New Mtb disc brakes are a huge upgrade for your bike. They look marvelous and are much more good in carrying out then rim brakes (V-brakes). But you have to spend some money to get the components you need. Check this out to get the knowledge to upgrade your bike with Mtb disc brakes.
The most prominent part is the wheel hub. You need a hub with one of the following mounting standard:
International acceptable (Is) (a six-bolt mount)
Centerlock (licensed by Shimano)
five-bolt (licensed by Hope)
three-bolt (licensed by Hope and another system licensed by Rock Shox)
there are many other mountings for mtb disc brakes...
The second thing you need is a Mtb fork respectively a frame with an Is or Pm mounting. Is means International acceptable and Pm means Post Mount. It doesn't no ifs ands or buts matter which mounting you have or your brakes have there are any adapters from one to the other mounting, so don't care about what type you have.
Of course you need a rotor (disc) for your Mtb disc brakes. This depends on the brake you got. Some manufacturers have special discs such as Avid, Hope, Hayes and Shimano. The best way is to get the disc the constructor suggests. You can choose in the middle of some disc sizes.
140 mm
160 mm
180 mm // 183 mm // 185 mm
200 mm // 203 mm // 205 mm
220 mm
These are to common ones but there are some others for special manufacturers. Anyway there are adapters for nearly every size.
