BMX Drivetrain Explained

BMX Drivetrain Quick Guide

Crank arm length: Measured from the centre of the crank axle to the centre of the pedal hole. Changing crank arm length affects pedal clearance, leverage and the size of the circle your feet travel through when pedalling.

Crank axle: The axle that passes through the bottom bracket and connects the crank arms. Common BMX crank axle sizes include 19mm, 22mm and 24mm.

Bottom bracket: The bearings the crank axle runs through. The BB needs to match both your frame's bottom bracket type and the diameter of your crank axle.

Sprocket: The front BMX gear attached to the cranks. Its tooth count works with the rear driver size to determine your gearing.

Chain: Connects the front sprocket to the rear driver.

BMX Cranks Explained

BMX cranks consist of two crank arms and a crank axle, along with the bolts and hardware required to hold everything together. Depending on the design, the axle may be separate from both crank arms or permanently attached to one of them.

You'll sometimes see cranks described as 2-piece, 2.5-piece or 3-piece. This describes how the crank arms and axle are constructed and assembled. Traditional 3-piece cranks use two separate crank arms and a separate axle, while many 2-piece designs have the axle permanently attached to one crank arm.

Different BMX cranks can also use different axle and crank arm interfaces, so replacement crank arms aren't automatically interchangeable between different makes and models.

BMX Crank Arm Length Explained

Crank arm length is measured in millimetres from the centre of the crank axle to the centre of the pedal hole. Common adult freestyle BMX sizes include 160mm, 165mm, 170mm and 175mm, but BMX cranks are available considerably shorter and longer than this.

Crank arm length can make a noticeable difference to how a BMX feels, particularly as modern freestyle frames have become shorter at the rear and riders have experimented more with shorter cranks.

Shorter Crank Arms

Shorter crank arms create a smaller pedalling circle and give your pedals and feet more clearance for a given bike setup. This can be useful on modern BMX frames with short chainstays and larger tyres, where heel and tyre clearance can become tight.

Sizes such as 160mm and 165mm have therefore become increasingly popular in freestyle BMX. Shorter cranks can also make sense on smaller BMX bikes and for younger riders, where a full-size adult crank arm may simply be too long.

The trade-off is that shortening the crank arm reduces the amount of leverage you have at the pedal.

Longer Crank Arms

Longer crank arms give you more leverage at the pedal and have traditionally been common on BMX bikes. 170mm and 175mm remain popular sizes and many riders simply prefer the feel they provide.

There isn't one crank arm length that's automatically correct for street, park, trails or race. Rider size, frame geometry, chainstay length, tyre clearance and personal preference can all influence what feels best.

What Crank Arm Length Should I Choose?

If you already have a BMX setup you like, your existing crank arm length is a good starting point. The length is often stamped or marked on the crank arm.

If you're getting heel or pedal clearance issues, moving to a shorter crank arm can be worth considering. If you prefer more leverage and have plenty of clearance, you may prefer a longer crank arm.

Find BMX Cranks by Crank Arm Length

KNOW WHAT CRANK ARM LENGTH YOU WANT? LET WALLER BMX FIND THE CRANKS THAT MATCH.

Our BMX Cranks collection allows you to filter the cranks we stock by Crank Arm Length, saving you from opening individual products to find out which sizes are available.

Our current range includes crank arm options across 125mm, 145mm, 150mm, 152mm, 155mm, 160mm, 165mm, 170mm, 175mm and 180mm, depending on the model and current availability.

This is useful whether you're replacing your existing cranks, looking for shorter cranks for a modern freestyle setup, building a smaller BMX or already know exactly what crank arm length you want.

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BMX Crank Axle Sizes

Another important measurement is the diameter of the crank axle. Common aftermarket BMX crank axle sizes are 19mm, 22mm and 24mm.

This measurement needs to match the internal diameter of your bottom bracket bearings. For example, 22mm cranks require a BB with bearings designed for a 22mm crank axle.

Don't confuse crank axle size with bottom bracket type. You could have a frame with a Mid BB and run 19mm, 22mm or 24mm cranks by fitting the appropriate Mid bottom bracket.

8-Splined and 48-Splined Axles

Some BMX cranks use a splined axle where the crank arm slides onto splines machined into the axle.

Entry-level cranks commonly use an 8-splined axle, while many aftermarket BMX cranks use or have traditionally used a 48-splined axle.

Not all cranks use the same system. Some manufacturers use their own axle and crank arm designs, such as cranks where the axle is permanently attached to one arm or designs that use their own locking or wedge system.

So even if two sets of cranks use the same diameter axle, that doesn't mean their crank arms or other hardware are interchangeable.

BMX Bottom Brackets Explained

There are two things you need to know when choosing a BMX bottom bracket:

1. Your frame's bottom bracket type – usually Mid, Spanish, USA/American or Euro.

2. Your crank axle size – for example 19mm, 22mm or 24mm.

You need both to be correct. If your frame has a Mid BB and your cranks have a 22mm axle, you need a 22mm Mid bottom bracket.

