Gearing Ratio

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

  1. The primary drive, the ratio between the engine (crankshaft) and the clutch or gearbox.
  2. The gear box it self, depending on the amount of gears this part has an equal amount of ratio to choose from
  3. 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 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 !

As you can see, the effect of adding a front tooth and keeping the rear the same has about 3 times more influence on the speed then  removing 1 tooth in the rear and keeping front the same. Changing only the rear with 1 tooth does not have a lot of effect, in combination with also changing the the front it either amplifies or weakens the total effect.
When you change more then just 1 tooth in front and/or rear, the results will increase significantly of course.

The exact increase or decrease for your bike can be calculated using the Gearing Commander tables.

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

Rear wheel Tire Size changes and their consequences

As the rear tire is a major part of the total bike gearing, changing its size does effect this gearing and thus effect speed and torque at certain RPM's. This is easy to explain: the chain (or drive shaft) is rotating the rear wheel at a certain RPM. Now when it is a small wheel in diameter it will have a small circumference and when rotated it will cover only a small distance. A bigger wheel will cover a bigger distance when rotated equally fast.

So changing for instance your rim size from 16" to 17" would mean a higher (top-)speed at the same RPM.
But not only changing the rim size influences gearing, also changing the width of a tire and leaving the rim size the same changes gearing. How come ? Well the tire circumference depends on the rim size and the height of the tire itself.

Now the height of the tire is a percentage of the width. A modern tire has these size marks on it 190/50/17 which means: the width is 190 mm, the rim size is 17" and the height of the tire is 50% of 190 mm (=95 mm). This tire has a circumference of: 1953.4 mm
Now assume you replace this tire with a 180/55/17 tire (same rim size, less wide and to compensate the height is bigger). Now this tire has a circumference of 1978.6 mm. Around speeds of 170 Mph (280 Km/h) this will cause a higher top speed of about 2 Mph (3 Km/h).  Not a big difference but still.

More info on Tire Circumference

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