v6 .5

Sprocket Diameter Calculations

When changing sprockets you normally do not change the number of theeth extremely and hence the size of the sprocket will not change a lot either. However sometimes all margings have (nearly) been used and is it important to know how big (or small) the new sprockets will be. For instance when you want to use a larger front sprocket but need to make sure it will fit or the same if you want to fit a smaller one, to make sure the chain does not rub the swingarm near the front sprocket. Also in the rear not all sizes will fit as often space is limited by a chainguard.

As sprockets are rarely listed with their actual diameter I have added a sprocket size calculator to the Gearing Commander. It is activated by loading your bike, adjusting the sprocket details and loading of the 'Sprocket Diameter Graph', using theat button below the table called "Top speed in Mph per gear for 49 final drive combinations at M.P.RPM in Custom setup".

This calculates sprocket diameters for both chain drive bikes and belt drive bike. However the results are to be interpreted differently. For both type of sprockets the so called 'pitch diameter' is calculated and displayed.


For chain sprockets that is the diameter which defined by the centerline of the chain rollers on the sprocket (red circle):

Pd=Pitch Diameter (smaller as actual sprocket diameter Sd)
Sd=Sprocket diameter (max. sprocket diameter)
Cd=Chain Diameter (diameter sprocket and chain together)

So what the GC shows is the calculated Pitch diameter. To check if your sprocket and chain will fit, you will have to add 1 x the height the chain to the 'Pd' to get the 'Cd' (chain diameter) !

For belt sprockets also the pitch diameter is calculated (blue dashed circle). It lies outside the sprocket, in the belt. Mostly at the height of the tensile cord.


Pd=Pitch Diameter (greater as actual sprocket diameter Sd)
Sd=Sprocket diameter (max sprocket diamater)
Bd=Belt Diameter (diameter of sprocket and belt together)

In this case if you want to check if your belt and sprocket will fit, you will have to add 2 times the height of the tensile cord to the top of the belt to the 'Pd' to get to the 'Bd'.

Sprocket Pitch Diameter graph

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