Gears, Sprockets and Speed
Gear & Speed Calculator
At 1000 rpm, an 18-inch tire would roll down the trail at over 70 mph, but connected to a motorcycle, that's "reduced" to around 7 mph, because motorcycles have an overall drive ratio due to the engine sprocket, clutch assembly, transmission and countershaft and rear sprockets. The engine sprocket and clutch basket establish a primary reduction ratio of 2.863636364 (63/22). The countershaft and rear sprockets establish a secondary reduction ratio of 3.571428571 (50/14). Together, they establish an overall reduction ratio of 10.22727273 (63/22 X 50/14). Coupled to the transmission, each gear creates an overall drive ratio. The top gear of a KLX-300, for example, has a gear ratio of 0.904761904, but coupled to the primary and secondary sprockets, top gear has an overall drive ratio of 9.253246746 (0.904761904 times 10.22727273). The chart below lists the approximate speed in each gear at 1000 rpm with a slightly worn Maxxis 110/100, circumference of 81.78 inches...
Kawasaki offers optional rear sprockets from 45 to 52 teeth but only the 14-tooth countershaft sprocket. After-market sprockets from 13 to 15 teeth, however, are available. Below are the approximate speeds in 6th gear for 24 combinations of countershaft and rear sprockets. The speed values are based on a Maxxis 110/100 with a circumference of 81.78 inches. The formula used to make these calculations is... HERE
|Pri/Sec Sprockets||Overall Drive Ratio||MPH in 6th Gear|
To calaculate speeds with a different tire, multiply it's circumference by 0.3645, then divide that value by the ratio of the two sprockets you are evaluating. A tire with an 85-inch circumference and a 13-tooth countershaft sprocket, for example, will give you 8.1 mph in top gear at 1000rpm...