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Here Are Some Considerations for Sprocket Changes

If you have a chain-drive street bike and want to change your gearing, here are a few things to consider.

For better low-speed acceleration, fit a smaller countershaft sprocket or a larger sprocket on the rear wheel. Want to increase fuel mileage and decrease engine wear on long highway runs? Just reverse that advice.

But before you rush to your dealer’s parts counter, do some arithmetic and take a few measurements to avoid problems later.

The purpose of changing sprocket sizes is to alter the relationship between engine speed on your tachometer and road speed on your speedometer.

Let’s say your motorcycle comes with an 18-tooth sprocket in the front and a 43-tooth sprocket on the rear wheel. The ratio is 43 divided by 18, which equals 2.39.

Say you want more acceleration and don’t mind making the engine rev higher at highway speeds. Switch to a 17-tooth front sprocket and a 45-tooth rear. Now the ratio is 2.65, which means you’ve increased the ratio by approximately 10.9 percent. And you can expect to see the same percentage difference on your tach at any given speed and gear.

In other words, if you previously ran 5,000 rpm at 65 mph in fifth gear, expect the engine speed to increase 10.9 percent, to 5,545 rpm, after the swap.

Once you have a new ratio in mind, take a look at your bike. Make sure there’s some extra clearance between your sprockets and the chain guard, the swingarm and the countershaft sprocket cover. Your dealer can probably tell you which sprocket combinations will work on your bike without other modifications.

The key here is that, unless you’re going racing, you probably want to make small changes. Remember, the engineers who designed your bike had a pretty good idea of what gear ratios work well with your bike at street speeds.

Note that changes made to the countershaft sprocket have a greater effect on overall gearing than changes to the rear sprocket.

In the example above, reducing the countershaft sprocket by one tooth changes your gearing by about 6 percent, while adding one tooth to your rear sprocket changes your gearing only 2 percent.

Finally, be aware that making significant changes to your gearing may require you to shorten or lengthen your chain. In any case, you should change your chain at the same time you change sprockets, so your stretched-out chain won’t prematurely wear out your new sprockets.