Tranny comparisons for frame mounted motors

I can top out at 40 mph with a "no power" Honda.
 
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nice . i would love to race ,
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49cc , uses bike gears will top 50 with wind at my back . lol
 
Down boy, put the tape measure away.

Yours may be faster, but on a bicycle, 40 mph is not slow. I never said mine was the fastest out there-

I was just was making the point that a good 4 stroke does not have "no power."

Oh, and are you running those fancy new invisible pedals I've been hearing so much about?
 
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Good rule of the thumb for the approximate overall reduction from engine crankshaft to rear wheel (26") is the following : Divide 1000 by engine displacement (cc) .For instance a 50cc engine we get 1000/50=20 reduction.To figure out road speed vs engine rpm,
S(peed)= 0.075xrpm/R.For instance, rpm 6000 rev/min ,
R(reduction)= 20, we get 0.075x 6000/20= 22.5 mph.These figures are conservative, on the flats you might get away with using 60 instead of 50 in the formula for a bit lower reduction .For a 50 cc engine, R would then become about 17.
 
question about equation

so if i shift gears ,on the back hub .in first gear its 34 teeth .i can shift down to about 12 teeth about the same diameter as my drive sprocket , no power down there but i can get going pretty fast with no resistance , does the math change ?
 
I'm not 100 % sure what exactly you are talking about.If you change rear sprockets like using a deraileur the overall ratio changes by the ratio of the sprocket sizes,that is by about 3:1 if you went from14 to 34t.The math gives the OVERALL ratio crankshaft to the rear wheel,which includes any engine internal or add-on gearbox+the rest of your drive train ratio all multiplied together.Lets say your engine has a 4:1 reduction and you need an overall ratio of around 16/20,then you would need an aditional reduction of 4 to5 with your chain drive with 12 fr/34 rr, you have a bit less than 3:1, so an overall reduction of 12:1 (assuming you had 4 in your engine.If you had 5:1 you would get about 14 (5x34/12=14.This would be OK for a displacement of 70cc or so.If you had a reduction of14 your speed at 5k rpm would be 0.0 75x5000/14=27 mph.Do you get it now?.
 
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robins 35 cc

ok, i have a robins 35 cc arriving in the mail very soon and i'm gathering all kinds of information from this great site. my immediate goal is to set it up on my mountain bike frame and use the existing gears and derailer to shift with. i'm having trouble finding a simple centrifugal clutch, no gear reduction, to match up with my engine. any information would be appreciated. i know it said early on in the thread that the first one mentioned should match up with the robins but has anyone had experience with a specific one?
 
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