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