View Single Post
Old 02-05-2011, 12:28 AM
highlander9's Avatar
highlander9 highlander9 is offline
Motorized Bicycle Senior Member
Join Date: Oct 2010
Location: Illinois
Posts: 74
Post It's late and I need answers (about choosing NuVinci freewheel)

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 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:
Low is like that. Its about as low as you can go while still being useful. This is a very good thing. It makes starting the bike very easy. It also makes hill climbing a breeze which is good because where I live is surrounded by some rather intense hills. San Francisco style intense.

The high end seems to be about perfect as well. On flat terrain the engine just starts to bog down when at top speed if I shift the hub all the way to the top end. Back it off just a hair and the motor is fine so I know it would benefit from a little more power.
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?

Cannonfish says in Cannonfish's skyhawk nuvinci:
Svc, the range of gear ratios on the Nuvinci is best visualized here: In practice, I can accelerate from a stop without pedalling, and with a basically unmodified engine, I top out on flat ground at somewhere between 32 and 35 mph (according to my speedometer). There's still plenty of gear-range above what my engine can push currently - if I get brave and attempt some porting/polishing soon then I suspect I could get it faster.
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.

Last edited by highlander9; 02-05-2011 at 12:54 AM.
Reply With Quote