Dong Fang DF 250 RTB Bobber Information

[barnone]

I wonder if a larger 28mm to 30mm would gain any increase in power on top end?

Are you talking about the DF carb? It is a Deni PZ30 so is already 30 mm.

[axa]

I have a new filter that allows me to reach 7k RPM without the sputtering and cutting out. Attachment 13361

http://www.ebay.com/itm/181180637913...84.m1439.l2649

hm, easy to wash, reasonably priced. though that material looks like its the type that will just crumble after a few weeks... (or after the first wash) let us know

[ProDigit]

I just wanted to see how much RPM you get per MPH, and how that works on your sprocket setup.

Meaning a 229cc engine, doing 7k RPM doing 70MPH, with a 15/41 setup could easily be lowered to a 35T sprocket, doing 6k RPM instead. Lower RPM, means better fuel economy.

Some people happily trade torque in for fuel economy, to gain engine life.

If my top speed would not exceed 60MPH anyway, I could calculate a good sprocket change.

For instance, If I would change the 15T front sprocket to a 17T, and the 41T to a 31T, I would be able to lower RPM from 7k RPM @70MPH, to 4k RPM @60MPH; which makes this bike quite the ECO bike!
If MPG was 75MPG before, now with the sprocket changes, it should be ~112MPG. This makes easy riders like me, extra happy, knowing they can go cross state with one tank of fuel!

The gears will be very tall, but the torque/acceleration not necessarily less.
Instead of 5th gear, you'll be running the same speed and RPM in 4th gear.

Acceleration will just be less at initial departure (the point where the clutch needs to grip is a bit harder to achieve), and final gear is basically a cruising gear, or an overdrive gear; where there will be little torque (no one is preventing you from upshifting one or two gears, and WOT the cr@p out of that bike, to overtake traffic).

I'm trying to figure out what setup you're running, so I could determine what sprockets to install...

MPG and CC's is directly related to torque. At 110+MPG, final gear should have very low torque, but if it's just to cruise around, high torque is not necessary.
In lower gears, even with the insane eco-sprocket change, the acceleration should be somewhat similar to a short geared bike.
Perhaps even better, since I'd have to shift less, thus less time lost in shifting.

[axa]

I just wanted to see how much RPM you get per MPH, and how that works on your sprocket setup.

Meaning a 229cc engine, doing 7k RPM doing 70MPH, with a 15/41 setup could easily be lowered to a 35T sprocket, doing 6k RPM instead. Lower RPM, means better fuel economy.

Some people happily trade torque in for fuel economy, to gain engine life.

If my top speed would not exceed 60MPH anyway, I could calculate a good sprocket change.

For instance, If I would change the 15T front sprocket to a 17T, and the 41T to a 31T, I would be able to lower RPM from 7k RPM @70MPH, to 4k RPM @60MPH; which makes this bike quite the ECO bike!
If MPG was 75MPG before, now with the sprocket changes, it should be ~112MPG. This makes easy riders like me, extra happy, knowing they can go cross state with one tank of fuel!

The gears will be very tall, but the torque/acceleration not necessarily less.
Instead of 5th gear, you'll be running the same speed and RPM in 4th gear.

Acceleration will just be less at initial departure (the point where the clutch needs to grip is a bit harder to achieve), and final gear is basically a cruising gear, or an overdrive gear; where there will be little torque (no one is preventing you from upshifting one or two gears, and WOT the cr@p out of that bike, to overtake traffic).

I'm trying to figure out what setup you're running, so I could determine what sprockets to install...

MPG and CC's is directly related to torque. At 110+MPG, final gear should have very low torque, but if it's just to cruise around, high torque is not necessary.
In lower gears, even with the insane eco-sprocket change, the acceleration should be somewhat similar to a short geared bike.
Perhaps even better, since I'd have to shift less, thus less time lost in shifting.

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

[ben2go]

Are you talking about the DF carb? It is a Deni PZ30 so is already 30 mm.

Yes.I was thinking it was a 26mm.

[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.

[barnone]

Thank you!
For sure, the 13T in the front is WAY too small!

I don't know how you can say the size of the 13 tooth front sprocket is way too small when the size of the front sprocket by it self means nothing. It is the combination of the two sprockets that matters.
I'm running a 13/30 right now on my 2012 DF with a top speed of about 65 MPH indicated at about 7k RPM now that I have an air filter that flows enough air to stop the cutting out. BTW, I am sitting upright and not tucked under.

Maybe you should start a different thread for the DF250RTC-B which looks like a neat bike. Also send an email to the seller to correct the 250cc to 229cc in their ad.

[barnone]

hm, easy to wash, reasonably priced. though that material looks like its the type that will just crumble after a few weeks... (or after the first wash) let us know

That is a two stage filter with the outer sleeve being a courser material to act as a prefilter. I have had similar filters in the past and they last just fine. Don't use too much filter oil (inner sleeve only) after a wash.

