Bicycle Anatomy
A bicycle is an integrated system where every component affects the others. Understanding what each part does—and how it connects to the whole—transforms maintenance from guesswork into informed repair. This guide names the parts, explains their function, and describes how they work together.
The Frame
The frame is the bicycle’s skeleton—a structure of tubes that connects every other component and defines the bike’s character.
The Diamond Frame
Most bicycles use the “diamond frame” geometry: two triangles sharing a common side. This shape is remarkably efficient at handling the complex forces of pedaling, steering, and braking while remaining light and stiff.
Front triangle (main triangle):
- Head tube: Short vertical tube at the front. Houses the headset bearings and accepts the fork’s steerer tube. Its angle (head tube angle) strongly influences steering behavior.
- Top tube: Runs from head tube to seat tube. Traditional frames run it horizontal; modern frames often slope it downward for standover clearance.
- Down tube: Largest-diameter tube, runs from head tube to bottom bracket. Bears the highest loads; often houses water bottle mounts and cable routing.
- Seat tube: Vertical tube accepting the seatpost. Measured center-to-top or center-to-center to determine frame size.
Rear triangle:
- Seat stays: Two thin tubes running from the seat tube to the rear axle. Provide vertical compliance and mount rear brakes (on rim brake bikes).
- Chain stays: Two tubes running from the bottom bracket to the rear axle. Must clear the chain and rear tire while positioning the rear wheel for proper handling.
Bottom bracket shell: The cylindrical shell where the down tube, seat tube, and chain stays meet. Houses the bottom bracket bearings that allow the crankset to spin.
Frame Materials
Each material offers different ride characteristics:
Steel (chromoly): Springy, repairable, durable. Heavier than alternatives but provides comfortable ride quality and can be welded if damaged. Corrodes if not protected.
Aluminum: Light, stiff, affordable. Doesn’t corrode but can’t be easily repaired. Fatigues over time; eventually cracks rather than bending.
Carbon fiber: Lightest and stiffest option. Can be tuned for different flex characteristics in different directions. Expensive; damage isn’t always visible; fails catastrophically.
Titanium: Light, corrosion-proof, excellent ride quality. Expensive and requires specialized welding. Extremely durable.
How Geometry Affects Handling
The frame’s angles and dimensions determine how the bike rides:
Head tube angle: Steeper angles (72-74°) create faster, twitchier steering; slacker angles (66-70°) create stable, slower steering. Road bikes are steep; mountain bikes are slack.
Seat tube angle: Affects pedaling position. Steeper angles (74-76°) position you over the pedals for spinning; slacker angles (72-73°) shift weight rearward.
Chainstay length: Shorter chainstays (405-425mm) make the bike more responsive but less stable; longer chainstays (430-460mm) add stability and carrying capacity.
Wheelbase: Total length from front axle to rear axle. Longer wheelbases are more stable; shorter wheelbases are more maneuverable.
The Fork
The fork holds the front wheel and enables steering. Its construction profoundly affects handling feel.
Fork Anatomy
Steerer tube: The tube that extends upward through the head tube. Threadless systems clamp the stem directly to the steerer; threaded systems use a quill stem inserted inside.
Crown: The horizontal piece connecting the steerer to the fork blades. Some forks have dual crowns for extreme suspension travel.
Fork blades (legs): The two tubes or arms extending down to the wheel. Their shape, material, and construction determine fork flex and compliance.
Dropouts: The slots or holes at the bottom of the fork blades where the wheel axle mounts. Quick-release dropouts have open slots; thru-axle dropouts have closed holes.
Brake mounts: Attachment points for the brake caliper—either rim brake bosses, disc caliper mounts, or both.
Rake and Trail
Rake (offset): The forward distance between the steerer tube’s centerline and the front axle’s centerline. More rake increases stability at the cost of responsiveness.
Trail: The distance on the ground between where the steerer axis would intersect the ground and where the tire actually contacts the ground. More trail creates stronger self-centering; the bike “wants” to go straight.
Fork rake and head tube angle together determine trail. Changing one requires adjusting the other to maintain intended handling.
Suspension Forks
Suspension forks contain springs (air or coil) and dampers (oil cartridges) to absorb impacts. Key terms:
Travel: How far the fork can compress (80mm-200mm+ depending on use) Preload: Initial spring tension; affects sag (how much the fork compresses under rider weight) Rebound damping: Controls how fast the fork extends after compression Compression damping: Controls how easily the fork compresses
The Headset
The headset is the bearing assembly that allows the fork to rotate smoothly within the head tube. It’s invisible when installed correctly—you only notice it when it fails.
Headset Types
Threaded headsets (traditional): The fork’s steerer tube has threads; a locknut and adjusting race thread onto it. Common on older and some steel bikes.
