converting a geared hub motor to a mid drive for a home made e bike with a small rear wheel

tim25

Finding my (electric) wheels
Nov 21, 2020
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0
Hi there,
I'm trying to build a low budget home made cargo e bike from the collection of old bits and pieces I have.
My first e bike used s a 250watt Hesc hub motor with a 36v battery. while it gave a great electric motor experience with the throttle and pas. it felt like pedalling was just for show and seemed like I was not able to add much as a rider. A lot of drag was felt from the hub at top speed pedaling.

Therefore for my next e bike I would like to try to use a mid drive motor setup, converted from a rear hub motor. I hope with the right home adapted freewheel system I should not experience such drag. And it will give me better hill climbing ability.
I don’t have the budget for a bb drive and like the idea of fabricating something. ( think scrapyard challenge)

I would love to have some feedback from some more experienced people on my ideas.

I am visualising a kind of extracycle using a 24 inch wheel at the front and a 20inch at the rear
A combination of a pashley millennium at the front with its great in built basket. And a 20inch mountain bike at the rear with a long and low rack for cargo.
This leaves a workable space in the centre behind the crank for a kind of mid drive using a converted hub motor.

I am aware that many have done similar builds before, but the reduction gearing is somewhat challenging.

a 250 watt motor with slow wind says it will have an rpm of 201. this seems not to far away from the 50- 100 range of my desired crank rpm.
I’m Thinking of using a cassette motor so I can try out a range of sprocket sizes and use that to either drive a freewheel crank or an intermediate freewheel setup.
That way I can access the mountain bike derailleur at the rear. No front derailleur.

I’m keen that I can add power with my legs and so need to get the gearing right .

I have only had discouraging remarks made about my different size wheel idea from non cycling friends, maybe their right?
On the other hand I get get stability with loads, smaller overall length and with the right gearing should be able to reach my desired 20 mph unloaded

I've been reading loads of posts on mid drive motors builds but not one of them quite like this yet

I would be greatful for any feedback before I go any further.
Thanks
Tim
 

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Wicky

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Feb 12, 2014
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"And it will give me better hill climbing ability."
"reach my desired 20 mph"

Can't see any major problem with the different size wheels - maybe cobble up an unpowered version first to get a feel of the arrangement.

You'll still need a bit of trial and error to see if your targets are achievable
 

tim25

Finding my (electric) wheels
Nov 21, 2020
24
0
Thanks Wicky thats a good idea!

Im keen to get the angle grinder out and play around with some bits!

when I've seen other examples of home made mid drives they often seem to use a 2 or 3 stage reduction process, using a combination of belts, pulleys and chains, looks rather complicated and
im wondering if by choosing a geared hub rather than a direct drive. such as this geared hub
AKM-100CST Cassette 36V250W EBike Rear Driving Hub Motor
spec says rpm 201 .

Does this mean i can use these in built gears as one of my stages? if so, from the motors smallish sprocket on the cassette to the smaller chainring drive might be pretty close to the correct range?

I may be over simplifying it due to my inexperience? I'm guessing i will have to suck it and see... then adapt from there. The principles are still a bit wooly in my mind.

The least home made engineered/butchered bits the better for my maintenance and reliability.
 

vfr400

Esteemed Pedelecer
Jun 12, 2011
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Basildon
You're going to need a 52T chainwheel unless you like pedalling like a BMX rider in the world championships.

Why don't you sort out your hub-motor? What you're describing isn't normal regarding drag. The HESC motors have a clutch that completely disengages the motor and internal gears from the hub, so drag from the motor isn't possible. Did you spin the wheel in the air to see how much drag it has? The power problem is because you were using a crappy controller. It would be sorted for £70 by a KT controller that uses current control instead of speed control for the PAS.

