Four degrees of freedom
You really don’t see many cantilever brakes on mountain bikes nowadays, though they’re still popular on cyclocross bikes (where they’re not expected to actually stop you, but simply slow you down). They were the forerunners of V-brakes and whilst they’ve both now been surpassed in terms of power and practicality by the disc brake, cantilevers are still to be found on the steeds of those who lovingly restore ’retro’ builds to their former glory – or who just still ride an old bike.
Take care to use the correct brake levers for cantilever brakes because they are not compatible with V-brake levers. V-brake levers are designed with a greater distance between the lever pivot and the nest that the cable nipple sits in. This means that more cable is pulled through the lever with V-brakes than with a cantilever brake lever. You can see the difference if you compare a V-brake lever to a cantilever brake lever. For a cantilever brake to work properly, the distance between brake lever pivot and cable nest needs to be around 30mm (11⁄8 inches).
Cantilever brake blocks usually last much longer than V-brake blocks, but they will wear through eventually. Change them every couple of years whether they’re worn down or not; the rubber hardens and ceases to work well after a while. This seems to be especially true if your bike lives outside or in outbuildings that get cold in the winter.
Check for wear by looking on top of the block for a wearindication line. Usually stamped in black writing on the black surface of the block, they can be hard to spot, and you may be able to feel the line with a fingernail more easily that you can see it. There may not be an indication line; in this case, replace blocks before they’ve worn down to the base of the grooves moulded into the blocks. Leave it too late and you risk wearing through to the metal bolt that the block is moulded around. The bolt will scrape the surface off your rims.
The brake block is held in place by an eye bolt. The stub of the brake block passes through a hole in the eye bolt, which in turn passes through a curved washer then through a slot in the brake unit. On the other side of the brake unit is another curved washer, then a nut. When you tighten the nut, it pulls the eye bolt through the brake unit, squashing the stub of the brake block against the first curved washer, and holding it securely.
This design means that when you loosen the nut on the end of the eye bolt, you can move the position of the brake block in different and useful ways. You can move the eye bolt up and down in the slot on the brake unit so that the brake block hits the rim higher or lower. You can push the stub through the eye bolt, moving the brake block towards or away from the rim. You can roll the stub in the eye bolt so that the block approaches the rim at an angle. You can also twist the eye bolt on the curved washers so that the front or the back of the brake block touches the rim first. We use this flexibility to get the block precisely positioned.
The vital adjustment for cantilever brakes is setting the position where the main cable splits into two, just above the brake units. The split can be made with a straddle hanger bolted onto the cable, with a straddle wire that passes from one brake unit to the other via the straddle hanger, or with a separate link wire, through which the main cable passes, then clamps to the brake unit. Either way, it’s important that the two sections of straddle cable, or the two arms of the link wire, are set at 90 degrees to each other. The best time to get this right is when fitting new brake blocks.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES