One limitation to the top speed potential of Honda Urban Express NU50 and Express SR NX50 is the final drive gearing, exacerbated by the small rear wheels. This addresses options to improve the final gear ratio.
The NU50 (and similarly the NX50) have a variated transmission which means there's a pair of pulleys and a belt driving the rear wheel.
Engine RPM goes up, the front pulley gets progressively tighter (=bigger front gear) and pulls the rear smaller. This means you continuously vary the gear ratio. And gear ratio * engine rpm gives your speed.
BUT! that variator maxxes out at some point. You can pin the bike at max RPM and not go any faster: you've got a set final gear ratio at max variation. You can't swap on a different sprocket to change final gearing the way you could on a chain drive. you're stuck. maxxed out.
SO... the question is: HOW DO I EXTEND THE GEARING TO HIGHER TOP SPEED
Increasing Top Speed
1st lets remember, you gotta have the power to push it. You can swap to higher gears on your bicycle but just because you can shift gearing to go faster for the same rpm, you're not gonna pedal up hill at 50mph...
OK so. 3 basic options:
- Belt - a wider belt (BX31 for NU50) is wider and longer. This makes it sit farther out on the pulleys and effectively gives you taller gearing (and maybe more range) the neat thing here is you still have the low range with the variator. The gains are entirely geometric, as long as you have enough power to push it, it will give you more top speed. I don't know the max radius differences or what that translates to as final gear ratio but people have claimed 5mph gains with belt alone. Rad.
- Engine - if you're limited by max gearing (and especially if limited by max power) you can get a better cylinder and exhaust. While this doesn't affect your gearing you can extend the engine to higher RPM, and high rpm for same gearing = higher top speed.
- Bigger rear wheel - the rear wheel is obviously part of the final gearing equation. You can't change gears in the transmission, but you CAN change "gearing" effectively w/ wheel size.
Unfortunately I think a 16" wheel's the biggest that'll fit... Fortunately many 16" rims exist, including from the NU50 front wheels. You need to lace up the larger rim to your stock hub.
The wheel size on mopeds is the diameter of the rim, figure in a 2" tire and the original 14" wheel has a 18" total diameter for a 57" circumference. That means every rotation of the wheel you go 57 inches. Swap on a 16" wheel (20" diam w tire) you've got a 63" circumference. So for same final RPM (after maxxed transmission and engine max RPM) you're going roughly 10% farther. That means your 30mph bike's now gonna go about 33 for the same RPM and transmission. If you're hitting 45 already with some higher rpm engine build and a better belt, that rear wheel change will push you up to almost 50.
Make sense?! hope so!