Calculate your topspeed

Which information do we need to calculate our highest theoretical speed :

(This of course gives only an indication and does not take losses as weight, wind, rolling resistance, belt slip, etc. in consideration)

Another (usually) uncertain value is your engine speed (this is your ‘N1’). In the absence of a tachometer on our motorcycles we have to “guess” to simulate a possible RPM speed.

N1 = front pulley speed (RPM)
D1 = front pulley outer diameter (mm)
N2 = rear pulley speed (RPM)
D2 = rear pulley outer diameter (mm)
Rf = reduction factor hub/gearing
Oa = outgoing shaft speed (RPM)
Wo = wheel circumference tire (in meters)
Km / h = outcome

‘Wo’ can be measured or calculated in two ways.
1: draw a line, drive your wheel exact one time around, draw the end line, measure how much distance it is.
2: measure from the heart of your wheel axle to the ground, multiply this by 2 and then by 3.1415.

The calculation/formula:

N1 X D1 / D2 = N2 / Rf = Oa X Wo X 0,06 = Km/u

An example:

An original Vespa Ciao moped.

N1 = 5500 RPM
D1 = 45 mm
N2 =?
D2 = 93 mm
Rf = 6.7 (1:6,7)
Oa =?
Wo = 1.72 (17-inch wheel with standard tire for the example, this size you would have to measure yourself)

To calculate N2:

N1 x D1 / D2 = N2
5500 X 45 / 93 = 2661.29

Divide this result by your hub/gearing reduction.

2661.29 / 6.7 = 397.21 (this is ‘Oa’)

This result is to be multiplied by your wheel circumference (‘Wo’) (in meters)

397.21 X 1.72 = 683.20 (these are the meters per minute)

Now we just have to convert to Km/h.
This is done by dividing by 1000 (one kilometer in meters) and then multiplying by 60 (minutes in one hour).

The standard (much easier) factor for this is multiplying by 0.06 (60/1000)

683.20 X 0.06 = 40.99 Km/h

Do you want the calculation to use with your vario, you can.
All you need to do is measure the diameter of your variobelt startposition and it’s diameter in max. variated position.

FYI: a Malossi Malossi vario with Malossi belt starts at about 40 mm and corresponds approximately to 80 mm. (This is your D1)

The difference between these two numbers (40 mm in the example)is about the same in decrease of your rear variator discs. (This is not 100% correct but the numbers are very close to)
At 100mm it would be about to 60mm, 90mm would be about to 50.
(This is your D2)

The rest of the calculation remains the same.

A vario example:

8000 RPM
Malossi vario @ 80mm
100mm @ 60mm discs
1:9 gearing
17 “wheel with tire (1.72)

8000 X 80 / 60 = 10666.67 / 1185.19 = 9 x 1.72 x 0.06 = 2038.53 = 122.31 km / h

Good luck!