Asking here because this seems to be the place for small wheels. On my 700 wheels, I stick to 36 spoke in the rear because I'm a heavy guy (200+), and tend to carry a lot. But I feel like I could go 32 on a 20" hub and still have just as strong of a wheel. I don't have a good enough understanding of the math to know that for sure. I just know that smaller wheels are supposedly stronger, the ones I've made certainly feel much more solid than my 700 wheels. But it still makes me nervous, so I thought I'd see how the experts feel about putting a lot of weight on a 32 spoke, 20 inch wheel, because 32 spoke is what I have to work with.
As someone contemplating the purchase of a folding bike, I've been wondering about this. Why is it that virtually all the 20" wheels I see on folders have 30+ spokes? There are rims drilled for 24 holes (Velocity Aeroheat 406), and I understand it's possible to build good wheels with 24 spokes and 36-hole hubs (http://sheldonbrown.com/rinard/36-24.htm). These should be roughly equivalent to 36-spoke 700c wheels, no? I tour on those and consider them overbuilt, or if you prefer, having a wide safety margin. Tempted to do this with a folder but nervous when no one else does... like I may be missing something.
That was how my '94 Bike Friday Pocket Rocket was built (451mm rims with 24 holes and 36-hole hubs with every third one left empty). Worked fine for several years but shortly before taking the bike on a loaded tour I broke a spoke in the rear wheel and was concerned that I might have more problems on the tour. I wasn't too confident about rebuilding it with the 24/36 combination so I replaced the rim with one that had 36 spoke holes for a more straight forward wheel build. The bike is still using the 24/36 combination on the front wheel and I have had no other wheel issues.
My Bike Friday Pocket Rocket has had the same 24 spoke front wheel and spokes on it since I bought it as a preloved bike in 1994. The rear wheel has the same rim and spokes but I changed the rear hub so I could get better more usable gearing. Roger
I broke a spoke on my new brompton a month after buying it. Replaced it myself. Ended up replacing two or three before the rim wore out at 12 months.
Just my opinion: it's virtually impossible to snap a spoke through tension. Breaking strength is around 300kg each spoke. Even if you hit a kerb head on the load would be taken by 3 or more spokes. All the failures i've encountered have been at the elbow. I reckon they failed through flexing. Because the lacing was not tight enough. I've now rebuilt the wheel twice and laced it as tight as I can get it. And not broken a spoke since.
So reducing the number of spokes isn't, to my mind, going to weaken the wheel. It will leave longer sections of rim unsupported. But since the rim is so small these sections are short.
Thanks for the input everyone. After reading the comments, I was feeling pretty confident that a 32 spoke wheel would suit my needs, and then I found the same hub in 36. That may be overkill for a small wheel, but there's a chance this hub could end up on my full-sized touring bike, where I definitely want 36 spokes, so I'm happy to have the option. I already have some 36 hole 20" rims kicking around, so all that's left is to find a source for really short spokes. Or have them cut.