Bicycle Mechanics - repeated spoke brakages

I bought a set of these (http://www.jensonusa.com/store/product/WH290B20-Shimano+Wh-Rs20+Wheels.aspx) wheels about 3 months ago and have already had 3 broken spokes. The first one made sense to me as i had noticed the spoke was bent while on a ride, i think i caught a stick in there or something, and it broke at that point before i got home that day. But the other two have been while riding on smooth pavement at relatively low speed. the second and third breakages were right next to the nipple.

Is it possible that the LBS i had do the replacement has not tensioned/balanced the wheel properly or do these wheels just have crap spokes in them? Is there anything else that can cause frequent breakages before i go and hassle the LBS about it?

You have explained the first spoke. The other 2 could be from the excess tension form the broken first spoke.

The other 2 could be from the excess tension form the broken first spoke.

That's my guess too. But also, how much to you weigh? And how are the roads out there? Those are pretty low spoke wheels. They might just not be suited for your riding.

You know how folks say a chain is only as strong as the weakest link? Well it's doubly true of spokes.

On a normal spoke the weakest spot is at the elbow, which has only 80% of the strength of the rest of the spoke. That's one of the main reasons to use double butted spokes.

Your wheels have direct pull (no elbow) spokes which eliminates this weak spot. So they'll break at the new weakest spot, which is at the root of the thread. They're especially vulnerable there because of the non-crossed lacing, which reduces resiliency and concentrates the stress and deflection at this one stress riser, leading to much shorter fatigue life. The reduced spoke count doesn't help either.

Sadly, had they considered it, building with butted spokes would have restored some of that resiliency, and greatly prolonged spoke life. Most good builders use butted spokes almost exclusively because of their contribution to longer wheel life, saving plain gauge spokes for special purposes like sprint track wheels.

The wheels weren't high cost, so you need to consider your options, but if the rim is good, I'd have a local builder rebuild the wheel using butter spokes.

You have explained the first spoke. The other 2 could be from the excess tension form the broken first spoke.

Nope.

Spokes fail due to fatigue, with the number of cycles dependent on mean stress and amplitude of the cycle with cycles occurring about 750 times a mile with the spokes unloading as they reach the bottom of the wheel.

All the spokes in a group see about the same mean stress (residual from the elbow forming operation, average tension) and variation (your weight, perhaps bending because they're too slack) so they fail about the same time.

You have explained the first spoke. The other 2 could be from the excess tension form the broken first spoke.

Not sure if i explained myself fully, i had the first breakage repaired straight away, the second breakage was a few weeks after that and also repaird immidately, the thirs breakage was today (another few weeks later). Surely any "excess tension" would only persist until the wheel is repaired? The process of replacing the space and re-trueing the wheel should involve balancing of tension as well?

As for stress levels, i weigh 70kg (about 140lb) so i'm not exactly a heavyweight. The road here are...patchy (i've updated my location to san francisco from melbourne), but none of these happens on bad sections of road. Though cheap, the wheels are brand new (<1000miles) so i would expect better longevity than that, certainly fatigue shouldn't be an issue yet. That said, replacing the broken ones has cost me ~$70 so far, almost 1/3 the value of the wheelset so they're getting more and more expensive the longerit goes on.

I guess i'll just describe the situation to the LBS and hope that they can do a better job when they fix this one. If another goes in the near future i'll look at other (more expensive) wheels or a dedicated wheelbuilder.