Rim
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Usually made of aluminum or carbon (or both), the rim is the circular part of the wheel. Road rims are made with a flat section on the outer edge to provide a smooth braking surface.
Wheel Hub
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This is the center of a bicycle wheel. It consists of an axle, bearings and a hub shell. The hub shell is typically where the spokes of the wheel attach. The axle is the portion of the hub that allows a quick release skewer to pass through. When purchasing wheels, the quality of the hub's bearings are directly related to the rolling resistance and performance of the wheel.
Spokes
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Spokes connect the hub of a wheel to the rim, and are loaded under tension. On the end of each spoke is a threaded nut called a nipple, which is used to adjust the tension of each spoke.
Quick-Release Wheel Skewer
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The skewer attaches the wheel to the bicycle by sliding through the axel of a wheel hub and is tightened inside the dropout. The quick-release lever allows the wheel to be taken on and off without disassembly.
Fork
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A bicycle fork holds the front wheel and allows the rider to maintain steering control over the bike. Carbon forks have become the standard in recent years because of the added shock absorption and comfort over other metal forks.
Head Tube
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Part of the bicycle frame, the head tube is where the front-fork steer tube and the headset fit. Longer head tubes give a rider more of an upright position, which is generally considered more comfortable. For a racing geometry, head tubes are usually shorter in length.
Headset
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A headset is a bicycle component that allows the handlebar and the steer tube of the fork to rotate. The bearings in a headset work directly with the inside of the head tube as a low friction point of contact.
Stem
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The handlebars connect to the steer tube of the bicycle fork by way of the stem. Most new road bikes use a threadless system that is compatible with a threadless headset and fork design. Stems are made in a variety of lengths, which alter your reach and position on the bike.
Handlebars
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On a road bike, these are usually called drop handlebars. Drop handlebars allow multiple hand positions: on the tops, the hoods (rubber grip above the brake levers), and in the drops (curved section).
Shifters
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A shifter on a new road bike is directly behind the brake lever. Road bikes made prior to the 1990s had shifters on the down tube or stem. The shifters are used to control a bike's gears in order to select a desired gear ratio. The left shifter typically operates the front derailleur. The right shifter controls the function of the rear derailleur, moving the chain up and down on the rear cassette.
Brake Levers
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Brake levers are mounted in front of the handlebars. On most modern component groups, the brake lever is integrated into the shifting mechanism. The left brake lever closes the caliper on the front wheel, and the right brake lever closes the caliper on the back wheel.
Brake Calipers
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Most road bikes operate on a caliper brake design mounted above each wheel. The force applied to the brake lever closes each caliper through the tension of a brake cable. The arms of the calipers extend to each side of the rim, contacting the rim surface with a brake pad.
Top Tube
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This is the portion of the frame that connects the head tube to the top of the seat tube. Most older and some newer frames have downward-sloping top tubes. Many newer models adopt a more compact geometry that has the top tube sloping upward. Compact frames are designed to allow for additional stand over clearance.
Down Tube
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The head tube is connected to the bottom bracket shell by the down tube. A derailleur cable usually fits on the underside of the down tube or, in some modern frame designs, inside the down tube for aerodynamics. On older bicycles, the shift levers may also be mounted here.
Seat Tube
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Part of the frame that holds the seatpost. Most seat tubes have mounts for a bottle cage and a braze-on mount for the front derailleur.
Seat and Seatpost
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The seatpost is the tube that extends from the seat tube section of a bicycle frame and attaches to the rails of a bicycle seat, or saddle. Most seatposts have minimum and maximum insertion points for safety reasons.
Front Derailleur
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Bicycle component that moves the chain side-to-side between chainrings. Front derailleurs are usually mounted by a clamp that matches the diameter of the seat tube or by a braze-on derailleur hanger, which attaches on the frame's seat tube with a mounting bolt.
Bottom Bracket
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The bottom bracket connects the crankset to the frame of a bicycle. The bottom bracket consists of a spindle and bearings that allow the cranks to rotate. The bottom bracket fits inside the bottom bracket shell of the bike frame. This point is where the seat tube, down tube and chain stays of the frame connect.
Crank
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Component of the drivetrain where the pedal attaches. Similar to a the belt on a car, a rider's legs use the arms of the crank to spin and move the bike forward. Sometimes the crank is also referred to as a crankset, which consists of the crank arms as well as the chainrings.
Chainrings
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Chainrings come in different sizes and are circular in shape. Chainrings have spaced teeth used to engage the chain. Modern road bikes usually have two or three chainrings. The smaller sizes are used for climbing while the larger chainrings are used to generate more speed on flat or downhill sections of road.
Chain Stay
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The chain stay is the part of the frame that runs parallel to the chain. It connects the bottom bracket shell to the rear dropout. Usually, a derailleur cable will run underneath the chain stay to the rear derailleur. The chain stay on the non-drivetrain side is usually connected to the opposite chain stay with a brace in front of the rear wheel.
Seat Stays
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The seat stay connects the seat tube to the rear dropout. Most bike frames use two parallel tubes that connect above the rear wheel. This is also the spot on the frame where the rear brake caliper is attached.
Dropout
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This is the spot on the frame where the front fork and rear triangles end. It is also where the axle of each wheel hub is attached with a quick-release wheel skewer. Dropouts can be horizontal or vertical, with the latter being the most common on newer road bikes.
Cassette
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Sometimes referred to as a cogset, this is a set of multiple sprockets that attach to the freehub body on the rear wheel. The cassette is the part of the drivetrain that works with the rear derailleur to provide multiple gear ratios. The smallest sprocket on the cassette provides the largest gear and vice versa.
Rear Derailleur
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The rear derailleur is a component that attaches to a pivot point on the rear triangle of a bicycle frame and moves the chain up and down the cassette. The components of a derailleur may be made up of aluminum, steel, plastic or carbon fiber, depending on the brand.
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