GX35 Reduction Ratio?

Discussion in '4-Stroke Engines' started by Tressie, Aug 21, 2013.

  1. Tressie

    Tressie Member

    What's a practical gear reduction ratio for a GX35 driving a 26" mountain bike?

    With the power cranked down to 200W for local regulatory conditions, I'm limited to around 25km/h top speed.

    I bought a kit with a 20:1 drive ratio, but it arrived with sprockets for 25:1 and I'm pretty sure that's not going to help.

    It's just that 5000 RPM at 25km/h seems excessive to me, and I think I'd be happier drawing the 200W at 4200 RPM like the setup I paid for.

    Any clues as to what I should be aiming at?

    Tressie :ack2:
     

  2. LR Jerry

    LR Jerry Well-Known Member

    Tressie I live in the mountains I have a 5 speed automatic setup. The reduction ratio is 45.83:1 to 21.56:1 I weigh 250 lbs. I use a Robin Subaru EHO 35 engine rated at 1.6 HP 8000 max rpm. The Honda GXH 35 is rated at 1.3 HP 7000 max rpm. 200 watts is between a 1/4 to 1/3 HP. With power that low what you have is an assist system instead of a propulsion system.

    If you are able to use your engine full power 25:1 could work quite nice for you. This is still dependent on your body weight and terrain.
     
  3. Tressie

    Tressie Member

    Hi LRJ,

    Oh, maybe I should give the 25:1 kit a go first. I guess I still have the GX25 centrifugal clutch to think about, and I'm sure I won't want that slipping anywhere above 20km/h (3900 RPM at 25:1, 3200RPM at 20:1). I guess I have a fairly large cross sectional area, and a pannier won't help with air drag much either. Okay, I'll give the 25:1 setup a shot and see how it goes.

    Thanks for the advice,
    Tressie :)
     
  4. LR Jerry

    LR Jerry Well-Known Member

    You must have a low rpm centrifugal clutch. My centrifugal clutch engages at 3500 rpm. 200 watts is just barely over 1/4 hp its too bad you can't use the full 1.3 hp your engine provides.
     
  5. Tressie

    Tressie Member

    Hi LRJ,

    Well it's just the 2-shoe clutch that comes bolted on the engine that I plan to start out with, and I have no clue where it kicks in, but 3500 RPM sounds like something I could use.

    Golly, do I have a 1.3 hp engine? Oh look, you're right. The GX35's 1000W is about 1.34 hp. Well, 200W was plenty for me on my first 'Happy Times' equipped bike. Except when one would encounter even the slightest gradient, that is. I can live with having to add a bit of extra pedal power, it's just all the 2-stroke noise and expensive lubricant that I'm hoping to avoid this time.

    Tressie :)
     
  6. LR Jerry

    LR Jerry Well-Known Member

    [video]http://m.youtube.com/watch?v=pGDGIIdn--s&desktop_uri=%2Fwatch%3Fv%3DpGDGIIdn--s[/video]

    Here is my bike.
     
  7. Tressie

    Tressie Member

    Hi LRJ,

    Gee, I'm absolutely intrigued by what you've accomplished there.

    How ever does your setup do the work of changing gears automatically? Some sort of microprocessor controlled servo arrangement, maybe? I bet the primary reduction gearbox has quite a high ratio of input turns to output turns.

    Golly, that's the only drive system of it's type that I've seen, and it really does give one food for thought. Nice work!

    Tressie. :)
     
  8. LR Jerry

    LR Jerry Well-Known Member

    The gearbox is Staton Inc 18.75:1. The way the bike shifts gears automatically is with an autoshift derailleur. IMAG0001.jpg IMAG0003.jpg IMAG0002.jpg
     
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