We all hear and read about motorcycle gearing, but what does it mean and how can it effect use? Gearing is a compromise. Speed, acceleration, handling, comfort and economy are all affected by the choice of final drive ratio. Motorcycle manufacturers spend huge amounts of money targeting each model to a specific market. But, no rider is average.
Most American cruisers are designed for long journeys on the open road in mind. Thailand too has a few long open roads, where low revs and comfortable cruising speeds could be important. But city riders have different needs. Street riders may want to move a vibration patch up or down the speed range, or gain better acceleration. Competitive riders alter gearing dependent on the length and design of a track to get the maximum performance from the motorcycle.
In simplistic terms, if 80km/h in sixth gear is 4,000rpm, fitting a smaller rear sprocket will make the motorcycle engine run at about 3,500rpm. With a smaller rear sprocket you can wind each gear out further before needing to shift. Your engine will work harder though, in a similar way to riding uphill. On a low powered motorcycle this will cost acceleration.Putting a larger sprocket on the back improves acceleration and perceptible power by making it easier for the engine to turn the final drive. But you will need to shift sooner and more frequently. At 80km/h in sixth gear your engine may now be spinning at 5,000rpm.
Trials riders need lower gearing to pull them up hills and they are unlikely to need high speeds. Engines produce less power at altitude, so mountain areas demand lower gearing to allow the engine to rev more freely.
Changing the front or rear chain drive sprocket is the simplest way to change gear ratios. Altering the number of teeth on the front or rear sprocket has different effects on the motorcycle's rpm and power delivery.
If you increase the number of teeth of the front sprocket, or reduce the number of teeth of the rear sprocket, you will reduce the rpm at cruising speed. This is called 'gearing up' and adds top speed to the motorcycle. It will also improve engine wear and fuel economy.
If you reduce the number of teeth of the front sprocket, or increase the number of teeth of the rear sprocket you will increase the rpm at cruising speed. This is called 'gearing down' and reduces top speed, but improves acceleration.
For example, if a front sprocket has four teeth and eight on the rear (artificial numbers), the fron sprocket spins twice to turn the rear sprocket once, making a 1/2 drive ratio or 0.5. Increase the rear sprocket to 12 teeth, while keeping a four tooth front sprocket, and the front sprocket spins three times to turn the rear sprocket once making a 1/3 drive ratio or 0.33.
More realistic numbers are 15 front and 34 rear. This give a final drive ratio of 15/45, which equals 3. Changing the rear sprocket for 43 teeth gives a final drive ratio of 15/43, which equals 2.867. This ratio will reduce the rpm at cruising speed. The smaller the number, the less rpm is needed at a given speed. Usually making a single tooth change on the front sprocket is sufficient to make a difference, while a more substantial change will be needed at the rear.