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What makes spokes break?

Spokes usually break on the right-hand side of back wheels. The back wheel takes more of your weight than the front, since you sit almost on top of it. Derailleur gears mean the cassette sits on the right-hand side of the hub, so the spokes on the right approach the rim at a steeper angle. They have to be tighter to keep the rim central in the frame, so they are the most vulnerable to breakage.

They are also the most awkward to replace; the cassette has to be removed to fit a standard spoke into the holes in the flange. These spokes frequently get damaged by the chain. A badly adjusted derailleur may allow the chain to slip into the gap between cassette and spokes. If you’re pedalling hard at the time (which is quite likely, since you were already in a low gear), the chain acts like a saw on your spokes, cutting through them. If you have to take the cassette off to replace a spoke behind it, inspect the others at the same time for chain damage. Replace any that have been cut or torn. It’s best to swap them one at a time, so that wheel tension and shape are retained. Tighten the nipple on each replaced spoke enough to support the rim before removing and replacing the next damaged spoke. The plastic spoke protectors that sit behind your cassette are ugly, but they do prevent the chain from dropping into the gap – always replace the spoke guard after fitting new spokes.

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Rim damage

Rim damage is frustrating, and often happens as a result of punctures. If the tyre deflates fast, there may not be enough time to stop before you’re running on your rims. This is particularly damaging if the wheel is heavily loaded or if you’re bouncing down a rocky hill. The rim sidewalls can dent, ticking constantly on the brake blocks and making the bike difficult to control during braking. Sometimes the whole rim gets a flat spot so that, as the wheel turns, and the flat spot passes between the brake blocks, the blocks rub on the tyre. Tyre sidewalls are very soft, and the brake blocks soon wear through them and the tube, causing blowouts. One of the advantages of disc brakes is that your braking isn’t affected by buckled wheels in the same way, but flat spots and bent rims will still weaken the rim.

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Bent sidewalls can be bent back as an emergency measure, although they will be weakened and should then be replaced as soon as is practical. A small adjustable spanner is an ideal tool; clamp it tightly onto the bulge in the rim and ease it straight. If the bulge is big, do it in several stages, working inward towards the centre from either side of the bulge.

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If the rim has a flat spot, check that brakes are clear of the tyre sidewalls. If not, adjust the blocks downwards, so there is clearance even when the tyre is at its lowest relative to the blocks. It’s important to check with the tyres pumped up because at higher pressures the sidewalls can bulge, throwing themselves into the path of the blocks. Once the wheel has a big flat spot, there’s little you can do to correct it – if the rim bends inward more than a couple of millimetres (1⁄8 inch), you’re looking at rebuilding the wheel with a new rim.

Toolbox

Tools for wheel repairs – post-crash wheel true:

  • Spoke key – individual ones are better than those that come on multi-tools, which can be awkward to use

Tools for wheel repairs – tools for replacing spokes:

  • Spoke key – as above
  • Tyre-levers – to remove and replace tyre
  • Pump – to reinflate tyre
  • Rear wheel – cassette tool plus chain whip to remove and replace cassette

Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Fitting a new spoke so that your wheels maintain their tension”