Threadless headsets (modern standard): The steerer is smooth; the stem clamps onto it. A top cap and star nut apply preload before the stem bolts tighten. Easier to adjust; allows quick stem height changes.
Integrated headsets: The bearings sit directly in the head tube without separate cups. Common on modern road and mountain bikes.
Headset Function
The headset performs two jobs:
- Allow smooth rotation of the fork/steerer within the head tube
- Prevent the fork from pulling out of the head tube under braking
Bearings can be loose balls (traditional) or sealed cartridges (modern). Cartridge bearings last longer with less maintenance but are replaced rather than repacked.
Signs of Headset Problems
Loose headset: Clunking when braking, notchy steering, visible play when rocking the bike forward with the front brake engaged.
Worn bearings: Grinding or rough feeling when turning the handlebars, especially with the front wheel off the ground. Steering may feel indexed—like it wants to point specific directions.
The Cockpit
The cockpit is everything you touch while riding: handlebars, stem, grips or tape, brake levers, and shifters.
Stem
The stem connects the handlebars to the fork’s steerer tube.
Threadless stems (modern): A faceplate clamps the handlebar; the body clamps the steerer tube. Length (40-130mm) and angle (positive, negative, or zero) adjust reach and height.
Quill stems (traditional): Insert into a threaded steerer tube; an expander wedge holds them in place. Height adjustable without changing the stem.
Stem length affects handling: shorter stems quicken steering; longer stems stabilize it.
Handlebars
Bar shape defines hand positions and riding posture:
Drop bars: Multiple hand positions (tops, hoods, drops). The “drop” refers to the curved lower section; the “reach” is how far forward the drops extend. Road, gravel, and touring bikes.
Flat bars: Single hand position, wide for leverage. More upright position, direct steering feel. Mountain bikes and hybrids.
Riser bars: Flat bars with an upward sweep for even more upright position. Mountain bikes and commuters.
Bullhorns/pursuit bars: Forward-pointing extensions. Time trial and track bikes.
Bar width matters: wider bars give more leverage and stability (mountain biking); narrower bars are more aerodynamic (road).
Grips and Bar Tape
Grips (flat bars): Rubber or foam sleeves, sometimes with lock-on collars for security. Provide cushioning and grip.
Bar tape (drop bars): Wraps the bars from end to end. Cork, foam, leather, or synthetic. Absorbs vibration and provides grip.
Controls
Brake levers: Activate the brakes. On drop bars, they’re integrated into the “hoods”—the ergonomic pods where you rest your hands.
Shifters: Change gears. Integrated brake/shift levers (brifters) are standard on drop bars. Flat bars use separate trigger or twist shifters.
The Saddle and Seatpost
The saddle supports your weight; the seatpost positions it. Together they’re crucial for comfort and power transfer.
Saddle Anatomy
Shell: The base structure, usually rigid plastic or carbon fiber.
Padding: Foam or gel on top of the shell. More padding isn’t always better—excess padding deforms under load and can cause pressure points.
Rails: The parallel rods underneath that clamp into the seatpost. Standard rails are 7mm round; oversized carbon rails require compatible seatpost heads.
Cover: The outer material. Leather (needs breaking in, long-lasting), synthetic leather (weather resistant), or textile.
Cutout/channel: Many saddles have a center groove or hole to relieve pressure on soft tissue.
How Saddles Work
Contrary to intuition, a saddle shouldn’t feel like a cushion. You sit on your “sit bones” (ischial tuberosities), which are designed to bear weight. A proper saddle supports these bones; the padding is just comfort.
Saddle width should match sit bone width—too narrow and you’re on soft tissue; too wide and the edges chafe your thighs.
Seatpost
Setback: The horizontal distance the saddle clamps behind the post’s centerline. Inline posts have zero setback; setback posts position the saddle 15-35mm rearward.
Diameter: Must match the frame’s seat tube. Common sizes: 27.2mm, 30.9mm, 31.6mm. Shims can adapt smaller posts to larger tubes.
Adjustment: The seatpost head clamps the saddle rails and allows fore/aft sliding plus tilt adjustment.
Dropper posts: Seatposts that lower on command (via a handlebar lever) for descending. Common on mountain bikes.
The Drivetrain
The drivetrain converts your leg power into wheel rotation. It’s the most complex system on the bike.
Crankset
The crankset consists of the crank arms (which the pedals attach to) and the chainrings (the front gears).
Crank arms: Connect pedals to the bottom bracket spindle. Length varies (165-175mm for adults); longer cranks provide more leverage but wider pedal circles.
Chainrings: The toothed rings that engage the chain. Single-ring (1x) setups are simpler; double and triple rings provide wider gear ranges.
Spider: The interface between crank arm and chainrings. Some chainrings bolt to spiders; others integrate directly with the crank arm (direct mount).
