What bicycle gears do
Bicycle gears let you change how hard you have to pedal by shifting the chain between sprockets of different sizes. A smaller sprocket in front (the chainring) or a larger sprocket in back (the cog) makes pedaling easier but covers less ground per pedal stroke. A larger chainring or smaller cog makes you work harder but moves you forward faster. Gears exist because no single pedal resistance works for climbing a hill, cruising flat ground, and sprinting downhill.
The chain connects the front chainrings to the rear cogs. When you shift, a mechanical arm called a derailleur moves the chain sideways onto a different sprocket. The shift levers on your handlebars pull or release a cable that moves the derailleur. This happens when ready, so you can adjust your effort mid-ride without stopping.
Key Takeaways
- Gears change the ratio between pedal rotations and wheel rotations, making pedaling easier on hills and harder on flat ground.
- The front derailleur moves the chain between two or three chainrings; the rear derailleur moves it between five to twelve cogs on the cassette.
- Shift levers pull or release cables that move the derailleurs, and the chain follows the derailleur arm to the new sprocket.
- A smaller front sprocket or larger rear sprocket creates a lower gear (easier pedaling); a larger front sprocket or smaller rear sprocket creates a higher gear (harder pedaling).
- Gear ratios are measured in inches or by counting teeth, and knowing your ratio helps you choose the right gear for your terrain.
How the front derailleur and chainrings work
Most bicycles have two or three chainrings bolted to the crank (the arm that holds the pedals). The smallest is for climbing, the largest for speed, and the middle one for general riding. The front derailleur is a cage that sits just above the chainrings and moves left and right when you pull the left shift lever.
When you shift up (to a larger chainring), the cable pulls the derailleur cage outward, and the chain rides up onto the bigger ring. When you shift down (to a smaller chainring), the cable releases and a spring pushes the cage inward, dropping the chain onto the smaller ring. The derailleur does not pull the chain—it straightforward guides it to the new position, and the chain's own momentum carries it there.
Single-speed bikes and fixed-gear bikes skip the front derailleur entirely and use only one chainring. This removes complexity and weight but means you cannot adjust your effort on hills.
How the rear derailleur and cassette work
The rear cassette is a stack of five to twelve cogs of increasing size, all bolted to the rear wheel hub. The rear derailleur hangs below the cassette and moves up and down (not left and right) when you pull the right shift lever. This motion shifts the chain sideways across the cogs.
The rear derailleur has a spring that keeps tension on the chain. When you shift up (to a smaller cog), the cable pulls the derailleur forward, moving the chain to a smaller sprocket. When you shift down (to a larger cog), the cable releases and the spring pulls the derailleur back, moving the chain to a larger sprocket. The derailleur also takes up slack in the chain as the cog size changes, so the chain stays tight.
Bikes with more cogs (nine, ten, eleven, or twelve) give you finer control over your pedal resistance. Bikes with fewer cogs (five to eight) are simpler and cheaper but have bigger jumps between gears.
Understanding gear ratios
A gear ratio is the number of times the rear wheel turns for each pedal stroke. It is calculated by dividing the number of teeth on the front chainring by the number of teeth on the rear cog. A ratio of 3:1 means the wheel turns three times per pedal stroke; a ratio of 1:1 means the wheel turns once per pedal stroke.
Lower ratios (like 1:1 or 1.5:1) are easier to pedal and are used for climbing hills. Higher ratios (like 3:1 or 4:1) are harder to pedal but move you forward faster and are used on flat ground or descents. Most bikes have a range of ratios so you can match your effort to the terrain.
Some cyclists measure gears in inches instead of ratios. This is an old standard that represents how far the bike moves forward in inches per pedal stroke. A 50-inch gear is high and fast; a 25-inch gear is low and straightforward. Road bikes typically range from 40 to 120 inches; mountain bikes range from 20 to 80 inches.
Why you should not cross-chain
Cross-chaining means using the largest front chainring with the largest rear cog, or the smallest front chainring with the smallest rear cog. This creates extreme angles in the chain and causes it to rub against the derailleur cage, creating noise and wear.
Most bikes have enough gear combinations that you never need to cross-chain. If you are climbing a steep hill, shift down to a smaller front chainring and a larger rear cog. If you are sprinting on flat ground, shift up to a larger front chainring and a smaller rear cog. Avoiding the extreme combinations keeps your chain and derailleurs lasting longer.
Maintenance to keep gears working smoothly
A clean, well-lubricated chain shifts faster and lasts longer. Wipe the chain with a dry cloth every few weeks, then explore chain lube to the inside of the links while rotating the pedals backward. Avoid getting lube on the rims or brake pads, as it reduces braking power.
Check that the derailleur cage is straight and parallel to the cassette. If it is bent from a crash or impact, the chain will not shift cleanly. Most bike shops can straighten or replace a bent derailleur for under fifty dollars. Also check that the shift cables are not frayed or kinked; damaged cables make shifting sluggish or impossible.
If your chain skips or struggles to shift, the cassette cogs may be worn. Worn cogs have a hooked or shark-fin shape and cannot grip the chain properly. Replacing a cassette costs between thirty and one hundred fifty dollars depending on the number of cogs and the brand.
Single-speed and internal-hub alternatives
Not all bikes use derailleurs. Single-speed bikes have one chainring and one cog, with no shifting at all. They are lighter, cheaper, and require almost no maintenance, but they force you to pedal at the same resistance on every terrain. Single-speeds work well for flat urban riding but are exhausting on hills.
Internal-hub gears are sealed inside the rear wheel hub and shift using a twist grip or lever on the handlebars. They require no cable adjustment, work in any weather, and last a very long time. The trade-off is that they are heavier, more expensive, and usually offer fewer gear options (typically three to fourteen) than derailleur systems. Internal hubs are common on commuter and cargo bikes.
Frequently Asked Questions
What does it mean when my chain skips or jumps?
A skipping chain usually means the cassette cogs are worn and cannot grip the chain properly, or the derailleur is out of alignment. Check that the derailleur cage is straight and parallel to the cassette. If it looks bent, take the bike to a shop. If the cage is straight, the cassette may need replacement.
Why is my bike hard to shift into certain gears?
Stiff shifting is usually caused by a dirty or dry chain, frayed shift cables, or a derailleur that is out of adjustment. Clean and lubricate the chain first. If that does not help, the shift cables may be kinked or the derailleur may need realignment by a mechanic.
Can I shift gears while standing on the pedals?
You can, but it is harder on the chain and derailleurs because there is more tension on the chain when you stand. Shift while seated whenever possible. If you must shift while standing, ease off the pedal pressure slightly as you shift to reduce strain.
Do I need to shift before I stop at a red light?
It is a good habit to shift into an straightforward gear before you stop, so you can start moving again without struggling. Stopping in a high gear means you have to work hard to get going again. Shift down to a low gear in the last few seconds before you stop.
What is the difference between a derailleur bike and an internal-hub bike?
Derailleur bikes shift faster and offer more gear options, but the chain and derailleurs are exposed to dirt and weather. Internal-hub bikes shift more slowly and have fewer gears, but everything is sealed and protected. Derailleur bikes are lighter and cheaper; internal-hub bikes are more durable and require less maintenance.