Wheels, Hubs and Axles

Telling the three rear hub types apart

Three designs have been used to let a BMX bike coast. They are not interchangeable, they do not share drivers, and knowing which you have is the first question in every rear wheel purchase.

Look at where the ratchet lives. On a cassette hub the ratchet mechanism is inside the hub shell and the cog sits on a small removable driver. On a freewheel hub the ratchet is inside the cog cluster itself, and the hub is a plain threaded shell that does nothing clever. On a freecoaster the mechanism is inside the hub and there is a clutch rather than a simple ratchet.

Look at the cog size. A modern cassette or freecoaster driver carries a very small cog, small enough that it looks wrong to anyone used to older bikes. A freewheel cog is much larger, because a threaded freewheel has to be big enough to screw over the hub shell threads.

Spin it. Forwards, a cassette clicks loudly and a freecoaster is close to silent. Backwards, a cassette drags the cranks round with it and a freecoaster lets the wheel go without the cranks turning at all. That last part is what a freecoaster is for.

Try to unscrew the cog. A freewheel will come off with a freewheel removal tool as one unit and leave threads behind on the hub. A cassette driver will not.

Cassette hub

The design that took over. Putting the mechanism inside the hub means the cog can be tiny, and a tiny cog is what makes a small front sprocket possible. Everything about modern BMX gearing follows from that.

The driver threads or splines into the hub body and can be removed. That matters twice over: it is the part that wears, and it is the part that sets your gearing. A worn driver in an otherwise good hub is a cheap fix, which is why a used cassette hub with a rough driver is still worth buying if the shell and bearings are sound.

Cassette hubs are made in both right hand drive and left hand drive, and the two are not the same hub flipped around. Drivers are threaded, and the thread direction has to suit the side it is on, so a right hand drive driver will not simply serve on the left.

What to check on a used cassette hub

Grab the rim at the top and rock it side to side with the wheel in the frame. Any knock is bearing play or a loose axle, and you want to know which before you buy. Spin it and listen for grinding rather than a clean ratchet. Then take hold of the cog and try to turn it backwards slowly: a healthy ratchet engages crisply, a tired one slips or feels notchy. Check the driver splines and the cog teeth for hooking, and look at the flanges for cracks radiating from the spoke holes, which is where hubs fail.

Freecoaster

On a cassette hub, rolling backwards drags the cranks backwards with you. On a freecoaster it does not. The clutch disengages when you roll back and re-engages when you pedal forwards.

The tradeoff is slack. There is a gap between starting to pedal and the hub picking up drive, and that gap is adjustable on most freecoasters by changing a spacer or a washer inside. Less slack means quicker engagement and a more direct feel. More slack means the hub is less likely to catch you out mid-trick. Riders have strong opinions and no two set them the same.

Freecoasters are mechanically busier than cassettes. There are more parts, they need servicing more often, and they are less tolerant of neglect. A freecoaster is not a good choice for someone who is never going to open it up. That is a real drawback and worth saying plainly rather than treating the hub as simply the better option.

For a used buyer the extra checks are: does it actually coast backwards freely, does it engage when you pedal, how much slack is there, and does the clutch feel smooth or does it clunk in. A freecoaster that does not engage reliably is a rebuild, not a bargain.

Freewheel

The whole ratchet is inside the freewheel body. That has one genuine advantage, which is simplicity, and two disadvantages that ended it. First, the freewheel has to be big enough to thread over the hub shell, so there is a hard floor on how few teeth it can have. Second, all the drive load is carried through that thread, right at the end of the axle, which is a bad place to be loading a BMX bike.

That second point is the practical one. Freewheel hubs bend axles. The drive load sits outboard of the bearing rather than inboard, and the axle takes it. This is the main reason cassette hubs replaced freewheels on anything ridden hard.

A freewheel unscrews with a freewheel removal tool that matches its splines or notches, and the tool is specific to the freewheel type. Freewheels tighten themselves under pedalling, so an old one that has never been off can be genuinely difficult to remove.

You cannot fit a freewheel to a cassette hub or a cassette driver to a freewheel hub. Different hubs entirely. If a bike has a freewheel and you want cassette gearing, you are buying a rear wheel.

There are two freewheel threads, not one, and that catches people out. Anything 16 tooth and up uses 1.375 inch x 24 TPI. Drop to 13, 14 or 15 tooth and the freewheel switches to an M30 x 1.0 metric thread, because there is not enough material left at that diameter for the larger thread. A small freewheel will not screw onto a standard hub and vice versa.

The larger thread is not a BMX thread at all. 1.375 inch x 24 TPI is the general bicycle freewheel standard, so a 16 tooth and up BMX freewheel and an ordinary single-speed bicycle freewheel share the same interface. If you are keeping an old freewheel hub alive, the parts pool is wider than the BMX shelf.

13 tooth is the floor, and it is a mechanical one rather than a market one. Below that there is no room left around the axle, which is precisely why cassette hubs exist: moving the ratchet into the hub shell is what allowed 9 and 8 tooth drivers.

Drivers and cog sizes

The driver does two jobs. It is the wear part that takes the drive load, and its tooth count is one half of your gear ratio. Change the driver and you change the gearing without touching the sprocket.

Cog size on the driver is the thing that separates a modern setup from an older one. Small drivers came in alongside small front sprockets, and the two go together: you cannot run a very small sprocket with a large driver without ending up in a gear nobody wants to pedal. That pairing is the micro drive idea, and it is covered properly in the drivetrain guide.

Drivers do not swap between brands. The spline pattern, spline count and driver diameter are design choices each manufacturer made independently. Some brands share a pattern by accident or by licensing, and there is no general rule you can rely on. Buy the driver for the hub you own.

To work out the size of the driver already on your bike, count the teeth. It is a small number and it takes ten seconds. Reading it off the driver body is unreliable because the marking, if there is one, may be the hub model rather than the tooth count.

Drivers currently sold run 8, 9, 10 and 11 tooth, with 9 the modern freestyle default. Sourced across Odyssey, Profile, BSD and Eclat. Confirmed

Drivers do not interchange between brands, with one exception worth knowing

This is the opposite answer to the pivotal seat question on the seats page, and it matters more.

Aftermarket drivers are proprietary per brand and often per hub model. A Profile driver does not fit an Odyssey Hazard, and neither fits a BSD. The spline and pawl or clutch interface is specific to the hub it was designed for, and no manufacturer publishes the spline geometry. So the answer here is two findings at once: there is no cross-brand standard, and there is no published specification either, which means you cannot even check compatibility on paper. You match the driver to the hub by name.

The exception is entry-level complete bikes. Hubs on many completes come from shared factories, Joytech and Novatec among them, and those generic hubs take generic replacement drivers that do cross between bike brands. So a driver for a Salt, Mission or Rant entry-level hub may well be the same part, because the hub is the same part with different branding.

That is a genuinely useful split for buying used: on an aftermarket hub, source by hub name. On a complete bike hub, the generic driver is probably right. Confirmed

One correction to a common assumption, including one this guide previously made. The driver does not thread into the hub shell. The driver body drops in. What threads into the shell is the internal ratchet ring, and manufacturers do not publish its thread diameter or pitch either.

Checked twice now across Profile, Odyssey, BSD and Eclat, technical archives and overhaul guides both. Neither the driver spline geometry nor the ratchet ring thread is published by anyone. The interfaces are proprietary and undocumented, which is why matching by hub name is not laziness, it is the only method available. Unpublished

Axle diameter, male axles and female axles

Axle diameter is the outside diameter of the axle itself. It decides which dropout slots the wheel will drop into, which pegs will fit, and what hardware you need.

The 14mm axle is the freestyle standard and the reason is straightforward: a bigger axle bends less, and BMX bikes get landed on. Race hubs run smaller and lighter axles because race bikes do not take peg loads or drop-in impacts, and weight matters more.

3/8 inch and 10mm are different sizes

This is the confusion worth spending a paragraph on. 3/8 inch is 0.375 inch, which is 9.525mm. A 10mm axle is 10.0mm. They are about 0.475mm apart, which is close enough that sellers use the terms interchangeably in listings and far enough apart that the hardware is not the same. No source we found endorses treating them as one size.

Neither is interchangeable with 14mm.

If a hub or axle is not marked, the honest listing says which one it measures with calipers, or says it was not measured. Guessing is how a buyer ends up with axle nuts that will not start.

Male axles

A male axle is threaded on the outside and takes nuts that thread onto it. The threads stick out past the dropout where you can see them, which is the identification and also the weakness.

Those exposed threads are the first thing to go on a bike that grinds, because they are the outermost part of the axle and they take the hit. Once flattened, the nut will not start, and you are into a thread file or a new axle. On a bike that never touches a ledge they are perfectly good, and they are cheaper and easier to replace than the alternative.

Checking a used one takes ten seconds: run a nut down the thread by hand on both sides. Threads that look acceptable and will not take a nut are common.

Female axles

A female axle is hollow and internally threaded, taking a bolt into each end with the end caps sitting flush or nearly flush against the dropout.

