Gearbox a problem?

evmatt · #1 05-06-2009, 02:00 AM

I just bought a 4 stroke kit from Kings Motor bikes and was lucky enough to end up with the "hoot" gearbox. I didn't have a chance to test the engine or gearbox yet. As I expected this kit needed a lot of custom fabrication...definately not a few hour bolt on installation.

It seems like the gearbox for the 4 stroke engines have quite the reputation on here. Where's the simple and reliable go kart clutch solution? I've seen 5/8" centrifugal go kart clutches on ebay for $30. Why can you not bolt this onto the engine and run a chain to the rear sprocket directly from the clutch?

Ghost0 · #2 05-06-2009, 10:16 AM

It is because a kart uses an eight inch diameter tire and we use 26 inch diameter tires. For the kart clutch to work you would need a rear sprocket about 20 inches in diameter to get the gearing right. That is the only purpose of the gear box for the 4 stroke is to get the drive ratio correct.

evmatt · #3 05-06-2009, 10:29 AM

I understand now so it would be entirely too fast?

HoughMade · #4 05-06-2009, 10:57 AM

You would never get it up to a speed the engine could comfortable operate at and there not nearly enough power to have any sustainable speed at that rpm level.....which is why they invented the gearbox. As problematic as some may be, the reduction is necessary.

Altering the gear ratio on a Chevette will not make it do 200 mph.

Cabinfever1977 · #5 05-06-2009, 01:51 PM

some use a belt and plastic rim thing about 20"s or bigger,
like a whizzer,then you wouldnt need a gearbox.

frictiondrive · #6 05-06-2009, 09:17 PM

From my experience a usable reduction ratio (at least to start with) is 6 to 1. This applies to both electric and ICE powered bicycles, mini bikes and go karts. 40 years ago, a single speed mini bike would have a 10 or 12 tooth clutch sprocket and a 60 or 72 tooth rear sprocket. For some reason, this works out very well with bicycles. If you go friction drive, you want a 1" Ø roller for average conditions, smaller for hills and no more than 1-1/4" Ø for high speed use. Just my $.02 worth...

Cris

Ghost0 · #7 05-07-2009, 12:16 AM

6 to 1 is a good start and about where most gear boxes are, however after that there is another 4 or 5 to 1 reduction to compensate for the 26 inch diameter wheel.

evmatt · #8 05-07-2009, 06:03 PM

Thanks for the answers. Looking at my 4 stroke kit I also noticed that a spinning clutch would be dangerous next to the riders leg without the gearbox housing, also the chain wouldnt line up because the gearbox sprocket has to be offset.