Driving the existing rear wheel via the largest cog

[marks]

Whilst I am still keen on a rear hubmotor (for the sheer simpilicty) I still watch this forum with much interest! I'm wondering if anyone has considered using the existing rear freewheel and cogs with an RC or "normal" style motor? I'm no engineer, but this seems like a really simple solution, I understand that there may need to be an additional gearing down but couldn't that also be part of the engine mount/block?

I'm not too good with drawing so I'll try and explain:
- Motor mounted under/behind the seat
- A guide is put in place to stop the de-railer from accessing the largest diameter cog on the rear wheel
- A chain is then run from the smallest possible toothed cog on the motor down to the largest diameter cog on the rear wheel

Any thoughts on this? It uses the existing freewheel of the rear wheel, so unless the cyclist pedalled backwards the motor wouldn't spin at all?

[fechter]

When the motor is running, the chain will be moving, so the pedals would be driven. This could be bad. They make freewheeling chainrings for the front, which might solve that problem. The derailleur might not be happy with the chain being driven, as the spring might fold up and cause the chain to go slack. A stiffer spring would help there. If the front freewheel has very little friction, it shouldn't be a problem.

[flyinmonkie]

Mark, that is basically what I am doing in my chopper build. It isn't quite as simple as you think though. You are right, you do have to gear it down to get the proper rpm and torque. Unfortunately, the rear freewheel or cassette turns as one unit no matter which sprocket you drive. So, if you are driving the rear wheel with the motor, the chain to the pedals will also turn, which means the pedals turn. Or vise versa, if you are peddling, the chain to the motor will turn, which will turn the motor. The freewheel or cassette only allows the rear wheel to turn independent of the sprockets to allow you to coast without peddling. To get around this you need to use 2 freewheels to separate the pedals from the motor at the rear hub. You will always drive the motor when peddling, so you need a freewheel somewhere in the drive of the motor, either at the motor or in the gear reduction some where. Check out my thread.

viewtopic.php?f=28&t=8492

FM

[marks]

Damn you are of course correct... it was such a simple solution

I guess this is why the cyclone and the elation type kits add a freewheel to the pedals, which isn't such a big addition but I guess that's not really new.

Is there a simple way of adding an additional freewheel onto the rearwheel?

I also guess you could adjust the derailer wire to stop the derailer from travelling the chain up to the "biggest cog" (this is in regards to the restricting the derailer bit)

[Rassy]

Marks, on my first project a few years ago I mounted a scooter motor just ahead of the crank and ran a chain from the motor to the largest sprocket wheel. It worked okay except the pedals and motor always turned together, unless coasting.

The point of my post here is that I was unable to shift to the middle sprocket wheel because there was not enough clearance for chains to be running on adjacent teeth. I think this would also be true on your rear gears, so you will probably lose your two lowest gears. The good thing is, with a motor, you won't need those low gears.

[marks]

Marks, on my first project a few years ago I mounted a scooter motor just ahead of the crank and ran a chain from the motor to the largest sprocket wheel. It worked okay except the pedals and motor always turned together, unless coasting.

The point of my post here is that I was unable to shift to the middle sprocket wheel because there was not enough clearance for chains to be running on adjacent teeth. I think this would also be true on your rear gears, so you will probably lose your two lowest gears. The good thing is, with a motor, you won't need those low gears.

Well that makes sense - I don't think I currently use those gears anyway, and you are right I would definitely be using the motor

[flyinmonkie]

Marks, like I said before have a look at my chopper build. The link is a few posts up. I am setting up 2 freewheels, or the equivalent on it.

FM

[etard]

After spending countless hours thinking about this same problem I think the easiest solution would be left side drive with a motorized (ICE bicycle part that attaches via spokes) sprocket and a freewheel on the motor sprocket. Or spend a little more dough and get a dual threaded (for freewheels) hub and put two freewheels on, but I'm still not convinced the left side freewheel would coast in the proper direction.

Although I think Earthnet64 has discovered the simplest solution of all by using a 148 tooth ring that attaches to the spokes and only requires one stage reduction.

Good luck!

[marks]

Hmm it really does seem like the easiest solution is to just buy the cyclone crank freewheel and then stick with the original idea.

[liveforphysics]

You will need additional reduction, or the motor will overheat and the bike wont accelerate.

I'm using a very very low RPM motor on my build, 130kv, and I still need a 12:1 reduction. Most hobby motors are going to be wanting to see something like 100:1 or 200:1.

With the right 3-4 stage planetary gearbox, it could be done in a neat compact package attached right to the motor, but the devil in that is finding what you need.

[JEB]

Try- http://banebots.com - lots of different ratios.

The largest sprocket on a freeweel rear wheel drive that I made by hand was a aluminum sprocket to using #35 chain 128 tooth, to a 12 tooth sprocket 10+to1!