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I'm always astonished that so many pilots simply don't know what to do when they should change the transmission ratio of their bike !
Usually, pilots try to reach the max power RPM (or slightly higher) at the quickest place of the track, and adapt their sprockets to reach this goal.
Here are the two main rules for the final transmission:
1. a small gearbox sprocket with fewer teeth gives the bike more acceleration but lower final speed (the engine reaches its upper RPM limit before)
2. a bigger rear sprocket with more teeth gives the bike the same effect than a small gearbox sprocket.
In fact you can calculate the ratio easily:
nb teeth rear sprocket
A small ratio gives more acceleration and a lower final speed
If the max power of your engine is at 13'000rpm and you reaches only 12'000, you should have a smaller ratio (smaller gearbox sprocket or bigger rear sprocket). In the other way, if you go over 13'000rpm a long time before reaching what should be the quickest part of the track, you should have a greater ratio (a bigger gearbox sprocket or a smaller rear sprocket).
I've personally the habit to calculate the ratios in percent
of the standard transmission, that lets me have a better feeling
of how big the difference of two ratios really is.
By example, if the standard transmission is a 16/37 or 0.432, this becomes 100%.
A shorter ratio will be less than 100% ( by ex. 15/37 gives 93.75%) and a longer ratio will be more than 100% (by ex. 17/37 gives 106.25%).
A small formula:
Ratio % = -------- -------- 100
where: Pnew = New gearbox sprocket number of teeth
Cnew = New rear sprocket number of teeth
Pstd = Standard gearbox sprocket number of teeth
Cstd = Standard rear sprocket number of teeth
It's useful to do a table with all the possible ratios with your available gearbox and rear sprockets.
Gearbox changes are used so that the engine is always in its best RPM range, especially when coming out of the corners.
The formula remains the same:
teeth mainshaft sprocket
nb teeth countershaft sprocket
and the effects too.
Usually the bikes where changes to the gearbox sprockets are easy, are sold with tables of the various gearbox sprockets combinations available.
It's a little bit more complicated to achieve a good gearbox transmission as the pilot should feel where the engine is under or over its better RPM range and changing a gearbox sprocket could help you in a corner but be disadvantageous in another one ...
Telemetry is of course the best method as it measures the RPM over all parts of the track.