Simple, open source middrive anyone can build

Larby

1 mW
Joined
May 10, 2022
Messages
12
Morning all,

Following much hunting around for what I want, I have concluded I need to build my own. I'm an engineer with experience in design, R&D and motorsport and have my full company workshop at my disposal including 3D scanning, CAD, rapid prototyping, profile cutting etc. So, I'm in a good place to develop something.

That said, I don't just want to develop something for me, I'd like to make it more of an open source group project and develop something any of us could put together with minimal and simple assistance.

My spec is to make a simple, fairly light, mid-drive assistance system which can be easily attached/detached.
I'd like to to preferably leave the original bike pretty untouched, save for maybe the chainring, so high-strength components don't have to be replaced with lower spec ones (such as square taper BB's, hubs etc).

I’m going to start off with a simple thumb throttle for testing, until I’ve got the drive setup sorted.
My plan is to then move to a torque and cadence sensed arrangement, to hopefully make the unit totally self contained and to also give the widest range of control options.

I should add that I'm (sort of) UK based, so I'd like it to have 'road legal' capability and I'm not chasing massive power figures. I envisage it as an assistance system, rather than something to save you pedalling at all.

So far, my theoretical spec list is as follows...

RC motor as the prime mover:
https://hobbyking.com/en_us/turnigy-sk8-6374-149kv-sensored-brushless-motor-14p.html?queryID=3a6165b0623055eb7629c4d7afed99ad&objectID=74469&indexName=hbk_live_products_analytics&___store=en_us

Simple VESC to power it:
https://hobbyking.com/en_us/turnigy-sk8-v2-80a-3-12s-single-motor-skateboard-esc.html?queryID=1f66f46c97b42d7b1a35c3a59ca64e5d&objectID=83138&indexName=hbk_live_products_analytics

NEMA 24 based planetary reduction drive, which will be linked via off-the-shelf gears or a belt drive for simplicity (something along these lines, there's all sorts available, recommendations of an affordable and readily available version welcome!):
https://www.ebay.co.uk/itm/203660888117?hash=item2f6b228835

Simple, compact and easily available battery:
https://hobbyking.com/en_us/turnigy-high-capacity-battery-20000mah-6s-12c-drone-lipo-pack-xt90.html?queryID=e4d73d973c9332952aba7c5ce6e53f96&objectID=78412&indexName=hbk_live_products_analytics

CA3 Cycle Analyst to bring it all together and give a large range of 'tuning' and input options, especially when I look to add torque/cadence sensing:
https://ebikes.ca/product-info/grin-products/cycle-analyst-3.html

In terms of the rest, I'm aiming to use simple profile cut steel/ally and 3D printing, so it could be easily recreated by anyone with the DXF/STL files.

Parts I could use help with:

Drive sprocket for the output from the gearbox.
I'd love this to have a freehweel, so the bike can be pedalled without the drag of the system if required. Something like a left hand sprocket/freewheel from a BMX? Alternatively, for now, just a fixed sprocket would do in the region of 13-15T.
Any ideas on where to get one with a simple shaft/key fixing or similar?

Freewheel for the chainring.
This one doesn't bother me, as I'd only use it for assist, so would simply bolt on another chainring on the outer mounting of my cranks. I'm sure some would like a freewheel though, and it'll become essential for cadence/torque sensed drive, so could do with concluding what to use.

I think that's about it for now. As I say, it's very much open project, so please do give any feedback/advice you think you can!
Remember that no one system will suit everyone, so I'm basing this on my personal needs primarily, which I'm sure will suit a bunch of other people. It just may not be 'your' idea of a perfect system.

Cheers, Larby
 
I already did that, sort of.

I think using a custom laser cut 100+ T 6.25mm pitch chain cog (will NOT cost much) on with 12T or whereabouts (10T is a bit noisy) will likely work for a single step reduction for a middrive that is regen brake capable, coupled with 6384 120kv motor.
A HDT5 belt drive will also work fine and is silent, but a bit finicky and a bit more draggy (but no chain maintenance) (see picture)

Currently I'm using 24v battery and 6374 motor identical to yours, belt reduction 20T to 180T, hence 9x in one step. It works and regen braking capable, but I'm using max 45A phase amps and let's say with weight of my monster of a recumbent and my own very considerable (lamentably) bulk, it does not have enough torque to even move me from a stop w/o pedal assist, but it can move me to 50 kmh WITH pedalling at full throttle (about 1 kw of added power)... but that's not it's raison d'etre.

r4k5ubch.jpg


Or you want a 'mid-drive' in a 'crank drive' sense?

