Dong Fang DF 250 RTB Bobber Information

[ProDigit]

you not will find the drivability as simple a comparison as that but if considering only sub 60 mhp indicated perhaps it's close enough for you.

first consider if investing in a front sprocket, rear sproket and new chain is really what you want to do cuz you will not be able to fit the 520 around a larger front.
if your like me and look to budget my bike mods , then you will find the Jtr279.30 tooth on amazon for only 17.50$ shipped to your door. that will give you a 30/13 ratio, the tallest possible with least possible work.
barone has and likes it. even in the depths of the Appalachians.

that said with my ratio, as I stated near 7k max rpm I best get about ~62 indicated in 5th. roughly about 50 in 4th, 38 in 3rd, 27 2nd, 18 1st.

not sure what that will tell you, you can do the same math using your current ratio and speeds

Thank you!
For sure, the 13T in the front is WAY too small!
Don't know if it's possible on the DongFang, but I've seen bikes that only needed a smile lid to be filed off for a larger sprocket to fit.

For comparison, I have a current bike, 127cc, running at 55MPH sustained, and 60MPH tops. I'm sure you can still optimize something to go faster on that bike, as 60MPH seems too low IMHO for a 200+cc bike!
Most 200cc scooters with CVT go 70MPH. That's a 10% loss that the CVT usually brings with itself.

The 127 I have is a MC-05-127 cc carbureted manually geared bike, comes stock with a 15/41 setup, and way too small gearing. I changed it to a 15/31T (too tall), but get 115MPG out of it, and it goes 50MPH easily all day long!
Recommended on that bike would be 15/34T setup, for better torque than 15/31, and would still get 105MPG with that sprocket.
I've tested out that bike with a couple of sprockets, and just like the DF, this 127cc engine is a Big Bore 110cc engine, with peak torque in the 6,5k RPM range (though I don't know if it's a push rod engine).
Changing sprockets from 41 to 31T (testing 36 and 34T sprockets too) does not increase top speed, but does lower RPM (and torque too).

Changing downto 34T or 31T does make top speed more susceptible to winds, while with 41T I'm running at max revs, and with a 36T I guess I'm in the torque band, and it sustains speeds much easier, even with heavy wind gusts;
When tailgating a truck, top speed could possibly exceed 60MPH (GPS verified), with a 34T sprocket.


Since I use that bike mainly in the suburbs, with 35-45MPH roads, (50MPH max speed on 45MPH roads here), I geared it really high with that 31T rear sprocket; as the bike is not good enough to go on highways or interstate (though I've done it before).
I don't mind going on the interstate with a bike that can maintain a sustained 70MPH, but 62MPH seems off...

I'm not getting identically the same bike. I'm getting the naked bike, RTC-B which is more aerodynamic.
They say the RTC and RTB use the same engine.
The RTB is the Bobber,
The RTC is the racing bike
The RTC-B is the naked bike, new for 2013 I believe.

But from what I'm hearing, that 13T sprocket is WAY too small!
And a top speed of 62MPH on the 229cc engine is VERY disappointing! Something must be wrong on your readouts!

I can't believe I would add 100cc, and still go just as fast as on my 127cc; although the 127cc is jetted a bit too lean, but it does have a manual choke on the handlebars. That manual choke can be opened slightly, to tune the a/f ratio (as the bike is jetted a tad too lean, opening the choke will cut a bit of airflow, and makes it run less lean), and when doing so, the bike increases in performance.

Strange but true, sometimes cutting airflow with the choke (by 10-25%) improves performance.
Sometimes increasing a jet size from say 100 to 105 might not improve performance at all; if the case is that the perfect jet size would be 102 or 103. I notice on my choke that the bike is very sensitive to that 'perfect' af setting. When you hit that perfect setting just right, the bike has noticeably a few HP extra from jetted too lean or too rich. Sometimes one millimeter off on the choke lever can mean 1-3 MPH slower top speed. And that perfect setting always changes, depending on the engine heat, fuel type, and throttle position.

A bike that's jetted too lean, can be compensated with a choke (if it's a manual, non step, choke, meaning where one can slide on or off the choke, and it does not have steps, but like a throttle can be slid open to any position).
If the bike is jetted too rich, there's nothing you can do with the choke to compensate. All you can do, other than jet it leaner, is play with the AF screw to let it run a bit leaner at idle, so that at WOT it might run a bit leaner too. But the AF screw only impacts the fuel mixture in a minimal way; and it could give problems idling on a cold day when you do.


I'm also very skinny (160LBS), and top speed (60MPH) is only gotten when sitting ducked forward, or tailgating a truck. When sitting upright, my 127cc only goes 55MPH.