Spoke length calculator for small motor wheels

[niosoliton]

Hi all,

I build wheels for e-bikes and pedal-power generators, usually 700cc and 26" wheels using meausred ERD, hub dimensions, and 12g spokes. That works well, giving accurate spoke lengths.

However, I recently built a radial 20" wheel and motor hub using the same spoke calculators, and the recommended was 87mm spoke (very short) was still too long by several mm. I triple checked various spoke calculators, checked measurements, and did the math myself on paper. The result was the same. Apparently this has been a problem in the shop previously with small wheels and short spokes. I suspect a small rim and short spoke with a big hub has tighter tolerances and is subject to discrepancies that usually wash out, such as inner/outer lacing, spoke flex at the flange, etc.

My question is: has anyone else had this problem, and can they recommend a solution in the form of a better spoke length calculator accounting for small wheels and large generator hubs?

My present solution is to find a length which works, and bias future spokes for 20" wheels the same amount, but I'd prefer an accurate calculator.

Thanks,
Nio

 

[grndslm]

I bought a rim from Sun one time, and the ERD listed on their website was off by like 10-20 mm. I should have measured the damn thing myself, but I'd never built a wheel before then. Why wouldn't Sun list the proper ERD? I'll probly never know.

 

[chisixer6]

I just built a 20" front bafung motor with radial..came out good, one of my better wheels. Next I did a cutie 100 type on a 305 16" rim, with a 1 cross pattern, as per Dogman writeup on 1 cross. It was tough, I used the spoke calc from WisIL hpv and check with spoke cal by prowheelbuilder, numbers matched.