BCD (bolt circle diameter): The diameter of the circle passing through the chainring bolt holes. Different BCDs require different chainrings.
Bottom Bracket
The bottom bracket (BB) contains the bearings that allow the crankset to spin. Several standards exist:
Threaded BSA/English: The traditional standard. Cups thread into the frame’s bottom bracket shell.
Press-fit: Bearings press directly into the frame without threads. Lighter but often creakier.
Square taper: The spindle has a square-profile taper; the crank arm presses onto it. Traditional, durable, heavy.
External cup/outboard bearings: The bearings sit outside the frame, allowing larger (stiffer) spindles.
Chain
The chain is the link between your power and the wheel. Each link consists of:
Outer plates: The flat sides you see Inner plates: The narrower plates inside Rollers: The cylindrical pieces that engage the chainring and cog teeth Pins: The rivets holding plates together
Chain width varies by the number of gears (11-speed chains are narrower than 8-speed). Don’t mix chain speeds with incompatible drivetrains.
Chains stretch over time—technically, the pins wear and the pitch lengthens. A worn chain damages the cogs and chainrings it engages. Measure chain wear regularly with a chain checker.
Cassette
The cassette is the stack of cogs on the rear wheel. A typical cassette spans a range (e.g., 11-32T means 11 teeth on the smallest cog to 32 teeth on the largest).
Gear ratio: Chainring teeth divided by cog teeth. 50/25 = 2:1. Higher ratios are harder to pedal but cover more ground per revolution.
Gear inches: An older way to express gear ratios that accounts for wheel size. Gear ratio × wheel diameter.
The cassette slides onto the freehub body and is secured with a lockring.
Freehub and Freewheel
The mechanism that allows you to coast (wheel spinning without pedals moving):
Freehub: The modern standard. A splined body attaches to the rear hub; the cassette slides onto the splines. Ratchets inside the hub body engage when you pedal.
Freewheel: The older standard. The ratchet mechanism is built into the cog cluster, which threads onto the hub. Heavier, weaker axle support.
Rear Derailleur
The rear derailleur moves the chain between cogs on the cassette.
Parallelogram mechanism: The derailleur body swings inward and outward on a parallelogram linkage, maintaining the chain’s angle to the cogs.
Jockey wheels (pulleys): Two small wheels that guide the chain. The upper pulley is the guide pulley; the lower is the tension pulley.
Cage: The arm holding the jockey wheels. Long cages handle large cassettes and big chainring differences; short cages are snappier but handle less range.
Limit screws: H (high) and L (low) screws set physical stops preventing the derailleur from throwing the chain off the cassette.
B-tension screw: Adjusts the gap between the upper jockey wheel and the cassette.
Clutch: Found on many modern derailleurs. Adds friction to the parallelogram movement, reducing chain slap on rough terrain.
Front Derailleur
The front derailleur moves the chain between chainrings. It’s a simpler cage that pushes the chain sideways. Many modern bikes (especially mountain and gravel) skip the front derailleur entirely, using single-chainring (1x) drivetrains.
The Wheels
Wheels are the contact point between bicycle and road. They determine weight, acceleration, durability, and comfort.
Hub
The hub is the wheel’s center. It contains bearings for the axle and (on rear wheels) the mechanism that transfers pedaling power.
Flanges: The wider sections where spokes attach. Higher flanges can make wheels stiffer but heavier.
Bearings: Allow the axle to spin freely. Cup-and-cone bearings are adjustable and serviceable; cartridge bearings are sealed and replaceable.
Axle types:
- Quick-release (QR): 9mm front, 10mm rear. Cam mechanism secures wheel in dropouts.
- Thru-axle: 12mm, 15mm, or 20mm diameter. Threads into the frame/fork for greater stiffness and alignment.
Rear hub specifics: The freehub body accepts the cassette. Engagement points determine how quickly the drivetrain responds when you start pedaling.
Spokes
Spokes are tensioned wires connecting hub to rim. A well-built wheel is a tension structure—the rim hangs from the upper spokes while the lower spokes support it from below.
Spoke count: More spokes (32, 36) means stronger wheels; fewer spokes (24, 28) means lighter but more fragile.
Spoke patterns: Radial (straight from hub to rim) is light but less strong. Crossed patterns (two-cross, three-cross) provide better load distribution and torque transfer.
Spoke nipples: The threaded fasteners at the rim that allow spoke tension adjustment.
Rim
The rim is the hoop that gives the wheel its shape and provides a surface for braking (rim brakes) and tire mounting.
Rim profile: The cross-sectional shape. Deep-section rims are aerodynamic but heavier; shallow rims are lighter but less aero.
Rim width: Internal width affects tire shape when inflated. Wider rims (21-30mm internal) support wider tires better; narrower rims (15-19mm) suit narrow tires.
