Ever wondered how your transmission compares to other

bikes? Gearheads like me spend sleepless nights pondering

these things, and gearheads with a lot of time on their hands,

well, let's just say Excel tables can get created. The table

below summarizes transmission ratios from a sampling of

KTM's from 1999-02 and my 2003 KX250.

The comparison starts with the primary drive, which is the

gear connected to the cranskshaft. The table below shows

that most of the bikes are initially geared down to about 1/3 of

crankshaft speed. From there, further "gearing down" takes

place at the main shaft and countershaft.

In the table below, the numerator in the Primary Reduction

ratio is the number of teeth on the sprocket attached to the

crankshaft. The denominator is the number of teeth on the

clutch sprocket. In the individual gear ratios, the numerator of

the ratio is the number of teeth on the sprocket attached to

the main shaft (same shaft on which the clutch spins) which

corresponds to what gear you're in. The denominator is the

number of teeth on the countershaft (same shaft that spins

the countershaft sprocket), again, corresponding to what gear

you're in.

bikes? Gearheads like me spend sleepless nights pondering

these things, and gearheads with a lot of time on their hands,

well, let's just say Excel tables can get created. The table

below summarizes transmission ratios from a sampling of

KTM's from 1999-02 and my 2003 KX250.

The comparison starts with the primary drive, which is the

gear connected to the cranskshaft. The table below shows

that most of the bikes are initially geared down to about 1/3 of

crankshaft speed. From there, further "gearing down" takes

place at the main shaft and countershaft.

In the table below, the numerator in the Primary Reduction

ratio is the number of teeth on the sprocket attached to the

crankshaft. The denominator is the number of teeth on the

clutch sprocket. In the individual gear ratios, the numerator of

the ratio is the number of teeth on the sprocket attached to

the main shaft (same shaft on which the clutch spins) which

corresponds to what gear you're in. The denominator is the

number of teeth on the countershaft (same shaft that spins

the countershaft sprocket), again, corresponding to what gear

you're in.

Transmission Comparisons

So how do these transmission variations affect how fast your

bike will go? It's pretty simple, really. Multiply the primary

gear reduction by the appropriate main/countershaft gear

ratio, then multiply that number by the ratio of the number of

teeth on the countershaft sprocket to the number of teeth on

the rear sprocket. Still with me? Good. Now take that number

and multiply it by the engine RPM's to get the RPM's at the

wheel. Figure out the distance (in feet) your wheel travels in

one revolution, multiply that number by the rear wheel RPM's

to get distance traveled per minute, then multiply that number

by 60 to get the distance traveled in an hour. That number is

feet traveled per hour, so divide by 5,280 to get miles traveled

per hour.

Simple.

But even simpler is using my handy spreadsheet to let the

computer do most of the work. All you have to come up with

is the number of teeth on the appropriate gears inside your

transmission.

Want to try out the spreadsheet?

Keep in mind that there are inefficiencies in turning engine

RPM's into ground speed, so don't expect your bike to go

quite as fast as what the spreadsheet calculates. But it

provides a relative idea of what gear ratios and sprocket

combinations do for overall speed.

bike will go? It's pretty simple, really. Multiply the primary

gear reduction by the appropriate main/countershaft gear

ratio, then multiply that number by the ratio of the number of

teeth on the countershaft sprocket to the number of teeth on

the rear sprocket. Still with me? Good. Now take that number

and multiply it by the engine RPM's to get the RPM's at the

wheel. Figure out the distance (in feet) your wheel travels in

one revolution, multiply that number by the rear wheel RPM's

to get distance traveled per minute, then multiply that number

by 60 to get the distance traveled in an hour. That number is

feet traveled per hour, so divide by 5,280 to get miles traveled

per hour.

Simple.

But even simpler is using my handy spreadsheet to let the

computer do most of the work. All you have to come up with

is the number of teeth on the appropriate gears inside your

transmission.

Want to try out the spreadsheet?

Keep in mind that there are inefficiencies in turning engine

RPM's into ground speed, so don't expect your bike to go

quite as fast as what the spreadsheet calculates. But it

provides a relative idea of what gear ratios and sprocket

combinations do for overall speed.