It's late and I need answers (about choosing NuVinci freewheel)

Discussion in 'Motorized Bicycle General Discussion' started by highlander9, Feb 5, 2011.

  1. highlander9

    highlander9 New Member

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    Here's how it looks --> 49cc HS 4 stroke + SBP shift kit + NuVinci hub

    I have been searching for the right freewheel to put on the NuVinci hub. I have called various places and read different threads, and they seem to be agreeing on using an 18 Teeth freewheel for such an application as mine.

    However, I am feeling dubious about the 18 teeth freewheel. The threads that I've read involving Nuvinci and shift kits have all been relative to the power specs of the 66cc skyhawk engine, which have different specs for the shift kit gears as well.

    My 4 stroke shift kit descriptions (using 18 teeth freewheel)
    • The shift kit for the 4 stroke has a 5:1 (gearbox),
    • 1.7:1 (gearbox output to jackshaft input),
    • 5.33:1 (jackshaft output to shift kit drive sprocket), .
    • 0.6:1 (shift kit driven sprocket to final driven freewheel sprocket),
    • and from thereon the NuVinci takes care of the rest with its 350% gear range, infinitely variable.
    The 2 stroke skyhawk 66cc shift kit descriptions (using 18 teeth freewheel)
    • The shift kit for the 2 stroke (66cc) has an unknown gear reduction in the engine shaft,
    • 1.7:1 (gearbox output to jackshaft input),
    • 4.4:1(jackshaft output to shift kit drive sprocket)
    • 0.5:1 (shift kit driven sprocket to final driven freewheel sprocket)
    • and from thereon the NuVinci takes care of the rest with its 350% gear range, infinitely variable.

    So what I'm trying to do is make sense of the information that's available about the 2 stroke nuvinci shift kit, and apply it to getting a answer to the problem of how many teeth freewheel I would need for my 4 stroke nuvinci shift kit.

    On a purely intuitive basis, I feel a 19 teeth or a 21 teeth would be more appropriate based on the two reports made by the gentlemen that own such Nuvinci usng SBP shift kits, albeit for their 2-strokes -- which doesn't say so much for my 4-stroke.

    The two motorbicycling.com members with the 66cc skyhawk Nuvinci shift kits have reported that with their current setups (username "Clotho" uses an 18t on his and username "Cannonfish" uses a 16t on his) they are unable to bring the NuVinci hub to the highest gear before it eventually would bog.

    Clotho says in Clotho's skyhawk nuvinci:
    My thought = Wouldn't he have been able to utilize all the available power from the NuVinci hub had he just used a larger freewheel, such as a 19 teeth one?

    and
    Cannonfish says in Cannonfish's skyhawk nuvinci:
    My thoughts: Same as before...

    Kinda tricky isn't it? Keep in mind, the 4 stroke is more "torquey" 2.0Nm/4500rpm, while the 2 stroke has more power at lower rpm (2.5hp/5000rpm) vs 4 stroke's 2hp/6800rpm.

    IDK, what's everybody else's take on this matter?

    Ok, thanks.
     
    #1 highlander9, Feb 5, 2011
    Last edited: Feb 5, 2011
  2. scotto-

    scotto- Custom 4-Stroke Bike Builder

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    It's not that big of a deal. Put a twenty tooth freewheel on to begin with and take it from there. It's not very hard to swap it out for a 22t or 18t if need be. Just be the guinea pig and let us know how it goes. Ain't rocket science.....just simple math and trial-n-error.
     
  3. highlander9

    highlander9 New Member

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    What's the initial gear reduction on a 66cc skyhawk engine does anybody know?? I believe there is a pretty big reduction at the engine shaft.
     
  4. Skarrd

    Skarrd Member

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    #4 Skarrd, Feb 5, 2011
    Last edited: Feb 5, 2011
  5. highlander9

    highlander9 New Member

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    I don't know if my calculations are correct, but I got 13.46:1 all the way to the hub for the Skyhawk and a 27.74:1 all the way to the hub for the 4 stroke. HMMM, continuing to thinking here. Because that's is quite a difference as far as the reduction goes.
     

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