Rim bed: The channel where the tire sits. Must be compatible with your tire type (clincher, tubular, or tubeless).
Brake track (rim brakes): The flat surface where brake pads contact. Must stay clean and true.
Tires
The tire is the only contact between bike and road. Its design affects grip, rolling resistance, comfort, and flat protection.
Bead: The edges that hook into the rim. Wire beads are heavier but cheaper; folding (Kevlar) beads are lighter and packable.
Casing: The fabric structure giving the tire shape. Higher thread count (TPI) means suppler, faster-rolling, but less puncture-resistant tires.
Tread: The rubber pattern contacting the ground. Slick for road; knobby for off-road. Compound hardness affects grip vs. longevity.
Size notation: 700x28c means 700mm diameter, 28mm width (road). 27.5x2.4 means 27.5” diameter, 2.4” width (mountain). ETRTO standard (e.g., 28-622) is unambiguous.
Tubes and Tubeless
Tube (clincher): A separate rubber tube holds air inside the tire. Punctures are fixable with patches. The simplest, most common system.
Tubeless: The tire seals directly to the rim; sealant inside plugs small punctures automatically. Lower pressures possible, reduced pinch flat risk, but more setup complexity.
Tubular: The tube is sewn inside the tire, which glues to the rim. Lightest, best feel, but expensive and harder to repair. Race use mainly.
The Brakes
Brakes convert your momentum into heat through friction. Everything else—lever design, cable routing, pad material—serves this physics.
Rim Brakes
Pads squeeze the rim to slow the wheel. Several types:
Caliper brakes: A single pivot point; the arms squeeze inward. Common on road bikes. Lightweight but limited tire clearance.
Cantilever brakes: Two separate arms pivot on bosses brazed to the fork/frame. Wide clearance for fenders and fat tires. Older touring and cyclocross standard.
V-brakes (linear-pull): Long cantilever arms with a cable pulling horizontally across. Powerful, common on hybrids and mountain bikes (pre-disc era).
Rim brake advantages: Light, simple, easy to inspect. Disadvantages: Performance degrades when wet; wears out rims over time.
Disc Brakes
A caliper squeezes a rotor attached to the hub. Two types:
Mechanical disc: A cable pulls a lever that pushes the pads. Simpler maintenance; requires manual pad adjustment.
Hydraulic disc: Fluid (mineral oil or DOT) transfers lever force to pistons that push the pads. Self-adjusting, more powerful, better modulation. Requires bleeding to maintain.
Disc brake advantages: Consistent wet/dry performance; doesn’t wear rims; better heat dissipation. Disadvantages: Heavier, more complex, harder to field-repair.
How Braking Works
Braking force depends on:
- Leverage: Longer levers multiply hand force
- Friction surface area: Larger pads grip more
- Friction coefficient: Pad material against rim/rotor
- Mechanical advantage: Cable pull ratio, hydraulic piston area
Maximum braking happens just before the wheel locks. Beyond that, you’re skidding—less control, more wear. Modulation is the ability to find and hold that threshold.
The Pedals
Pedals are your power interface—where force enters the system. Design affects efficiency, comfort, and control.
Platform Pedals
Flat surfaces for any shoe. Pins or grip patterns prevent slipping. Best for: commuting, casual riding, learning.
Clipless Pedals
Despite the confusing name, clipless pedals lock to a cleat mounted on special shoes. “Clipless” distinguishes them from older toe-clip systems.
How they work: A spring-loaded mechanism grabs the cleat when you step down; you release by twisting your heel outward.
Benefits: Power throughout the pedal stroke (you can pull up as well as push down); foot always optimally positioned; no foot slip on rough terrain.
Types: Road cleats (large, 3-bolt) offer more contact area; mountain cleats (2-bolt) are smaller and walkable.
Pedal Anatomy
Body: The platform you stand on. May be plastic, aluminum, or carbon.
Spindle: The axle that threads into the crank arm. Note: left pedal is reverse-threaded (tightens counter-clockwise).
Bearings: Allow the body to spin on the spindle. Loose balls or sealed cartridges.
How It All Connects
The bicycle works because every system connects to every other:
- Frame holds headset, which holds fork, which holds front wheel
- Frame holds bottom bracket, which holds crankset, which holds pedals
- Chain connects crankset to cassette on rear wheel
- Derailleurs position chain across chainrings and cassette
- Brake levers pull cables that actuate calipers that squeeze rims or rotors
- Seatpost fits into frame, holds saddle, positions rider
- Stem clamps handlebars to steerer in headset
Understanding these connections helps you diagnose problems. If shifting is bad, the issue might be the derailleur, the cables, the shifter, the hanger alignment, or the wheel position. Knowing how the systems connect lets you isolate the fault.