You can see the clutch in the pictures in this thread. It's contained between the ring gear and side-plate, so the ring gear and the motor internals don't move when freewheeling. That's the same as the AKM/Q-series motors:

If you want to go ahead with using a hub-motor as a crank-drive, you need a free-wheeling crankset that you can get from Cyclone (Eclipse are UK agents) for about £75. I have a couple that I never used if you can pick up from Telford. I also have a GNG mk1 that has had very little use (50 miles max) that comes with a freewheeling crank for £100 if you want it. Here's what I wrote about it when tried it, with some photos:

40946
 
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tim25

Finding my (electric) wheels
Nov 21, 2020
24
0
hi vfr400,
unfortunately that hub was stolen with the rest of the bike after a very short trial period, perhaps it could have been set up better?

it was quite a heavy ride without the motor 23kgs but no particular friction points. When i used the motor it seemed the peddling added very little improvement to performance. I used a kt-t06s which seemed to work ok on the electric side of things.

I wasn't altogether sold on that experience so have gone down the mid drive rabbit hole as it seemed like an worthwhile improvement....
i can see the attraction of just sticking a 20inch hub motor wheel on the back and sticking the batteries and controller in the space in the middle, forget all the free wheel/jackshaft fabrication, reliability problems and get out and ride!
 

tim25

Finding my (electric) wheels
Nov 21, 2020
24
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ha ha, I like the image of a bmx rider in the world championships, but probably not what I'm after... i saw on ebay some 55 teeth chain ring sprockets with freewheels that fit. I thought i could use that as the basis of my homemade freewheel crank .

where you thinking the 52 tooth sprocket would be for the pedals to drive so they are more powerful than the motor?
 

vfr400

Esteemed Pedelecer
Jun 12, 2011
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Basildon
hi vfr400,
unfortunately that hub was stolen with the rest of the bike after a very short trial period, perhaps it could have been set up better?

it was quite a heavy ride without the motor 23kgs but no particular friction points. When i used the motor it seemed the peddling added very little improvement to performance. I used a kt-t06s which seemed to work ok on the electric side of things.

I wasn't altogether sold on that experience so have gone down the mid drive rabbit hole as it seemed like an worthwhile improvement....
i can see the attraction of just sticking a 20inch hub motor wheel on the back and sticking the batteries and controller in the space in the middle, forget all the free wheel/jackshaft fabrication, reliability problems and get out and ride!
Just in case you didn't see it, I edited my post above.

Sounds like you didn't setup your KT controller properly. there's a setting to switch between speed control and current control. Many people on the forum that theirs were supplied set to speed control, which is daft because current control is their best feature. You will need such a controller if you use a hub-motor as a crank-drive, otherwise you can't control the power with pedal assist. Speed control would give the same power on each level with different crank speed limits, like 30 rpm, 40 rpm, 50 rpm, 60 rpm and 80 rpm. In other words, the first three levels wouldn't give any power at all at normal pedal speed.
 

tim25

Finding my (electric) wheels
Nov 21, 2020
24
0
well, yeah you may be right about the controller setup...i never delved into those settings. i was able to use the pedal assist power settings 1-5. I had thought to buy a similar controller again, if i went down the hub 2 mid drive option.

Thanks for the link to your testing of the gnc drive. It looks like it could work well for my purpose and allow me to try different wheel sizes. I would also rather run at 36 volts and have a good street machine, does it have pas built in? it looks like a 10 ah battery would suffice.

How does one replace the belts, are they a common pattern?

Im not in your area but could pay for a courier to pick up. would that work for you?

may i ask why you only put 50 miles on it?
 

vfr400

Esteemed Pedelecer
Jun 12, 2011
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3,338
Basildon
I used to test every new ebike motor that came out to satisfy my curiosity and to provide content for the ebike forums. Since I found ebike nirvana with a Q128C, I don't do it so much now.

The GNG motor uses a standard pedal sensor and controller, so you can choose whatever you want. The supplied controller was a speed control type, so no much good. That's why it gave the impression of massive power compared with other systems. It was better wiith the throttle, but useless with PAS. It would wheelie if you started in first gear with PAS or too much throttle, so I had to make sure I started in a higher gear. Any crank-drive needs current control. It should be fine with a KT or any other current control controller.

