Exploding spokes
I have an Xtracycle that I originally bought to haul groceries (we live sans car in a small rural town), but this summer I realized it would make for a decent two-person commuter vehicle. The route is 1.5 miles up two hills with an 8-10% grade and about 100' total climbing.
We commute by me pedalling, and my wife riding on the Xtracycle extension, and our lunches and gear in the panniers. After around two months of commuting this way, a spoke blew and we walked the bike home. It was the spoke the wheel reflector was attached to, so at the time I thought it was the reflector that broke the spoke (the reflector got obliterated in the process).
I replaced the spoke, and fast forward two more months ahead to last night, and another spoke blew. Between my wife and the backpacks, there's probably around 140 extra pounds on the rear wheel, much less than what I figured would cause spoke to keep breaking.
The wheel is fairly new (around 2 years old). For now, I put a BionX wheel on the rear to take the strain, but I was wondering what people use for beefy rear wheel that can take a cargo load.