It really should boil down to what size do you want to use, and what effort are you willing to expend to get that size the way you want.
In my case, the space at the rear of the 3Eman would only allow a 20" tire, so that made the selection of the 20" cast wheel pretty straightforward. But esthetics called for a wide (4 inches or so) tire. The rim width of 1 1/4" really would not work for such a wide tire. So I bought a 20" bike from Good will for $5, stripped the rims off it and epoxy and steel pinned those rims onto the cast wheel to wind up with a rim width of 3 3/4". Was it a pain in the neck to get everything straight and true? Absolutely! Will it work to meet all my needs? Absolutely, tho I am admittedly concerned about inner tube stretch tears.
So What does that have to do with your problem of spokes stretching under load? Just this. If you want a sturdy 26" wheel powered by a MP3, get rid of the thin spokes. Here is one way to do it. Buy a 20" cast MP3, which has an outer diameter of about 16". Get a 26" rim of the width you require, which will have an inside diameter of about 19". All you need to do is bridge the roughly 1 1/2 " gap with some bolts, and then you'd have a stiff, strong 26" wheel. You could drill and tap the required number of holes with staggered offsets into the casting. For the sake of argument, lets pick a # 6 coarse thread button head cap screw as a fastener to join the rims. These have a proof strength of about 650 lbs, so they are more than hefty enough to do the job. You could probably get away with #2 screws, but you won't enjoy drilling and taping that many small holes in aluminum.
Another way would be to epoxy sheet metal straps from rim face to rim face on both sides. A 1 1/2" strap of .065 aluminum, JB Welded to the rim would have a shear strength of over 1000 lbs per strap. If you used 9 straps per side you would have a tremendously strong and rigid wheel assembly. You could completely join the rims with solid sheet, as well. I'm confident you can come up with even more methods to join the rims. Think how pretty the wheel would be if you used black walnut blocks to space them apart!
Would any of these be easy to do? No. Getting the resultant wheel to run true would take a lot of patience and attention to detail at every step. Could you do any of these, at home, with commonly available tools? Yes.
My Dad used to tell me " stop wishing and start thinking how to do it". I encourage you to do the same.
And let me know what you come up with.
TTFN,
Dennis
