I've broken and repaired dozens of spokes on cheap wheels. I weigh 200 lbs, and used to regularly carry 40 - 50lbs of gear around. Bike wheels are incredibly strong for their weight, but they (sadly) aren't indestructible as you're finding out. Spokes are just bits of stainless steel, and they'll fatigue given distance ridden. Specific stuff you want to look for:
- Spoke tension should be roughly even all the way around. (If your rim is damaged it gets bent out of shape . . . a common fix for this problem is to correct the bend in the rim by loosening/tightening spokes . . . which means you can't ever get even tension, which means you'll start getting regular spoke failures)
- Low spoke tension tends to be worse than high spoke tension. I think that lower tension spokes move and bend more as the wheel revolves which fatigues the spoke much faster.
- Fewer spokes in a given rim will lead to a weaker wheel. If you're regularly hauling around 200+ lbs on a bike, you probably should be riding a 36 spoke rear wheel. Most bikes these days come with 32 or fewer spokes.
- The rear wheel breaks spokes much more often than the front wheel, partly because it's carrying more weight, partly because it's got uneven tension since it has to be dished to accommodate your cassette.
You absolutely do not need a truing stand to replace a spoke! Here is what you do:
- Flip your bike upside down.
- Take the wheel off.
- Take the tire and tube off the wheel.
- If it's the rear wheel, take your cassette off (sometimes you can get away without taking the cassette off depending on which spoke is broken)
- Remove the broken spoke if it's still stuck in the wheel and throw out the old spoke nipple
- Put a small amount of grease on the threads of the spoke
- Replace the spoke in the wheel (Look at the other spokes to see the pattern of how it fits in there, most wheels are built so that each spoke crosses three other spokes) and thread a new spoke nipple on
- Put a bit of masking tape on the new spoke you've added so you don't lose track of it.
- Put the wheel back on the bike
- Adjust the brake pads so that they're almost touching the rims and spin the wheel. The wheel will bulge out in the area that you've broken your spoke and rub the pads.
- Keep tightening the new spoke until this bulge is mostly gone and the wheel is spinning easily between the pads. When you flick the spoke with your finger it should ring out at roughly the same note as other spokes on the same side of the wheel.
Stuff that you'll need:
- Cassette removal tool (10 - 15$)
- A wrench
- Chain whip (or you can make do with a rag, but it's possible to build a chain whip out of an old chain and a bit of wood and they're much easier to use)
- A spoke wrench that is the correct size for your nipples (10 - 15$)
- A spoke of the correct length (maybe a buck?)
- A new spoke nipple (.30$)
Takes about 15 - 20 minutes to replace a spoke once you know what you're doing. Why do it on your own? Because you learn to true your wheel!
Truing wheels is something that you'll regularly need to do if you cycle a lot (probably a good idea at least once a year), particularly if you carry heavy stuff. If you have rim brakes, you will have poor braking if your wheel is out of true. It's much faster and easier to do it yourself than depend on going to a shop. I'd put truing a wheel up there with changing a tire as essential skills for any cyclist.