Ultra-lightweight motorcycle using fixed 3-speed hub?

Discussion in 'General Questions' started by Ollie, Dec 9, 2014.

  1. Ollie

    Ollie Member

    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?


  2. professor

    professor Active Member

    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.
  3. darwin

    darwin Well-Known Member

    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............................
  4. darwin

    darwin Well-Known Member

  5. Ollie

    Ollie Member

    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:

    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?
  6. KCvale

    KCvale Motorized Bicycle Vendor

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

    Ollie Member

    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.
  8. darwin

    darwin Well-Known Member

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

    Ollie Member

    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.
  10. KCvale

    KCvale Motorized Bicycle Vendor

    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.
  11. Ollie

    Ollie Member

    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:


    Screen shot 2014-12-12 at 15.26.59.png
  12. fattirejack

    fattirejack Member

    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.
  13. butre

    butre Well-Known Member

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

    Ollie Member

    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 :)

    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!).
  15. KCvale

    KCvale Motorized Bicycle Vendor

    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.
  16. Ollie

    Ollie Member

    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?
  17. KCvale

    KCvale Motorized Bicycle Vendor

    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.

    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.
  18. Ollie

    Ollie Member

    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.
  19. KCvale

    KCvale Motorized Bicycle Vendor

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

    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.