The most common arrangement for mechanical disc brakes is that used by the Shimano callipers shown in the pictures.
The position of the outer pad is controlled by the cabletension, while the inner pad remains stationary.pull the brake lever, the outer pad is pushed onto the rotor,When you which flexes sideways against the stationary pad and becomes trapped between the two. The distance between the rotor and the pads is adjusted separately on each side. The brake lever barrel-adjuster is used to set the gap between the outer pad and the outer surface of the rotor, while a bolt on the back of the brake calliper allows you to finetune the gap between the stationary pad and the inner rotor surface.
The gap on either side is critical – too much space, and you’ll have to pull the brake lever all the way back to the bars before the brakes bite. Too close and the pads will rub as you ride, slowing you down.
Step 1: If the outer pad rubs on the rotor, use the barrel-adjuster to release cable tension. If the barrel adjuster has a lockring (the XT lever in the picture doesn’t), roll it a couple of turns anticlockwise. Turn the barrel adjuster into the lever (clockwise as you look along the brake cable) to loosen the cable, increasing pad clearance. Turn clockwise to move the pads closer to the rotor.
Step 2: You may run out of barrel-adjuster range, so that the barrel falls out of the lever or won’t screw any further in. If this happens, reset the barrel to the centre of its range, then undo the cable clamp bolt [A] at the brake calliper and make a course adjustment. Reclamp the cable firmly, then return to step 1. Test. Once you’re satisfied, reseat any lockrings clockwise back against the lever body.
Step 3: If it’s the inside pad that’s rubbing on the rotor, the stationary (inside) pad will need adjusting. This usually requires a 3mm or 5mm Allen key, or a Torx T25, which will have to be threaded in through the spokes. Normally, clockwise moves the pad closer, anticlockwise increases the gap, but there are exceptions – watch the pad as you adjust to confirm the effect. Quarter-turns are enough to make a difference.
Alternative arrangements for centring the pads over the calliper do exist, although the above method is now becoming standard since it’s the easiest and most accurate way to set the clearance between pads and rotor.
Some older callipers have an integral adjuster, allowing you to wind the whole calliper sideways to adjust the gap between the pads. Turn the thumbscrew slowly. Once you get close, small adjustments make a big difference. Pull the brakes on hard between each adjustment to settle everything into place.
If the stationary pad is not adjustable, you will have to move the whole calliper sideways to set the gap between stationary pad and rotor. Then use the cable to adjust the clearance on the outside of the rotor. For Post Mount callipers, undo the two calliper mounting bolts just a little — just enough so you can move them. Ease the calliper sideways, checking the gap beside the inside of the rotor nearest the wheel. This needs to be approximately 1mm (1⁄16 inch). Hold a piece of white card on the far side of the calliper to make it easier to see the gap. Retighten the bolts firmly. You may need to readjust the outer-pad spacing after this.
International Standard (IS) mounts are trickier. Estimate how much more gap you need for the pad to clear the rotor. Undo the calliper fixing bolts and add an equal number of shims between each bolt and the calliper. With normal washer-shaped shims, you need to remove the bolts completely. Shimano tunning fork-shaped shims can be slipped between calliper and frame without removing the bolt. Add shims until the inside pad clears the rotor, then retighten the calliper fixing bolts firmly. You may need to adjust the outer pad spacing after this.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES