Drivetrain, Chains and Sprockets

What a BMX drivetrain is made of

Four parts, one loop.

The sprocket bolts to the cranks and is the front of the drivetrain. Its tooth count is the top half of your gear ratio. How it mounts to the cranks is the single most common fitment problem on this page, because there is more than one way to do it.

The chain connects the two. It has a width, which has to suit the teeth it runs on, and a length, which has to suit the frame and where the axle sits in the dropout.

The driver or freewheel is the rear of the drivetrain. Its tooth count is the bottom half of the ratio. On a cassette or freecoaster hub it is a small removable driver. On an older hub it is a threaded freewheel cluster. Which one you have is a rear hub question and it is covered in the wheels guide.

The chainline is not a part. It is whether the sprocket and the driver sit in the same plane, and it is what decides whether the drivetrain runs quietly or throws the chain when you land on it.

Everything else on this page is a detail of one of those four.

Chain width, 1/8 inch and 3/32 inch

Both sizes use the same pitch, which is the distance from one pin to the next. What changes is how wide the gap between the inner plates is, and therefore how thick a tooth the chain will accept.

1/8 inch is the single speed width and it is what BMX has mostly run. It is wider, heavier, and stronger, and it accepts a thicker tooth. Almost every BMX sprocket and driver is cut for it.

3/32 inch is narrower. It comes from geared bikes, where a narrow chain is needed to sit between closely spaced sprockets, and it appears in BMX on lighter setups where the sprocket and driver are cut thinner to match.

Which one do you need

Match the chain to the teeth, not to a preference. A 1/8 inch chain will physically sit on a 3/32 inch sprocket because the gap is wider than the tooth, but it sits loosely and can wander sideways off the tooth under load. A 3/32 inch chain on a 1/8 inch sprocket is worse, because the tooth is too thick for the gap and the chain rides up on the teeth rather than seating.

Neither combination is something we would build a bike with. Buy the chain that suits the sprocket and the driver you are actually running, and if you are changing to a lighter sprocket check what width it is cut for before assuming your existing chain carries over.

The other reason chain width matters is peg and frame clearance. A wider chain is a wider chain, and on tight frames that is occasionally the thing that decides it.

Here the names are honest. A 1/8 inch chain really does measure 3.175mm inside and a 3/32 really is 2.381mm, both exactly their fractions converted. That is not something to take for granted on this site, given the 24mm spindle that is really 23.81mm and the 22.2mm bar clamp that is really 22.225mm. On chains, the nominal name is the actual dimension.

Outer width is the number that decides clearance, and it is the one nobody quotes. A half-link 1/8 inch chain is up to 10.2mm across against 9.4mm for a standard one, so a chain that fits a tight chainstay in one form may not in the other. Worth measuring if you are running a short back end.

Boxed chains ship at 96 to 112 links and get cut to roughly 72 to 86 on a BMX, depending on chainstay length and gearing. Buy the chain expecting to shorten it.

Half links

A standard chain is made of alternating inner and outer links. You can only join it inner to outer, which means the chain can only be shortened or lengthened two half-links at a time. On a bike with an adjustable axle position, that is fine: you take the chain to the nearest whole link and slide the axle to take up the slack.

The problem shows up when the axle adjustment runs out. Take a link out and the chain is too short to reach with the axle all the way forward. Leave it in and the chain is too slack with the axle all the way back. That is exactly the gap a half link fills.

Half link chains, where every link is a half link, exist too. They let you adjust in half-link steps anywhere in the chain, at the cost of a chain built entirely from a less common link shape.

When you need one

You need a half link when the frame's chainstay length and your gearing land the chain between two whole link lengths, which happens most often on short frames and on unusual gear combinations. You do not need one just because you have a short frame. Fit the chain, see where the axle sits, and only reach for the half link if the answer is genuinely between two positions.

A single half link fitted into an otherwise normal chain is the common approach and it is fine. It is slightly weaker than a normal link in the sense that any single point in a chain is where it will fail, but this is not a reason to avoid one on a correctly set up bike.

Gear ratio and gear inches

Ratio is the arithmetic and it is easy: sprocket teeth divided by driver teeth. A higher ratio is a harder gear, meaning more speed for a given cadence and more effort to accelerate. A lower ratio is easier to spin up and runs out sooner.

The useful thing about ratio is that it is what actually matters. Two completely different sprocket and driver combinations with the same ratio give you the same gear. A large sprocket with a large driver and a small sprocket with a small driver can be identical to pedal. What changes between them is ground clearance, weight, chain length and how much the small parts wear.

Gear inches

Gear ratio on its own does not let you compare bikes with different wheel sizes, because the same ratio on a bigger wheel covers more ground. Gear inches fixes that by multiplying the ratio by the wheel diameter in inches, which gives you the diameter of a direct-drive wheel that would give the same gear.

The number people quote is a rough one, because it uses the nominal wheel size rather than the actual rolled diameter of the tire you have fitted. It is a comparison tool, not a measurement. What it is genuinely good for is comparing your current bike to a bike you are thinking about, on the same arithmetic.

We are not printing a full gear inch chart, because it is trivial arithmetic once you have the wheel figure and the two conventions above: tooth count divided by cog count, multiplied by wheel diameter in inches. Do it with the figure that matches whatever chart you are comparing against.

A 20 inch BMX wheel is not 20 inches, and that is why gear charts disagree.

The rim is ISO 406, and a modern 2.25 to 2.40 inch street tire puts the rolled diameter at roughly 19.4 to 19.6 inches, not 20. So there are two conventions in circulation and both are in use:

The gap is about 2.5 percent. That is small enough to ignore for choosing a sprocket and large enough to explain why two charts give different gear inches for the same tooth counts. If you are comparing gear inches between sources, check which wheel figure each one used before concluding one of them is wrong. Sourced to Sheldon Brown and Park Tool against race charts from Rennen. Confirmed

25/9 is the default across modern freestyle completes, giving about 55.6 gear inches on the nominal convention. If a used bike has something else, somebody chose it deliberately.

Micro drive

This is the change that shaped the modern BMX drivetrain, and it only became possible because cassette hubs put the ratchet inside the hub. A threaded freewheel has to be big enough to screw over the hub shell, so there is a floor on how small it can be. A cassette driver has no such floor, and once the rear cog could shrink, the front sprocket could shrink with it and keep the ratio.

Three things follow from smaller parts.

Ground clearance. A small sprocket sits higher and is much less likely to catch on a ledge or a rail. On a bike ridden on obstacles that is the main reason to do it.

Weight. Less material in the sprocket and less chain to wrap it.

Wear. This is the honest tradeoff. Fewer teeth means fewer teeth sharing the load at any moment and a tighter wrap angle on the chain. Small drivers wear faster than large ones. That is not a reason to avoid micro drive, it is a reason to expect the driver to be a consumable.

The compatibility part

Sprocket and driver have to be chosen together. Dropping a small sprocket onto a bike with a large driver gives you a gear low enough to be unusable. Fitting a small driver behind a large sprocket gives you a gear high enough to be unusable. Work out the ratio you want first, then pick a pair that gives it.

Chain length changes too. Smaller sprocket and smaller driver means a shorter chain, so a micro drive conversion on an existing bike usually means removing links and possibly a half link to land the axle correctly.

Micro drive is a description, not a size class. There is no ISO definition and no trade association standard behind it. The term was coined for setups smaller than the traditional 36/13, specifically 25/9 and down, at a time when that was unusual. Sub-28 tooth setups are now simply standard freestyle gearing, which makes micro drive an era word more than a specification.

In practice it means a front sprocket of about 28 teeth or fewer and a driver of about 10 or fewer, most often 25/9, 28/10, 23/8 or 22/8. But because no one owns the definition, a listing saying "micro drive" does not tell you the tooth counts. Ask for the numbers.

On whether small drivers wear out faster, the honest answer is that nobody has published the test. Manufacturers acknowledge that an 8 tooth driver concentrates the same load over fewer teeth and turns more times per wheel revolution than a 10, which is a sound mechanical argument. What does not exist anywhere in Odyssey, Eclat, BSD, Profile or KMC's published material is wear life data or mileage figures comparing them. So the reasoning is available and the numbers are not.

Drivers and cog sizes

Two separate questions get asked about drivers and they have different answers.

"What size driver do I want?" is a gearing question. Pick the tooth count that gives you the ratio you want with the sprocket you have, or the ratio you want with a new pair. The driver is the cheaper half of the pair to change, so it is often the one that moves.

"What driver will fit my hub?" is a fitment question, and the answer is almost always a driver made for that hub by that manufacturer. The spline pattern, spline count and driver diameter are design decisions made independently by each brand. Some patterns coincide. There is no rule you can rely on, and the failure mode is expensive.

If you have an older hub with a threaded freewheel instead of a driver, none of this applies. A freewheel screws onto the hub shell and is a self-contained unit. You cannot fit a driver to a freewheel hub or a freewheel to a cassette hub. Changing between them means changing the rear hub.

Driver wear

The driver is a wear part and it is meant to be. What you are looking for on a used one is hooking on the tooth profile, where the teeth have worn into a curved hook shape rather than a symmetrical tooth, and any slipping when the ratchet is loaded. A hub with a worn driver and a good shell is still a good hub, because the driver comes off.

