Rear hub motor for a small urban bike

[Paulamellamo]

Hello,

I'm product design student and I'm working right now on an e-bike project. Bad is that I know little about electric bikes apart of what I've been reading on internet since few months ago, so I though it would be good idea to ask more experienced people.

So, the thing is that I need to select a rear hub motor (better geared) for a 20" wheel bike, max. 250W, obviously small, for city riding (in Madrid there might be some hills to climb), maximum speed on 25km/h, as permitted, for a single speed bike with belt transmision... and I think that's all.

I've been looking some chinese hub motor (8fun, cute) but I'm not very sure without a second opinion about them, plus more specific information is hard to find.

Thank so much, and sorry about my english

 

[docw009]

The Bafang/8Fun SWXH weighs about 2.7Kg and diameter is about 120mm from opposite spoke holes. I paid $89USD, but ordered motor only, as shipping with a rim was very expensive. It's rated at 250W, but I run it with a 17 Amp controller. I'm using tapered spokes which are 14 gauge in the center. Many ebikes use 12 G or 13G.

It's small, but the finish is rough. Maybe a Cute motor looks better.

For a single speed belt drive, maybe you should consider the Xiongda 2 speed motor. If your project is competing with other bikes, that might be something to make it stand out from the others.

 

[docw009]

Well, I thought it's for a school project, so it needs to be heavy on innovation, even if unproven. Anybody can pull stuff out of a parts bin and build a boring e-bike.

 

[docw009]

Maybe Paul or Paula can engineer a solution that keeps the Xiongda from eating up gears too. You were right to point to the thread, but I would hope the OP would do the research.

He or she is "a product design student working right now on an e-bike project". If it's for a grade, I would think some more creativity is needed. If it's a personal ride, yes, look for reliability.

I bought the Bafang because I wanted a little motor for a lighter weight bike. It's a nice motor for a pedal assisted bike at 36V. I use under 10 wh/mile at 15 mph in the lowest pedal assist mode, so I can almost ride for an an hour on a 180WH lawn tool battery. Top speed is around 18 mph at 36V, maybe 22 mph at 52V. It's quite likely that it might eat the gears at 52V, so I bought a spare set. The wiring on the Bafang uses the standard phase and Hall sensor colors. It connected up and worked. A "boring" motor. It does whine, because I don't have a sine wave controller.

I would think the Cute 100's are good motors too for this purpose.

 

[dogman dan]

Your starting point is to look at the belt drive. If you cannot unscrew the belt cog from the rear hub and screw it on a hub motor, then you will have to fabricate something to do the job. Modify a single speed screw on freewheel for regular bike, to have belt drive gear on it. then that can attach to the rear motor in the normal way.

The boring answer is too simple. front hub motor. That's how a non engineer does a belt drive e bike.

The fun way might be to drive the rear wheel with a second belt drive on the left side of the original rear wheel. Then you get to have some fun, and create something. This is the favored way people put a string trimmer gas motor on a bike.

 

[LewTwo]

I have a little Bafang hub motor as well: SXWU
However I am lazy and went the simple front installation (throttle only, no e-brakes, etc). At 36 volts performance is similar to what docw099 lists. The cable for this motor exits the side of the hub. It is also under 2Kg in weight. In many respects it is similar to the Cute 100. For a 'low power' bike a front mounted hub motor is sometimes a good alternative.

 

[Paulamellamo]

First, thanks so much for the interest and opinions.

I think the point is to complete the product designed with components which fit on the frame and at least seem somehow workable. So no need to be too much of realistic or specific on mechanical features or difficulties. Basically we have to give something that looks good and could work in some point. We are on a contest for a real e-bike makers company, but if the idea wins and they want to use it, surely, they will remake it and adapt it into a real industrial context.

So probably I'll choose the 8fun model based on that. But when I'll get to build my own e-bike, sure, I'll take into consideration all the information given. Thanks again for the help.

(And it's Paula, by the way)

 

[mechaniker]

I would say that paula should join the thread and give more details because at the moment there is not so much to go on.
Beltdrive is a bit difficult because you need a frame that you can open and close or you need to a dropout like Riese&Müller use.
Anyhow more information of this project is necessary to give serious advice and she need to give more detail
which sense has a project that work somehow? I think a project should have a real goal or should be something interesting

 

[Paulamellamo]

But if you are still willing to discuss other aspects and help me meanwhile (even if it's not totally engineering matters). I have many more questions about e-bikes components...

Rear sprockets: I need 19Teeth or 20 max. to have a decent "development" (?... not sure if it's traduced like that, like how much you move in relation of how much you pedal) with such a small wheels. I was looking at carbon drive (same brand we have belt and crankset) but when they say "fitment" or "mounting style" I'm not very sure what they mean, I guess something related with how it fits with the motor and toecap... so I'm afraid non of them will fit with a motor as the ones before as they mostly say "Rohloff spline" or stuff like that.

Here are the models I talk about
http://www.gatescarbondrive.com/products/rear-sprockets

 

[Paulamellamo]

Yeah, I know it has no big sense, put is how we have to work with it. I can't get to be expert on bike engineering in few months and having thousend of other stuff around to deal with (especially not in last 2 weeks) I wish I could, but is not the case.

About the belt, yeah we are aware of that. We are going to make one of those which you can open on the back and adjust, something like like this
http://www.mountainflyermagazine.com/img/upimages/2013_NAHBS/NAHBS_13_BR/Gates_Carbon_Awards/Boo_IMG_9059.jpg
Does it looks ok?

And I dont even know, what folder means, so is going to be somehow hard to follow the thread, sorry.

 

[mechaniker]

on a folder it is easier to have a dropout like this from "bernd´s" bicycle. they are one of the best folder´s that I know about and with this dropout you don´t need to build a complicated frame with lock inside.
btw, this is an e-bike with two very small motors on the front-rim legal 250Watt 25km/h or higher if you like

http://bernds.de/alle-faltraeder/

 

[999zip999]

Got to love the cargo bike with a basket in front. The belt is going take up all your time. Can it be a chain ? And baskets all trikes need a basket. How about battery storage as part of the frame ? Have you ridden an electric trike ? Go find one and ride it. This my answer some questions and open up others. Good luck.