Ultra-lightweight motorcycle using fixed 3-speed hub?

[Ollie]

Hey guys, I have an idea for an ultra-lightweight 3-speed bike that will technically be a motorcycle, but here in Brazil will still be in the motorized bicycle category as it's engine will be smaller than 50cc. I wonder if you can see any major errors with my idea or if you can think of any improvements?

I've been trying to work out a way to get gears that work directly with the engine on the left-hand side of the bike to avoid using a jack-shaft or SBP shift kit (weight-saving). I discovered that Sturmey Archer has a fixed 3-speed hub (the S3X) which I reckon I could flip over so that the engine drives the hub directly and can make use of the gears. There would be no need for a freewheel pedal chain or chainrings (more weight-saving) and I intend to bolt a different sprocket to the existing one to get the gear range right. Because the gear hub is fixed, it should be able to run in reverse, although I'd put a few spot-welds on the cog and lock-ring to stop it undoing itself.

I intend to use the Hua Sheng 49cc 4-stroke engine that is sold here as it's got a pull-start and won't need to be jump-started by pedalling like my 2-stroke. I'll stick all this onto a steel frame & forks with V-brakes and the fattest tyres that will fit. What results should be a very light 3-speed motorcycle that looks like a bicycle (cops don't stop me at road checkpoints on my motorized bicycle) and has low gears to get up some of the steep hills around here.

Any thoughts, suggestions or apparent flaws?

Ollie

 

[professor]

I guess you have to use the hub as a jackshaft, which is what I did. The issue is with the freewheel aspect of the hub. It needs to be oriented as it is within the back wheel. I had to use a primary jackshaft to shift the engine's output to the right side so it would output to the hub. Then the hub drives a left side sprocket on the wheel.

I have been using SA 3 speed oldies for a few years and they are great.
You let off the throttle to shift.
The clickshift is great too.
However they likely would not live in a rear wheel driven by and engine.

 

[darwin]

50cc motor/tranny on a bicycle frame just like a scooter or a small MC. Why does the drive chain need to be on the right side? Problem lies in a sprocket attached to the wheel hub on the left side and mounting the motor in the V. There's bicycle hubs where a disc is mounted on the left side, use that as a sprocket mount.......there's a way if you really want to. There's those frames that look like lowriders that will take a 50cc Honda clone w/o modification. Just rambling............................

 

[darwin]

Ollie KCvale is who you should be talking to. He'll know he builds some nice bikes and knows his stuff. http://kcsbikes.com/KCsBuilds.asp

 

[Ollie]

@professor:
The S3X hub is fixed, so it doesn't freewheel and should be able to run in reverse. Here's the info on the SA website:
http://www.sturmey-archer.com/products/hubs/cid/3/id/47.html

@darwin:
That's a good method for a singlespeed, I've bolted a chainring to a disc hub before and it worked well. What I really need is gears though, so I can get going from a stationary position and still have the gearing for a decent top speed. I had a look at KC's website, nice bikes! His jackshaft/gear hub combo whs what I first considered trying for a geared bike. Is there a way to tag him like you do on facebook so that he sees this thread?

 

[KCvale]

I've been trying to work out a way to get gears that work directly with the engine on the left-hand side of the bike to avoid using a jack-shaft or SBP shift kit (weight-saving). I discovered that Sturmey Archer has a fixed 3-speed hub (the S3X) which I reckon I could flip over so that the engine drives the hub directly and can make use of the gears.
Any thoughts, suggestions or apparent flaws?
Ollie

Yep, it won't work, no bike hubs will function running backwards (counter-clockwise like the engine), only Clockwise.

 

[Ollie]

@KCvale:
Thanks for the comment, but is that the case with a fixed gear hub? It doesn't have a freewheel, it can't coast. Why would that not function in reverse? You can ride a fixed gear bike backwards if you want to, why wouldn't an engine do the same? Sorry for all the questions, but it doesn't make sense to me.

 

[darwin]

Yep, it won't work, no bike hubs will function running backwards (counter-clockwise like the engine), only Clockwise.

What if you turn the wheel around? Cassette to the left side?

 

[Ollie]

@darwin:
Yeah, that's what I was thinking. Turn the wheel over so the hub's sprocket is in line with the engine's sprocket. Because fixed hubs have no freewheel and function the same forwards or backwards, it should work as far as I can see.

 

[KCvale]

Of course you can run a fixie on either side, but it has no gears.
The whole point was to get gears right?

Well, ALL shifting hubs freewheel and simply can't be powered in reverse.
One other note about fixies is they no place on a motorized bike unless you want your legs beat to death.

I have explored about everything there is to try to get reasonable left side shifting or right side clockwise output engines and it simply is not to be, except with electrics that is, man it's cool to hook a motor output directly to the bike drive train without a jackshaft.

And for putting a cassette and derailleur on the left, sorry I can't help but chuckle, if you can't pedal spin the wheel and shift backwards normally, it won't change on the left hand side.

Once again, everything bicycle is designed for clockwise right side operation.
Every gas engine is counter-clockwise left side output.
A jackshaft just mirrors the output on the other side with clockwise rotation where you want it.

When you see a bicycle with left side pedal power grab it as it would really be a coop.
Personally I am still hoping to get a 4-stroke base that takes the JS out and outputs clockwise just above the pedals so it just 2 chains which is the same as any direct drive.