Quote:
Originally Posted by ProDigit
We now want to combine high top speed, with great MPG.
We can do this, by creating an extra overdrive; and by making our second to last gear, the same ratio as our last gear.
In the example above, our Honda Rebel's 4th gear would need to have the same speed per RPM as our modified 5th gear.
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This is pretty much what I did with my bike Metric (VT250C).
The only power/torque curves for this model on the internet are from Honda's factbook site (in Japanese). Due to it's small size and lack of labelling the curve only gave a rough idea of how far I could change the ratios.
As a result, I relied on personal observations during my commute. The bike has no tacho, so it was "seat of the pants" estimations.
The bike would take the whole climb through the mountains in top gear, except for one long steep hill (Lapstone hill) where 5th was
just too tall.
I used this hill as my 4th gear target.
Using the gear ratio list in the owner's manual, I calculated that changing from 14/38 to 15/36 gave me a 4th gear that was just a bit lower than the original 5th. The sprockets had to be ordered from Japan.
Trying 15/38 first, the bike performed well and I saw an MPG improvement.
Going to 15/36 was even better on the motorway at 110kph and climbing Lapstone hill in 4th is fine, but 5th is now too tall for some of the other hills through the mountains, so I find myself using 4th more than I expected at first.
I'm happy with the new gearing, even though I'm not getting as much improvement in MPG as I'd hoped.
My next step would be aerodynamic improvements to take better advantage of the tall gearing.
