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.

I use an IGH with a MTB freeride rim...between the beefy rim and symmetric build it's bomber and hasn't needed any attention on my Dummy. I've carried two adults on the back as well as my GF and 100L cooler full of beer and ice.

In terms of your wheel issues...was the wheel machine built and if so did a wheel builder go over the wheel at all? It's possible the components were up to the job, but poorly tensioned.

If you don't want to go the IGH route I'd get a 36H heavy duty MTB freeride rim and get an expert wheel builder to build it up for you.

Aaron's in Seattle has a few modified wheels for loading up Big Dummys , and etc

48 hole tandem rear hub and a thread to disc adapter, narrowed to 135.


so the stopping can happen in a timely manner too.

And a re drillred 16 extra holes in Rohloff flanges ,
32 2 cross, 16 radial spokes into a 48 hole rim.

When a spoke breaks, it places tremendous stress on the spokes next to it. In many cases, this is not a problem but in most cases on cargo bikes or heavily loaded bikes the two adjacent spokes to the broken one will be seriously weakened. On my bike, it is always the drive side spokes that break and I replace them in sets of 3. It could also be a problem with the wheel build causing strain on the spokes, but I couldn't tell without looking at the wheel and even then, i might not see a problem.Vik is correct in stating that a symmetrical wheel will be much stronger. Also, if it is a machine-built wheel, it should at least be re-tensioned and possibly rebuilt by someone who knows how to build wheels.

My Big Dummy came stock with 36-spoke Salsa Gordo rims and Deore hubs. I've had a total 500lbs on it so far... that weight includes me. I don't recommend that much weight (neither does Surly), but I don't think the wheels will be a problem.

+1 - plus by the time a spoke breaks the whole wheel has been run in a poorly tensioned state for a while so likely you'll have several spokes break one after another now. If the hub and rim are appropriate to your needs I would get the whole wheel rebuilt replacing all the spokes.

I was expecting the second broken spoke to either be next to or opposite the first one, but instead it was about 90 degrees. The wheel was machine built, but then retensioned by the LBS, at least so they claim. We've had nothing but problems with the bike the wheel came from through. It was a new Trek 7500 that we bought as a commuter bike, but in the first couple months the front shock had rusted beyond recognition and the SRAM derailleur had completely seized. For an $850 bike I expected more, considering that I've been riding my similarly priced Diamondback Topenga Comp for 10 years in all weather, and didn't have to even replace the chain, cassette and sprocket until after 4 years, and the derailleur and most everything else is still the original equipment.

The wheels were also disappointing when I went to go replace the first broken spoke, as the shop didn't have a spoke in that thread and diameter, explaining that Trek sometimes uses non-standard threads. If they can't take an extra 120 pounds on smooth road, I really doubt they could ever take the rigors of mountain biking and single track, which is theoretically what this bike was designed for. Now it is just sitting in the basement, unusable until we become motivated enough to chip away all the rust. :(

I have had excellent luck with a 38 spoke RhinoLyte rim on my bike. They're a bit heavy, but completely bomb-proof. I guess I could buy another, although the idea of buying a tandem rear wheel also sounds like something worth looking into.

I am reading Mark Beaumont's "The man who cycled the world". He recounts how he had 2 (was it 3) spokes break at the beginning of his journey. In Poland, he had the wheel completely rebuilt. The mechanic replaced all the spokes with thinner ones and loosened the tension on them. He explained that the spokes were too over-tensioned to bear the weight of his load. I haven't finished the book but he's made it to Pakistan (which is as far as I have read) and did not have any more broken spokes.

Never in my life seen, or heard of a 38 spoke wheel. As for your shop's explanation for not being equipped to replace a spoke, well let's just say it makes no sense.
Some wheels that come complete from a wheel manufacturer use proprietary spokes that may have unique threading, but I doubt very much if your bike has anything other than standard garden variety spokes that any half decently equipped bike shop should either have in stock, or be able to make one by cutting a longer one to length and rolling threads onto.
Heck, I'm equipped to do that in my own garage.

I expect it was a 36 holer, evenly divided by 4 as spokes are grouped in 4s.
(typically)
Cannot do is a weak excuse from that LBS, it reads..
LBS in town, here, has a Hozan thread rolling tool to make weird length spokes
out of longer ones .
to get the tourists back on the road down the coast.