Motorcycles come from the factory with gearing that is a compromise designed for typical or average conditions. Many manufacturers gear for rapid acceleration to give them bragging rights rather than offer final ratios that allow for relaxed highway cruising. Other motorcycles may be geared so you can hardly ever get into top gear. Fortunately, there are ways to tailor your bike’s gearing to your preferences.
Gasoline engines operate at high crankshaft speeds, so it’s necessary to use gear reduction to match engine rpm to load and road speed. “Taller” gearing (also called numerically lower) allows the engine to run at a slower rpm for a given road speed, which reduces noise, engine wear, and fuel consumption and allows a higher top speed (if the engine has enough torque). “Shorter” (numerically higher) gearing can make your motorcycle quicker off the starting line but may reduce top speed and mileage while increasing engine noise and wear.
Motorcycles typically come with one of three basic final drive systems: chain, belt, and shaft drives. Chains and sprockets wear out relatively often and require replacement, and replacement time is an excellent opportunity to change sprocket sizes to modify gearing. You may need a chain with a different number of links to compensate for the number of teeth that are added or deleted.
Motorcycles driven by a belt and toothed sprockets can also be regeared at home using the same principles as a chain-drive model, except the availability of sprockets with a different number of teeth and matching belts is more limited and costly. Shaft-drive motorcycles can be regeared using the same formulas, but gears are significantly more expensive, must be installed in matched sets, and require careful assembly and setup that is beyond the average DIYer. Therefore, shaft drives should be modified by a dealer or specialist.
The number of teeth is usually stamped on the sprockets, or you can simply count the teeth. To determine final drive ratio, divide the number of rear-sprocket teeth by the number of front-sprocket teeth. For example, if the rear has 45 teeth and the front has 15 teeth, you have 45 ÷ 15 = 3.0:1.
Now let’s calculate percentage of change. Say you want to slow the engine down by 10 percent, so instead of revving 5,000 at cruising speed, it will drop down to 4,500 rpm. (5000 – 4500 = 500, and 500 ÷ 5000 = 0.10 or 10 percent) A 10 percent reduction of the original ratio of 3.0 is 0.30, so 3.0 – 0.30 = 2.70. This is the ratio you’d want to try for.
There’s an excellent ratio chart online at www.rebelgears.com/ratiochartview.html. According to the chart, the rear sprocket that gives the closest-to-desired ratio (without changing the front) is 41 teeth (41 ÷ 15 = 2.73). However, you can get even closer by changing to a 43-tooth rear sprocket and a 16-tooth front sprocket for a ratio of 2.69 (43 ÷ 16 = 2.69).
A larger rear sprocket and/or smaller front sprocket gives you shorter gearing and more bottom-end acceleration. A larger front sprocket and/or smaller rear sprocket gives you taller gearing for lower-rpm cruising. Using the 3:1 ratio as an example, changing one tooth on the countershaft is equal to three on the rear sprocket (this varies depending on your stock gearing).
The cheapest way to change gearing is to replace the front sprocket with another having one tooth more or less (these are usually readily available). You can’t go much smaller on the front sprocket because the chain will have trouble bending around it and will have less contact area. Changes greater than one tooth generally require a new chain (or belt). Also, space limitations usually prevent you from going too large on front sprockets. If you can’t get a sufficient change with just the front sprocket, changing the rear sprocket allows you to vary the number of teeth to get the desired ratio.
If you change gearing, your motorcycle’s speedometer might be affected. First, determine which wheel the speedometer measures, front or rear. If the speedometer runs off the front wheel, no changes are needed; it will still read speed exactly as before. If the speedometer runs off the rear wheel, changes may be in order. I’ve found that most motorcycle speedometers have a certain amount of “optimism” built in; that is, they typically indicate you are going between 5 and 10 percent faster than you actually are. Therefore if you change to taller gearing within that range, your speedometer will actually become more accurate. You can easily check your speedometer’s accuracy with a GPS.
If you change gearing significantly, you’ll probably want to do something to correct the speedometer and odometer errors that result. Mechanical speedometers can be modified by an automotive instrument specialist. If you have an electronic speedometer, SpeedoHealer is a device that enables the speedometer and odometer to be corrected. It can also adjust for changes in tire sizes and profiles.
Text and Photography: Ken Freund