Spokes usually break as the result of a crash, but they also break from being worn out. When you look at the wheel, it usually has one area with a big buckle. Run your fingers gently over the spokes and one may come away in your hand. It’s important to fix it as soon as possible. Wheels rely on even spoke tension for strength. A broken spoke weakens the entire wheel structure, which then falls further out of true quickly.
Replacing broken spokes
If you have a broken spoke, remove the wheel, then the tyre, tube and rim tape. For rear wheels, remove the cassette. Locate the broken spoke and remove it. If it’s broken near the head, pull the spoke out through the nipple hole and push the head out through the flange of the hub. If it’s broken at the nipple end, push the nipple out and weave the spoke back so that you can pull it out of the hub. You may have to bend it to get it out.
Measure one of the other spokes to determine the length you need. Make sure you measure one from the same side of the same wheel as the broken one – lefts and rights can be different lengths. The spoke length is measured from inside the elbow where the head curves over to the very end, which will be inside the rim – look at adjacent spokes from the outside of the rim to estimate how far they protrude through the rim. The replacement spoke must be no
more than 2mm (1⁄8 inch) longer or shorter than the others.
It’s vital to weave your spokes back in in the right order. Look at your hub: you’ll see that alternate spokes are ’heads in’ and ’heads out’. ’Heads out’ spokes on the flange nearest to you appear as circles because you can only see the heads (perhaps with the manufacturer’s logo stamped on them), whereas ’heads in’ appear through the flange so that you can see the elbow of the spoke, which then points off toward the rim. Your new spoke must follow the pattern.
’Heads in’ spokes are the easiest to fit. Start from the far side of the hub, post the spoke across the hub and through the hole. It will now dangle on the outside of the wheel, with the head between the flanges. Pull it gently all the way through so that the head of the spoke butts up the inside of the hub. Wiggle it around so that the spoke points toward the rim. It’s important to ease the elbow of the spoke gently through the hole in the hub. The hole is only just big enough for the spoke, so that it can’t shift about in use, which can make it awkward to get the elbow of the spoke seated in the hub hole without bending it.
“It’s vital to weave your spokes back in in the right order”
The spoke pattern
The pattern repeats every four spokes, with all the ’heads in’ spokes on each side of the rim radiating out in the same direction. Pick out the next similar spoke and use it as a guide. Your new spoke crosses three others on its way to the rim. The first cross is over the adjacent spoke – the ’heads in’ one. These two spokes are very close so that the hub flange is between the two spokes. The new spoke passes over the next spoke it meets, but then has to be woven under the third. Try to curve it gently, rather than bending it, and avoid scratching the rim with the sharp end of the spoke. Line the spoke up with the empty hole in the rim, checking that the adjacent spokes in the rim head off to the opposite side of the hub.
’Heads out’ is trickier – and more common! Post the new spoke a little way through the near side of the hub. Don’t push it all the way through yet – it will end up stuck between the second and third spoke crossings on the opposite side of the wheel. Curve it gently outwards from the hub and guide the end of the new spoke out of the far side of the hub, beyond the spoke crossings. It will need to be quite curved, and so will not slide easily through the hole in the hub. Push the head through with one hand, while maintaining the curve on the spoke with the other.
As with ’heads in’ spokes, the pattern repeats itself every four spokes. Count along three from your new spoke, in either direction, and use this one as a guide. Your new spoke crosses the adjacent spoke at the hub, passes under the next one it meets, and then has to be woven so that it passes outside the third. Line the spoke up with the empty hole in the rim, checking that the adjacent spokes in the rim head off to the opposite side of the hub.
For both types, put a drop of oil on the thread, post the nipple through from the outside of the rim and thread the nipple onto the spoke. Take up the slack with a spoke nipple, and true the wheel.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Wheel-truing: the science of keeping your wheels in balance”