning, and brake pad selection, so it's possible to make them perform quite well.
Cons are that if the rims are wet or muddy, they don't work well, and if you ride in mud or wet sand, you'll wear out your rims fairly frequently. They're expensive to replace.
If you meant cantilever brakes, they have an additional con - the leverages involved in the brake lever and the brake itself aren't that good, so you have to really haul on a cantilever brake to get good stopping power.
Disc brakes are more expensive, more complicated, and, when they're working, better at everything. My current ones (Avid Elixir) have been problem free for the nine months or so that I've owned them. My previous ones (Avid BB5) had only marginally better stopping power than V-brakes in dry conditions, sometimes quit working in muddy conditions, and the pads didn't last very long.
Both Disc and Rim brakes work on the same pricipal which is that two brake pads clamp a disc (rotor on disc brakes, rim on rim brakes) to slow your bicycle. Rim brakes can be very powerful just like disc brakes, for instance Magura HS-33 hydro rim brakes are every bit as powerful and any hydro disc brake.
Disc brakes tend to out perform rim brakes in three main areas though
-On disc brakes, the rotor is up away from the elements so especially in wet and muddy conditions they tend work much better.
-Disc brakes also dissipate heat better, especially away from the tire. Rim brakes can heat the rim up so much on a long descent that your tube can blow causing a wreck (though this rarely happens) where as disc brakes do not heat up the rim.
-Disc brakes do not depend on wheel trueness to work correctly, so if you slightly bend a rim on a ride, you won't either have a dragging brake or have to disconnect your brake to let the wheel roll.
if you're interested in disc brakes, I went from v-brakes to Avid BB5's to Avid BB7's. the Sevens are a HUGE improvement over the Fives! if you're' going to go to discs, spend the extra $$ on BB7's, unless you want to go for hydraulics. i have not used hydros because cable disc brakes work well enough for me and hydros are a bit more complicated.
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