[ProDigit]

I don't know how you can say the size of the 13 tooth front sprocket is way too small when the size of the front sprocket by it self means nothing. It is the combination of the two sprockets that matters.
I'm running a 13/30 right now on my 2012 DF with a top speed of about 65 MPH indicated at about 7k RPM now that I have an air filter that flows enough air to stop the cutting out. BTW, I am sitting upright and not tucked under.

Maybe you should start a different thread for the DF250RTC-B which looks like a neat bike. Also send an email to the seller to correct the 250cc to 229cc in their ad.

It's a common error for those engines...
I guess they're meeting the emission standards for a 250cc engine, but not for the 229cc.

Concerning the gear ratio, you're still 1000RPM above the optimal setting, meaning for every 200-300cc bike, there's an optimal 100:1 RPM over MPH ratio, meaning at 60MPH you should be doing 6k RPM, not 7K. That way you can go another 5MPH faster.
Your gear ratio is about the same as my 150cc scooter with CVT, and that one accelerates like a beast, and also tops out at 62MPH. I can only imagine with 1/3rd more cc's! (CVT eats upto 10% of power compared to geared bike).
I'm saying the front sprocket is too small, because it is. You can't go much lower on the rear without having the chain eat up the rear fork. You need to increase the front if you want to make your ratio a bit higher.

From the numbers you told me, optimal gear setting for your bike (best acceleration/MPG/top speed) would be a 15/31 sprocket setup. 15/30 is bad because of the rear sprocket teeth being a multiplier of the front sprocket teeth, causing uneven tooth wear.
It would drop RPM from 7k to 6k, which is a lot better for the engine.

If I would have geared your bike, from the numbers I see, I would have equipped it with a 17/30 setup. It would not be a dragster, but it would be rideable in traffic, accelerate just fine, do 5,5k RPM at top speed, perhaps suffer some top speed, but it would be in the 110-125MPG range.

But should I sell that bike as a dealer, I would equip it with a 17/36T setup, which is about the same as a 15/31 or a 13/27.5T (yeah, just between 27 and 28T on the rear)
With that sprocket you'll not only lower RPM, and increase MPG, but you hit top speed right in the torque band, allowing you to go even faster (at least 5MPH faster, perhaps 70-72MPH tops).

No wonder top speed figures didn't make sense to me!

I can't believe they actually equipped those bikes with a 13/41 (or 13/43 for the RTD, which is a rebel type) stock.
I don't know what front sprocket the RTC series have, but the RTC racing bike has a 46-48T rear sprocket, which is just insane, but at least it has a lot of room for improvement on the rear!

[axa]

It's a common error for those engines...
I guess they're meeting the emission standards for a 250cc engine, but not for the 229cc.

Concerning the gear ratio, you're still 1000RPM above the optimal setting, meaning for every 200-300cc bike, there's an optimal 100:1 RPM over MPH ratio, meaning at 60MPH you should be doing 6k RPM, not 7K. That way you can go another 5MPH faster.
Your gear ratio is about the same as my 150cc scooter with CVT, and that one accelerates like a beast, and also tops out at 62MPH. I can only imagine with 1/3rd more cc's! (CVT eats upto 10% of power compared to geared bike).
I'm saying the front sprocket is too small, because it is. You can't go much lower on the rear without having the chain eat up the rear fork. You need to increase the front if you want to make your ratio a bit higher.

From the numbers you told me, optimal gear setting for your bike (best acceleration/MPG/top speed) would be a 15/31 sprocket setup. 15/30 is bad because of the rear sprocket teeth being a multiplier of the front sprocket teeth, causing uneven tooth wear.
It would drop RPM from 7k to 6k, which is a lot better for the engine.

If I would have geared your bike, from the numbers I see, I would have equipped it with a 17/30 setup. It would not be a dragster, but it would be rideable in traffic, accelerate just fine, do 5,5k RPM at top speed, perhaps suffer some top speed, but it would be in the 110-125MPG range.

But should I sell that bike as a dealer, I would equip it with a 17/36T setup, which is about the same as a 15/31 or a 13/27.5T (yeah, just between 27 and 28T on the rear)
With that sprocket you'll not only lower RPM, and increase MPG, but you hit top speed right in the torque band, allowing you to go even faster (at least 5MPH faster, perhaps 70-72MPH tops).

No wonder top speed figures didn't make sense to me!

I can't believe they actually equipped those bikes with a 13/41 (or 13/43 for the RTD, which is a rebel type) stock.
I don't know what front sprocket the RTC series have, but the RTC racing bike has a 46-48T rear sprocket, which is just insane, but at least it has a lot of room for improvement on the rear!

Is it just me or can everyone else make heads or tails of this?

maybe you should start that RTC thread...