Street riders prefer female for one reason: a smooth bolt head slides over a ledge where an exposed male thread catches and gets destroyed. That is the whole argument and it is a good one for anyone who grinds.

The corresponding weakness is that the damage moves inside where you cannot see it. A rounded bolt head or a stripped internal thread is a bigger job than a flattened male thread, and an axle with a snapped bolt in it is worse again. Check that both bolts come all the way out and go back in before buying a used female hub, rather than judging by the outside.

Hardware is not shared between the two. Nuts for a male axle are useless on a female axle and the reverse. Peg fitment differs too, because a female setup runs a bolt through the peg into the axle, which changes thread engagement and which pegs and spacers work.

Axle nuts and bolts

The hardware itself is the most commonly missing part on an incoming used wheelset, along with stripped or rounded examples of the same. Assume a used wheel needs new hardware until you have checked it, and count what is there before you agree a price.

Thread pitch is not something to order on the nominal size alone. 26 TPI is the commonly quoted standard for BMX axle nuts, and female bolts are also sold as 24 TPI. Both are in circulation. Take the old nut or bolt with you, or match against the axle in hand, because a fastener that starts and then binds two turns in is the wrong pitch rather than a dirty thread.

Hub spacing and over locknut dimension

This is the measurement people confuse with axle diameter, and the two answer completely different questions. Axle diameter asks whether the axle drops into the slot. Over locknut dimension asks whether the hub fits between the dropouts at all.

Measure it across the outermost faces of the locknuts, not across the flanges and not including the axle sticking out past them. On the frame side, measure between the inner faces of the dropouts with the wheel out.

Two things about that table are deliberate. We are not saying old school rear spacing was the same as modern, and we are not saying it was different. The forum threads that discuss it could not be read and no technical reference we found covers BMX. If you are fitting a modern hub into an old frame, measure the frame.

And a 14mm axle in a 110mm frame is completely normal. The two numbers are describing different things and neither constrains the other.

Hub end caps

Hub end caps and spacers set the over locknut dimension, and on some hubs they can be changed. That is how one hub gets sold to suit more than one spacing, and it is also how a hub arrives at your door set up for something other than what you have. Count and keep every spacer that comes off a used wheel.

End caps are not a generic part. Profile, Odyssey and BSD caps do not interchange across hub models, and often not across generations of the same model. Fitting the wrong one causes bearing drag if it sits too deep, or side to side axle play if it does not seat, and both feel like a worn bearing to anyone diagnosing by feel.

So a used hub missing an end cap is a hub waiting on a specific part, not a cheap fix. Check for crushed or burred cap shoulders too, because a deformed cap binds the bearing the moment the axle bolt is tightened.

Wheel dish

Wheel dish is how centred the rim sits between the locknuts. A wheel built for one frame can sit off-centre in another, and a wheel rebuilt badly can be off-centre in every frame.

Every used wheel that comes through here goes on a truing stand and gets its dish checked before it is listed. That is not perfectionism, it is because an off-centre wheel is what makes a wide tire rub one stay and not the other, and the buyer will blame the frame or the tire rather than the wheel.

You can do a rough version without a stand. Drop the wheel into the frame and look at the gap either side of the tire. If it is uneven, swap in a wheel you trust to find out whether the problem is the wheel or the frame, because a bent stay gives you the same symptom.

Rims, wall construction and bead seat diameter

Look into the rim through the valve hole or at a spoke nipple. On a single wall rim you can see straight through to the tire bed and the nipples sit in that same surface. On a double wall rim there is a closed inner box section, and the nipples pass through a hole in the outer wall into a cavity.

Single wall rims dent and go out of true easily under real riding. They are fine on a bike that is ridden gently and they are what most complete bikes at the cheap end come with. Double wall is the practical minimum for anything that gets jumped, and it is what almost all BMX rims sold on their own are. Triple wall exists, is heavier again, and shows up on bikes that get very hard use.

A dented rim is worth noticing early. Look along the braking surface, spin the wheel and watch the rim against the brake pad or a fixed point, and look at the sidewall for flat spots and for cracks radiating from spoke holes. Cracks at the spoke holes are the end of the rim.

ISO bead seat diameter

ISO bead seat diameter is the actual measured diameter of the rim where the tire bead sits, and it is the only reliable way to know a tire and rim will go together. Nominal sizes like "20 inch" are trade names rather than measurements, and there is history in bicycles of two sizes sharing a nominal name and not fitting each other at all.

In practice, BMX freestyle and street is well behaved here, because the wheel nearly everyone rides is a genuine ISO 406 and stays that way. The exception inside BMX itself is race, where narrow tires are sold to the ISO 451 standard instead, a different bead seat that does not interchange with 406. Outside BMX, the reason to know the term is for everything else that comes through in a job lot: cruisers, smaller kids' bikes, and anything unusual. The ISO number is marked on most tires as a pair of figures, the width and the bead seat diameter.

BMX wheel sizes

Five sizes turn up, and the ISO figure is what separates them rather than the inch name. 20 inch is ISO 406 and is nearly everything on the freestyle side. 24 inch cruiser is ISO 507, 22 inch is ISO 457, 18 inch is ISO 355 and 16 inch is ISO 305, all in the table above and all sourced to ISO 5775.

We stock 20 inch adult freestyle and mid school almost exclusively, so treat the other four as identification help for a job lot rather than as things you will find here.

ISO 406 is the number that matters, and it is a standard rather than a convention, which makes it one of the few figures on this site you can rely on absolutely. A "20 inch" BMX wheel is a 406mm bead seat with a tire on top, which is also why it rolls about 19.5 inches rather than 20, as covered on the drivetrain guide.

Effective rim diameter

ERD is a different number from bead seat diameter and it is easy to confuse them, which is the reason this sits directly under BSD.

Bead seat diameter is measured where the tire sits, and it decides whether a tire fits. Effective rim diameter is measured at the spoke nipple seats inside the rim, and it decides how long the spokes need to be. One is about rubber, the other is about spokes, and they are never the same figure for the same rim.

We sell wheels rather than build them, so the practical use here is knowing which number a seller is quoting. If someone gives you a rim diameter that is not 406, they are probably quoting ERD, and it tells you nothing about which tire will go on. Ask for the ISO number stamped on the rim or printed on the tire instead.

If you are having a wheel built, the builder needs the ERD of your specific rim, and rim makers publish it because it changes between models. It is not something to estimate from the nominal size.

One trap: ISO 451 also exists. Some narrow BMX race and older wheels use a 451mm bead seat, and 406 and 451 tires do not interchange in either direction. If a tire will not seat, check the number printed on the sidewall before blaming the tire.

Valve hole size

Rims are drilled for one valve type or the other, 8.5mm for Schrader and 6.5mm for Presta, both in the table above.

A Presta tube will physically pass through an 8.5mm Schrader drilling but should not be run bare in one. Use a grommet. The hole is nearly 2mm oversize for the stem, so the valve gets pushed sideways every time the tire moves on the rim, and that is how a stem tears at the base.

Tires, tubes and what actually fits

This section covers the tire and tube rather than the rim they mount to, which is covered above. The two meet at the bead seat and the valve hole, both already defined, so start there first if a tire will not seat or a tube will not pass through the rim.

ISO 406

ISO 406 is the bead seat diameter, in millimetres, of the rim used on nearly every 20 inch freestyle and street BMX wheel, and it is the number that actually defines "20 inch" on this kind of bike rather than the inch figure printed on the sidewall.

This is the same figure already given in the rim dimensions table above, restated here because a dedicated entry for the number itself is what a reader following a size code lands on. A tire or rim marked with a size ending "-406", for example 47-406, is built to this bead seat, regardless of what inch figure sits next to it. See ISO bead seat diameter for what the measurement itself means and BMX wheel sizes for how 406 compares against the other four ISO diameters BMX-adjacent bikes use.

The wheel size that shares the "20 inch" name and does not share this bead seat is ISO 451, next.

ISO 451

ISO 451 is a second, larger 20 inch bead seat diameter, 45mm bigger than the 406 that nearly all freestyle and street wheels use, and it does not interchange with 406 in either direction. A 451 tire will not seat on a 406 rim and a 406 tire will not seat on a 451 rim, at any pressure.

Where 451 actually shows up on a BMX bike, rather than in the standard generally, is narrower: Tioga sells its Fastr-X and PowerBlock BMX race tires in both 1-1/8 inch and 1-3/8 inch widths built to ISO 451, sold and marked that way by retailers including Dan's Comp. That is a race application, on narrow tires, not something to expect on a freestyle or street wheel. Sheldon Brown's tire sizing reference separately lists 451 as covering "juvenile lightweights" and "BMX for light riders" alongside some folding bikes and recumbents, consistent with a narrower, lighter duty use than the 406 standard.

The practical result is the same either way: a 451 wheel and a 406 wheel are two different diameters wearing the same "20 inch" name, and no amount of lever work seats a tire built for one onto the other. If a tire will not seat, read the number printed on the sidewall before assuming the tire is faulty or the rim is out of true.

Tire Size Marking

A used tire with the box long gone still has its size stamped or moulded into the sidewall, and reading it correctly is what stops a 406 tire going into a 451 order or the other way round.

