Tech: Motorcycle Final Drive Ratio

Final Drive Ratio
Photo by Brian J. Nelson

Motorcycle final drive ratios are changed by many motorcycle riders for many different reasons. A few of the reasons riders may alter their gearing are to increase top speed on a straightaway, get increased acceleration from a corner or from a start or to increase the efficiency of the motorcycle by increasing miles per gallon. Most riders know that changing the front sprocket tooth count will produce a large change and, conversely, changing the tooth count on the rear sprocket will have a smaller effect change on final drive. What many riders do not know is how to calculate final drive ratio and determine the effect that a change will have.
The equation for final drive ratio is: (Rear tooth count) / (front tooth count) = drive ratio
For example: We will assume that these sprockets are on our stock motorcycle. Our example front sprocket has 15 teeth and the rear sprocket has 42. 42 stock rear sprocket / 15 stock front sprocket = 2.8 Therefore, the final drive ratio is 2.8

Increasing Low End Torque

Knowing the equation for the final drive ratio, let's take the stock motorcycle gearing and aim for more drivability off the corner. To do that, final drive ratios need to be increased, which can be done by increasing the rear sprocket size or reducing the front sprocket size. Reducing the front sprocket size is not recommended due to the decreased sprocket diameter will affect the chain by making it have to bend at an increased angle. Increasing the chain angle more will in turn make the o-rings on the chain wear out sooner. It should also be noted that there is a sacrifice in top end speed for increasing torque. For this example, we will add 3 teeth to the rear sprocket.
The new final drive ratio is: 45 rear sprocket / 15 front sprocket = 3 Therefore our new final drive is 3 If we do the same equation with the old rear sprocket and a 14 front sprocket you will get the same final drive ratio. 42 rear sprocket / 14 front sprocket = 3

Increasing Top End Speed

The opposite can be done to increase top end speed, but torque on low end will be sacrificed. For this the front sprocket tooth count can be increased or the rear sprocket tooth count can be reduced. The space that the front sprocket can take up is limited, so ensure there is enough space in the engine compartment for the sprocket and chain to move without obstruction. 42 rear sprocket / 16 front sprocket = 2.635 Therefore our new final drive is 2.635

Calculating the Percentage of Change

On average, riders won't notice a change in final drive ration that is less than 3%. Also, if the motorcycle determines vehicle speed off of the front sprocket, it's important to understand this equation in order to determine the impact on the speedometer. If the motorcycle being modified will be ridden on the street, it is important to get the speedometer recalibrated following any change in final drive ratio. Pre-modification and final drive ratios are used to determine the percentage of change: Low end torque final ratio 3, stock final ratio 2.8 3 / 2.8 = 1.07 1.07 - 1 = 0.0714 0.0714 x 100 = 7.14 Final drive percentage change is 7.14%
High end MPH final ratio is 2.635, stock final ratio is 2.8 2.635 / 2.8 = .941 .941 / 1 = -0.059 -0.059 x 100 = -5.89 final drive percentage change from stock was -5.89

Important Notes

When increasing final drive ratio by swapping out a sprocket, remember that using a sprocket with more teeth than the existing sprocket on a motorcycle will require a chain with a higher link count to compensate for the longer path which the chain must take and, conversely, using a sprocket with fewer teeth in order to reduce final drive ratio will require a chain with less links. It may also be noted that stock sprockets are made of steel for durability, but when replacing stock sprockets with lighter aluminum sprockets, some longevity may be sacrificed.

Courtesy: http://www.amaproracing.com