At the bottom of this
page is a Javascript form you can use to calculate MPH at a selection
of reference RPM when you know the
motorcycles tire diameter in inches, ratios of the primary drive gears,
choose your specific transmission gears and sprocket
sizes.
For simplicity's sake
we'll be using the wheel diameter in the calculation rather than a much
more accurate "roll-out"
of the rear wheel (measurement of the distance covered in one
rotation). Using just the diameter doesn't take
into account tire
pressure, sidewall flex and the load on the bike - all of which affect
how far you go in one
rotation of the rear wheel (as does the speed you are going). The
next version of the page should have a reversed chart (RPM at specific
MPH), and a space for the more accurate "roll-out" numbers.
So far, I'm limiting my
efforts to 49-72cc street legal Honda horizontal singles. I still
need a few transmission numbers for USA-type street-legal bikes
(CT70, SS50, XL70, ZB50). If you've got them - send them
along. Links to online service manuals accepted as
well.
©2006 Mike Gladu - all
rights reserved
1969-1973 C-Bikes
|
C50
|
C50M
|
C50M-K1
|
C65(M)
|
C70
|
C70M
|
CL70
|
|
Primary
|
3.722
|
3.722
|
3.722
|
3.300
|
3.722
|
3.722
|
3.722
|
|
1st Gear
|
3.364
|
3.364
|
3.272
|
3.364
|
3.364
|
3.364
|
2.692
|
|
2nd Gear
|
1.722
|
1.722
|
1.823
|
1.722
|
1.722
|
1.667
|
1.824
|
|
3rd Gear
|
1.190
|
1.190
|
1.190
|
1.190
|
1.190
|
1.190
|
1.300
|
|
4th Gear
|
n/a
|
n/a
|
n/a
|
n/a
|
n/a
|
n/a
|
0.958
|
|
Final Drive
|
3.000
|
3.000
|
2.857
|
3.154
|
2.786
|
2.786
|
3.615
|
|
Sprockets
|
13/39
|
13/39
|
13/37
|
13/41
|
14/36
|
14/39
|
13/47
|
|
Wheel
|
22
|
22
|
22
|
22
|
22
|
22
|
22.5
|
CT-Z Bikes
|
CT70
|
S50
|
SS50
|
S65
|
SL70
|
XR70
|
ZB50
|
|
Primary
|
3.722
|
3.300
|
4.312
|
3.300
|
3.722
|
3.722
|
|
|
1st Gear
|
3.272
|
1.765
|
3.000
|
1.765
|
2.692
|
3.272
|
|
|
2nd Gear
|
1.722
|
1.300
|
1.765
|
1.300
|
1.823
|
1.722
|
|
|
3rd Gear
|
1.190
|
1.190
|
1.400
|
1.190
|
1.300
|
1.190
|
|
|
4th Gear
|
n/a
|
1.043
|
1.136
|
1.043
|
0.958
|
n/a
|
|
|
5th Gear
|
n/a
|
n/a
|
0.958
|
n/a
|
n/a
|
n/a
|
|
|
Final Drive
|
2.333
|
3.154
|
3.154
|
3.308
|
3.384
|
2.400
|
2.600
|
|
Sprockets
|
15/35
|
13/41
|
13/41
|
13/43
|
13/44
|
13/44
|
15/39
|
|
Wheel
|
17
|
22
|
22
|
22
|
19.5
|
19.5
|
15
|
For example, a 1982
USA-type Honda C70 Passports has a 22"
rear wheel, a 4.058 primary
gear reduction, 1.19 ratio in third gear, a 36 tooth rear and a 14
tooth countershaft sprocket.