Sprocket bolt patterns

This is the older of the two main ways to mount a BMX sprocket, and it is what you will meet on old school and a lot of mid school bikes. The crank arm carries a spider, which is a set of arms radiating out from the spindle, and the sprocket bolts to the ends of those arms.

The two numbers that define it are the bolt count and the bolt circle diameter, which is the diameter of the circle the bolt centres sit on. A sprocket with the right bolt count and the wrong circle diameter will not go on. A sprocket with the right circle diameter and the wrong bolt count will not go on either.

Bolt patterns in bicycles generally are a well-documented mess with many standards, and BMX has its own history within that. What matters for a used part is that you cannot tell by looking, so a bolt-on sprocket has to be matched to the specific spider it is going on. If you are selling one, measure the bolt circle and count the bolts, and say both.

Sprocket guards

A sprocket guard is a disc that sits over or beside the sprocket to protect the teeth during grinds. It has to match the sprocket, both in size and in how it mounts, so it is not a universal part. On bolt-on setups it usually shares the sprocket bolts. Guards are frequently missing from used sprockets, and a sprocket sold with its guard is worth more than one without.

The useful finding here is that the old standard was not a BMX standard at all. Old school 5-arm BMX spiders used 110mm BCD, which is the same bolt circle as road and touring cranks, so 110 BCD road chainrings physically interchange with them. If you are restoring something old school and cannot find a BMX sprocket, the road parts bin is open to you.

Modern is the opposite. A single-bolt freestyle sprocket matches no road or mountain standard. It locates on drive pins spaced relative to the spindle bore, and that arrangement is specific to BMX and often to the brand. Nothing from outside BMX will fit, and not everything inside BMX will either.

On bolt threads, two are in circulation on modern drive bolts and you cannot assume which you have: M8 x 1.25 and 5/16 inch x 18 TPI both appear across brands, and they are not interchangeable despite being close in diameter. Legacy 5-bolt spider hardware uses the standard fine-pitch chainring bolt, M8 x 0.75. Check the bolt you have before ordering a replacement.

The two drive bolt threads split by where the crank was made

This turns out to have a rule behind it, and it is a useful one.

Imperial thread on US-manufactured cranks, metric on Asian-manufactured ones. That is not a BMX standard, it is two manufacturing traditions meeting in the same parts bin, and it explains why both threads are in circulation with no one to arbitrate.

The practical version: if the crank was made in the States, expect 5/16 x 18. Otherwise expect M8 x 1.25. They are close enough in diameter to start and then destroy a thread, so check before forcing anything.

Spline drive and sprocket bore

Spline drive removes the spider entirely. The sprocket has a splined hole in the middle, the spindle has matching splines, and the sprocket slides straight on and is retained by the crank arm. Fewer parts, less to work loose, and a stiffer connection.

The catch is that spline drive is a system rather than a standard. The sprocket has to match the splines on that specific spindle, from that specific crank system. It is not a size you can measure and shop for generically.

Sprocket bore

For sprockets that are not spline drive, the hole in the middle has to suit the crank spindle passing through it. BMX freestyle cranks use 19mm and 22mm spindles mainly, with 24mm found mostly on race setups.

The practical takeaway from that table is the adapter. A sprocket bored larger than your spindle is usually usable with the correct adapter, and a sprocket bored smaller than your spindle is not usable at all. So a sprocket bored for 24mm is the flexible one and a sprocket bored 19mm is the restrictive one. When you buy a used sprocket, find out the bore and find out whether the adapter is included, because a sprocket without its adapter is not a complete part.

The other detail worth carrying: a 24mm spindle is really 15/16 inch, which is 23.81mm. Calipers reading 23.8 on a used spindle mean it is correctly sized, not worn.

Chainline

This is the part of a BMX build people skip, and it is the reason a drivetrain that is made of correct parts still does not work properly.

Picture the bike from above. Draw a line through the centre of the sprocket, parallel to the frame. Draw another through the centre of the rear cog. If those two lines are the same line, the chainline is good. If one is inboard of the other, the chain is running diagonally for its whole length.

What sets it

At the front, the sprocket's position is set by where it sits on the spindle, which is set by the bottom bracket spacers, which side the sprocket is mounted on the crank arm, and the crank system itself. Bottom bracket kits ship with a spacer set precisely because chainline needs setting up.

At the rear, the driver's position is set by the hub, the hub's spacers, and where the axle sits in the frame.

In between, the frame decides how much room there is. A frame with a tight chainstay yoke or a short chainstay constrains where the chain can be.

Setting it and checking it

The straightforward check is to sight along the chain from behind the bike with the wheel in the dropouts and the chain tensioned. A chain that is running true looks like one straight line. A chain that is off looks like it steps sideways somewhere along its run.

If it is off, the adjustment is almost always at the bottom bracket, moving spacers from one side of the spindle to the other to shift the sprocket inboard or outboard. That is what the spacer kit is for. Second place to check is the rear, where hub spacers can move the driver, and third is whether the wheel is sitting square in the dropouts, because a wheel pulled over to one side puts the driver out of line as surely as anything else.

A left hand drive conversion changes chainline completely, because the sprocket moves to the non drive side and everything about the setup has to be redone. That is not a bolt-on job.

Chain tension and tensioners

Too tight and you are loading the bearings in the hub and the bottom bracket continuously, which wears both and makes the bike feel dead to pedal. Too loose and the chain slaps, wears the teeth, and comes off when you land.

The adjustment method is to loosen both axle nuts or bolts, pull the wheel back until the tension is right, make sure the wheel is square in the frame, then tighten. Getting it square is the part that takes the time. Pull one side back further than the other and the wheel sits crooked, which puts the chainline out and makes the tire rub.

Tensioners

A chain tensioner holds the axle position so it does not creep forward under pedalling load. Two kinds turn up.

External tensioners bolt to the dropout and push against the axle from behind, with an adjuster screw setting the position. They are the common type, they are cheap, and they are fiddly to set evenly on both sides.

Internal tensioners sit inside the dropout slot, between the front of the slot and the axle. They are tidier, they do not stick out to be caught on things, and they only fit frames whose dropouts are built for them.

Tensioners are more necessary on some frames than others. A frame with thin dropouts and a smooth slot lets an axle creep. A frame with thick dropouts, a serrated slot face and correctly torqued hardware often does not need anything. If your axle is creeping forward, check the torque first before buying a tensioner.

Chain wear

A worn chain has stretched, which really means the pins and bushings have worn and the effective pitch has grown. A stretched chain no longer sits correctly on the teeth, and it accelerates wear on both the sprocket and the driver, which is the expensive part.

The rough field test is to pull the chain away from the front sprocket at the three o'clock position. On a healthy chain it barely lifts off the tooth. On a worn one you can lift it clear enough to see daylight under it. There are dedicated chain wear gauges that give a proper answer, and they are worth owning if you are going through a lot of used bikes.

The practical rule for a used bike: if the chain is visibly worn, replace the chain and inspect the driver and sprocket, because a chain that has been run past its life has usually taken the teeth with it.

The 0.75 percent figure is general bicycle practice rather than BMX specific, measured over twelve links with a chain checker, and it is the single-speed threshold. Derailleur drivetrains are replaced earlier. Past 0.75 percent the chain starts hooking sprocket teeth, which is how a worn chain turns into a worn sprocket and driver.

Chain play is workshop guidance, not a specification. Aim for some slack. A chain tensioned like a guitar string binds the driver and wears everything faster than a loose one.

On rear axle torque, nobody publishes a figure and this page will not invent one. Odyssey, Profile, BSD, Sunday and Shadow Conspiracy were all checked, manuals and spec sheets, and none states a torque for a 14mm axle nut or a 3/8 inch axle bolt. A figure does circulate informally in the race world, and it is deliberately not repeated here, because an unsourced number for the fastener holding your rear wheel in is worse than no number. This is the same situation as crank bolt torque on the cranks page, and it gets the same treatment.

Three passes have now failed to find a published rear axle torque figure, across Odyssey, Sunday, Profile, BSD, Cinema and Shadow Conspiracy. Manuals and spec sheets both. This is the same result as crank bolt torque on the cranks guide, and at this point it is a characteristic of the industry rather than a gap in this research. Unpublished

Drive side and non-drive side

Hub guards are the common trip. A drive side guard is cut to clear the driver and a non-drive guard is not, so the two are not swappable, and a listing that says "hub guard" without saying which side is only half an answer. Ask before buying.

The other one is grind wear. On a bike that grinds, the non-drive side axle hardware takes the worst of it, because there is no sprocket or chain in the way and nothing else to hit first. When you are assessing a used rear wheel, look hard at the non-drive end of the axle, the nut or bolt on that side, and the guard if there is one. The drive side often looks better than the bike has actually been treated.

Sprocket guards

Guards fit reasonably well across brands provided the hardware is right. The part that has to match is the hat washer bore, which follows the spindle: 19mm, 22mm or 24mm, the same set covered under crank spindle diameter. Get that wrong and the guard does not sit concentric with the sprocket, which is worse than no guard.

The second check is clearance. Frames with a wide or heavily flared chainstay yoke can leave less room than a guard needs, so a guard that fits the cranks may still foul the frame. Worth offering the guard up before committing, rather than assuming that matching the spindle is the end of the question.

On a used guard, expect the outer edge to be ground away, which is what it is for. What matters is the mounting area and whether the bolt holes have been pulled oval.

Questions