BMX Guide
BMX Bike Anatomy
A BMX bike is a single speed, 20 inch bike built from about 20 named parts: here is each one in order from the front wheel to the rear wheel.
Updated 3 min read717 words
A BMX bike is a small, single speed bike with 20 inch wheels, one gear and, usually, one rear brake. It is made from roughly 20 main parts that fall into four groups: the frame and steering, the wheels, the drivetrain, and the contact points where your body touches the bike. This guide lists every part in order from the front wheel to the back, with a link to each parts article.
Front end
- Front tyre. The rubber that grips the ground. BMX tyres are about 1.75 to 2.4 inches wide.
- Front rim. The metal hoop the tyre sits on. Most are aluminium and double walled for strength.
- Front hub. The centre of the front wheel. It holds the bearings and axle and attaches to the forks. Some hubs have bolts or axle nuts that can carry pegs.
- Forks. The two legs and the steerer tube that hold the front wheel and connect to the frame. The fork ends where the axle sits are called dropouts.
- Headset. A set of bearings that lets the forks turn inside the frame's head tube.
- Stem. Clamps the handlebars to the steerer tube. Top load stems have a faceplate in front. Front load stems open from the front.
- Handlebars. Usually a four piece or two piece shape with a crossbar for strength. Rise, width and backsweep decide how the bars feel.
- Grips. Rubber or foam sleeves that cover the bar ends. Many have a flange (a raised collar) so your hand does not slip off.
- Brake lever and cable. The lever mounts on the bar. A steel cable runs down to the rear caliper. See brakes.
- Gyro detangler. An optional part that lets the bars spin 360 degrees without twisting the brake cable. Brakeless riders do not need one.
Frame and seat
- Frame. The main structure. Its tubes have set names: the head tube holds the headset, the top tube runs back from it, the down tube runs to the bottom bracket, and the seat tube holds the seatpost. Seat stays and chain stays run back to the rear dropouts. The measurement that matters most is top tube length. See what size BMX bike do I need.
- Seat and seatpost. The seat is small and usually padded. The post slides into the seat tube and is held by a clamp. Many riders set it very low.
Drivetrain
- Bottom bracket. The bearings in the frame's bottom bracket shell that let the cranks spin.
- Cranks. The two arms that connect your feet to the bottom bracket. Three piece cranks (two arms and a spindle) are the usual upgrade over one piece cranks.
- Pedals. Wide platform pedals, either plastic or metal, with a pin or edge for grip.
- Sprocket. The toothed ring on the right crank. Common sizes run from about 25 to 28 teeth.
- Chain. Links the sprocket to the rear cog. See how to fix a BMX chain.
Rear end
- Rear hub. Either a cassette hub, which has a small cog and rolls with a ratchet, or a freecoaster hub, which lets you roll backwards without the cranks moving.
- Rear rim and tyre. Often the same as the front, but some riders run a wider tyre or a stronger rim at the back.
- Hub guards. Metal or plastic covers that protect the axle ends and the hub from grinds and knocks.
- Rear brake caliper. A U brake or similar mount on the seat stays that squeezes the rim. Also covered under brakes.
Common groupings
You will hear parts described in groups. This table shows the ones used most often.
| Group | Parts included |
|---|---|
| Cockpit | Bars, stem, grips, headset |
| Wheelset | Hubs, rims, spokes, tyres |
| Drivetrain | Cranks, sprocket, chain, bottom bracket |
| Contact points | Grips, pedals, seat |
| Safety | Brake, lever, cable, gyro |
Which parts matter most
If you are building or upgrading on a budget, the frame, forks, cranks and wheels do the most to decide how strong the bike is. Grips, seat and pedals are cheap to swap and make a large difference to comfort. For a full buying walk-through, read how to choose your first BMX. If you want to put a bike together yourself, read how to build a BMX bike from parts.
Related
Frequently asked questions about BMX Bike Anatomy
How many parts does a BMX have?
A typical bike has around 20 main parts, from the frame and forks to the chain and tyres. Counting every bolt, bearing and nut, the total is far higher.
Which BMX parts wear out first?
Tyres, grips, chains, pedals and brake pads wear fastest. Sprockets and rims follow. Frames and forks last years if they are not bent or cracked.
Do all BMX bikes have brakes?
No. Many street and park riders remove the brake and ride brakeless. Race bikes and most beginner bikes keep one rear brake.
Related articles
Frame Geometry
Frame geometry is the set of measurements, such as top tube length, head tube angle and chainstay length, that decides how a BMX frame fits and rides.
4 min readForks
A BMX fork holds the front wheel and steers the bike, with offset, steerer size and dropout width setting how it fits and handles.
3 min readCranks
Cranks are the two arms that connect the pedals to the bottom bracket spindle, turning a rider's leg power into rotation of the sprocket and chain.
3 min readHow to Build a BMX Bike from Parts
Build a BMX from parts by checking compatibility first, then fitting the headset, forks, bars, bottom bracket, cranks, wheels and chain in a set order.
5 min readBMX Maintenance Basics
Check your bolts, chain, tyres, headset and cranks weekly, and give the whole bike a deeper service monthly, to keep a BMX safe and quiet.
3 min readCheap vs Expensive BMX Bikes
Expensive BMX bikes buy stronger chromoly steel, sealed bearings, better heat treatment, lower weight and resale value, while cheap bikes suit casual use.
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