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grindel2
05 April 2016, 1336
Hi everyone,

I've got a '78 suzuki GS400 that I intend to convert. I have been intrigued by the enertrac hub motor. Does anyone have an opinion on how it's efficiency numbers compare to something off of the goldenmotors website or any other website that shows performance curves for their motor. I am able to calculate battery needs based on logged data to and from work based on those performance curves, but it doesn't look like that is available for Enertrac's motor. My thought is to get a handle on the total power required for a trip to and from work and double it, making assumptions based on rolling resistance, Drag coefficient, and total build weight.

Anyway, I have a ton more questions, but I guess they should be in other threads.

nedfunnell
08 April 2016, 1200
Hi everyone,

I've got a '78 suzuki GS400 that I intend to convert. I have been intrigued by the enertrac hub motor. Does anyone have an opinion on how it's efficiency numbers compare to something off of the goldenmotors website or any other website that shows performance curves for their motor. I am able to calculate battery needs based on logged data to and from work based on those performance curves, but it doesn't look like that is available for Enertrac's motor. My thought is to get a handle on the total power required for a trip to and from work and double it, making assumptions based on rolling resistance, Drag coefficient, and total build weight.

Anyway, I have a ton more questions, but I guess they should be in other threads.

Hi, welcome.

Regarding your battery/power needs- you could get started with some basic numbers that others have achieved with their builds. Electric motor efficiency does vary, but not wildly. Many other design decisions and factors will drive what kind of total efficiency your build will get. For example, I just did a ride today that came in at 74 Wh/mi. It was all in-town riding under 40mph with some mild hills. Yesterday, though, I went for a longer ride on a state highway over steep hills, with speeds up to 60mph. It was very windy. I used 127Wh/mi on that ride. The motor powering my bike wouldn't have made a big difference, as it was the speed, elevation, and aerodynamics that made the biggest difference. People often cite 80-100Wh/mi for cruising around. I've regularly hit 120Wh/mi, though, on my unaerodynamic GPR-S and my not-small self on top of it. I'd design for 120Wh/mi, add some of the wiggle room you mention, and pick your batteries based on what you can find after that. Many people here end up using Leaf or Volt cells anyway.

grindel2
11 April 2016, 1241
Thanks for the reply. My math seems to work out fairly well. My simulations yielded ~10Ah used if the pack voltage stayed at a constant 96 V (Voltage sag is not simulated yet) for a roughly 8mile trip. The thing that got me thinking about the efficiency of the hub motor is that at 10RPM/V, there has to be some tradeoff or else it seems like a lot of other motors would be built that way. You're asking a lot more out of each rotation of the motor. It is just a gut feeling that it will result in an efficiency hit even if the motor controller is able to run at a lower frequency and you factor in the efficiency of the chain drive. What is everyone else's feeling?

I wish you hadn't mentioned the Leaf/Volt cells, I had written them off thinking that they would be hard to get and there would be quite a bit of work to make them viable for DIY, it seems for me if it isn't turnkey enough then by the time I am confortable enough to make a decision the situation has changed.

Thanks much.

Spoonman
12 April 2016, 0333
The trade of that you mention is current, and accordingly, heat - there's a ton of both generated by these machines.
The other trade off is unsprung mass which makes your speed and terrain aspirations something that you need to examine a little more closely than you would otherwise.

As to the Leaf cells, they're easy to get if you're in the states, not so much otherwise unfortunately; but they are, hands down, the best value on the market at the moment, and incredibly well built to boot.

grindel2
12 April 2016, 1405
OK, I will look into leaf cells. Is http://hybridautocenter.com/ the only place selling them?

I just didn't know if anyone could give me a rough idea in real world numbers of the efficiency differences between hub motors and some equivalent chain drive motor all other things being equal. It is replacing a big drum brake so I don't know if unsprung mass will be as big of a factor as it might be on a more modern bike. Maybe I should head over to the motors section...

Thanks again