Wheel Strength

[PpPete]

Yes - the cassette is in the way so the drive side spokes are shorter and at a different angle to the others. Because of this, the spoke tensions on that side have to be a lot higher.

Spokes almost never fail in tension. They usually fail in fatigue, usually on the non-drive side, which have less tension and so more movement.

If your LBS is just replacing single spokes and re-truing the wheel the tensions are going to be all over the place and you'll keep on popping them.
As mentioned above even tension around the wheel is the key to long lasting spokes. Learn to replace your own spokes and true up wheels. There's a "sticky" in the know how. Save money AND get better results.

 

[Gravity Aided]

My experience agrees with Colin J, although what PpPete is also, to my knowledge, quite true. The agreement between these two views, is, in my opinion, in the need for even spoke tension on both sides of the back wheel, higher tension on the drive side due to the shorter length and flatter angle of the spokes. When I have had failures, they always seem to occur on the drive side. And at 6'4" and 20 stone, believe me, I've had failures. Good spokes, even tension, and knowledge of the repair/truing process help greatly in avoiding failure in the future, as does a tensiometer.

 

[Spoked Wheels]

I agree with the comments above.

The key is the correct amount of tension and even tension for each side of the wheel.

When a spoke is loose then as the wheel turns the spoke head moves inside the hole on the flange causing premature fatigue. Additionally, other spokes have to take more of their share of the load which also has a negative effect on the strength of the whole wheel. Spokes begin to loose tension and so on.

I found that two of my LBS at least, maybe more, when they replace a spoke, they true the wheel by adjusting the new spoke and neighbouring spokes. That is not half the job. A properly done job would require the new spoke brought up to the correct tension while the wheel is laterally and radially trued and making sure the rest of the spokes also have the correct tension and event tension on both sides ( as close as possible - rims are not perfect so we have to allow for that but most of the time it's possible to get the tension pretty even) - Stressing the wheel is a good idea as some spoke twist could have been introduced while truing and equalising the spokes tension around the wheel.

 

[Davidc]

Can't add much to the above, but would say that if I have a wheel (36 spoke) where more than 6 have been replaced then if the rim's still good it gets rebuilt (i.e. a complete new set of spokes).