Doug,
I think there are many reasons why some people break spokes and others don't. Here are a few, in no particular order.
- Wheel width (or OLD). Current design is for 135 mm (mountain and touring bikes), or 130 mm (road). Previous standards have been 130 mm, 126 mm and 120 mm. The wider the OLD, the stronger the wheel... especially with a higher number of gears. A 126-mm wheel with an 8-speed
freewheel (there were a few of them) is fairly weak, because the right spokes are nearly vertical.
- Beefy rear triangle. If you look at a current "loaded touring bike" such as the Trek 520 or the Cannondale T800 or T2000, you'll notice that chainstays and seatstays are much wider than on a touring bike of the late 1970s or on a current road bike (compare Al with Al and steel with steel). If the rear triangle isn't still enough, there might be vibrations or shimmy transferred to the wheel. This typically may become a problem with heavily loaded rear panniers or a BOB trailer.
- Inside derailments. As you said, if the chain derails into the spokes, it will gauge them. It's not a serious problem if you were applying
light pressure on the pedals, but could be if you were standing on the pedals at that time, as the chain will dig in more forcefully.
- Riding technique. Standing on the pedals is harder on the bike than spinning all the time. One just needs to do it a few times on a flexible 1970-1980 loaded bike to feel it.
- Riding technique II. Lifting ones weight from the saddle helps ease out bumps.
- Wide tires. Within reason, of course.
- But the most important source of problems is poorly built wheels. I don't know your bike shop in Minot ND, but I'm often more worried about large-volume shops, especially during the high season, as they have many temporary workers around. It's quite likely that your wheels were built or maintained by a true bike mechanic. As for your machine-built wheels, ping the spokes and you'll have an idea how well they are built. It's likely that adding 1/2 to 1 full turn of the spoke wrench to each spoke will help, but read Sheldon's page first. Url:
http://sheldonbrown.com/wheelbuild.html
Regards