by spinningmagnets » Sun May 20, 2012 10:19 pm
I don't think a friction-drive is for everyone, but it appealed to me enough that I researched the experiments here on ES and made some choices for a drive that was a long-term robust solution for me.
The motor-shell drives (Kepler/Adrian) are wonderful. They are the smallest and lightest a drive can be. I don't want anything I say to sound as though its a negative against them (its like a discussion of geared-hub vs Direct-Drive hub, each has its strengths).
When I saw how Keplers drive could be attached to the seat-post, it meant that when coupled with a lever-style clamp, I can easily attach and remove the drive from a bike, or swap to another bike (I have an aluminum 20-inch BMX inside my RV).
The 63mm motors have a larger diameter rotor (the magnets in the shell) compared to the 50mm motors, and that gives them a leverage advantage, meaning they produce more torque per a given watt (bog down less under load). Plus the larger mass of copper in the 63mm motors make them more capable of some abuse when tackling the occasional hill.
Having the roller be a separate component than the motor-shell was a conscious choice. This way the motor doesn't need a skirt bearing, and that means that any 63mm motor will work. I admit that this choice adds cost and size/weight, but I feel the choices it provides are well worth the downsides. The 63mm kVs available are 150-295, and when compared to practical voltages, this is perfect.
If the roller is housed and supported on both ends with bearings, there is no deflection pressure on the motor-shaft. I wanted a system that would last, regardless of abuse. Rollers can be had from 5/8-inch diameter up to 1-1/2 inch. I liked EVTodds design using a roller-clutch bearings to allow the roller to remain in contact with the tire (no external clutch, or lever-linkage).
A smaller diameter roller allows the same speed from higher volts/RPMs, and sometimes higher volts means that you can support the same load with fewer amps. I've tried a couple and I like the hill-climbing ability of the one-inch roller (climbs well while drawing fewer amps).
I'm very happy with the 63mm Exceed 295-kV motor. I had tried it using 24V on a 1-1/4 inch roller to get 20-MPH. It climbed OK, but I was uncomfortable with the amount of heat on a steep hill. Warm is good, hot is bad. I now use 36V on a one-inch roller for 22-MPH. I feel I can throw anything at it, steep hills, etc...I haven't tried 44V-48V, but that should result in around 28-MPH.
Castle Creations ESCs have data-logging, and that verified what Todd had posted. Peak amps on hard acceleration were 60A, and constant draw was around 10A. The Castle ESC I tried tops out at 28V, so I tried the Hobbywing HV-70A to reach 36V (capable of up to 48V). That proved to be the final piece of the puzzle. If I was starting out from scratch, I would just get the Castle HV-80A...since at low volts I had a problem with a loss of sync with the "affordable" generic ESCs. I sincerely wish the cheap ESCs had worked well, but they didn't for me.
Low amps across the RPM range when under load is always a good thing, but it also means that the battery is not limited to only using LiPo for a high-C-rate.
Hobby King has 63mm motors with kV of 250 that have skirt bearings (if you wanted a shell drive). It should provide 50-MPH at 44V, but only on flat land. Once you put a 50-MPH load on it (or a mild hill), the motor and controller will get hot. Much less copper mass than the fattie 80mm motors.
i wanted to make a cam-type manual "clutch system" engagement lever for it, but i never got around to it. at the moment, its in pieces. ill be getting a new motor to have it back together soon, because my experiment with high kv/small drive rollers didnt pan out so well 
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