Stock Chinagirl Riders- Got a minute?

Hi gang
I want to find out the very best gearing for a single speed, stock (more or less) Chinadoll engine. The first thing I tried to look up was the torque peak.
There doesn't seem to be any hard data available, so I thought it might be just as good to ask guys about their bikes' performance with their own set-ups.
Since so many factors have an effect on top speed, I would like to ask you a few questions.

1) displacement 66 or 50
2) wheel size
3) rear sprocket tooth count
4) top speed reached

To keep the data solid for the basic CG, I'd like to see the numbers from people with: engine broken in, no trick porting, no piston grinding, no high compression heads, and no expansion chambers. All those things would skew the data too much. Feel free to mention what carb or manifold you have on it, all info is useful.

Things that are allowed: RT, CNS or NT carbs, extended exhaust pipes as long as there is a muffler, and the different aftermarket CDI's are all ok.

Next we could gather data on engines that are warmed up a little, but I'd like to get sort of a baseline with the stockers (or nearly so) first.

Anybody that chimes in will be held in great esteem for at least 2 full days.
;-)

I'll start-
66 cc
29"
40 T
28.6 mph
Using the ratio computer program from this forum, I think my torque peak is at about 5,250 RPM. I have the z-type manifold, though, so I may have moved that peak down the rev range a bit. My engine may not be fully broken in with only 180 miles on it, but it's close.

Whaddayasay- wanna help me out here?

-Slogger

Thanks for all the info, Cannonball.
I made a chart for sprockets and RPM at speed and a good guess for your current peak RPM would be about 7000.
A fair guess at peak torque would be about 6400. Much higher than mine.
And that's pretty good for only 100 miles. That stretched exhaust must be working.
Adding the z-manifold might slow ya down some on top, but it will really pull.

djpass7 said:

66cc GT-5
44 tooth
RT Carb
26 inch wheels
32.48 mph

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You're a little hotter there dJ. 7500 Peak, your torque peak, (educated guess) may be around 6700. I bet your bike pulls low hills at about 28mph?
Good runner you have there. Thanks for the input!

maniac57 said:

--- Adding the Lightning HD CDI: again, very little top speed gain, but much better pull from lower rpm with much smoother feel and a happier engine everywhere in the band.
Now I'm running a mildly ported 66 with all the same mods, and it has now maxxed out at 32mph on a 38 tooth sprocket, but will pull higher once it's broken in. MUCH better low end makes it by far my favorite motor so far.
My opinion is for a bone stock motor, I'd stick with a 36 tooth in flat places, and a 44 if there are fairly steep hills to deal with.
The 36 reduces vibrations and revs and makes the engine feel nicer to ride unless you have a weak sister engine (which DOES happen) or a lot of hills.

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Thanks, Maniac. That 38 toof makes her work a little harder, but it sure is sweet for cruising around, I bet. The 44 (on a 26) is a pretty good all around choice, since so many guys want to reach higher rpm and use all the power it can make. Being geared too tall can actually hurt top speed when taken to extremes, the engine can't get to the meat of its power band.

LandSpeedRecord said:

Not sure if this helps but here is mine. 66cc China girl from gasbike.net fully port matched but very little material taken away except on the intake and exhaust that i brought out to match the ports, cleaned the castings on the transfer ports, kit came with a billet head and I have it on right now, stock nt carb with the jet soldered and drilled, stock airbox cut away with just the filter element left, piston filed to match intake port, rubbed cutting oil on the rings, bearings, primary gears, and chain and rode about a mile with the spark plug out before ever starting it. The kit shipped at 23lbs and the bike is an aluminum frame fixie style 700c road bike that weighed 22lbs and i have lightened the whole package about 3lbs further. I have about 3/4 of a gallon of break in premix run thru it so far. It runs best with the end cap off the stock exhaust but is pretty loud. It will hold 40.6 mph on flat roads and I have seen just over 43 on a slight downhill. I have the stock 44 tooth sprocket on it. Pulls strong low to mid but i am having intermitent vibration up high right now that i have to figure out.

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I'm keeping this in case I need a recipe for a 40+ motor bike! Thanks!
That's an interesting break in trick, too.
Downhill don't count (haha) but 40.6, if it's a 26" would put you off my chart! You be screamin there. I could find out if I could remember how many teeth on the clutch plate. A calculated rpm came out 9456 RPM. It's vibratin because it's churning out mucho power. It wants to fly apart. I'd try a 40 tooth because it will last longer.

BTW, that's a racer! You're outtahere! ;-)



<kiddin>

djpass7 said:

Yes its running pretty good. I gained almost 7 mph when I completely gutted the exhaust. I cut the muffler 1 inch down from where the header tube goes into the muffler. Then I cut out all the baffle making a straight shot from motor to muffler. I drilled the end cap to a 3/4 inch hole and threaded in the SBP muffler. Best mod for a stock chinadoll IMHO.

P.S. I just pulled it apart and ramped/skirted piston, polished piston, polished combustion chamber, decked head way down, and decked the jug about .5mm. I'm going to try to see what my new top speed is today. Should run like a beast.

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Ah, a heavily modded muffla.. Thanks for the info, dude.
Let us know how your grindings turned out.
;0)

LandSpeedRecord said:

Slogger, I'm on a 700x35 tire so it's quite a bit larger around than a 26. It is still spinning pretty well though. I am doing some more work and see if I can get it to pull a 36 tooth rear. Hope your questions are getting answered, I'm always glad to help if I can.

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It's all appreciated, LSR.
You're RPM would be a touch lower than I thought, no big deal.
Thanks!

djpass7 said:

I didn't get anymore top speed, but my cruising range went up about 2 mph. the difference is barely noticable.

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When an engine runs as well as yours already did, your improvements will naturally be less dramatic, you know?

Glad to hear you got something out of the work. A 2 mph faster cruise is pretty good step for a chinagirl.


Thanks for the update.

Thanks, Boxcar- Nice looking bike you have there, the chrome pipe looks cool.
At 34.5 your engine peaked at about 8000. That's really cranking.
I'm starting to think that on stock engines, those longer exhaust systems (sans expansion chambers) make better power than the little short deal the engines come with.
The 44 tooth is a good all around sprocket. If the engine is torquey enough, a 40 tooth will let your engine do a more relaxed cruise and maybe even go a little faster.

djpass7 said:

I believe that it has more, but the balance of the crank is pretty bad at those rpms. My hands are almost falling off the handlebars.

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One time, during break in, something came over my engine and it began to run really strong and accelerated right up to 29 mph (40 tooth, 29er) but it was vibrating with big high amplitude pulses that felt like it would break a mount or something so I let off.
It did it one other time but not anymore, now it only reaches 27 without a small downhill to get past the hump. I might have blown a seal or something, but it sure runs nice right up to that point, no vibration really at all.
These things are weird. If it shook like that at high speeds, I wouldn't go there. I really don't need too much power around here anyway.
The cops are watching most the time.