It shows the calculated pitch- and chain/belt diameters of both the front- and rear-sprockets in 3 setup's.
The input used to calculate the sprocket pitch diameter comes from the data entered in the fields for the Final Drive (Front & Rear) and the pitch of either the chain or belt in the 3 setup's on the main page. To calculate the Chain-diameter, the height of the chain is used, based on the chain type which is derived form the entered pitch. To calculate the Belt-diameter, it is assumed that the bike has a 14mm pitched belt of which the thickness is known. If a belt-pitch other than 14mm is entered, no Bd can be calculated and it will show as being 0 in that particular setup.
What the graphs show is a visualisation of the calculated dimensions of front and rear sprockets in 3 setup's, showing the calculated size either in inch or mm. As this is the calculated pitch diameter, the actual sprocket outer diameter will be different.
Also the more important Chain diameter (Cd)
or Belt diameter (Bd) is shown.
On this page you can see the differences
between the various sprocket diameters:
For chain sprockets, the actual outside sprocket diameter (Sd) will be slightly bigger as the calculated (Pd) one as the teeth are higher as the pitch diameter. However, the outer sprocket diameter is not important, what is important is the 'chain diameter' (Cd), that is the outside diameter of the chain when on the sprocket. It is the calculated pitch diameter (Pd) plus the height of a chain. For chains of the types 415, 420, 428, 520, 525, 530, 532 and 630 the (approximate) height of the chain is known and the Cd will be calculated. As chain heights for the same chain type slightly vary between manufactures, please measure yours and verify teh caculation results.
For belt sprockets the actual
outside sprocket diameter (Sd) will be slightly smaller as the
calculated pitch diameter as the teeth of the belt sprocket do not reach
up to the pitch diameter. Also here this Sd is not important when
your sprocket will fit. What is important is the 'belt diameter' (Bd),
that is the outside diameter of the belt when on the sprocket,
which is the pitch diameter plus 2 times the height from the tensile cord
to the top of the belt. For belts with a pitch of 14 mm this height from
tensile cord (= pitch diameter) to the outside of the belt is about 2mm.
Hence for 14 mm pitched belts the Gearing Commander can calculate the
Bd. If a different pitch is entered (so not being 14mm) the Bd for that
setup is not calculated and set to 0. That is indicated at the bottom of
For most sprocket changes the difference in calculated pitch diameter between your current installed and the custom one considered, will indicate how much more space you need for your new sprocket as you will probably mount a chain or belt with the same height you currently have. If also changing the chain- or belt-type, the differences between the Cd or Bd will indicate how much more (or less) space is needed on the bike for the new setup to fit.
When considering a smaller front sprocket this will also to determine how much the chain will drop towards the swing arm so you can see if they will touch: half the difference between current and custom calculated Cd.