18 speeds is a common drivetrain configuration for modern bicycles, with options for a double chainring paired with a 9-gear cassette or a triple chainring with a 6-gear cassette. Understanding how to use different gear ratios to ascend hills, quickly accelerate and keep your muscles moving at a good cadence is a great way to master your bicycle. You can also work to preserve the integrity of your chain by avoiding gear ratios that put the chain under additional stress.
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The Driving Force
Your 18-speed bicycle uses two derailleurs to change between different gear ratios, which allow you to shift into lower gears for climbing and higher gears when you're moving faster. The front derailleur picks up the chain and moves it gently between your chainrings, and your rear derailleur uses an arm to maintain tension while it slides the chain up and down the cassette. The rings are different sizes, but the spring in the derailleur arm keeps the chain taut no matter which gears you choose.
All Downhill From Here
When you're moving at a quick pace down a steep incline, a higher gear ratio will slow down your rate of pedaling. Even though you're pedaling slower, you're transferring more power to the back wheel because the higher gear ratio causes the back wheel to rotate more times with each pedaling stroke. The inverse happens when you shift down; you'll need to use less power to turn the back wheel once in a lower gear ratio because the back wheel spins more often with each stroke. This makes ascending a hill significantly easier.
Your bicycle is capable of using all of its gears, but some ratios put more stress on the drivetrain than others. The highest gear on your lowest chainring and the lowest gear on your largest chainring can spell trouble, since the chain will be held between the innermost chainring and the outermost gear on your cassette, or vice versa. This pulls sideways on your chain slightly, which isn't the end of the world, but over time it can cause extra wear on your chain and your drivetrain; it's best to avoid these two particular ratios. Try to shift up to a higher chainring or down to a lower chainring before you reach the end of your cassette.
The best way to use the 18 gears on your bike is to try to maintain a constant cadence. Since shifting into higher or lower gears changes the speed you'll go at a given pedal stroke, the most efficient way to ride your bike is to keep your pedaling cadence between 60 and 90 rotations per minute using higher and lower gears. This cadence is great for an aerobic workout and for keeping your drivetrain under minimal stress. To avoid getting stuck in a gear that is too low or high for the incline you're on, shift before you reach the bottom or the top of a large hill, preemptively getting the bike prepared for the road ahead.