Ultra-lightweight motorcycle using fixed 3-speed hub?

[Ollie]

@KCvale:
From my understanding, the Sturmey Archer S3X hub is a fixie hub WITH gears. It allows you to change the ratio while maintaining a fixed gear. Check out the start of this video, the guy demonstrates it:

https://www.youtube.com/watch?v=AJmUds2YQ74

 

[fattirejack]

Ollie I'm with you, don't know if it will work or how long it will last. I would like to know how it turns out for you. Just remember not to shift under torque or you might have a grenade on your rear hub. Keep us posted. I really think your on to something.

 

[butre]

@KCvale:
From my understanding, the Sturmey Archer S3X hub is a fixie hub WITH gears. It allows you to change the ratio while maintaining a fixed gear. Check out the start of this video, the guy demonstrates it:

https://www.youtube.com/watch?v=AJmUds2YQ74

View attachment 55141

the hub itself is fixed gear, but it takes a freewheel.

 

[Ollie]

@fattirejack:
That's good advice, an engine with a manual clutch may be a good idea so that I can disengage and be sure that there's definitely no torque during the shift. Thanks for the encouragement

@butre:
Yeah, you can put either a fixed sprocket or a BMX freewheel on it. That'll be handy if it doesn't work out, as I have the option to use it like a normal 3-speed hub (provided it hasn't broken!).

 

[KCvale]

@KCvale:
From my understanding, the Sturmey Archer S3X hub is a fixie hub WITH gears. It allows you to change the ratio while maintaining a fixed gear. Check out the start of this video, the guy demonstrates it:

https://www.youtube.com/watch?v=AJmUds2YQ74

View attachment 55141

I stand corrected.
Now that is a nice find with marginal potential!

Since all the gears are planetary with straight teeth it shouldn't care which way it turns, which is unlike anything else I've seen.
That must because it provides no mechanical advantage, 3rd gear is simple 1:1, second is a .75 reduction, and first is a .623 reduction which can be a good thing though I like the underdrive/overdrive aspect of regular shifters.

You may have to make a ~35T sprocket (do the math) but that should be pretty close to give you a low gear and a high gear compared to direct drive 44T.

That's all well and good if you don't have pedals.
A pull start 4-stroke with foot pegs could use that, put a freewheel on that sprocket to save the coast drag.

Pedals on the other hand are going to be a major issue.

How the heck are you going to mount a ~18-22T freewheel pedal sprocket here?

It's just the thin smooth outer shell of the hub.
I suppose you could fashion some sort of 'rag joint' mount to as small as sprocket as you can get and then your BB to freewheel pedals (like a jackshaft kit has) so your legs don't get beat to death, but with ~$150 for the none laced hub alone and $100 for a freewheel BB before even considering costs to 'make it go' this seems totally impractical for anything but a foot peg bike with it's own means to be started to me.

 

[Ollie]

@KCvale:
Yeah, I was really surprised that something like this existed! I imagine the highest gear being a direct drive will suit my design. Most of my time will probably be spent cruising in 3rd; I just need the two lower gears to start from a complete stop and to slowly climb hills (which I won't need to do that often).

I agree that it wouldn't be worth the effort for a regular motorized bicycle with pedals, but luckily the law here only relates to engine size and top speed (<50cc & <50km/h). If 1st gear is low enough to start me from a complete stop then I doubt the highest gear will get me past 30mph, but if it does I'll get a bigger drive sprocket and have even lower climbing/starting gears. This isn't meant to be a speed machine, just a handy (and legal) bike for getting around the city.

I was thinking of getting the largest sprocket available (18t I think), drilling holes in it and bolting on a BMX chainring as a larger sprocket. I'm not sure how I could attach a larger sprocket to a freewheel though, any ideas? And is it possible to get a BMX freewheel that runs in reverse?

 

[KCvale]

So you are going with a 49cc 4-stroke and foot pegs right?

Which hub did you get, the 120mm OLD or 130mm OLD?

You can get screw on freewheels with a flange and bolt holes for a sprocket.
SickBikeParts.com has some freewheels and big sprockets.
http://www.sickbikeparts.com/catalog/index.php?cPath=21&osCsid=gvfekamotnkf3n86e82sa5ehp6

It's doubtful that freewheel will fit your hub but you can check.
Your main problem is whatever your screw on there it is going to want to unscrew as soon as put power to it.
Best of luck on it and I look forward to seeing how you do it.

 

[Ollie]

@KCvale:
Yes, that's the plan. I haven't bought the hub yet, it's not available in Brazil. As it costs over $50, it will receive the ridiculous 100% import tax if I get it mailed, so I'll get my friend to bring one from England when he visits next year. What's the difference between 120mm OLD and 130mm OLD? Is that hub/axle width?

I think I'll go with the fixed sprocket, that way it won't want to unscrew itself. To save money, I'm considering using my Raleigh with a 2-stroke engine that I already have and just replacing the rear wheel. I'd have to push it along in 1st to start it, but that would reduce the budget significantly; all I'd need to pay for would be the hub and for someone to build it into a wheel here in Brazil. If it works out, I'd consider spending some money on a 4-stroke engine.

 

[KCvale]

@KCvale:
What's the difference between 120mm OLD and 130mm OLD? Is that hub/axle width?

Beats me. 130mm is 5.1". Outside L Diameter? Whatever L is.

I think I'll go with the fixed sprocket, that way it won't want to unscrew itself.

You couldn't run a 2-stroke with a freewheel anyway or you would have no way to start it.

It looks like you would always have problems starting it anyway without any pedals.
Go spend a day running your current 2-stroke bike around without using the pedals at all.