I have a similar reduction gearbox using a 3d printed adapter on a left-side crank drive. It heats up very considerably during operation and seems quite lossy - I've been using it as crank drive primary reduction, using a 3d printed adapter to, again, 3d printed (GF nylon) fixed cog.
Frankly, I think you should step up a notch and use helical gear reducer.
https://aliexpress.com/item/32966975500.html

Not much more expencive, but much more impressive specs - at least on paper...

Previously (again, see picture), I've been using two-step reduction using a belt as first reduction and 3d printed adapter (PLA) on a 12T cheap freewheel, but it didn't work very long when trying to pump about kilowatt of power thru it. It held up fine up to 500watts of human power when working in reverse - using as a pedal generator trainer during the winter.

It should be possible to have an adapter to a freewheel machined on a lathe for a reasonable sum I think.
 
Hi Balor,

I think we’re heading in slightly different directions, but I see what you mean. It’s great to see someone else using RC kit.

I’m most interested by your comment about it having no torque from a standstill. What ESC are you using and is the motor sensored?

I do plan to use a production Nema 24 type planetary reduction drive. The 3D printing bit was just for casings etc.

I don’t want to put the gearbox inline with the motor, as it would become too long to fit between the pedals.
As such, I’ll run them alongside each other and join with a belt/gears, which also opens options for an easily adjustable primary drive ratio.

Thanks for your reply and interest!
 
Cyclone.tw and sickbikeparts both have a range of various freewheel adapters etc that may be of some use.
 
Larby said:
Morning all,

Following much hunting around for what I want, I have concluded I need to build my own. I'm an engineer with experience in design, R&D and motorsport and have my full company workshop at my disposal including 3D scanning, CAD, rapid prototyping, profile cutting etc. So, I'm in a good place to develop something.

That said, I don't just want to develop something for me, I'd like to make it more of an open source group project and develop something any of us could put together with minimal and simple assistance.

My spec is to make a simple, fairly light, mid-drive assistance system which can be easily attached/detached.
I'd like to to preferably leave the original bike pretty untouched, save for maybe the chainring, so high-strength components don't have to be replaced with lower spec ones (such as square taper BB's, hubs etc).

I’m going to start off with a simple thumb throttle for testing, until I’ve got the drive setup sorted.
My plan is to then move to a torque and cadence sensed arrangement, to hopefully make the unit totally self contained and to also give the widest range of control options.

I should add that I'm (sort of) UK based, so I'd like it to have 'road legal' capability and I'm not chasing massive power figures. I envisage it as an assistance system, rather than something to save you pedalling at all.

So far, my theoretical spec list is as follows...

RC motor as the prime mover:
https://hobbyking.com/en_us/turnigy-sk8-6374-149kv-sensored-brushless-motor-14p.html?queryID=3a6165b0623055eb7629c4d7afed99ad&objectID=74469&indexName=hbk_live_products_analytics&___store=en_us

Simple VESC to power it:
https://hobbyking.com/en_us/turnigy-sk8-v2-80a-3-12s-single-motor-skateboard-esc.html?queryID=1f66f46c97b42d7b1a35c3a59ca64e5d&objectID=83138&indexName=hbk_live_products_analytics

NEMA 24 based planetary reduction drive, which will be linked via off-the-shelf gears or a belt drive for simplicity (something along these lines, there's all sorts available, recommendations of an affordable and readily available version welcome!):
https://www.ebay.co.uk/itm/203660888117?hash=item2f6b228835

Simple, compact and easily available battery:
https://hobbyking.com/en_us/turnigy-high-capacity-battery-20000mah-6s-12c-drone-lipo-pack-xt90.html?queryID=e4d73d973c9332952aba7c5ce6e53f96&objectID=78412&indexName=hbk_live_products_analytics

CA3 Cycle Analyst to bring it all together and give a large range of 'tuning' and input options, especially when I look to add torque/cadence sensing:
https://ebikes.ca/product-info/grin-products/cycle-analyst-3.html

In terms of the rest, I'm aiming to use simple profile cut steel/ally and 3D printing, so it could be easily recreated by anyone with the DXF/STL files.

