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Mechanical disc brakes

Mechanical discs are often thought of as a halfway-house between V-brakes and hydraulic discs. They’re not as powerful as hydraulic brakes, but they’re much cheaper. The simplicity of cables may also be appealing, though hydraulics aren’t any more complicated to maintain.

Shimano mechanical disc brake calliper

Shimano mechanical disc brake calliper

The advantages of not using the rim as the braking surface apply equally to mechanical and hydraulic discs, though hydraulics escape the drag issues that affect V-brakes and mechanical discs equally once the cables start to get wet and dirty. Since mechanical disc callipers all work in slightly different ways, there isn’t space here to show you how to strip and service each and every one but the general method is the same.
Refer to your neatly filed manual for full instructions.

There are two designs of mechanical disc brake. The first, and most common, consists of one moving and one stationary pad. The moving pad, mounted on a piston mechanism, pushes against the rotor when the brake lever is operated at the bar. This bends the rotor slightly so that it’s pushed against the static pad and trapped between the two, stopping the rotor from turning and slowing the wheel. When you release the brake lever, a spring acting on the moving pad returns it to the calliper body so the rotor and the wheel are free to move again.

Mud, dust and salt can work through the seals, getting trapped inside the mechanism. This makes the piston action sluggish, so it’s well worth taking the time to learn how to strip them down. Mechanical disc brakes respond well to being stripped, cleaned and reassembled.

Although all callipers work in slightly different ways, they’re so similar that it’s possible to generalize. Most brakes have a selection of small washers separating the internal components. It’s vital that these all go back in the same order. By laying everything you remove out in a line, you can avoid the classic moment of discovering a stray washer after reassembling the calliper. For jobs like this, set the parts out on a clean sheet of paper – you can draw diagrams on it as you go along to remind yourself which way round the parts went.

It’s worth disconnecting the cable and removing the calliper completely from the bike, so that you can lay the brake on a flat surface and see properly what you’re doing. If you can remove the pads by wiggling them out of the calliper slots, do so now. The stationary pad, the one nearest the wheel, may be held in place with a little steel washer. The teeth on the inside of the washer grip the post on the back of the pad. Use a small screwdriver to lever off the washer. Take care not to damage it.

Disassembly from here will vary, so make diagrams of what you’re doing as you go along. Remember to make a note of which direction parts like seals face. Remove the actuation lever by undoing the bolt in its centre. Take out any seals. Remove the innards of the brake carefully, there’s a spring inside pulling the pad away from the rotor. You’ll find a spiral groove inside the body of the calliper – this is what pushes the piston against the rotor when you rotate the actuation lever and is normally the part that gets gummed up with mud. Clean the groove carefully. Cotton
buds are perfect. Wipe the inside of the calliper body and the other internal components clean, and reassemble according to your diagram. Don’t regrease the piston head – the grease melts when the calliper body heats up, creeping onto the brake pads and contaminating them.

The actuation lever bolt needs a strip of Loctite to stop it from vibrating loose. Retighten firmly, then test the action of the brake. Squeeze the actuation lever, pinching the barrel-adjuster and cable stop together while looking into the calliper slot. The brake pad should move smoothly across the slot, springing immediately back when you release the actuation lever. If the actuation lever won’t rotate easily, check that you’ve not left out a washer on reassembly. If the pad doesn’t spring back smartly, your spring is misaligned or there’s still dirt in the spiral groove.

See also bike maintenance tips, tricks and techniques “Disc brakes : introduction to rotors”