Mechanical vs hydraulic disc brakes
Although not all disc brakes function in the same way. At a very basic level they all feature pistons that push either one or both brake pads onto the rotor. The differences come with the way that lever force is transferred to the calliper and brake rotor.
At one end of the spectrum we have cable actuated brakes, sometimes referred to as mechanical disc brakes. These function in much the same way as a rim brake, relying on a braided steel cable to move the pistons. The upside of this design is it works in conjunction with your normal, rim brake compatible shifters. Just like rim brakes, they can suffer from cable contamination and are on the whole notoriously fiddly to set up perfectly without rubbing.
In contrast hydraulic disc brakes use a sealed, fluid filled system as the means of actuation. This allows the highest level of braking consistency due to reductions in friction and the fact that both brake pads can move in and out as required. The downside is the considerable expense of dedicated shift levers and system and the comparable lack of simplicity for home mechanics.
However the performance of the the hydraulic system renders the mechanical system nearly redundant. The majority of mechanical brakes only marginally outperform the calliper brake.
Rim brakes vs disc brakes - what’s the difference?
Traditional rim brakes apply the braking force on the outer edge of the wheel. A disc brake focuses forces on a smaller rotor, situated towards the centre of the wheel.
What are the advantages of a disc brake?
Power - Disc brakes generate an incredible amount of stopping power, usually far more than is necessary to adequately stop a road bicycle. This allows the rider to apply much less force to the lever before the bike comes to a halt. Less hand strength leads to a decrease in muscle fatigue, especially on longer descents.
Disc brake power can also be customised by exchanging the disc rotor (the metal braking surface) for rotors of differing diameters. A larger rotor equates to more powerful brake, useful for larger riders or heavier bikes.
Modulation (control) - Pulling with a specific amount of strength on the brake lever of a rim brake can result in wildly inconsistent results. When you pull on a disc brake equipped lever, resultant braking force is much more consistent. This reliability allows you to accurately judge how much force you need to apply in order to achieve the expected result. So despite it being easier to lock up a wheel with a disc brake, the feedback at the lever means you are far less likely to do so.
Disc brake advocates often state that disc brakes actually make you faster as technically you can spend less time ‘on the brakes’, allowing you to brake later before a corner and increasing the time spent at higher velocities.
Reliable, all weather braking - When you apply a rim brake in wet weather there can be a split second delay before you start decelerating. This delay is due to the brake pad displacing water and road grime from the rim to enable sufficient contact and friction. As such the actual braking force you apply in the dry might not stop you in the wet. If you have carbon rims, this is further exagerated . The location of the disc rotor, plus the generally protected calliper position normally results in very little impact to a disc’s wet weather performance.