It’s easy to forget that the rims are just as much a part of the braking system as the brake blocks. Every time you brake, you’re forcing your brake blocks against your rims. Powerful, controllable braking depends upon the condition of both blocks and rims. Whenever you brake, you wear both surfaces.
Rim design is subject to two competing demands. When you’re trying to go faster, it helps to have rims that are as light as possible. Your wheels are spinning around as well as along, so saving weight on them makes the bike feel substantially faster than saving the same weight on a static part of your bike like the handlebars. So ideally, the rims should be as thin as possible so that they don’t weigh much. Light wheels mean it’s much easier to make your bike accelerate, as well as to make it change direction when you’re moving fast.
If this doesn’t make sense to you, take a wheel off your bike. Hold each end of the axle and move the whole wheel up and down. Then spin the wheel and do exactly the same thing again. Even though the weight of the wheel hasn’t changed, you’ll find it harder to move it where you want it to go when it’s spinning.
But when you’re trying to slow down, you need the rim material to be thick because the action of braking wears it out – and you don’t want the brake blocks to wear through the rim. The deal is that rim manufacturers make their rims light so you buy them, but they expect you to keep them clean so they wear as slowly as possible and to inspect them regularly so that you can replace worn rims before they blow on you.
Having a rim sidewall blow suddenly is very alarming. People can think they’ve been shot – there’s a loud bang and suddenly they’re lying on the ground, like in the movies. Because of the pressure inside the tyre, the sidewalls don’t give way gracefully. Over time, the sidewall gets thinner and thinner. One part of the sidewall gets too thin to hold in the tyre. Then you brake suddenly – the moment of reckoning! Once one section of the rim starts to give way, it cannot support the next section, so within a fraction of a second most of your sidewall is ripped off. This punctures your tube, the resulting mess usually jams on your brake and you fall off the bike.
“The resulting mess usually jams on your brake and you fall off”
Rims also give way when you’re pumping your tyre up. The extra tyre pressure on the inside of the rim sidewall is all it takes for the rim to finally give way. This is just as alarming and may also shower you with rim shrapnel. Some newer rims come with indicator marks that show when the rim is worn out. The rim will have a small hole drilled from the inside, but not all the way through. The position of the hole is marked by a sticker on the rim with an arrow pointing to where the hole will appear. As you wear away the sidewall, the bottom of the hole appears from the outside; you can see your tyre through it. Time to get a new rim. Another type of rim indicator consists of a groove milled all the way around the braking surface of the rim. When the rim is worn away to the base of the groove, it is worn out and should be replaced. To help you see it, the bottom of the groove will be a different colour than the sidewall of the rim; for example, a silver rim will have a black groove in it and a black rim will have a silver groove.
If you don’t have a wear-indicator, check the condition of the sidewall by running your fingers over it. It should be flat and smooth, without deep scours and ridges. Check both sides because one sidewall may be far more worn than the other. Curvy, bulging or scarred rims are due for replacement. If they look suspect, ask your bike shop for an opinion (you know straight away once you’ve seen enough of them). If you find any cracks in the sidewall when you inspect, stop riding immediately.
It’s also worth checking the join where the two ends of the rim meet. It’s usually directly opposite the valve hole. Goodquality rims will have a milled sidewall. The wall is made slightly too thick and welded together in a loop. The surface is then ground off flat. Cheaper rims are simply pinned together, relying on the spoke tension in the built wheel to push the joined ends properly together.
Sometimes, the ends don’t meet exactly, making a bump in the rim that knocks against the brake blocks. Small imperfections can be filed flat, but if the join protrudes by more than 0.5mm (1⁄50 inch), take the wheel back to your bike shop for inspection because overenthusiastic filing will just weaken the joint. Also check for cracks around the spoke holes and the valve hole. These are less dangerous but still mean the rim should be replaced.
Rim sidewalls can be made to last longer without increasing their weight by covering the brake surface with a hard ceramic coating. This is expensive, but it radically reduces the speed at which the brake blocks can wear the sidewalls. Since the rim is much harder than normal, it’s necessary to use harder brake blocks too, specifically designed for ceramic rims.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES