JRH is building me a wheelset with a CroMotor in the back wheel, the front wheel will be on a 20mm through-axle DH hub. This is my first foray into using moped rims/tires on ebikes, so I learned a lot from JRH as he helped me sort out the configuration.
I was really glad for his expertise, because without it I would have asked for a really dangerous wheel build. The strongest spokes and strongest rim don't necessarily make the strongest wheel. After checking with him, I thought I'd post what I learned, in case anyone else knows as little as I did. This is in no way intended to be a guide - I have insufficient experience - but to highlight some of the issues involved. Pretty much everything below I picked up from John, except for anything that is wrong.
The front wheel bicycle hub is the weak point in the build. The RockShox Boxxer fork's 200mm front travel will help with hard bumps. However, John is still worried about the hub flange, so he wanted to use 13ga spokes "to encourage spoke breaking vs hub failure." The smaller spoke size also dictated a lighter steel rim, as the heavier alloy rim required 10ga/11ga spokes.
The rear rim is 17" to suit the 13kv of the CroMotor. Since the motor has more top end than I need, I wanted to go wih a small wheel to maximize accelleration, keep the motor cool, etc. I'll swap in 144v packs for testing/exhibitions (maybe even <gasp> LiPo
), but the rest of the time I'll probably keep them at 72v or 100v. I considered a 16" rim, but if I choose to run this at 72v nominal (roughly 79v for LifePO4) top speed would only be 46mph. Besides, I can run a Gazelle tire on the rear rim.
John likes the Michelin Gazelle M62 Moped Tires, I read a lot of good reports about them, and the are 'p' speed-rated to 93mph, so it seemed like a slam-dunk to use them on both wheels. However, I wanted my front wheel to be larger than the rear wheel - the Norco A-line comes with a 230mm fork, My Boxxer forks 'only' have 200mm, so I can use a little extra height to correct the rake angle. However, Gazelle moped tires are only made in 17"/18" diameters, so an 18" moped rim was the largest I could go.
That said, guess what? Good luck finding an 18" moped rim to fit that 18" moped tire, 16"/17"/19" are your choices - and a heavier 18" Motorcycle rim is too much weight for a bicycle front hub, plus as mentioned earlier it would need heavier spokes that would make the front hub flange fail first. When we thought the motor was 9.5kv, John bought a 19" rim and pair of 19"x2.5" Deestone tires/tubes at my request, So I decided to just go ahead and use that combo on the front wheel anyhow.
Which brings me to the mixed bag of nutty calculations used to describe tire diameters. Some tires are rated just as a diameter, I guess like bicycle tires. Other have ratings with 2 numbers, like automobile tires. Lets use 110/80 as an example - 110 is the width in mm. so the tire is 110mm wide. 80 describes the height of the tire, but it is a percentage of the width, not mm, so in this example 110mm*80%=88mm tall.
It is important to note that the width is measured across the widest part of the tread, which is not neccessarily the widest part of the tire, so if you buy a 110mm tire to fit a 120mm space, you still might find the tire rubs. In addition, I already knew from bikes that a wider the rim will also spread the tire wider. It seems that practical experience with the tire rim-combo at hand is the only way to be sure it will fit, so it seems like a good idea to leave some wiggle room when sizing the tires.
Most of the Moped tires seemed to be 2.25", 2.50", 2.75", and 3.0". The Boxxer forks have something like a 4.5" gap - room to fit any of them. However, the rear of the Norco A-line is about 3.5" where the treadline on a 16" or 17" rim meets the frame. I was concerned that 3" might be too tight a fit, especially since I might need even more wiggle room when shimming the Cromotor's 135mm axle into the Norco's 150mm dropouts, so I was more inclined to go with 2.75"/2.50". On the other hand, I wanted as much diameter as possible to cushion the heavy hub motor. On the other other hand, each extra .5" of tire width adds 1" of diameter, which decreases the accelleration I want, and increases top end I probably won't use. So while I would have liked to use a 16" rim with a 3.25 (22.5" diamter) wide tires for maximum padding, they might not have fit. The 17" rim with 2.75" tires is the same 22.5" diameter, plus I can use the Gazelle.
More details I got from John: Wheel Diameter: bicycle rims are measured at the top of the tire, but Moto/Moped rims are measured at the top of the rim. Rim Width: Bicycle wheels are measured between the rim's outer surfaces, while the moped rim is measured to the inner surfaces.. Typical moped rims are 1.35", my ebike-preferred Alex DX-32 rims are 38mm(1.5"), but both are roughly the same size. In theory, my 19" front wheel with 2.5" moped tires should be the same size as a 24" (19"+2.5"+2.5") front bicycle using Alex DX-32's and 2.5" Maxxis hookworms.
Thanks to Saint JRH!