If you are familiar with motorcycles, you must be familiar with common terms like sprockets, gears, gearing and sprocketing. These elements might not sound like much to everyone but they play a very significant role in how the power of your motorcycle is transferred to the tarmac. Even with the same amount of power on paper, there can be significant difference on how two exactly same motorcycles with different gearing perform. Most manufacturers tailor the gearing to suit the motorcycle’s characteristics. A commuter will have short gearing while a highway tourer will have tall gearing. While the gearbox and final drive setup is meant to suit a large number of riders, some choose to alter the setup to suit individual driving style. Here we discuss these parts and how you can alter the final drive setup to tailor the motorcycle to your needs.
See that large toothed wheel on which the chain runs? It is called sprocket. Most motorcycles rely on sprocket and chain setup for final drive. Some motorcycles rely on shaft drive or belt drive system, but there aren’t many. Usually the front sprocket is smaller and the rear sprocket is larger.
Sprocketing means changing the size of the sprocket, whether front or rear or both. This will affect the final drive and thus the gearing of the motorcycle. This is a very simple modification that can give impressive results if done right. The size of the sprockets can be anything sensible as long their ratio is same. It is the ratio that makes a difference. The gearing value is a ratio of teeth on the rear sprocket to the number of teeth on the front sprocket. This ratio can be used to alter the acceleration or top speed of the motorcycle.
A Smaller sprocket in the front or rear sprocket in large results in higher torque (more distant from the center of sprocket) being transmitted to the rear wheel and thus better acceleration. However, this will reduce the top speed of the motorcycle because the rear wheel will have fewer revolutions for the same engine speed now. You might have noticed the huge rear sprockets on the motorcycles of stunt riders. This is because they need insane acceleration for the wheelies and drifts. Moreover, they don’t need a high top speed so this is a sacrifice they can make.
At the front, if the sprocket is larger or smaller at the rear, this would cause a drop in acceleration but it will boost the top speed. The torque you get on the rear wheel is reduced (the perpendicular distance from the center is reduced) but that allows the rear wheel to run at high speed. This allows each gear to be used for a wider range of speeds and theoretically the top speed goes up significantly. We said theoretically because in case of small engines, you might not be able to use the top gear properly after switching to a smaller rear sprocket. If you tend to drive around a lot on highways, this setup would be great. Even if you don’t get a significant boost in top speed, you will get better fuel efficiency as the engine will run at less rpm for the same vehicle speed. But as we said already, it might not work very well on very small engines or if you overgear too much. It is recommended to alter the final gearing by 10 percent at the most.
To calculate the final gearing, find the ratio of number of teeth on rear sprocket to number of teeth on front sprocket. Like if on a motorcycle, the front sprocket has got 15 teeth and the rear one has got 45 teeth, the final gearing value is 3. Now if you switch to a 48 teeth sprocket at the rear, you get a final drive of 3.2 which would be undergearing and would give a high acceleration. If instead you switch to a 42 teeth sprocket on the rear, you get a final drive of 2.8 which would give you overgearing and would increase the top speed of the motorcycle. If you are planning to change both the sprockets, you should calculate the final drive ratio on the stock setup and on the setup you are going to choose to make sure the new setup is right.
Changing wheel size
The size of rear tyre can also alter the gearing of the bike. A higher profile increases the overall gearing of the motorcycle and thus it increases the top speed. This is because the high profile would increase the diameter of the wheel on the same width of rim. You must have noticed that performance motorcycles come with wheels of smaller diameter. They usually have 16 or 17 inch wheels. This is because they need to be very quick and they can make up for the speed with high engine rpms as compared to street bikes. Adventure bikes come with large wheels for obvious reasons but they need to be quick too. That is why along with large wheels, they also have large rear sprockets.
If you are willing to make your motorcycle quicker, switching to smaller wheel would help but you will have to settle with lower top speed. The diameter of the front wheel doesn’t make a difference in the speed or acceleration. However it would alter the speedometer reading. If you switch to a larger wheel, the indicated speed will be lower than what it actually is and in case of a smaller wheel, the indicated speed will be higher. If you are willing to make changes to the final drive of your motorcycle, you need to keep all these factors into account to attain the desired result. It would be tough to calculate the final drive while keeping all these factors into consideration but there are various tools available online that can help you simplify the job. There are some other methods to alter the gearing of vehicles. If you are looking to seriously alter the gearing of your motorcycle, you can also consider modifying the gearbox.