Steeper Gearing

The little green Honda Express NC50 will do about 50 MPH but to do this the engine has to spin about 13K RPM, which is about double the stock configuration RPM. I've also gained a couple MPH by going to a bigger tire, from a 14x2.25 to a 14x2.50. The 14x2.50 tire is about the limit, because cases protrude in front of the tire. The options at this point for more speed is either to continue to spin the motor faster or modify the internal gearing.

Inside the transmission case, from the crankshaft there is a 14 tooth sprocket on the clutch that drives a chain to a 30 tooth counter shaft sprocket. Looking at the counter shaft sprocket it seems to have enough metal to tolerate cutting all the teeth off and making a smaller sprocket.

I only have a small mini-lathe to cut a sprocket, but it looks like it is possible with patience if I buy a rotary table for my lathe.

There are a bunch of different sites that describe how sprockets are made and the geometry of sprockets. One site I liked a lot, since I could easily understand the geometries:

http://www.gizmology.net/sprockets.htm

Measuring the internal chain:

• Roller Width measures 4.75mm
• Roller Diameter measures 4.97mm, probably meant to be 5mm
• Roller Pitch varies from link to link, but the outside to roller to outside of the roller looks like 13.47mm. The actual pitch would be the outside roller to roller measurement minus 1/2 the roller diameter on one end and minus 1/2 the roller on the other end, or 13.47 - 4.97 = 8.5mm pitch.

The NC50 Parts Manual lists the chain as Part Number 23100-147-003, Chain Drive (DID270H-79L). 

Looking up that chain, DID270H:

http://www.did-coltd.com/english/products/pdf/s_102-103.pdf

• Roller Width is 4.75mm
• Roller Diameter is 5mm
• Roller Pitch is 8.5mm

From Ebay I was able to order a handful of "GENUINE DID 270H OIL PUMP DRIVE CAM CHAIN CONNECTING PL LINK RIVET". 

From Amazon and Ebay, it looks like there are a number of small chain rivet tools. I am not sure yet which to buy, or what tool is a good one.

If I cut all the teeth off the counter shaft, it looks like I have about a 75mm diameter blank to work with.

From the gizmology site:

Outside Diameter = P × (0.6 + cot ( 180° ÷ N) )

Original counter shaft sprocket has 30 teeth.

Outside Diameter = 8.5 * (0.6 + cot( 180 / 30 )) = 85.9720978609

Measuring the sprocket, I get about 84.37mm

If I do 26 teeth, that gives me:

Outside Diameter = 8.5 * (0.6 + cot(180 / 26)) = 75.1037981132

It looks like I will attempt 26 teeth.

The overall gearing ratio is given in the manual as 14.22 to 1. If we look at the stock gears, the primary sprocket on the clutch has 14 teeth:

The counter shaft has 30 teeth on its sprocket and 11 teeth on its gear:

And the output shaft has 73 teeth on its gear:

This gives a gear reduction of (14 / 30) * (11 / 73) = 1 / 14.22 or one revolution of the tire for every 14.22 revolutions of the motor.

If we cut the countershaft gear down to a 26 tooth sprocket the gear reduction would be

(26 / 14) * (73 / 11) = 1 / 12.3246753247

With a 2.5 inch wide, 14 inch tire, and assuming that I have enough horsepower to still pull to 13K RPM at 52 MPH with the steeper gear ratio, this would give a top speed of

52 * 14.22 / 12.32 = 60 MPH

Having enough HP to still pull 13K RPM is a pretty big "if". The HP is falling off pretty seriously at that RPM and friction, aerodynamic drag, effects of the oil being thrown around in the crankcase due to the increase in output shaft gear speed are increasing. Also, I am assuming the current engine will pull to 52 MPH. I know with the 2.25 inch wide tire, the NC50 would GPS on flat ground at 50.5 MPH, but I haven't re-GPS'd it with the larger tire.