The ISO/ETRTO format is two numbers with a dash: width first, bead seat diameter second, both in millimetres. A 47-406 tire is 47mm wide on a 406mm bead seat. Schwalbe, one of the tire manufacturers whose own technical documentation explains the system, states plainly that the designation is unambiguous and maps directly to rim size, which is the entire reason the format exists.

The older inch markings are where the trap is, and it is a real one on 20 inch wheels specifically. A decimal inch figure such as 20 x 1.75 is a 406 tire. A fractional inch figure such as 20 x 1 3/8 is a 451 tire. Both say "20 inch." Neither says which bead seat it is built to, and the two are 45mm apart and do not interchange. Fractional and decimal notation happen to sort into the two different standards, which is not obvious unless you already know it.

When a used tire only has an inch marking and no ISO code, measure the rim it is currently mounted on rather than trust the inch figure, or compare against ISO 406 and ISO 451 above.

Wire Bead Tire

A wire bead tire uses a loop of steel wire around the edge of the tire to hook it onto the rim, instead of a woven fibre. It is heavier at the rim, the worst place to carry extra weight, and it does not fold flat. It is also the cheaper construction, and it is what most used tires coming through a job lot will be.

The wire keeps the tire round rather than collapsible, so a wire bead tire has to be stored and shipped as a full circle. None of that affects fit: a wire bead and a folding bead version of the same tire model use the same casing and the same size code, and either one seats on the same rim. The difference is weight, price and whether it packs flat, covered in full at Folding Bead Tire.

Folding Bead Tire

The bead is aramid fibre, commonly known by the brand name Kevlar, woven into a loop instead of steel wire. That saves weight exactly where it matters most on a spinning wheel, at the outer edge, and it lets the tire fold down small enough to carry as a spare. Neither of those things changes what it fits: a folding and a wire bead version of the same tire model share the same casing and the same ISO size code, and either seats the same rim the same way.

The tradeoff is cost. Folding bead tires are consistently the more expensive version of a given tire model, which is the whole reason wire bead tires still exist on complete bikes and budget listings.

Bead Hook

The bead hook is the small inward lip inside the rim wall that the tire bead locks behind under pressure. It is what keeps a clincher tire on the rim rather than letting the bead walk off the wall as pressure rises, and on a standard hooked rim it is there on both sides.

Every rim sold here is a hooked rim, hooked being the traditional clincher design BMX has always used. Hookless rims exist elsewhere in cycling, mostly road and gravel, where the rim wall is straight rather than lipped and the tire and rim have to be matched to a tighter tolerance to hold pressure safely. No hookless BMX rim turned up in this research, and none is stocked here.

The hook is part of what a rim's rated maximum pressure depends on: a worn or damaged hook seats a tire less securely at the same pressure that was previously safe, which is one reason a cracked or heavily dented rim wall gets pulled rather than sold, whatever the rim otherwise looks like.

Tire Seating

A properly seated tire has its bead sitting evenly in the rim all the way round, with the moulded ridge line near the sidewall running parallel to the rim edge rather than dipping in and out. A tire that looks round on the bike but was seated unevenly will still run out of true, and no amount of truing the wheel itself fixes it.

Check seating before blaming a wheel for hop or wobble. Spin the wheel and watch the gap between the tire's moulded line and the rim edge as it passes the brake pad or a fixed point. If the gap wanders, deflate fully, work the bead back off the low spot, and reinflate a few PSI at a time while working the tire round by hand so it seats evenly rather than snapping into place on one side. A tire that will not seat evenly after that is more likely mismatched to the rim (see ISO bead seat diameter) than simply stubborn.

Casing TPI

TPI is threads per inch in the tire casing, and it is a tradeoff rather than a quality scale: a higher number means a lighter, more supple tire, and a lower number means a stiffer, more puncture-resistant one for the same rubber. Kenda's own tire glossary describes TPI as an indicator of casing quality where "higher TPI tires are lighter weight and provide a more supple ride," while a lower TPI casing typically carries thicker rubber and better puncture protection at the cost of weight and suppleness.

No BMX tire manufacturer found during this research publishes a TPI figure for its BMX-specific tires. Casing TPI is a real, well documented spec on other categories of bicycle tire, and the tradeoff it describes applies to BMX tires too, but the actual number for any given BMX tire model is not something Odyssey, Kenda, or the other brands checked put in their spec sheets.

Tire Pressure Range

Pressure range is set by the tire casing and confirmed against the rim it is designed to run on, which is why the figure is printed per tire rather than published once for BMX as a category. A tire rated to a given maximum is not necessarily safe to that number on every rim, since the rim's own bead hook and wall have to hold that pressure too.

These two rows are the same tire model sold in two different pressure ratings, which is the clearest illustration available that pressure is a per-product figure and not a spec you can guess from wheel size or tire width alone. Treat any number quoted without a specific tire attached to it as unverified.

Durometer

Durometer is the rubber hardness rating on the Shore A scale, and a lower number is a softer rubber compound. A softer compound grips better and wears faster. A harder compound rolls faster and lasts longer but breaks traction sooner, which is why two tires that look identical can feel completely different under the same rider.

No BMX tire manufacturer found during this research publishes a Shore A durometer figure for its tires. The tradeoff durometer describes is real and well documented for rubber compounds generally, but no specific BMX tire model's rating could be sourced from a manufacturer spec sheet, a retailer listing, or a technical reference.

Tread Pattern

Tread pattern is a surface decision, not a fitment one: slick, semi-slick and knobby tires in the same size code all seat on the same rim. A slick has no tread and rolls fastest on pavement and smooth park surfaces. A knobby has raised lugs and grips loose or uneven ground, at a real cost in rolling resistance. Semi-slick splits the difference, usually a smoother centre strip with some tread toward the edges for cornering grip.

The one place tread pattern changes something other than grip is effective width. A knobby tire's lugs sit outside the casing's actual width, so two tires marked the same width can measure differently once mounted, which matters if clearance is tight against a frame or fork.

Sidewall Marking

A tire's sidewall carries most of what you need to identify it with no box or listing to go on. The ISO/ETRTO size code is the one that actually decides fit. Alongside it you will typically find an inch size, the pressure rating, and often a rotation arrow if the tread is directional. A brand and model name are usually moulded in as well, which is the fastest way to identify a used tire if it is still legible.

Wear and UV exposure fade sidewall markings on used tires before the tread itself is worn out, so a tire that has been sitting in the sun for years may only give up the ISO code and lose the rest. When a marking is illegible, measuring the mounted rim's bead seat diameter directly is more reliable than guessing from what is left of the print.

Skinwall Tire

A skinwall tire has a tan, unreinforced sidewall instead of a black one, and the tan colour is the natural, uncoated casing rather than a dye. Because the sidewall has no added protective layer, a skinwall tire is more vulnerable to sidewall cuts and UV cracking than the black-wall version of the same casing, which is a real durability difference and not just a look.

Skinwall construction is strongly associated with a particular period in BMX and is often used to identify an older bike at a glance, but the association is a visual convention rather than a rule with a hard date attached, and skinwall tires are still made and sold new today for riders who want the look. Treat a skinwall tire as a styling and durability fact about that specific tire, not as dating evidence on its own.

Directional Marking

A directional tire has a rotation arrow moulded into the sidewall, and mounting it backwards means the tread pattern works against you rather than for you on braking bite, cornering grip, or both, depending on the design. The arrow shows which way the tire spins when the bike moves forward, not which way to thread the valve or any other convention.

Used bikes and used wheels get this wrong constantly, because a tire swapped from side to side or refitted after a repair is easy to put back on facing the wrong way. Check the arrow against the direction the wheel actually turns before assuming a directional tire is mounted correctly, front or rear.

Inner Tube Size Range

A tube is rated for a range of tire widths, not one exact width, because the rubber has to stretch to fill whatever tire it ends up inside. Run a tube outside its rated range and it either sits folded and prone to pinching at the narrow end or stretched thin and prone to blowouts at the wide end.

The Kenda row is not one tube covering that whole span. It is several distinct SKUs at the same 20 inch wheel size, each rated for its own narrower slice of that range, which is the normal pattern: manufacturers split wide and narrow tire widths into separate tube models rather than selling one tube that stretches across all of them. Match the tube to the tire width you actually have, printed on the tire's own sidewall, not to the wheel size alone.

Tube Valve Type

Schrader and Presta are the two valve types, and the tube has to match both the rim's valve hole and the pump head you own. Schrader is the wider, car-tire-style valve with a spring-loaded pin inside. Presta is the narrower valve with a threaded locknut at the tip. The two are drilled to different hole sizes, covered in full at Valve Hole Size.

A Presta tube physically passes through a Schrader-drilled hole but should not be run there bare, since the hole is oversize for the stem and will saw at the base of the valve as the tire moves. Going the other way does not work at all: a Schrader valve will not pass through a Presta-drilled hole.

Which one a given wheel takes is a property of the rim, not something to choose freely, so check the drilled hole before ordering a tube rather than assuming.

Valve Stem Length

32mm is the common stem length on a standard 20 inch BMX tube, whether Schrader or Presta, long enough to clear a normal BMX rim and reach a pump head with room to attach it. Longer stems exist for deeper rims or for riders who want more room to get a pump chuck on: Dan's Comp lists a 48mm option, and Kenda's own 20 inch Schrader tube uses a 35mm stem rather than 32mm.

BMX rims are shallow enough that stem length rarely stops a tube fitting, unlike on a deep aero rim, so this matters mostly for making sure a pump head has room to seat and lock rather than for clearance.

Tube Gauge

Gauge is the wall thickness of the tube's rubber, and a thicker wall trades weight for puncture resistance.

No comparable wall thickness figure for a thorn-resistant or heavy-duty 20 inch BMX tube turned up during this research. Thorn-resistant and heavy-duty tubes are real, commonly sold products, and they are consistently marketed as thicker and heavier than a standard tube, but none of the retailers or manufacturers checked published a millimetre figure for that thicker wall the way Kenda does for its standard tube.

The tradeoff is straightforward even without a second figure to compare against: more rubber in the wall means more weight at the rim and a stiffer tube, in exchange for resisting punctures a standard-gauge tube would not survive. A thorn-resistant tube is a reasonable choice for a bike that sits outside or gets ridden through debris, and dead weight to carry for a bike that does not.

Rim Tape

Rim tape covers the spoke nipple holes inside the rim bed so the tube sits on a smooth surface instead of directly against the nipples. Without it, or with tape that has slipped or split, a tube punctures from the inside at a nipple hole, which looks identical to a normal flat until you pull the tube and find the hole lined up with a spoke.

Tape has to be wide enough to cover the full internal width of the rim bed with a little overlap up each wall, so a narrow rim can run narrower tape than 30mm and a wide rim needs it. Using tape that is too narrow leaves the tube exposed to the nipple holes at the edges, which defeats the point of taping the rim at all. When re-taping a used wheel, measure the rim's own internal width rather than assuming 30mm fits everything.

Tubeless BMX

Alienation's TCS (Tubeless Compatible System) is a matched rim-and-tire system built to 406mm bead seat, confirmed both by Alienation's own product pages and by an independent review of the TCS Mischief rim, which lists its diameter as "406 (20 inch)." TCS rims use a different internal well shape than a standard rim to hold the tire bead without a tube, and Alienation's own FAQ is explicit that the parts are not interchangeable with anything else: a TCS tire fits a normal rim, but only with a tube, and a normal tire fits a TCS rim, also only with a tube.

Setup needs all four TCS parts together: the rim, the tire, TCS-specific tape applied in two layers, and a Presta valve with a removable core for adding sealant. Alienation's FAQ also warns that the rim's valve hole is drilled for the TCS setup specifically and that drilling it out removes the option of converting that wheel back later.

No second BMX-specific tubeless system, from any other brand, turned up during this research. That does not rule one out, only that this pass did not find one, so treat "Alienation is the only BMX tubeless system" as this research's limit rather than a confirmed fact about the whole market.

Spoke count and lacing

Spoke count is easy to establish and easy to get wrong by miscounting. The reliable method is to count nipples around one side of the rim and multiply, or count spokes leaving one flange and multiply by two. A 48 spoke wheel looks visibly busier than a 36 and the flanges are drilled with noticeably more holes.

More spokes spread the load over more points, which is stronger, and add weight and build time. 48 spoke wheels were the heavy-duty answer for a long time and turn up on old school and mid school bikes constantly. Modern rims and hubs are strong enough that 36 spoke does the job for most riders, which is why it is the default now.

Three cross lacing

Three cross means each spoke crosses three others on its way from hub flange to rim. It is what almost every BMX wheel is built with, in both 36 and 48 spoke counts.

The crossing is what lets the wheel take drive and braking load, because a crossed spoke pulls tangentially on the flange rather than straight out of it. That matters more on BMX than on most bikes, given what a grind does to a rear wheel.

Lower cross counts exist and change the spoke length required, which is the practical consequence: lacing pattern and spoke length are the same question asked twice. For a used wheel this is identification rather than a decision. Count the crossings on a spoke from flange to rim if you need to know what you have, and do not assume a rebuild will reuse the spokes if you change the pattern.

J-bend spokes

Spokes on BMX wheels are J-bend, meaning the head has a bent elbow that hooks through the flange from the inside. Straight-pull spokes exist elsewhere in cycling, use differently drilled flanges, and are not interchangeable with J-bend in either direction.

For a used buyer that is one less thing to worry about, because a BMX hub will be J-bend. Where it matters is inspection. The elbow is where spokes break, so when you are checking a used wheel, look at the flange holes for spokes that have gone slack, for bright wear marks around the elbow, and for any hole that has been elongated. A wheel with one replaced spoke is normal. A wheel with several is telling you something about how it has been ridden.

Spoke nipples

Nipples are the small threaded fittings at the rim end that hold spoke tension. Brass is the durable choice and alloy is the light one, and on a used wheel the difference shows up as corrosion and as rounding.

We sell wheels rather than lace them, so the relevant part is what to look for. Rounded nipple flats mean somebody has already fought this wheel with the wrong size spoke key or with too much tension, and a rounded nipple cannot be adjusted without replacing it. Green or white corrosion around an alloy nipple means it may be seized to the spoke, and turning it hard will twist the spoke rather than tension it.

Look through the valve hole or along the rim bed for nipples that have pulled into the rim, which is the rim failing rather than the nipple.

Working out spoke length for a rebuild

You need four numbers to calculate spoke length: hub flange diameter, hub centre to flange distance on each side, rim effective diameter, and the lacing pattern. Then a spoke length calculator does the arithmetic. There is no shortcut and there is no universal BMX spoke length, because hub flange dimensions vary between hubs.

The honest practical route on a used wheel is to measure a spoke that is already in it, if the wheel is currently built and correct. That gives you the answer without touching a calculator, and it is why it is worth measuring before you strip a wheel rather than after.

Pegs and what decides whether one fits

Axle diameter first. A peg is bored for the axle it goes on, and the two sizes on BMX are 3/8 inch at the front and 14mm at the rear on most modern bikes. A peg bored for 14mm will slop about on a 3/8 inch axle, and one bored for 3/8 inch will not go on a 14mm axle at all. Some pegs ship with a reducer sleeve to cover both, which is worth checking for when buying used, because the sleeve is the part that goes missing.

Then male or female. On a male axle the peg sits over the protruding thread and the nut goes on outside it. On a female axle a bolt runs through the peg into the axle end. Those need different lengths of thread engagement, so a peg and hardware that worked on one setup is not automatically right on the other. See male axles and female axles above.

Then clearance at the dropout. The peg has to sit flat against the dropout face, and anything in the way stops that. On the rear that is usually a hub guard or a chain tensioner. On the front it is the fork dropout itself, and forks vary in how much flat face they give you.

Peg length and material are rider preference rather than fitment, so this guide does not publish a length range. What is worth saying is that a peg is only as good as what is holding it on, and the axle, the dropout and the hardware are all doing that job together.

Hub guards, drive side and left hand drive

Hub guards are not generic. They fit a specific hub design, or a specific family of hub designs, and they come in drive side and non drive side versions because the two sides of a rear hub are shaped differently. They also have to suit the axle diameter, and on female axle setups the bolt arrangement changes what works.

The reason riders want them is simple: a grind that lands on the hub flange rather than the peg will chew spokes and flange metal. A guard turns that into a replaceable part.

Drive side and non-drive side guards

The two sides take different parts and it is not a preference. The non-drive side guard is a plain disc covering the flange and spoke heads. The drive side guard has to clear the driver and the chain, so it is cut differently, and a non-drive guard fitted on the drive side will foul the chain.

That makes a listing saying only "hub guard" half an answer. Ask which side, and ask which hub it came off, because guards locate on the hub body or the axle in a way that is usually specific to a hub family.

Checking a used one is quick. On plastic guards, look at the press-fit retention tabs, which are the ears holding it on. Those crack, and once cracked the guard spins or comes off entirely. On aluminium or steel guards, check the threads and look for deep gouges on the inner face that would bind against the hub body. A guard scarred on the outside has simply done its job. One damaged where it mounts has not.

Left hand drive and right hand drive

Right hand drive is the conventional arrangement, with the sprocket, chain and driver on the right. Left hand drive puts all of it on the left instead.

Riders run left hand drive to keep the drivetrain out of the way of the side they grind on, or because they prefer to grind on the right and would rather the chain was not there. The change is not cosmetic and it is not a flip.

A left hand drive setup needs a left hand drive rear hub, or a hub that can be converted with the correct parts, and a left hand drive driver, because the driver threads into the hub and thread direction has to suit the side. It also usually needs the front sprocket moved to the other side of the cranks, which brings chainline and sprocket mounting into it.

The practical warning for a used buyer: a left hand drive wheel is not a right hand drive wheel turned around. If a listing offers a wheel without saying which it is, ask, because the wheel will physically go in the frame either way and only drive correctly one way.

Questions