Re: 43mph 48cc stock internals + 1500 miles later
Facts:
1) If going downhill or have wind at your back the argument is moot. I can hit 60 mph if the wind is strong enough or the hill is steep enough with the engine not running.

2) Numbers don’t lie. With a given ratio, Speed (mph) and engine RPM are directly related and cannot be disputed (unless wheel is spinning (not making full contact to ground) or clutch is slipping).

3) Resistance is what will keep you from achieving your intended goal of RPM or MPH (again both directly related).
Resistance = Going uphill, Against wind, Weight, Drag (wheel bearings, fat versus skinny tires, drive chains and sprockets …. etc.). And with only about 2.5 hp ? and ? torque It’s our biggest enemy.

4) Bicycle wheel diameters: An important calculation Uto determine. The size of your wheel will determine how many times it needs to go around to achieve a distance of 1 mile.
Calculation example: For a 20” dia tire. It would be 20 x Pi (3.141593) / 12 = 5.2360 ft or …….. 1 revolution of a 20” tire travels 5.2360 ft. Since there is 5280 ft in 1 mile if you calculate 5280 / 5.2360 you get 1008.4034 or …………..

………….. a 20” tire has to turn 1008.4034 times to travel 1 mile. Shortcut # used later (1008.4034 / 60 = 16.8067)
………….. a 24” tire has to turn 840.3361 times to travel 1 mile. Shortcut # used later (840.3361 / 60 = 14.0056)
………….. a 26” tire has to turn 775.6949 times to travel 1 mile. Shortcut # used later (775.6949 / 60 = 12.9282)
………….. a 27” tire has to turn 746.9655 times to travel 1 mile. Shortcut # used later (746.9655 / 60 = 12.4494)


Ok back to ratios : Using a standard 2cycle China girl the internal gears are 20 (crankshaft) to 80 (clutch) and motor exit sprocket has 10 teeth, Rear wheel sprocket varies from about 32 to 48 teeth.

For this example (my bike) I have a 26” tire with a 40 teeth rear wheel sprocket and with my big but of 220 lbs I can achieve a top speed of about 31 mph (checked with a GPS with +\ error of about .3 mph).
Since most of us know (assume) how fast we are traveling in mph let’s calculate to figure what RPM our motor is turning at that speed.

20 (crankshaft gear) x 80 (clutch gear) x 10 (motor output sprocket) x 40 (rear wheel sprocket) = .0625 (1 revolution of the motor = .0625 revolution of the rear wheel)

Since the wheel is traveling in MPH ”miles per hour” and the motor is in RPM’s are per “minute” we need to adjust our wheel speed to revolutions per minute to do that we simply divide by 60 (60 minutes in 1 hour)
Which is why I created the “shortcut wheel numbers above. OK back to our calculation …

20/80 x 10/40 = .0625 (revolution of the rear wheel per 1 revolution of the motor)

12.9282 (adjusted tire rev per min in 1 mile) / .0625 = 206.8512 (# of revs motor has to turn in 1 min per mile an hour)

206.8512 x 31 MPH (your actual speed) = 6412.4 (actual motor RPM @ 31 MPH)


So back to Ahmedy’s example: China girl standard engine with 26” wheel and a 36 tooth rear wheel sprocket:

20/80 x 10/36 = .0694
12.9282 / 0.0694 = 186.1661
186.1661 x 43(mph) = 8008.141 (motor rpm)


You can go smaller with the wheel sprocket but if you go too small you may run out of the horsepower or torque to achieve the motor rpm’s & mph (again directly related) you want.

Bottom line is: If ya wanna go faster either reduce the resistance or increase the horse power or torque.
__________________
Law of Logical Argument:
"Anything is possible if you don't know what your talking about".
Last edited by langolier; 01212016 at 06:29 PM.
