Speed vs RPM for Grubee GT5A & 26" tires

[sbest]

I have no tach so I have had no idea what RPMs I was turning.
Cold weather and snow ruined my bike riding so I did math.
My tires are 26"x2.3" and measure an actual 26.5" high.
I have the "as issued" 44t rear sprocket, and 10t front.
The engine gear ratio is 20:82.

Much of you who have built "80cc" kits have the same specs.
My tires are a bit oversize but we are only talking 200 rpm at max speed.

So the math is:
tire-dia x pi x 25.4mm/inch / 1000 = distance of wheel rotation in meters
speed in kph / 60min/hr x 1000m/km / distance of wheel rotation = wheel RPM
(wheel sprocket / shaft sprocket) = 44t/10t = 4.4
(clutchgear teeth / drivegear teeth) = 82t/20t = 4.1
Wheel RPM x (wheel sprocket / shaft sprocket) x (clutchgear teeth / drivegear teeth) = Engine RPM

So, as first mounted and new, I got 40kph top speed or 5700rpm
As it wore in, speeds picked up to 50kph or 7150rpm
With some tuning and head work I can often see 60kph or about 8600rpm
my strongest torque is at about 35kph or 5000rpm
torque drops off fast under 30kph or 4300rpm
9000rpm is 63kph and I touch it occasionally but the vibes are terrible.

Aiming for 50mph? 80kph is about 11500rpm
60mph? 100kph will buzz this combo to 14000rpm!
Even with a 36t sprocket if I can still spin 8600rpm I will be lucky to see 73kph.

Steve

 

[cannonball2]

That's the deal with a single speed build. I still run an old stock CG build with a 36T sprocket. My calc shows 37mph@7000, 48@9000. I imagine my engine might make 6-6500rpm and that's a pretty vibey range. I am running the 36t to cruise in the "comfortable" range vibe wise around 25mph. Works very well because I live in the flats. I doubt it would be much of a hill climber.

However I have a 3spd IGH stock CG build that tops over 40 and loafs along rpm wise around 35mph. Gearing is the only way to have your cake and eat it too.
In my opinion with the common engine kits the single speed is most rideable/tolerable in the 20-25mph range that's what they were designed for . Sure you can make em so faster. Vibe wise its not a pleasant experience.