The foundation of every bicycle is its frame. Most bikes for beginners and intermediate riders are built with hi-tensile steel tubes. However, on more advanced bikes, you'll often find frames constructed from 4130 chromoly steel, which is lighter and sturdier. Besides the materials, the frame's geometry also significantly impacts a bike's handling. This becomes a more critical factor as you progress to higher-level bikes. If you'd like to delve deeper into frame geometry, check out our Frame Buyer's Guide for more information.

Following the same logic as the frame material, chromoly offers advantages over hi-tensile steel. It's lighter yet delivers superior strength, making it a preferred choice for advanced bikes. Within the frame itself, look for CNC-machined steerer tubes for enhanced toughness. Additionally, tapered legs contribute to weight reduction and create a more streamlined aesthetic.

Your handlebars set how the bike feels to steer more than any other part you can change. Cheaper bikes run hi-tensile steel bars, while anything above entry level uses chromoly, often multi-butted so the tube is thicker where it is stressed and thinner where it is not. Rise (how tall they are) and width are the numbers that change your riding position, and taller riders generally want more of both. Bars come as two piece or four piece, with four piece being the modern standard. Our Handlebar Buyer's Guide goes through it properly.

The stem clamps your bars to the fork and comes in two shapes: front load, which is the common choice for street and park, and top load, which sits lower and is popular with riders who want the bars a touch further back. Reach is the measurement that matters, usually between 48 and 53mm, and a shorter reach makes the front end feel quicker. Look for forged alloy rather than cast on better stems. There is more in our Stem Buyer's Guide.

The headset is the set of bearings that lets your fork turn inside the frame. Almost every modern BMX uses an integrated headset, which drops straight into the head tube with no cups to press in. Sealed cartridge bearings are what you want, since they keep water and dirt out and stay smooth far longer than the loose ball bearings found on the cheapest bikes. See our Headset Buyer's Guide for the detail.

Grips are cheap, they wear out, and they change how the bike feels straight away, which makes them the easiest first upgrade. The main choice is flanged (with a lip against your hand) or flangeless, then compound: a softer rubber grips better and wears faster. Check whether bar ends are included, because they are what stops a bare bar tube coring a hole in the ground, or in you.

Cranks take more abuse than almost anything else on the bike. Entry level bikes use one piece or two piece steel cranks, while anything serious runs three piece chromoly with a sealed bottom bracket. Spindle diameter steps up with quality, from 19mm through 22mm to 24mm, and arm length is usually 165 to 175mm, with shorter arms suiting smaller riders and more technical riding. Our Crank Buyer's Guide covers the standards.

The bottom bracket is the bearing set your cranks spin in, and it has to match two things: the shell in your frame (Mid is the modern standard, with Spanish, American and Euro still around) and the spindle diameter of your cranks. Get either wrong and the parts will not fit, so check both before you buy. Sealed bearings are standard on anything decent. Our Bottom Bracket Buyer's Guide explains which is which.

The sprocket is the front gear bolted to your cranks, and together with the rear driver it sets your gearing. Most complete bikes run 25 tooth at the front with a 9 tooth driver, which is the modern standard and gives a good balance of pedal speed and ground clearance. Guard sprockets add a solid outer ring that protects the teeth on grinds. There is more in our Sprocket Buyer's Guide.

BMX chains are 1/8 inch, which is wider and stronger than the chains used on geared bikes. The main choice is a full link chain or a half link chain, where half links let you adjust wheel position in much finer steps, which is handy for getting the rear wheel exactly where you want it. Better chains use stronger pins and plates and stretch less. See our Chain Buyer's Guide.

Pedals come in plastic or alloy. Plastic is lighter on your shins and cheaper to replace, alloy lasts longer and grips harder through metal pins. The thread size has to match your cranks: 9/16 inch for three piece cranks, 1/2 inch for one piece cranks, and they are not interchangeable. Our Pedals Buyer's Guide has the rest.

Wheels are rims, hubs and spokes, and all three matter. Single wall rims are found on entry level bikes, while double wall rims are far stronger and are what you want as soon as you start dropping off things. At the rear, cassette hubs have replaced freewheels on anything above entry level, and a freecoaster lets you roll backwards without the cranks turning, which suits riders learning fakie tricks. Our Hub Buyer's Guide goes deeper.

Tyres change how the bike rides more than most people expect. Most 20 inch BMX tyres are between 2.1 and 2.4 inches wide: wider gives more grip and cushioning for park and trails, narrower rolls faster and weighs less. Tread ranges from near slick for skatepark to knobbly for dirt. Pressure matters just as much, and the range is printed on the sidewall. Our Tyre Buyer's Guide covers sizes and pressures.

Most BMX bikes run a rear U-brake, and plenty of street riders remove it entirely and ride brakeless. If you want to spin the bars, you need a gyro (also called a detangler), which splits the brake cable in two so it can rotate without wrapping up. Brake feel comes mostly from the pads and the lever rather than the calliper, so those are the cheap upgrades worth making first.

Seats mount in one of three ways: pivotal, where a bolt runs down through the seat into a splined post, railed, which uses a clamp on two rails and gives more adjustment, and combo, where seat and post are one piece and cannot be separated. Pivotal is the most common on modern bikes. Post diameters are usually 25.4mm or 27.2mm, so check your frame before buying a post on its own.

Pegs bolt onto your axles so you can grind ledges and rails. Chromoly pegs are the strongest and can be run bare, while alloy pegs with a replaceable plastic sleeve slide better and cost less to keep running, since you replace the sleeve rather than the peg. Most modern pegs suit a 14mm axle and include an adaptor for 3/8 inch axles, and remember they are almost always sold individually rather than in pairs.

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