You can run it at any voltage. It felt about right at 36v. At 48v, the crank would spin 30% faster, which would be OK for not pedalling, or it would suit someone with a high cadence.
 

tim25

Finding my (electric) wheels
Nov 21, 2020
24
0
Thanks for your reply and advice, having researched the gnc gen 1, and your words about my possible previous current/speed setup I dont think its right for me, although it sure looks fun !
Im also concerned about the legality of using on the road and it seems like it might draw unnecessary attention to me ! Im keen on the concept of ebike nirvana, who makes the q128 c ?
 

vfr400

Esteemed Pedelecer
Jun 12, 2011
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Basildon
Im keen on the concept of ebike nirvana, who makes the q128 c ?
That's a good question. As far as i know, they're unbranded AKM128s. You can get them from BMSBattery, Greenbikekit, Topbikekit and others. Recently, they've started selling branded AKM128s, which are identical. There are three versions: the C has the cassette spline and the other two are for freewheel gears. The H version has a higher internal gear ratio, so can handle more power than the cooking version (no suffix). Both freewheel versions have a wider stator than the C version, so are slightly heavier. The C version also has a higher internal reduction ratio so has similar capability to the wider cooking version.

Any cassette motor with a high internal ratio would be equally as good. The AKM128 has 12.6:1 and 10.5:1. Normal motors have 5:1. Bafang have recently released a range of motors with high internal ratios too. They're all catching on now.
 
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tim25

Finding my (electric) wheels
Nov 21, 2020
24
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Thanks for your reply,
I have searched around but cant find an exact match...Do these look like a suitable pair available from Top bike kit ?

AKM-128HCST 38V800W Rear Driving Hub Motor Cassette motor For ebike

T09S 36V/48V500W Torque Simulation Sine Wave Controller with Julei Waterproof connector
http://www.topbikekit.com/t09s-36v48v500w-torque-simulation-sine-wave-controller-with-julei-waterproof-connector-p-760.html,

I've read in other searches they are a quite small form factor and wouldn't stand out too much as i am looking for a bike that will be on uk main roads and not looking for trouble.
 

vfr400

Esteemed Pedelecer
Jun 12, 2011
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The motor is the same size as most ebike hub-motors. note the size of the controller is a bit bigger than most, so you're going to have to think about how and where you're going to install it. Don't forget to tick the LCD, harness and other stuff with that one.
 

Nealh

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Aug 7, 2014
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vfr400

Esteemed Pedelecer
Jun 12, 2011
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I never noticed before that there were two versions. i wonder what the difference is apart ftom the 800w one only being 201rpm. and more expensive. when they originally brought out H versions of the Q100 and Q128, they had a higher reducyion ratio than the cooking versions. I wonder if their listings are wrong about the 10:1.
 

tim25

Finding my (electric) wheels
Nov 21, 2020
24
0
The motor is the same size as most ebike hub-motors. note the size of the controller is a bit bigger than most, so you're going to have to think about how and where you're going to install it. Don't forget to tick the LCD, harness and other stuff with that one.
Thanks yes, i figured its bigger than the previous one i bought from TBK, but i should have enough room under/over or around my racks, and yes good value for all the bits and pieces!
 

tim25

Finding my (electric) wheels
Nov 21, 2020
24
0
I never noticed before that there were two versions. i wonder what the difference is apart ftom the 800w one only being 201rpm. and more expensive. when they originally brought out H versions of the Q100 and Q128, they had a higher reducyion ratio than the cooking versions. I wonder if their listings are wrong about the 10:1.
Thanks for the Link Nealh
 

Nealh

Esteemed Pedelecer
Aug 7, 2014
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I wonder how much the 800w will take 35a ?

The 500w is the one I bought years ago from BMSB as a 201rpm, we think BMSB have the labelling wrong as it topped 20mph so 260rpm.

I noticed the 800w cst model about a year ago, surprised know one else picked up on it.