The angle at which the cable connects the two brake units is critical since it determines how powerfully the brake blocks are forced onto the rim when the brake cable is pulled.
Readjusting your cable angle will help you stop more quickly, but you may have to readjust your brake block positions. If you’re fitting new cables and blocks, adjust the cable angle first, then fit the brake blocks to match. The brake unit is attached to the frame by a single fixing bolt so that the brake unit can rotate around the bolt. The cable attachment points on the tops of the brake units move in a circle around the fixing bolts. This means the cable must be set up to pull the brake units around this circle – there’s no point in pulling them in any other direction. This brake cable angle is best estimated by setting the sections of cable that connect the brake units so that they’re at 90 degrees to each other – a right angle or square corner.
There are two different styles of cable fitting: link wire and straddle wire. Adjusting the cable angle is slightly different for each; adjusting brake blocks and cable tension is identical. To work out which you’ve got, check the last section of cable, where the single cable comes from the brake lever and splits into two cables, which each serve one brake unit.
The older method was to bolt a straddle-hanger onto the cable, then run a short piece of separate cable from one side of the brake unit over the hanger, bolting it onto the other brake unit. The straddle-hanger is usually a metal triangle with a pinch bolt, but there was a fashion for purple anodizing and wacky shapes. Luckily, V-brakes put a stop to all that. I like this style because it is easy to adjust the height at which the cable splits in two. The downside is that if the brake cable breaks, the spring in each unit pulls down the straddle wire onto your tyre, where it can get trapped, lock your wheel and throw you high over the bars.
The link wire style was designed by Shimano to prevent this from happening. In this design, the straddle-hanger and straddle wire are replaced by a link wire. This is a V-shape set-up, with one arm of the ’V’ a short section of cable with a nipple at the end, and the other a short section of outer casing. The brake cable from the lever runs down to the link wire, through the section of outer casing, then bolting onto one brake unit. The other arm of the link wire hooks onto the other brake unit.
Both types of fitting tend to suffer from fraying just beside the cable clamp bolt. This part of the cable always gets squashed by the clamp bolt, and is then kinked and released every time you operate the brakes. Over time, the separate strands of the cable snap, reducing the number of strands carrying the load, therefore increasing the stress on each one. Replace cables as soon as they start to fray and inspect the clamped area regularly. Use these steps to set your straddle or link wire angle to 90 degrees, maximizing your mechanical advantage for powerful braking.
Step 1 : Too low – with the link wire at this angle, your brakes feel spongy. Loosen the nut on the back of each brake unit and pull the brake blocks back from the rim. Undo the cable clamp bolt and pull through enough cable so that the link wires are at 90°. Retighten the cable clamp bolt and reset the brake blocks just clear of the rim.
Step 2 : Too high – with the link wire at this angle, your brakes won’t be particularly powerful. Undo the cable clamp bolt, release the cable until the angle between the link wires is 90°, retighten the cable clamp bolt and readjust your brake blocks so that they sit closer to the rim.
Step 3 : Just right! The pictures show a link wire type, but the principle is the same for straddle wire types. Play with the position of the straddle-hanger until the two sides are at 90°. Ideally, the brake block studs should be central on the eye bolt, with an equal amount showing on either side.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES