The straight facts on the Dong Fang Bobber

Hi, Ive been riding motor cycles since I was about 12, Minibikes from age 7, I'm 50 now. First bike was a Honda SL 100 enduro," Dual sport" to the younger crowd. And Ive had some damn fast stuff too through the years. Anyway, point being I know a little about bikes and what they are made of. This little bobber (Dong Fang 250 RTB) has a OHV 229cc 5 speed ATV engine. The reason I say ATV engine is it is very close ratio, geared way to low for a street unitized powerplant. After some math and seat of the pants figuring I pulled those useless stock sprockets off (must change final ratios) and settled on a 15 tooth front sprocket, and 38 tooth rear. A 16 tooth front sprocket made the gearing a little to tall. If I weighed 100 lbs I would have kept the 16 tooth. But being 250 and 6'3 I needed the 15 tooth up front instead. You will have to modify the front sprocket cover to accommodate the larger than stock front sprocket. I modded mine with a right angle grinder and a steady hand. Then as said put a 38 tooth out back. My chain did not need the length changed. That was a pleasant surprise. Now the Bobber can be used on a country 2 lane highway without eccessive revs or getting ran over because you can't go above 45 mph. Now, next important issue for most people is power. The Dong Fang 250 bobber has a 229cc OHV design. With the little stock carb that it comes with my seat of the pants Dyno says its only putting out 12 HP, 14 HP tops. That's because of the ancient OHV (over head valve) design engine. Should have been OHC (over head cam). Much better HP per cc. But in these engines the OHV is more durable than the racier OHC design. That's probably why you see 90 million motorcycles with these same exact powerplants in India and other parts of Asia and lets not forget Central & South America. Now What is good about this machine. Once broke in and necessary mods done, This thing rides rock solid and unshakable. It will inspire confidence in corners as well as going straight down the road. I very seldom slow up much before entering a corner it corners so good. IMO because of the flat track racer layout of the frame. Only a real Dumb Dumb can wreck this bike IMHO:icon10: Next, The brakes are great too. The fairly light weight and big tires for the bikes size make the single disc front and rear practically throw you over the handlebars (stops on a dime). Now for the hard numbers after the necessary sprocket changes. It will cruise about 55 mph all day long. I can reach 70 mph (without headwind) with my big tall butt on it. A smaller person would fare even better on this bike. The acceleration is acceptable also with this gearing. Good strong pull for its size. Now for the last number someone may be concerned with that would buy a small machine such as this. With this gearing it gets 80 mpg at 48-52 mph in rolling hills. Mileage checked with two other vehicles. So this is confirmed, yes 80 MPG ! This is why I won't put a bigger carb or rejet it now :clap: Oh, And last but not least I can ride with a buddy with a $30,000 Harley or whatever and the Chinese Dong Fang Bobber gets all the attention at gatherings. Go Figure , But I eat it up ! :mwink: I know this was a long post but I hope I may have answered some questions about this bike that there really is not that much meaningful info on. Feedback or alternate opinions are certainly welcome. Cheers

Thanks for the info and input; 'barnone's' thread has everything and anything one wanted to know about these, and Iwas pleasantly surprised to not only see him post, but that he is getting a new DF!

Yea, But do you really want to look through 3000 pages ! LOL I wrote what was absolutely necessary before the bike is even a motorcycle to be ridden on the road in my book. But after you do what needs to be done the bike is worth buying again as you have said.

Which bobber did you get? Is it the 2010 original model, or one of the newer ones with the peanut tanks? I just finished reading the whole thread on these. Sounds like your gearing is about spot on on your bike. I don't have this bike but am thin king it over. I'm also considering one of the Skyteam 125cc bikes like the CT-70 clone, the PBR, or even the big balloon tired Trex.

If I were you I would go for this bike ( Bobber) . I have a vintage CB125 Honda and it only gets probably 5 more miles per gallon. And the 229 cc motor in the Bobber has more balls being bigger. Unless one of the bikes you mentioned has dual purpose capabilities and you need that. I would still go for this bike because it would probably cost about the same as any of the others you spoke of and Has Got to Be Twice as Cool ! :taz: And handles twice as good as a standard style motorcycle (the DF 250 China girl not the CCW Heist) . The fork rake is chopperish on the Heist so I don't know if that would handle as well AFAIK. But don't get this bike (DF 250) if you have crummy roads all over your area. This bike has a true hard tail but still rides supprisingly smooth with lower than normal pressure in the back tire. And lastly, Mine must be the old one because it has the weird custom looking tank. If I wanted to put a peanut tank I would. I have an unused early 70s Harley peanut tank out in the shed. Still has the Harley logo on it. :naughty:

Do you have any pictures of you on the bike? thinking about getting one, but at 6"5 not sure how I would feel on it. I ride my dads 1991 sportster all the time, and its a slighty small bike, but im super comfortable on it. Already have plans for one of these if I get it (drag bars, rebuild the rear so it sits lower, and drop the seat) . Im planning on the newer one with the springer and peanut tank. Any pictures would be helpful, thanks.

