How to drive ebike using outrunner?

raayyymond

1 µW
Joined
Jan 18, 2018
Messages
4
So I have a Turnigy Sk3 6374, and looking into using it to drive a new eBike I'm working on. However I've come into a mechanical engineering dilemma of frustrating sorts.

The bike that I'm converting is a mountain bike with front suspension, 21 speeds, a front disc brake, and back disc brake. Considering this, there isn't much room to put anything such as an extra sprocket. I also would like this eBike to support regenerative braking, and I've already got everything sorted out on the electrical side. But as for mounting the motor, I don't have the option of doing mid-drive or something like this.
337102d1203717505-new-brooklyn-machine-works-dh-bike-sr8-2.jpg


This is because it wouldn't support regenerative braking as the back wheel would freely spin forward despite any braking from the motor. Any ideas?
 
Several people have put a second drive chain on the left side of the bike, driving from the motor to a sprocket mounted to the 6 bolt holes for the disc brake.
Obviously the sprocket has to be on a flange that moves it inward slightly, and the disc ends up moving outwards slightly, so it would depend on the frame and the tyre size etc, but it has been done.

From you post I assume it is a hardtail frame.. no rear suspension. If so, then the chain can be tensioned by moving the motor and fixing it in position with correct chain tension, and would not need any form of chain tensioner, so the chain would work equally well for regen as for forward drive.

I have also seen people who fixed a sprocket to the right hand side of the rear hub inboard of the rear cassette and ran a second chain to the motor inboard of the "normal" chain. The large inner sprocket was fixed directly to the hub so would work in regen as well as drive, while the cassette sprockets outside it were still on the freewheel. Possibly one or more sprockets would be removed from the cassette to give room for the fixed sprocket inside them.

Either would be increasingly difficult as the width of the rear tyre increased... and both would require machining skills.

You will probably need a 2-stage reduction to get the motor spinning fast, where it will be efficient and keep cool, rather than trying to develop power at low RPMs with low voltage and high current, which would be inefficient and generate a lot of heat in the windings.

For example, if you have the 149kV version of the motor on 36 Volts and are aiming for 30mph (50kph) top speed you would need about 14 to 1 gearing reduction, which would be difficult to achieve in a single chain drive (Reduction for the 196kV version would be around 19:1).

Good luck with the project,
Dave
 
Drum said:
I have also seen people who fixed a sprocket to the right hand side of the rear hub inboard of the rear cassette and ran a second chain to the motor inboard of the "normal" chain. The large inner sprocket was fixed directly to the hub so would work in regen as well as drive, while the cassette sprockets outside it were still on the freewheel. Possibly one or more sprockets would be removed from the cassette to give room for the fixed sprocket inside them.

Thank you for your help. There doesn't seem to be any room at all to mount a sprocket behind the disc brake even with some sort of flange, but the other idea you gave me sounds feasible. Would you know by any chance where people get these larger inner sprockets that are attached directly to the hub? The only ones I could find are all freewheel, and therefore wouldn't work for my regenerative breaking.
 
You can look up "sprotor" whcih is a brake rotor that is also a sprocket; that's one option. Might have to custom-make it if the size you need isn't available anywhere.

You can also use a front dual-disc hub, with a longer rear axle in it, as a rear hub, and bolt a sprocket onto it on the right side and your brake rotor on the left. No freewheel that way (also only one gear in teh rear unless you build a non-freewheeling cluster to bolt on. Would require either rebuilding your wheel with the new hub, or finding a complete wheel you can just swap in.


If you dig around in the many threads in the Non-Hub-motor section, theres' dozens of DIY builds over the years with ideas you could use (it's just a lot of reading). If the title has Lightningrod, Tangent, BBS, GNG, Cyclone, etc in it you can skip those, as they probably are just about the commercial products, though there might still be info in some of those you could save them for last.


Keep in mind that regen thru the chain is probably going to be even harder on the drive parts than hard acceleration would be, so you may end up replacing them even more often than otherwise, so nothing fails during braking (which would suck).

You'll also want a straight chainline without a derailer or tensioner, because the braking forces will pull on the bottom of the chainline where the acceleration pulls on the top.
 
Drum said:
Several people have put a second drive chain on the left side of the bike, driving from the motor to a sprocket mounted to the 6 bolt holes for the disc brake.
Obviously the sprocket has to be on a flange that moves it inward slightly, and the disc ends up moving outwards slightly, so it would depend on the frame and the tyre size etc, but it has been done.
Actually, back to your idea about having a sprocket behind the disc brake... I think I might've found a person who could be able to mill these.

https://endless-sphere.com/forums/viewtopic.php?f=28&t=48113

I'll be it, the post is 5 years old, but the guy still seems active. Fingers crossed!
 
raayyymond said:
Drum said:
Several people have put a second drive chain on the left side of the bike, driving from the motor to a sprocket mounted to the 6 bolt holes for the disc brake.
Obviously the sprocket has to be on a flange that moves it inward slightly, and the disc ends up moving outwards slightly, so it would depend on the frame and the tyre size etc, but it has been done.
Actually, back to your idea about having a sprocket behind the disc brake... I think I might've found a person who could be able to mill these.

https://endless-sphere.com/forums/viewtopic.php?f=28&t=48113

I'll be it, the post is 5 years old, but the guy still seems active. Fingers crossed!

If that person can't make it, search up user recumpence on this board. He makes them as well.
 
Back
Top