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Sprocket and Chain (or Belt) 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 the 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)

To check if your sprocket and chain will fit you need to check for the  Chain diameter (Cd) as that is the biggest one.

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

To check if this sprocket with belt will fit you need to check for the Belt diameter (Bd) as that is biggest one.

For belt driven motorcycle mostly belts with a 14mm pitch are used. For those belts the total belt height and the tensile cord part thickness is known, it is about 4 mm (0.158"). Only for these bikes (14mm pitched belt) the Gearing Commander can calculate the Belt diameter. For bikes with another pitch NO Belt diameter (Bd) will be calculated and it will be shown with the same size as the Pitch diameter (Pd).   In that case the graph Legend will show:
"NOT all Belt Diameters (Bd) calculated as the belt height is unknown for belts other than 14 mm pitch"

Sprocket Pitch Diameter graph

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