One of the points used to sell V-brakes was that changing the blocks would be easier than with cantilever brakes. This is slightly misleading – changing blocks is not difficult, but can be fiddly. You may find yourself wishing you had smaller fingers.
The key is to set up the brake units so that they’re parallel and vertical before fitting the brake blocks into the units. Most new brake blocks come with a new set of curved washers but occasionally you’ll have to reuse the old ones. It’s a good idea to clean your rims at the same time so that the new brake surfaces can get maximum grip.
Step 1 : Undo and remove Allen key nut on the end of the old brake block stud, then wriggle out the old block and its curved washers. The new brake block comes with fresh washers, but keep old ones as spares. Now look at the position of the brake units. They should be parallel and vertical (A). Get the position of the units right before you fit new brake blocks. If they’re not parallel, undo cable pinch bolt and pull in or release cable. Retighten the pinch bolts.
Step 2 : You may find the units are parallel but pointing off to one side. If so, use the balance screws (A) at the bottom of each unit to even out the spring tension. This screw is normally a slot head but might be an Allen key. Choose the side that sits closer to the wheel and move the screw half a turn clockwise. Pull and release the brake to settle the spring and repeat until brake arms are even. Detailed explanation of the balance screw.
Step 3 : Check whether the brake blocks are designed for fitting in a particular direction. Any arrows should point forwards, and the shape of the block should follow the curve of the rim. Each block comes with a collection of curved washers to space and angle the block. Their order of use varies from bike to bike and depends on the distance between the brake unit and the rim.
Step 4 : There should be a domed washer on the inside of the brake unit with the flat side facing the brake unit, and a cup washer between the dome and the brake block. Choose either the thick one or the thin one so that the block sits close to the rim, but not touching. A gap of about 2–3mm (around 1/8 inch) is ideal.
Step 5 : The adjustment does not need to be perfect at this stage, just approximate. With the stub of the brake block sticking out through the slot in the brake unit, fit the other domed washer, flat side against the brake unit. Then fit the remaining cup washer, followed by any flat washers. Finally, loosely fit the Allen key nut.
Step 6 : You will find that with this arrangement you can alter the angle of the brake block, and also slide it up and down in the slot in the brake unit. Set the position of the brake block so that when you pull on the brakes, the block hits the rim with the fixing bolt at 90° to the surface of the rim. The block should be level, not higher at the front or back. None of the block should hang over the top or bottom of the rim.
Step 7 : ’Toeing-in’: the front of the block (B) should be 1mm (1/16 inch) closer to the rim than the back, facing the same direction as the bike. Toeing-in helps stop your brakes squealing. Position the block and tighten the fixing bolt firmly. Check you cannot twist the block; the bolt must be firmly secured! Fit the other block the same way. The washer arrangement should be the same on either side, but may be different between front and back brakes.
Step 8 : You will probably have to adjust the tension in the cable again to get the correct gap between brake blocks and the rim. For big changes, undo the cable pinch bolt again, pull through or let out cable, and tighten the pinch bolt. For a more subtle change, use the barreladjuster on the brake lever. Roll the lockring (if there is one) away from the brake lever. Now roll barrel-adjuster anticlockwise to bring the brake blocks closer to the rim.
Step 9 : Roll barrel-adjuster clockwise to move brake blocks away from the rim (C). Turn lockring (D) so it wedges back up against the body of the brake lever. (Some barrel-adjusters don’t have lockrings!) Pull the levers firmly to check the brake action. Brakes need to be fully locked when the lever is halfway to the bar. If you run out of barrel adjustment, reset the barrel centrally, make a rough adjustment with the cable pinch bolt and use the barrel-adjuster to fine-tune again.
Readjusting balance screws
Finally, you will probably need to readjust the balance screws. Turn the balance screw clockwise to pull that side brake block away from the rim, but remember that this also pulls in the opposing brake block towards the far side of the rim. Pull and release the brake levers frequently as you adjust the balance screw because they have to settle into place every time. For a more detailed explanation of how to adjust your balance screw. Check every nut and bolt to make sure each one is tight. Pull on the brakes firmly, and check that the wheel locks up. Spin the wheel and watch the brake blocks – if the wheel isn’t completely true, you might find that the tyre rubs on the brake block as the wheel spins. Readjust the brake block position if necessary.
Choosing new brake blocks
This V-brake block set-up, using a threaded stud with curved washers, is used almost universally, making V-brake blocks completely interchangeable between makes and models. This might seem unremarkable, but the situation with disc brake pads is completely different. Every make and model requires a specific pad – and nothing else will do. The interchangeability of V-brake blocks has helped to keep the price down, since each manufacturer knows you can go elsewhere for replacements. Good makes include Aztec, Fibrax and Shimano. Longer or fatter brake blocks won’t give you more braking power but are more durable. Slots cut in the surface of the block can help channel water away, but they can also collect grit if not cleaned regularly. Ceramic-coated rims need matching ceramic-specific brake blocks, which are harder than standard ones. Normal ones will wear away very quickly, as will ordinary rims if you use them with ceramic blocks.
“Fatter brake blocks won’t give you more braking power but are more durable”
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES