The Ten Master Guides
Pillar 01
Bottom Brackets and Spindles
The four shell standards, what each one measures, and which spindle and bearing combination fits.
16 sections · 9 questionsPillar 02Frame ID, Materials and Geometry
What the frame is made of, how it is put together, and which measurements decide whether it fits you.
16 sections · 9 questionsPillar 03Wheels, Hubs and Axles
Which rear hub you have, which axle it runs, and whether the wheel will fit the frame you own.
13 sections · 9 questionsPillar 04Drivetrain, Chains and Sprockets
Four parts have to agree with each other: sprocket, chain, driver and chainline. Here is how they fit together.
13 sections · 9 questionsPillar 05Cranks and Pedals
Three crank types, one spindle number that matters, and the pedal thread that ruins an arm if you get it wrong.
15 sections · 8 questionsPillar 06Brakes, Cables and Detanglers
Mounts, straddle cables, detanglers, and the parts that go missing off a used frame.
15 sections · 9 questionsPillar 07Headsets, Forks and Stems
What the head tube decides, what the steerer decides, and why the stem is the last thing to worry about.
27 sections · 10 questionsPillar 08Handlebars, Grips and Cockpit
Four numbers describe a set of bars, and changing one quietly changes the others.
14 sections · 8 questionsPillar 09Seats, Posts and Clamps
The seat interface, the post diameter and the clamp are three separate fitment questions, and mixing them up is what makes seat shopping confusing.
13 sections · 7 questionsPillar 10Era, Dating and Restoration ID
The era words are useful. The year ranges attached to them are repeated everywhere and sourced almost nowhere.
11 sections · 9 questionsThe A-Z Parts Glossary
Every term links into the guide section that covers it. All 152 entries are live.
A
- American Bottom BracketA bottom bracket standard using a large unthreaded frame shell with bearing cups pressed into each side.
- Axle DiameterThe outside diameter of a hub axle, commonly 14mm on modern rear hubs and 3/8 inch on older and lighter builds.
- Axle Nuts and BoltsThe threaded hardware that clamps a wheel into the dropouts, either nuts on a male axle or bolts into a female axle.
- AheadsetA trade name for the threadless headset design that is now the common freestyle standard.
- Adjustable ConeA threaded cone on a loose-ball hub or headset that sets bearing preload.
B
- Bottom Bracket Shell WidthThe measured width of the frame's bottom bracket shell between its two faces.
- Butted TubingTubing with thinner walls in the middle and thicker walls at the ends to save weight without losing strength at the joints.
- Brake MountsThe welded posts on the seat stays or chain stays that a U-brake bolts onto.
- Handlebar BacksweepThe angle the bar ends are swept back toward the rider.
- Sealed Bearing SizesThe numeric code stamped on a cartridge bearing that gives its bore, outside diameter, and width.
- Bead HookThe lip inside the rim wall that retains the tire bead under pressure, absent on some rim designs and part of what caps safe pressure.
- 990 MountsThe post spacing standard most BMX U-brakes and frames are built to, and the number needed before a used brake bolts to a used frame.
C
- Cassette HubA rear hub with an internal ratchet and a removable driver that carries the sprocket-side cog.
- Chainstay LengthThe distance from the bottom bracket centre to the rear axle.
- Chain SizeSingle speed BMX chain is normally 1/8 inch wide, with narrower 3/32 inch chain used on some setups.
- Crank Spindle DiameterThe diameter of the axle running through the bottom bracket, with 19mm, 22mm, and 24mm all in circulation.
- Chromoly (4130)A chrome-molybdenum steel alloy used for stronger and lighter frames, forks, and bars.
- Casing TPIThreads per inch in the tire casing, which trades suppleness against puncture resistance and sidewall support.
- Compression CapThe cap whose bolt pulls the fork up to preload the headset bearings, and which is not a structural clamp.
- Crown RaceThe bearing race seating on the fork crown, which has to match the headset's bearing angle and diameter or the front end never adjusts properly.
- CrossbarThe tube bracing a four-piece bar between the uprights, adding stiffness and giving a mount point for a number plate or cable.
- Cross-ThreadingStarting a fastener off-axis so it cuts across the existing thread, the failure that ruins crank arms and bottom bracket shells.
- Cable Pull RatioHow much cable a lever pulls per unit of hand movement, which the brake has to be designed for or the lever either hits the bars or the pads never bite.
D
- Dropout Slot WidthThe width of the axle slot in the frame or fork ends.
- DriverThe removable splined body on a cassette hub that the cog is part of or bolts to.
- DetanglerA rotor unit that lets the bars spin while keeping the rear brake connected.
- Wheel DishHow centred the rim sits between the hub locknuts.
- DurometerThe rubber hardness rating, which trades grip against wear life and is why two identical-looking tires behave differently.
- Directional MarkingThe rotation arrow on a directional tire, which decides which way round it mounts and is routinely ignored on used builds.
E
- Euro Bottom BracketA threaded bottom bracket that screws into a threaded frame shell, common on older race frames.
- Expander BoltThe long bolt that pulls a wedge up inside a quill stem to lock it in the steerer tube.
- Effective Top TubeThe horizontal top tube measurement used to compare frame size between brands.
- Effective Rim DiameterThe diameter a rim measures at the spoke nipple seats, used to calculate spoke length.
- Hub End CapsThe spacers or caps at each end of a hub axle that set the overall width.
F
- FreecoasterA rear hub with an internal clutch that lets the bike roll backwards without turning the cranks.
- FreewheelA threaded cog cluster that screws onto an older rear hub rather than driving from inside it.
- Female AxleA hub axle that is internally threaded and takes bolts rather than external nuts.
- Fork OffsetHow far the dropouts sit ahead of the steerer tube centreline, also called rake.
- Front Load StemA stem with a removable faceplate that clamps the bars from the front, as opposed to a top load.
- Folding Bead TireA tire with a flexible aramid bead that folds for shipping and saves rotating weight, the lighter and more expensive option.
- Four-Piece BarBars made from four tubes with a crossbar, the common freestyle construction, with more welds and a different feel under load.
G
- Gyro TabsThe small welded tabs on the head tube that a detangler bolts to.
- Gear RatioThe relationship between front sprocket teeth and rear driver or cog teeth.
- Grip Diameter and FlangeThe inner diameter and end style of a grip, including flanged and flangeless types.
- Sprocket GuardA plastic or alloy disc that protects the sprocket teeth during grinds.
- Frame GussetA reinforcing plate welded at a high-stress joint, usually at the head tube or bottom bracket.
H
- Headset StandardsThe set of head tube and cup designs, including integrated, semi-integrated, and older threaded types.
- Head Tube AngleThe angle of the head tube relative to the ground.
- Hi-Ten SteelHigh tensile steel, heavier and cheaper than chromoly, common on entry-level bikes.
- Hub GuardA disc that sits against the hub to protect the flange and spokes during grinds.
- Headset Bearing AngleThe contact angles on a cartridge headset bearing, which decide whether a bearing physically seats in a given cup and race combination.
- Headset Stack HeightHow much height the assembled headset adds above and below the head tube, changing bar height and usable steerer.
- Headset SpacerSpacers between top race and stem that set bar height and take up spare steerer, which still have to leave the stem fully clamped on the tube.
I
- Integrated HeadsetA headset where the bearings drop directly into a machined seat inside the head tube with no pressed cups.
- ISO Bead Seat DiameterThe ISO measurement of the rim diameter at the tire bead seat, the true way to match tire to rim.
- Integrated Seat ClampA seat clamp built into the frame's seat tube rather than a separate collar.
- Internal Chain TensionerA tensioner that sits inside the dropout slot to hold axle position under load.
- ISO 406The bead seat diameter of a standard 20 inch freestyle and street rim, and the number that actually defines 20 inch.
- ISO 451The larger 20 inch bead seat diameter used on some race and fractional-sized wheels, which will not interchange with 406 in either direction.
- Inner Tube Size RangeThe span of tire widths a tube is rated for, and why a tube outside its range either folds or over-stretches.
- Internal Compression BoltA compression system running inside the steerer rather than pulling on a star nut, which is what a hollow-steerer fork needs.
J
K
L
- Spoke Lacing PatternThe pattern spokes follow between hub and rim, commonly three cross on BMX.
- Left Hand DriveA setup where the sprocket, chain, and driver sit on the left side of the bike.
- Brake Lever ReachHow far the lever blade sits from the bar, sometimes adjustable by a screw.
- Loose Ball BearingsIndividual bearing balls running in cups and cones rather than in a sealed cartridge.
- Lock-On GripA grip clamped by a collar rather than held by friction, needing bar end room for the clamp and no glue.
- Left-Hand ThreadA thread tightening anticlockwise, used where normal rotation would loosen a part, and why a forced pedal or cup comes out damaged.
M
- Mid Bottom BracketA pressed-in bottom bracket standard using a shell smaller than American, with bearings pressed straight into the frame.
- Mid-SchoolThe loosely defined era of BMX between old school and modern, marked by specific part standards.
- Male AxleA hub axle threaded on the outside that takes nuts.
- Micro DriveA drivetrain using a small front sprocket with a small-tooth-count driver.
N
- Spoke NippleThe small threaded fitting that joins a spoke to the rim and sets tension.
- Non-Drive SideThe side of the bike opposite the sprocket and chain.
- Nylon Pedal BodyA plastic-bodied pedal with a metal spindle, lighter and cheaper than an alloy body.
- NeckOld-school slang for the stem, still used in listings for period parts.
O
- One-Piece CrankA crank where both arms and the spindle are a single bent steel piece, also called Ashtabula.
- Over Locknut DimensionThe measured width of a hub across the outer faces of its locknuts.
- Old SchoolThe earlier era of BMX, generally covering the sport's first generation of bikes and parts.
- Oversize Bar ClampA larger-than-standard bar clamp diameter used on some stems and bars.
- One-Piece to Three-Piece ConversionFitting three-piece cranks into a frame built for one-piece, using an adapter kit in the American shell, the most common upgrade question on old frames.
P
- Pivotal SeatA seat and post interface using a splined face and a single bolt through the seat.
- Pedal Thread SizeThe thread size of the pedal spindle, which differs between one-piece and three-piece crank setups.
- PegsTubes that bolt onto the axle for grinding and standing.
- Press-Fit Bottom BracketA bottom bracket where bearings or cups press into an unthreaded shell.
- Pinch BoltA bolt that clamps a slotted part, such as a stem on a steerer or a seat clamp on a post.
Q
R
- Rim Wall ConstructionWhether a rim is single, double, or triple wall in cross section.
- Removable Brake MountsBrake mounts that bolt on rather than being welded to the frame.
- Right Hand DriveThe conventional setup with the sprocket, chain, and driver on the right side.
- Handlebar RiseThe vertical height from the bar clamp area to the top of the grip area.
- Frame ReachThe horizontal distance from the bottom bracket to the head tube top.
- Rim TapeThe strip covering the spoke nipple holes inside the rim, without which the tube punctures on the nipples rather than from outside.
- Railed SeatA seat on twin rails clamped by a railed post, the main alternative to pivotal and not cross-compatible with it.
S
- Spanish Bottom BracketA pressed-in bottom bracket standard with a shell narrower than Mid.
- Spline Drive SprocketA sprocket that mounts directly to splines on the crank spindle rather than bolting to a spider.
- Seat Post DiameterThe outside diameter of the seat post, with more than one size in common use.
- Standover HeightThe height of the top tube above the ground at its lowest useful point.
- Sprocket Bolt PatternThe number and spacing of mounting bolts on a bolt-on sprocket.
- Sidewall MarkingThe full set of stamped markings on a tire sidewall and how to read them, which is how a used tire is identified with no packaging.
- Skinwall TireA tire with an unreinforced tan sidewall, both a period identifier and a real durability difference against a black wall.
- Steerer Tube DiameterThe outside diameter of the fork's steerer, which has to match both the headset and the stem clamp.
- Steerer Tube LengthHow much steerer the fork has, which caps how many spacers fit under the stem and whether the fork suits a longer head tube at all.