Yea Bowman, I have the exhaust taken off right now putting a baffle in it. I am done with it so I will put it on bike tomorrow. I will have my girlfriend shoot a pic of me riding it and I'll post it for you. I may look a tad gawky on it but It doesn't feel cramped riding it like some small bikes. I hate that. I have a 75 CB 125 I have to get rid of because of that.

Okay cool, thanks alot. Did you have the same issue most people on here have had with getting the bill of sale and mso or whatever its called? Started reading barnones thread, but there were way to many pages to stay interested. Lol

Not really, I didn't have any trouble ? The process took a little longer than I thought it would. But I just did what they told me and it all worked out. Probably took a couple weeks after I received bike before I could title it here in FL. . Just a little scary waiting to get the stuff to make the bike legal . Everybody knows ,When you can't make a bike legal all you have is an expensive boat anchor

Thanks for providing us with the hard evidence numbers.
How does acceleration from a dead stop go with a 16/38T setup?

I am a bit disappointed in the low MPG, I had hoped to be able to get more out of it.

My 127cc gets almost 120MPG riding at 40MPH in final gear.
I knew the stock D250-RTX (RTB and RTC) are geared too short, so I was planning on getting a new sprocket on it anyway.

Are 16T front sprockets easy to find?
Are there any 17T front sprockets out that would fit?

What's the lowest rear sprocket that would fit?
You could measure it on average, with a rope, from the front sprocket to somewhere in the middle of the rear sprocket.

Here's an example how:

http://img32.imageshack.us/img32/5238/szv6.png

By measuring the radius of the sprocket, compare that to the teeth, and then measuring the smallest radius possible using the rope, you can apply r/R * T;
Or, in above example:
R=4in (Radius of stock sprocket)
T=40T (tooth of stock sprocket)
r=3in (Radius measured of smallest possible sprocket)
t= ?T (Tooth count of smallest sprocket)

t=r/R*T = 3/4*40 = 30T

In the above example, the smallest sprocket would be 30T.

Should t (smaller sprocket tooth count) be calculated to 29,9 or 29,1, then the smallest sprocket would be a 30T sprocket (rounding up).
Should you increase front sprocket, it may be possible to decrease rear sprocket even more.

As an example, let's assume a 17T front sprocket would fit with some grinding, and that because of the increase in the front, the forks will no longer pose an issue with the chain, no chain gliders necessary anymore, and you could reduce the rear sprocket to 30T; then you could calculate MPG in top gear pretty easily.
You currently have a 15/38T setup, and get 80MPG riding 40,
Going to a 17/30T setup in final gear the fmla will be roughly like this:

80MPG * 17T / 15T * 38T / 30T = ~115MPG; if the final gear does not rev the engine too low at 40MPH.

115MPG means much too low torque. Lucky you got 5 gears, not 4, so you probably need to rev the engine in high revs to accelerate normally with traffic, but once in 5th gear 115MPG is attainable

You could also calculate avg speed increase per RPM like this.
Eg: If you're riding 40MPH with a 15/38T setup, you'd be riding 40*17/15*38/30 MPH with a 17/30T setup, or 57.5MPH when cruising at the same RPM.

You could also calculate RPM drop, if you're going 3k RPM at a certain speed, you'd be doing 3000*15/17*30/38 RPM, or from 3k RPM to ~2090 RPM at the same speed.

I would like some info on how high RPMs are riding 40MPH with your current sprocket setup?
Also what the largest front sprocket would be and smallest rear, and where you ordered the 16T sprocket?

Well, this is an old thread now. But I thought I might come back and update it about a year after I started the thread. Turns out the Bobber must have completed the brake in process at about 800-900 miles on the OD. It only has 1600 miles on it now. The reason I'm stating this is because the Bobber motor started running a good bit more powerful at about that point. It is much more usable now with its new found power . So I decided to up the gearing (sprockets) and go one number drill bigger on the main jet than stock. The bike ran even better with the jet drilled. Now, with my modified front sprocket cover I'm going to try to stuff a 17 tooth 520 pitch sprocket in there. I like the big chain (520). Its been hard but I've managed to find all the gearing changes I've experimented with in 520 pitch sprockets. I should be able to cruise at 65 mph all day. And go an honest 70 + top end.