You crazy to think 8000RPM on these

G

goofyfoot2001

New Member
These engines are doing no more than about 2000RPM WOT. Using any of the gear calculators found on the Internet you have to be careful because in those calculation the drive gear (front one) is larger on a bike and not smaller like on these babies. So you can't divide the larger gear by the smaller one. You have to do it the other way or you will come out with like the speed of sound or there bouts at 2000 RPM on a 44T sprocket. So use this formula for MPH estimate (and if I'm wrong please tell me why):


1. Gear development (in) = (engine sprocket teeth ÷ rear Sprocket Teeth) X wheel size (in) X 3.14
So 24.83 Inches of travel for each turn of the engine drive sprocket = 2.07 Feet of travel

2. Speed MPH = Gear Development Feet x RPM X (ratio=360/(2*pi*radius))

So in my case 28.3 MPH = 2.07 x 1200 x .0114 which is about right for my top speed so my 1200 RPM guess is probably close.
 
my engine idles at 1k.

grubee gt5 specs 2.5hp at 5k.

modified motors wrap up over 8 grand.

this has been confirmed with tachometers (like the "tinytac.")

something's wrong with your calculations.
 
Hey goofyfoot, the gear calculator works fine - as long as you input 20 and 82 on the first lines.

That's right, there's a 4.1:1 gear reduction built into the chinagirl. Forgot that, did you? :p

8000RPM.jpg
 
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When you talk about engine RPM, you're talking about the crankshaft, not the final drive.
Your drive sprocket might only be spinning @ 1200 but you must consider there are gears involved between the crankshaft and the sprocket.
As Aleman explains, remove the engine right side cover and check the reduction between the small gear, which spins with the crank, and the large one that drives the sprocket. There's your RPM reduction and the problem with your calculations.
Tom
 
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A stock 66cc china two stroke will wind out between 6500 and 7k rpm. When I put all the correct numbers in and what I know to have been my most consistent top speed, I get 6800. 8k is definitely not achieved without some modifications, but even stock engines are not far off. The gear ratio calculator that we use here was also developed by a member of either this forum or the other forum, so its not a regular bicycle gear calculator that we are putting the numbers into incorrectly.
 
You know, I just thought about that. A bicycle gear calculator would give you pedaling speed. It doesnt have anything to do with the drivetrain/gear ratio of the engine side of things. if you are using a bicycle gear calculator to come up with engine rpm's, no wonder you think its off lol.
 
The gear calculator most of us find so handy is fully capable of figuring out pedal cadence and MPH, too.

Enter crank sprocket in first line, rear sprocket in second, tire size and pedal RPM. Most people cruise between 50 and 90RPM.


Here's an example, using typical Schwinn single-speed gearing with 90RPM pedal cadence:

Schwing.jpg
 
I know our gear ratio calculator is capable of calculating pedal gears too, but this guy thinks that calculating the pedal gears will give you the speed/rpm of the engine. Yet he provided what he thinks is an accurate formula for calculating ratio and rpm, which I am inclined to assume he pieced together based on a few minutes of googling.
 
I stand corrected in not knowing that the Chinese are reducing wages and gear ratios! But, all that really matters is how fast that tiny little gear up front spins. Has anyone tried modifying the cover and making that one larger? one tooth will give you 2 mph better.
 
goofyfoot2001 said:
I stand corrected in not knowing that the Chinese are reducing wages and gear ratios! But, all that really matters is how fast that tiny little gear up front spins. Has anyone tried modifying the cover and making that one larger? one tooth will give you 2 mph better.
Click to expand...

It easiest to change the rear drive sprocket. I calculated my gearing with a 36T rear sprocket. I've come to the conclusion these motors don't like running over 7000 rpm. They can run 8000, but won't last long doing so.

36T sprocket
26x1.95 tire 80.25" circumference


RPM MPH
1000 5.2
1200 6.2
1400 7.2
1600 8.3
1800 9.3
2000 10.3
2200 11.4
2400 12.4
2600 13.4
2800 14.5
3000 15.5
3200 16.5
3400 17.6
3600 18.6
3800 19.6
4000 20.7
4200 21.7
4400 22.7
4600 23.8
4800 24.8
5000 25.8
5200 26.9
5400 27.9
5600 28.9
5800 30.0
6000 31.0
6200 32.0
6400 33.1
6600 34.1
6800 35.1
7000 36.2
 
Hi all. Ok, most kits come with a 44 tooth rear sprocket. This is an easy way to get a "ballpark only" idea of RPM but will only work if u have 26" wheels & 44 tooth rear sprocket. Multiply speed in MPH by 227. This will give u roughly ur engine speed in RPM. I say rouhly because tire size & pressure r contributing factors. U could do a little math to apply this type of formulae to any size wheel or sprocket size. Cheers
 
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