All braking systems have four basic parts: brake lever, brake unit (the part that does the stopping), brake pads (the replaceable contact point), sometimes called brake 'blocks' and the cable (or for hydraulic systems, hose and fluid) that connects the lever to the brake unit.
It is usually possible to improve braking performance by adjusting or upgrading just one component and it's not necessary to replace everything. Just make sure that the new product you choose is compatible with your existing parts or frame. For example, if you are upgrading from cantilever brakes to V-brakes, you will need a special V-brake lever too.
There are four popular types of brake. All work a little differently, and each has its own advantages and disadvantages. The main considerations for each type are outlined below.
These are the original mountain bike brakes. They fit onto special 'bosses' attached to the frame on either side of the wheel. With cantilevers, any size tyre can be accommodated, but height adjustment is limited so they must be fitted to suit a particular rim size (but since all mountain bikes use the same size rim diameter, this is not an issue).
A 'straddle cable' runs over the tyre and connects the two halves of the brake. In turn, this is pulled up vertically by means of a yoke or 'straddle bridge' connected to the lever cable, and the two brake arms arc in towards the rim.
They work well in off road conditions, are lightweight, relatively inexpensive and do not clog with mud too easily. However, they are not as powerful as other brakes and they can be complicated to set up correctly. Cantilever brakes are still popular with road tourists and cyclo-cross riders, but on mountain bikes they have been superseded almost universally by V-brakes.
V-brakes have now replaced cantilevers as the standard brakes on mountain bikes. They're inexpensive and simple to set up. They are similar to cantilever brakes in that they're comprised of a unit on either side of the rim and they use the same bosses on the frame as cantilevers. However, V-brake arms are longer and sit vertically.
Their extra length makes them more powerful and their upright profile means that you're unlikely to catch your heels on the rear ones or your knees on the front ones. The cable from the brake lever directly connects the two arms, and this brings them together in the same plane as the rim, adding to their power. The travel on a V-brake is different to that of a cantilever and so needs a dedicated V-brake lever. Their only disadvantage is that they can clog up with mud a bit more easily than cantilevers.
There has been enormous development of disc brakes in the last few years. They are now standard equipment for downhill riders and are finding their way onto many cross country and free ride bikes too.
Mountain bike disc brakes are very similar to those used on motorbikes. The brake unit is attached to the bottom of one side of the fork (or seat stay). A disc is mounted on a special hub and this runs between the brake pads on the unit. Many of these brakes use hydraulic fluid (just like motorcycles) as opposed to cables to connect the lever to the unit. Mechanical (cable operated) units are easier to install and cheaper than their hydraulic cousins but they lack the same level of performance and modulation and may need more regular maintenance.
Disc brakes are very powerful. The braking occurs away from the rim so there is no problem with heat build-up on long descents (which could cause a tyre to expand and explode off a rim); nor do riders have to worry about a slight buckle in the rim catching on the brake blocks. Similarly, disc brakes operate well in all weathers as the braking surface is away from most mud and water, and it is cleaned more rapidly by the brake action. Frame clearance is improved too, as there is nothing around the rim or tyre except for the frame itself.
Hydraulic brakes have made a small impact on mountain bikes. They do offer very powerful braking but improvements with V-brakes and disc brakes have left them out on a limb. Mounted either side of the rim they are operated by hydraulic fluid instead of cables. The brake pad moves horizontally towards the rim and provides very powerful, controllable stopping.
Hydraulic brake systems definitely need a dedicated brake lever and so are usually sold as complete sets (or at least, you have to buy a separate matching lever if you want to use your brakes!), but you can mix and match most cantilever and V-brake systems.
However, due to the different travel distances of each style it is a good idea to purchase the specific lever, as they will have been designed to work together. You can buy specialist brake levers, sometimes made out of carbon fibre or designed to save weight, particularly for cross country racing, but these can be very expensive - and it's worth remembering that in the event of a crash your brake levers often take the brunt of the impact.
Mountain bike brakes take a huge amount of abuse as their constant application in muddy, dusty or wet conditions all combine to reduce their efficiency and contribute to accelerated wear. For safety - and more enjoyable riding - maintain your brakes regularly.