Dong Fang DF 250 RTB Bobber Information

[ProDigit]

aye I run 17 36 ? or a custom 35 I don't recall , with a 428 HD chain , but my back tires also about 5" shorter so needs to spin faster and increases my torq , for flatland I find it ideal

so your fella's gearing shouldn't be quite that tall or u never be able to use 5th gear

Yeah, I guess the limiting factor in using 5th gear would be, how many MPH does your bike do at 2,5k RPM?
Ideally you will want to run between 2,5 and 3k RPM. With a 100:1 RPM/MPH ratio, you'll still be running only 25MPH to 30MPH, which is ridiculously slow for a final gear; but then again it's a small engine!
If you run it below 2-2,5k RPM the engine might start lugging, but at 2,5k to 3k RPM even my 127cc can pull the weight.

At 25MPH in final gear, you might as well depart from 2nd gear, and immediately shift in 5th gear, I mean...

Most roads in flat florida don't require torque, and I'm doing just about the same gear ratio on a 127 motorcycle, and a 150cc scooter.
The 127 is a bit on the slow side with these gearings, understandable, the engine is very small too.

Most roads in flat florida also allow you to ride between 35MPH and 40MPH. If you're cruising, you will want to do that in 5th gear. With that 100:1 ratio, that'll mean your RPMs will run 3500-4000 RPM, for just cruising at 35-40MPH.
Ideally you will want to reduce those RPMs when cruising, and upshift into 4th to reach top speed.

I know the math is a bit simplified, but it comes very close to the real thing.
If fast acceleration is not your thing, but comfort riding, and economy (good MPG) then you will want your engine to run at ~3k RPM when you're doing 35MPH.

The engine won't have no problem pulling that off, since there's not a lot of wind resistance below 40MPH, so not a lot of torque is needed to cruise around.

I know I'm castrating a perfectly great accelerating bike into something that's barely puffing along, but that's my aim, to maximize my fuel efficiency on bikes like these; and in the process extending engine life as well!

If you're into high revving engines, fast acceleration, this might not be your kind of pie. But to each his own!


Another thing, getting gas mileage on the highway of 80+MPG is extremely hard!
Those 100+MPG numbers you basically only get when taking it easy, going 35-40MPH, without too many stops.

Any slower, and you'll have to downshift, causing lower MPG; any faster and the wind drag increases so much that MPG automatically suffers from it.

On my 127 I get ~96MPG @ WOT 60MPH, ~104MPG doing 50MPH, and 118MPG doing 35-40MPH.
I guess if you're eco-cruising, it makes most sense to stay between 35 and 45MPH.