Why do my spokes keep breaking?

Answer 1

Probably because your spokes are too loose. Spokes must always be in tension (i.e., with a force trying to stretch them out) as the wheel rolls. Otherwise a spoke will flex when it's at the bottom of the wheel and unloaded and be pulled tight again when it's at the top and under load again, once each time the wheel goes 'round. Eventually the spoke will fatigue and break just like any metal will if you repeatedly bend it back and forth.

You're breaking spokes on the right side (drive side) of your rear wheel, aren't you? The rear wheel supports most of your weight, so it's almost always a rear spoke that breaks. And, because the rear rim must remain centered while the rear hub must be offset to the left to make room for the gears on the right, the shorter right side spokes have to provide almost all of the support in the rear wheel - the longer left side spokes just roll around and look pretty.

So, find the loose spokes and tighten 'em up.

(0) If you haven't replaced all the broken spokes, then do it now. Otherwise the weight that broke those spokes will be supported by even fewer spokes, and there's a greater chance of breaking yet another spoke. (I know, it seems obvious, and yet...)

(1) Inspect the spokes. If you've ever thrown the chain into the spokes, some of them will be mashed flat or have little nicks carved into them near the hub, and maybe have an unnatural bend to boot. These will probably be the first spokes to go, and they're the only spokes that could break when you tighten them. Often a half-dozen or more will get mashed when you throw a chain, so if you see damaged spokes go learn how to adjust the low limit screw on your rear derailleur.

(2) Grab each pair of spokes and squeeze. They should barely budge. (The left rear spokes may give a bit more, but that's to be expected.) If you can squeeze 'em together, they'll have to be tightened. (Which will probably mean tightening or loosening some of the others, too, to keep the wheel true.)

(3) Tap each spoke lightly with a wrench. The tighter the spoke, the higher the tone. Ting, ting, ting, tong - that's a loose spoke. Tighten it up.

(4) No loose spokes? Hmm. Sit on the bike, and tap the spokes at the bottom. (Unless you're a gymnast, you'll need a friend to help you.) Turn the wheel, sit back down, and continue until you've found the loose spokes. ("Loose" here is relative - spokes tight enough for a 150 lb rider may be too loose to support a 240 lb rider.)

Replace broken spokes with thick (2.0mm) spokes (they're 25% stronger than 1.8mm spokes), and use straight spokes instead of butted spokes. Butted spokes are thin in the middle and thick on the ends - they're lighter but weaker than straight spokes.

If you're heavy you need more spoke tension (tighter spokes). If you jump your bike off of loading docks for fun, you need a clue and (a lot) more spoke tension. If your wheel is tight and you're still breaking spokes, you need a stronger wheel. For $50 you can replace your 32-spoke (or fewer!) stock wheel with a 36-spoke wheel. More spokes means a stronger wheel, though those extra spokes will add almost an ounce of weight to your bike. For $80 you can get one with a double-wall or "box" rim, so that the rim itself can take more of the load off of the spokes.