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Gear Ratios

Many factors impacts on a bike's power delivery and performance such as number of cylinders, bore/stroke ratio, is it 2 or 4 stroke, carb's vs. fuel injection, turbo charging, etc. This section simply attempts to describe the important role of gear ratios.

The overall gearing of a motorcycle consists of 3 major parts:

The primary drive, the ratio between the engine (crankshaft) and the clutch or gearbox.
The gear box it self, depending on the amount of gears this part has an equal amount of ratio to choose from
The Final drive, (or secondary) ratio between the number of teeth on the Rear sprocket and the Front sprocket

All these ratio combined results in the Total Gearing Ratio of the drive train:

Changing any of these parts or changing the rear wheel tire's dimensions will effect speed and / or torque results. All of these changes can be simulated by using the GC (gearing calculator) by changing involved gearing fields. To be able to do this correct, all ratio, sizes and RPM's are necessary input.

Primary Drive
The primary drive is the ratio between the engine's crankshaft and the incoming shaft of the gear box or clutch. Usually this ratio can not be easily altered unless you perform major surgery on your bike.... But if you do, also change the Primary Drive Ratio field in the GC accordingly.

The gear box
The gear box is the only gearing part that can change its ratio very easily and that is by you changing the gear while you drive. Normally a gear box is dedicated for an engine or bike so you do not have to change gear ratio in the gearbox itself. But sometimes the stock gears are not suited for all circumstances and manufacturers offer a way to change the internal gear box ratio by either offering different cogwheels or by offering a so called cassette gearbox that can be swapped easily. That is why in the GC you can change the gear ratio of each gear by changing the white gear box ratio fields for each gear.

Final Drive changes and their consequences
The Final Drive is sometimes also referred to as 'the product of all ratios involved' but I prefer to call that the 'Total' or 'Overall' ratio. In our example the Final Drive ratio is the ratio of the final part of the gearing, the ratio between Rear and Front sprocket. This is also referred to as 'Secondary Ratio'.

Below a table containing the possible final drive gearing changes and their results on Speed and Torque.

As a rule of thumb, changing the front sprocket with 1 tooth, amounts to the same effect as changing the rear sprocket with 3 teeth. This does not exactly apply to every bike but as a rule of thumb it will do and explains the results in the table below.

Change		Result On
Front Sprocket	Rear Sprocket	Final Drive Ratio	Speed	Torque	Chain length needed	Rear wheel Adjustment
+1	- 1	down	++++	----	shorter	<->
+1	same	down	+++	---	longer	>- -<
+1	+1	down	++	--	longer	>- - - - - - - - <
same	- 1	down	+	-	shorter	<- - ->
same	+1	up	-	+	longer	>- - - <
- 1	- 1	up	--	++	shorter	<- - - - - - - ->
- 1	same	up	---	+++	shorter	<- ->
- 1	+1	up	----	++++	longer	>-<

LEGEND:
Sprockets:        '+1' means adding 1 tooth and a '-1' means removing 1 tooth
Results   :         '+' means the speed (or torque) will go up, '-' means the speed (or torque) will go down
                       Where as '++++' means a 4 time as big affect of course.
Wheel Adjust.:   <-> means move rear wheel backwards, >-< means move rear wheel forwards
                       Where <- - - - - - - -> means move it more then <- -> of course.
                       Rear wheel adjustments assuming you keep the current chain !

Total or overall ratio
Combining all these individual ratio like they are in a motorcycle, results in the total drive train ratio or the overall ratio. Just multiply all the ratio to get the Total Ratio: Primary ratio x Gear Box ratio x Final Drive ratio

Note: There is much more to gearing ratios than covered above. Your are advised to visit The Gearing Commander website - www.gearingcommander.com. This is web-site dedicated to this topic. It has a simple Gearing Calculator (GC) pre-set with the information of many bikes. It also allows you to recalculate the effect of possible changes as described above.

Most bikes these days electronically calculate speed by gear position and engine revs or a gearbox speed sensor. Thus, if you change the gearing, the speedo will be out. The answer is to calculate the error and re-mark the speedometer. If your speedo’s digital you’ll need a pretty gymnastic mind

On the road
Lower gearing will give a more ‘lively’ ride. You’ll need less revs to get off the line and first gear will be over more quickly. The front wheel will be more likely to leave the ground too as the extra thrust accelerates you faster.

Higher gearing will give a more relaxed ride, with cruising speed at correspondingly lower revs. More clutch slipping will be needed from a standstill and first gear will feel ‘longer’. Acceleration in top gear may feel rather sluggish.

Below a short video illustrating gear ratios at the most basic level:

Sources: The Gearing Commander, Damon I'Anson