- Star NutA sprung nut driven into the steerer giving the top cap something to pull against when setting headset preload on a threadless front end.
- Split Crown Race RingA split race that clips onto the fork crown rather than being pressed, common on modern BMX and the piece most often missing from a used fork.
- Stem ReachThe horizontal distance from steerer centre to bar clamp centre, moving the bars forward or back independently of bar sweep.
- Straddle Cable HangerThe piece carrying the straddle cable above the brake, whose height changes the mechanical advantage of the whole brake.
- Seat Clamp DiameterThe outside diameter of the seat tube at its top, which is what a clamp must match, and a different number from post diameter.
T
- Top Tube LengthThe measured length of the top tube, the headline size number on a BMX frame.
- Threadless HeadsetA headset used with an unthreaded steerer, where a stem clamps the steerer directly.
- Threaded HeadsetAn older headset design where the top race threads onto a threaded fork steerer.
- Three-Piece CrankA crankset made of two arms and a separate spindle.
- Tire Size MarkingHow a tire states its size on the sidewall, where decimal and fractional markings of the same nominal diameter are different standards.
- Tire SeatingBringing the bead evenly onto its seat so the moulded line sits parallel to the rim, which is what stops a tire running out of round.
- Tire Pressure RangeThe minimum and maximum pressures marked on the sidewall, which are a property of the tire and rim together rather than rider preference.
- Tread PatternSlick, semi-slick or knobby, which is a surface decision rather than a fitment one but changes effective width on the same casing.
- Tube Valve TypeSchrader against Presta, which has to match the rim's valve hole and the pump, and is a different question from the hole's size.
- Tube GaugeThe wall thickness of a tube, trading weight against puncture resistance, and the difference between a standard and a thorn-resistant tube.
- Tubeless BMXWhether a given BMX rim and tire can run without a tube, which requires both a compatible bead seat and a sealed rim bed.
- Top Load StemA threadless stem clamping the bars from above, giving a different bar height and clamping feel than a front load stem on the same fork.
- Two-Piece BarBars made from two tubes, lighter with fewer welds, distinguishable by eye and both a durability and an era marker.
- Thread PitchThe distance between thread crests, which makes two same-diameter threads incompatible and is the root of most stripped parts.
- Torque SpecificationThe tightness a fastener is designed for, which matters more on alloy and titanium parts than on the steel they replace.
- Thread InsertA helical or solid insert restoring a stripped thread in a frame or crank, the repair deciding whether a damaged part is scrap.
- Tripod Post SplineA third seat mounting interface using a three-point pattern, compatible with neither railed nor pivotal parts.
U
- U-BrakeThe standard BMX rim brake, mounted on two posts either side of the wheel.
- Unsealed BearingsBearings running without a cartridge seal, usually loose ball with cups and cones.
- Unicrown ForkA fork where the legs bend in to meet the steerer directly, with no separate crown.
- Handlebar UpsweepThe angle the bar ends rise from the horizontal cross section.
V
- V-BrakeA linear pull rim brake mounted on posts, uncommon on BMX but found on some builds.
- Valve Hole SizeThe drilled hole diameter in a rim, sized for either a Schrader or a Presta valve.
- Vertical DropoutA dropout with a vertical slot rather than a rearward-facing horizontal slot.
- Valve Stem LengthHow far the valve protrudes past the rim wall, which has to clear deep sections and be long enough to take a pump head.
W
- WheelbaseThe distance between the front and rear axle centres.
- Wheel SizeThe nominal wheel diameter of a BMX bike, with 20 inch standard and other sizes used for cruisers and smaller riders.
- Handlebar WidthThe overall width of the bars measured end to end.
- Wedge StemA quill stem locked in place by an angled wedge pulled up by the expander bolt.
- Wire Bead TireA tire with a steel wire bead, heavier, cheaper, and not foldable, which is what most used tires will be.
X
Y
Z
- Zero Stack HeadsetA semi-integrated headset where cups press into the head tube and the bearings sit inside it.
- Zero Offset Seat PostA seat post with the clamp directly above the post centreline rather than set back.
- Zinc-Plated HardwareBolts and small parts with a zinc coating for corrosion resistance.
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