The pursuit of performance for a motorcycle enthusiast is a perennial obsession. Irrespective of the engine capacity or the power output, a passionate petrolhead’s desire to get better acceleration and speed from their motorcycle is almost unquenchable. Increasing the power output of an engine itself is a complicated process, there are various ways in which that power and torque are laid down onto the tar, thereby improving the performance of the motorcycle. One of the easiest ways to improve the acceleration or top speed of a motorcycle without tinkering with the engine is to change its sprocketing. In this article, we will simplify what motorcycle sprocketing is and how it affects the performance of your motorcycle. A note of caution before we go any further though. Manufacturers carefully choose the sprocketing of their stock motorcycles based on customer expectations and after benchmarking their products with competition. Altering the sprocketing of a motorcycle may, thus, adversely affect its performance on specific parameters and may also void its warranty. So if you ever decide to play around with the sprocketing or gearing of your motorcycle, do it only with the full knowledge of the subject and assuming full responsibility for your actions.
What is sprocketing, or gearing?
A motorcycle may feature a variety of final drives, including shaft, belt, or chain drive systems. However, since sprocketing is applicable only to chain drive systems, we will keep the discussion in this article limited to motorcycles featuring chain drive systems.
Sprocketing is the final drive ratio of a motorcycle. A chain driven motorcycle comprises two sprockets, one on the side of the engine, called the countershaft sprocket, and another one at the rear wheel. The final drive ratio of a motorcycle is arrived at by dividing the number of teeth the rear sprocket of a motorcycle has by the number of teeth on its front sprocket. For example, if a motorcycle has a front sprocket with 15 teeth and a rear with 45 teeth, its final drive ratio will be 45/15 = 3. That number signifies that the front sprocket will have to rotate three times for the rear sprocket to rotate once. Final drive ratios differ based on what the motorcycle is expected to do. A higher final drive ratio will lead to better torque and quicker acceleration lower down the rev range as a rule of thumb. Conversely, a lower final drive ratio will lead to slower initial acceleration, but a higher top speed.
In general, a smaller front sprocket, and/or a bigger rear sprocket will translate into a higher final drive ratio, and lead to better acceleration at the cost of top speed. On the other hand, a bigger front sprocket and / or a smaller rear sprocket would denote a lower final drive ratio and would lead to better top speed, at the cost of quick initial acceleration. In essence, sprocketing is a tool to manipulate how the engine rpm translates to road speed, by increasing or decreasing the torque served at the rear wheel at any given rpm.
When you want to alter the performance of your stock motorcycle, you can play around with its gearing to change its performance characteristics. If you want quick bursts of acceleration, you would typically be looking at higher gear ratios. However, if it’s a long-legged, high fuel-efficiency oriented cruising experience on the highway you are looking at, you would want to go down a notch on the final drive ratio.
Lower or higher final drive gear ratios are also sometimes addressed as tall, or short gearing, respectively. Taking the example, we cited above for reference, if we want better acceleration, you have to either increase the sprocket teeth at the rear or reduce the number of teeth at the front sprocket. That will make the final drive ratio figure go above three, and in this case, it will be termed as relatively ‘short’ gearing. Similarly, if one wants better top speed, even if it comes at the cost of quick acceleration, we will have to either increase the number of teeth on the front sprocket or reduce the number of teeth at the rear sprocket. This will take the figure below three of the final drive ratio below three will be relatively ‘taller’. Do note that the terms tall and short gearing can be somewhat confusing, as a smaller number represents a taller gear ratio while a higher number denotes a shorter gear ratio.
For the small and medium capacity motorcycles, going with a higher final drive ratio, or short gearing, generally improves acceleration and makes them more exciting to ride. Most production bikes are generally sprocketed slightly on the taller side to offer good fuel efficiency and top speed along with decent acceleration. Hence, shorter gearing is the way forward if you are looking for a spurt in acceleration. For applications where a very high amount of torque is required at very low engine speeds, like in motorcycle stunting, where performers need to lift the front wheel at the slightest wring of the throttle, extremely short gearing with a tiny front sprocket and a huge rear unit is employed. These motorcycles are suitable for stunts, but don’t have a high top speed, and it’s an apt compromise, for the latter is not required of a stunt bike.
How much is too much?
Even with minor changes to sprocketing, you can experience a significant difference in the performance characteristics of your motorcycle. A tooth less or more upfront, and a tooth or two more or less at the rear are generally good enough to bring about a big difference in the performance of everyday motorcycles. In terms of ease of replacement, changing the front sprocket by a tooth is the easiest thing to do, as the front sprocket is relatively inexpensive and easier to replace. The rear sprocket is bigger, more expensive, and cumbersome to replace, as you have to remove the rear wheel to have it replaced.
Unless you need to prepare a purpose-built bike, don’t tinker around with the sprocketing of your everyday motorcycle too much. An advantage of not making significant changes with sprocketing is that the chain tension can be readjusted, and you don’t have to go for a different sized chain altogether. Also, with relatively more minor changes, the difference in performance won’t cause too much of a negative impact on the engine and other mechanicals.
Another essential thing to note here is that if your motorcycle’s ECU picks the readings from the transmission, the changed gearing may lead to an inaccurate speed display on the speedo. However, if your motorcycle’s speedo is the mechanical type, where a cable connected to the front wheel calculates the speed, the gearing changes won’t make any difference. A bigger or smaller front wheel doesn’t make a difference to the sprocketing of a bike. A bigger or smaller rear wheel, however, does amount to a change in gearing, as the torque required to rotate a bigger or smaller wheel differs hugely. Hence, a bigger rear wheel would mean taller gearing, meaning lower initial acceleration and higher top speed. Conversely, a smaller rear wheel would translate to quicker acceleration and lesser top speed or short gearing.