Dyi electric bike kit

Joined
Mar 23, 2020
Messages
7
Hi,
I'm trying to add a motor to a bike. I'm on a rather tight budget so my idea was to use a 895 dc motor which is quite strong (380w at 24v). However this motor has high rpm (3000-6000), would it be better to use a geared motor like the 37GB-550 (300rpm)? Do electric bicycles need low rpm?
thanks
 
Joined
Oct 19, 2012
Messages
799
Location
Wakefield, UK
A 26" wheel will do 194 ish rpm for 15mph. A 380W motor is going to top out around 15 to 20 mph plus whatever you add by peddaling. You need to gear a motor appropriately for that. If your motor wants to do 3000 rpm you may well need a jackshaft to gear it down and on a typical bike there just isn't room. Much will depend on the sprocket size on the 3000 rpm motor as to whether you can gear it down or obtain a better sprocket size for it. Don't forget to consider the pitch of the chain required for that motor sprocket. Many do not take std bike chain. I have a high rpm motor of 600W / 24V on a trike currently under construction. The motor and large sprocket came off a Schwinn scooter. I've had to employ the scooters very large sprocket on a jackshaft to gear it successfully for a 26" wheel. The jackshaft drives a left hand sprocket I had to fabricate on the wheel. The motor takes a much smaller pitch chain but the jackshaft then converts to std bike chain on the opposite side.

 
Joined
Oct 19, 2012
Messages
799
Location
Wakefield, UK
The red one has no sprocket with it - can one be obtained for it? It's also incredibly tiny to be making a claim of 1760W. A shaft diameter of 5mm is going to snap if asked to move a bike plus rider even if the motor could move it. It seems wholly unsuitable for the job. The other one will work if you gear it right. You can not specify one cog in isolation as it will / may depend on:-

Motor speed
Expected top speed of bike
Driven wheel size
Number of teeth on motor
Number of teeth on driven wheel.

Then there's the variables you need to choose such as do you live in a hilly area or a flat area? Do you want the bike geared for top speed or to get up hills better? You'll also need to allow for a freewheel in the motor drive so you aren't driving the motor as well as the bike when pedalling. Are you mounting it mid drive (ie attached to the front cogs - easier to mount / harder to implement a freewheel) or to the front or rear wheel? If driving the rear wheel do you intend to drive it to one of the existing cogs or to a left hand side cog that will need adding to your existing wheel. Is your wheel suitable for such a drive? Will you put the freewheel on the jackshaft or driven wheel? The 250W one you linked is likely to use 25H chain or similar meaning one of the jackshaft cogs (the big one) needs to be for this size chain too. Can you position the jackshaft so that you can get an unimpeded chain run both to the motor and the driven wheel? When calculating gearing you also need to remember that the quoted motor speeds are no load ones. In reality it'll top out at less than that when moving a bike.

There are a lot of variables and unless all are known any advise can only be general. As well as the motor you will need a bare minimum of a 25h or similar chain and a large sprocket. You may also need a jackshaft with a freewheel and an extra bike chain plus the ability to make and mount (weld usually) such hardware. Don't forget to factor in a controller and throttle too to say nothing of batteries. It may be cheaper to get a new hub motor and lace it into your wheel negating the need for any other sprockets, chain or fabrication. Old non-working electric bikes are regularly sold on ebay. Usually it's the batteries that have died on them. If so you'd then have a hub motor plus controller plus throttle to use. The only "hard" part would then be putting that hub into your wheel and that's frankly a LOT easier than it appears at first glance.
 
Last edited:
Joined
Mar 23, 2020
Messages
7
The red one has no sprocket with it - can one be obtained for it? It's also incredibly tiny to be making a claim of 1760W. A shaft diameter of 5mm is going to snap if asked to move a bike plus rider even if the motor could move it. It seems wholly unsuitable for the job. The other one will work if you gear it right. You can not specify one cog in isolation as it will / may depend on:-

Motor speed
Expected top speed of bike
Driven wheel size
Number of teeth on motor
Number of teeth on driven wheel.

Then there's the variables you need to choose such as do you live in a hilly area or a flat area? Do you want the bike geared for top speed or to get up hills better? You'll also need to allow for a freewheel in the motor drive so you aren't driving the motor as well as the bike when pedalling. Are you mounting it mid drive (ie attached to the front cogs - easier to mount / harder to implement a freewheel) or to the front or rear wheel? If driving the rear wheel do you intend to drive it to one of the existing cogs or to a left hand side cog that will need adding to your existing wheel. Is your wheel suitable for such a drive? Will you put the freewheel on the jackshaft or driven wheel? The 250W one you linked is likely to use 25H chain or similar meaning one of the jackshaft cogs (the big one) needs to be for this size chain too. Can you position the jackshaft so that you can get an unimpeded chain run both to the motor and the driven wheel? When calculating gearing you also need to remember that the quoted motor speeds are no load ones. In reality it'll top out at less than that when moving a bike.

There are a lot of variables and unless all are known any advise can only be general. As well as the motor you will need a bare minimum of a 25h or similar chain and a large sprocket. You may also need a jackshaft with a freewheel and an extra bike chain plus the ability to make and mount (weld usually) such hardware. Don't forget to factor in a controller and throttle too to say nothing of batteries. It may be cheaper to get a new hub motor and lace it into your wheel negating the need for any other sprockets, chain or fabrication. Old non-working electric bikes are regularly sold on ebay. Usually it's the batteries that have died on them. If so you'd then have a hub motor plus controller plus throttle to use. The only "hard" part would then be putting that hub into your wheel and that's frankly a LOT easier than it appears at first glance.
Hi, thanks for reply.
So the motor speed would depend, i was thinking of 2 possibilites, one was to use a 895 dc motor (https://it.aliexpress.com/item/33035170979.html? ) which has a maximum torque of 10kg (380w) (i've seen videos of people propelling bikes with these) but has rpm of 3000 to 6000 so i was thinking of attaching a 80t 25h sprocket to it, and a 12t 25h sprocket to the wheel, which would go attached to a toothless freewheel or a clutch bearing (this because could not find any 25h freewheel, only 410h) in this way i'd get a significant reduction but still the small cog would spin around maybe 400- 800, would it be ok?

