My recent obsession with hub motors started in a funny way as a result of seeing, and starting a conversation with ZEV scooters about their Trail model scooter-something. (It looks like a Honda Ruckus, kind of, but is a completely different frame.) My first interest in them, however, started a while ago with Sparky- my first electric conversion- my son’s BMX bicycle. He still hasn’t really forgiven me.
1. First thing I’ve learned. Ask your son before you chop up his bicycle.
I picked up a cheapo, not working, Chinese scooter for short money on CraigsList, and ripped it apart. It actually was a great project that taught me a whole lot about motors and EVs in general, mostly, what voltage does. I took the basic 36V scooter motor, I think rated at 500W, and added batteries. I got the thing up to 84V, and over 30mph.
2. More volts makes the motor spin faster.
3. Too many volts makes the motor burn up.
I’d been messing with trying to fit this little BMX frame with a motor and batteries and really, there’s just no way to do it and make it pretty. Then it dawned on me. Take the hub motor from the scooter and put it into the BMX. Problem solved.
4. Hub motors are easy to install, for the most part, and take up very little space in the build.
The only hitch was the wheel diameter. On a scooter, or BMX frame, those little tiny wheels- 8-13″ or so- are what you get, game over. On the one hand, gearing becomes a moot point, because you have none. The diameter of the tire acts as your final gear reduction- and that’s a fixed size. The performance of the bike will ultimately be a function of how much load the motor can take, how much torque it can muster, and what it will spin at- the RPM/volt rating. So you have a few factors at play that make the predictions pretty much impossible- weight, load, aerodynamics, and the inherent gearing. In a lot of ways a conventional motor has most of these unknowns as well, but you can, by tweaking the final gearing of the chain or belt drive, fine tune stuff after.
For instance- My project bike- a Honda VF500F with a Mars ME0709 running at 72V- accelerates really well, but just stops pulling at about 65mph. The calculations suggest that it’s because it’s simply reaching the top RPM limit of the Mars ’09 motor I have. This leads me to think that it could handle more load, and if I geared it a bit higher I could go faster- not as quickly, but the top end could, maybe, pull around 80mph. Like that. With a hub motor, I don’t have that option.
Enter the EnerTrac. EnerTrac suggests the use of a 110/90 X 18 tire- that is, an 18″ wheel. Keep in mind, what we’re talking about is the outer diameter of the tire- about 23″-25″ actual, depending. However, you can mount the motor in a range of a 16″ to 18″ wheel- give them your wheel, or spec the size, and they’ll string it up for you. Want to mess with different builds and ratios? Change the diameters.
In the real world, that’s where working with the EnerTrac guys can save you a lot of time and money. Work with them, try to figure out what you want to do, and they can help you plan out the build. (Before I get slammed for only mentioning EnerTrac’s willingness to advise you, ZEV does the same thing, but really all they can help you with is the other specs of the motor- voltage, controllers, like that because they can’t change the tire or wheel diameter. You have only one choice- a 130/60-13.)
5. But still… with a hub motor, the thing you have to understand is you have very little control over the ultimate gearing of the bike- and controlling the performance is about controlling the other factors of the build.
In making the decision between the only three products that I can find available right now, the EnerTrac, the ZEV and the Kelly, I’m back to my basic decision process that I used making the choice of my Mars motor. It’s my first build. There are a whole lot of people that have done it before me. I’m not lucky enough to think I’m going to out-think and out-engineer a bunch of guys who actually know what they’re doing. So, my basic advise for anyone thinking about using a hub wheel?
6. Don’t try to re-invent the wheel. (oh fer…) Go with a tried-and-true.
Take a look at what’s out there and work with what you see actually works. The EV Album site is great for this, although you’ll need a few days to go through the enormous pile of stuff there, but you want to see what works? Look at what’s been done.
For me, building a motorcycle, that means EnerTrac. I want to run a standard wheel-frame-geometry configuration. Although I suspect that the ZEV 130/60-13 scooter wheel with a big-wide patch and a slightly larger wheel in the front on a road-racing frame may work, I’m certainly not going to bet a few thousand dollars and hundreds of hours on it. I’m going to look at the Volta or any of the (many) amateur builds and start there. If you’re looking at something that looks more like a scooter, it only makes sense to start with a scooter motor- the ZEV. Same for the Kelly- there are more than a couple builds on blogs and groups using those motors.
At this stage in the game, I’d go out on a limb and say that these motors are three different animals. It’s not like you can say you’d like to build a bike and then look at 12 different types and models to choose from. Someday, maybe you will, but at this point you choose your motor based on what you want to build- motorcycle, scooter or, uh, something else.
By the way- having more to chose from, and that being some bigger, more powerful options, or different diameter drive diameter is, literally, right around the corner. From what I hear. From the guys who build them. Just sayin’.
7. My Final Conclusion on hub motors? I’m currently planning my next build around a hub motor.
I’d like to make it modular, as I’ve discussed here, to the extent that’s possible, as I’ve discussed here, and I’m looking for a mid ’70s vintage 250-350 frame- ideally a Yamaha TZ350, or a TZ aftermarket, but I’m looking at Rickman frames too. All that aside, the hub motor will be the core of the build.
Why? It’s more efficient. (No power loss through bearings and drive train.) It gives you crap tons of space in the frame for batteries. (More range.) It’s simple.
It’s the future of electric motorcycles.