Parts I could use help with:

Drive sprocket for the output from the gearbox.
I'd love this to have a freehweel, so the bike can be pedalled without the drag of the system if required. Something like a left hand sprocket/freewheel from a BMX? Alternatively, for now, just a fixed sprocket would do in the region of 13-15T.
Any ideas on where to get one with a simple shaft/key fixing or similar?

Freewheel for the chainring.
This one doesn't bother me, as I'd only use it for assist, so would simply bolt on another chainring on the outer mounting of my cranks. I'm sure some would like a freewheel though, and it'll become essential for cadence/torque sensed drive, so could do with concluding what to use.

I think that's about it for now. As I say, it's very much open project, so please do give any feedback/advice you think you can!
Remember that no one system will suit everyone, so I'm basing this on my personal needs primarily, which I'm sure will suit a bunch of other people. It just may not be 'your' idea of a perfect system.

Cheers, Larby

two projects and members of this forum come to mind.

https://www.revelpropulsion.com/

and

https://www.lightningrodev.com/

it seems like your idea is unique but you might find these projects informative.
 
Larby said:
Hi Balor,

I think we’re heading in slightly different directions, but I see what you mean. It’s great to see someone else using RC kit.

I’m most interested by your comment about it having no torque from a standstill. What ESC are you using and is the motor sensored?

I do plan to use a production Nema 24 type planetary reduction drive. The 3D printing bit was just for casings etc.

I don’t want to put the gearbox inline with the motor, as it would become too long to fit between the pedals.
As such, I’ll run them alongside each other and join with a belt/gears, which also opens options for an easily adjustable primary drive ratio.

Thanks for your reply and interest!


Regarding torque - The problem with single stage reduction, large wheel and low phase current. I have the cheaper variety of vesc and motor is sensored. It does result in about 20 nm of torque after reduction, but again given overall weight of ~170 kg - this is just not enough get me moving. It is possible to nearly double the current, but I'm not willing to go up, cause I do not have a temp sensor and this is more about efficiency and regen braking potential for scrubbing speed (again, using pretty low amps) than raw power/torque.

For high torque I have crank drive. You have a pretty interesting idea regarding using the planetary reduction drive as a separate second stage reduction, I give you that!
But than, you will lose 'shaft support' aspect which is pretty important I think... but that can be either omitted (but take a hit to bearing longevity) or have it in a separate arrangement. I'm not sure, however, that that input shaft is designed with ANY lateral forces in mind. it might work, but depending on how it is arranged you may strain or break things in the long run, unless you add shaft support to input shaft as well. Output shaft, obviously, have very hefty bearings by default.

Technically, since you do not need *high* power/torque, you might as well use shorter reduction gearbox and shorter motor.

If you don't use a road crank, I think you'll have just enough space.
 
maiz said:
two projects and members of this forum come to mind.

https://www.revelpropulsion.com/

and

https://www.lightningrodev.com/

it seems like your idea is unique but you might find these projects informative.

Thanks for that! I've already looked through those, along with pretty much every other project I can think of, to make lists of the methods, pros & cons of each system.
If there's any one project this is similar to, it's GKASEV's build. This was a massive inspiration and while mine will be a little different in a few ciritical aspects such as mounting, his overall execution is very similar. So much so, I've messaged him to check he doesn't mind me getting so close and to ask if he'd like to be involved.
https://endless-sphere.com/forums/viewtopic.php?f=6&t=115176

Cheers, James
 
I would go with a big fat pancake motor.

Motors like the style you picked are made for spinning 10-20,000rpm making very small shaft torque. When you want to spin a large wheel slowly at high torque, it leaves the most shaft power transformation to become useful.
 
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