Or another option was these small already geared down motors such as the JGB37-555 or the JGB37-550, which offer a torque of about 50kg at certain rpm ( http://www.aslong.net/en/h-pd-64.html ). But i'm not sure they don't seem very strong.

Top speed would be 25km/h for now. the size of the wheel i think it's just average, like a normal bicycle , sorry i can't find anything to measure now.
Number of teeth on the 895 motor i think will be 10, because i thought that the less the better, to get more rpm reduction?

Number of teeth on the driven wheel 80 (the biggest i could find)
It's rather hilly, but there is also plains. I'd like it to be able to carry my weight (55kg) and the groceries (10kg).
Looking at it i thought i could simply somehow attach the freewheel on here where i marked:
https://ibb.co/wcR6bnH
Otherwise i would get a double sided hub from china, the thing is i think the bike in question has a cassete, so i'd have to remove that entirely and replace it with some other freewheels.
Using a jackshaft doesn't seem very easy.
I cannot weld but i can frankenstain(improvise) something, maybe to hold things tightly with thick iron wires.
The electronic part would not be hard for me to program as i have some background of using Arduino and other things. For batteries i was going to go for lithium polymer, hoping that they are not fake,
https://www.ebay.it/itm/DC-12V-1800-20000-6800-9800mAh-Rechargeable-Li-ion-Battery-with-Charger-Adapter/323882204936?hash=item4b68e23f08:m:mfG2EaacF7mzoxwzbwc6wwQ
 
Joined
Mar 23, 2020
Messages
7
The 895 dc motor that runs at 12v 3000rpm 5kg/cm torque and at 24 6000rpm 10kg/cm torque . I can gear it down with a ratio of 8 to 1, so the rpm would become 375 at 12v and 750rpm at 24v. The bike will go at to maximum of 25km/h, after that the motors will stop working (because of the law). but even just 12v 375rpm is about twice as much as the rpm that the wheels will have at 25km/h. So i'm worried this will not be efficient/ effective at all? I mean will it even be able to exert some torque in this scenario?
 
Joined
Oct 19, 2012
Messages
799
Location
Wakefield, UK
Without a jackshaft you only have the motor to wheel ratio to gear the bike. At the power you are looking at you need to gear for around 200 rpm at the wheel. A 3000 rpm motor would need a 15:1 ratio and you are not going to get that. The faster motors get even worse. If you can't weld I'm at a loss as to how you would attach a freewheel to that hub. A flip flop or double sided hub will allow you to attach one but then you're needing to attach a monster size sprocket to that freewheel. I have no idea how well an electric motor will perform if considerably under spinning. The theory is they develop most power at half their no load speed. Those motors don't look very waterproof either.

I don't know the batteries you link to but China has a bad reputation on 18650 batteries, claiming 6000, 9000, 12000 mah and more when the reality is the best South Korean units are giving a true 3500 mah. Tests on some 6000 mah claimed units showed less than 500 mah (yes 500 not 5000!) in reality and weighed half of a good unit. I'd expect the claims for them to be a blatant lie given their size and price.
 
Joined
Mar 23, 2020
Messages
7
Yes you have a point i think i'm going to go with this: 350w dc motor 3000 rpm . It has a gear ration of 9.7:1, the size of the wheel is 26.7 inches ( i measured it mannualy), so if i'm not wrong should give me about 25km/h at 2000 rpm and 250w which is perfect because it is the exact legal limit. So i don't think that with this setup i would need and further gearing?
Yes it's true about the batteries but these are lithium polymer, it's not those cilindrical cells, they use less volume and at least the different sizes they have seem to make sense with one another. Anyhow if they are fake i can get the money back through paypal.
Just one more thing could you tell me if any of these one way bearings would work?
I was going to get this: single speed freewheel , do you think a 25h will fit in it?
Otherwise as an alternative (i'd have to attach a cog to them):
https://www.ebay.it/itm/CSK12PP-CSK40PP-Sprag-Clutch-One-Way-Bearing-With-Internal-External-Keyways/153107586906?
https://www.ebay.it/itm/Bicycle-Toothless-Flywheel-Freewheel-Metric-Thread-98mm-Outer-Diameter-for-Bike/274142072394?
 
Joined
Oct 19, 2012
Messages
799
Location
Wakefield, UK
If you use a 12t cog as per the motor an 18T freewheel would seem ideal to top a 300rpm motor out at 200 rpm at the wheel and 15mph. If you use a 12T one you're keeping the motor nearer it's max power at 15mph but further from it at slower speeds. Which is best I do not know. Pretty much all freehweels use the same threads but if putting one onto a flip flop hub you want a specific LHD one. They have a slightly different thread. A RHD std frewheel used in reverse on the left will fit but will want to unscrew as it drives. Unfortunately LHD freewheels are somewhat more expensive. Make sure the hub you use has threads for a LHD on the left side and not just regular threads for a disc brake adapter. Many hubs have two sets of threads on the left for both.
 
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