Mid BB

Mid is the most common bottom bracket type on modern freestyle BMX frames. The sealed bearings press directly into the frame's BB shell and normally use an internal spacer between the bearings.

Spanish BB

Spanish uses smaller press-fit bearings than Mid. It appeared on a number of BMX frames but is much less common on current freestyle frames. Replacement Spanish BBs are still available for frames that use the standard.

USA / American BB

USA or American uses a much larger BB shell and is commonly found on older BMX bikes and some entry-level bikes. Depending on the setup, these can use traditional loose-ball hardware or sealed bearing cups.

Euro BB

Euro bottom brackets use threaded cups that screw into the frame rather than bearings being pressed directly into the BB shell. Euro BBs are particularly common on BMX race frames.

If you're also choosing a new frame, our BMX Frames & Geometry Explained guide covers BMX frame sizing, geometry and compatibility.

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BMX Sprockets Explained

The sprocket is the front gear on a BMX and attaches to the cranks. BMX sprockets come in different tooth counts, materials and designs, including sprockets with built-in or removable guards for riders who grind.

Common freestyle BMX sprocket sizes include 25T, 28T and 30T, although plenty of other sizes are available.

The sprocket size needs to be considered alongside your rear driver size because the two together determine your gearing.

BMX Gear Ratios

25T front with a 9T rear driver is one of the most common modern freestyle BMX gear combinations.

Other combinations can give a similar gear ratio. Examples include 28T/10T and 30T/11T.

Increasing the front sprocket size or reducing the rear driver size gives you a harder gear. Reducing the front sprocket size or increasing the rear driver gives you an easier gear.

If you're changing gearing, remember that a different sprocket size can also affect the chain length required and the clearance around the frame.

Sprocket Bore and Top-Hat Spacers

Standard BMX sprockets commonly have a larger centre bore and use a top-hat spacer to reduce the bore to match the crank axle.

For example, a sprocket may include different top-hat spacers for use with 19mm or 22mm crank axles. Exactly what is required depends on the sprocket and cranks, so always check the product specification rather than assuming every sprocket includes the spacer you need.

Spline Drive

Some BMX cranks use spline drive. Instead of the sprocket being driven by the normal sprocket bolt arrangement, a compatible spline-drive sprocket fits directly onto the splines of the crank axle.

If you're running spline drive, both the cranks and sprocket need to be compatible with that system.

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LHD and RHD BMX Drivetrains

RHD (Right-Hand Drive) means the sprocket, chain and rear hub driver sit on the right-hand side of the bike. LHD (Left-Hand Drive) puts the drivetrain on the left.

Some BMX cranks can be run either LHD or RHD, while others may be drive-side specific. Your rear hub also needs to be set up for the same drive side.

For freestyle riders, drive side can be particularly important when grinding. Riders who grind predominantly on the right may choose LHD to move the chain and driver away from the side taking the most abuse, while riders who grind predominantly on the left may choose RHD for the same reason.

BMX Chains Explained

BMX chains are supplied long enough to work with a range of sprocket sizes and frame setups, then shortened to the correct length when fitted.

The quality and construction of the chain varies, with stronger chains generally using more substantial plates, pins and construction designed to cope with the loads of BMX riding.

Standard BMX Chains

A standard chain uses alternating inner and outer links. They're simple, reliable and available at a wide range of price points.

Half-Link Chains

A half-link chain allows finer adjustment of the rear wheel position than a standard chain. This can be particularly useful if you're trying to get the rear wheel as far into the dropout as possible and run the frame close to its slammed chainstay length.

You don't necessarily need a full half-link chain to achieve this on every setup – compatibility and the type of chain you're using matter – but half-link chains are popular with BMX riders who want finer adjustment of their rear wheel position.

Chainline and Chain Tension

Your front sprocket and rear driver should line up as straight as possible. A poor chainline can make the drivetrain noisy, increase wear and cause the chain to run poorly.

Don't overtighten the chain either. It needs enough tension to stay securely on the sprocket and driver, but the drivetrain should still rotate smoothly without excessive binding.

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BMX Drivetrain Compatibility Checklist

Before ordering new drivetrain parts, check the following:

Crank arm length – check what you're currently running and whether you want to stay the same, go shorter or go longer.

Crank axle size – commonly 19mm, 22mm or 24mm.

Bottom bracket type – Mid, Spanish, USA/American or Euro.

BB size – the bearing needs to match your crank axle, for example a 22mm Mid BB for 22mm cranks in a Mid BB frame.

Sprocket fitment – check the sprocket bore, top-hat spacer required and whether you're using a standard or spline-drive setup.

Gearing – check your front sprocket and rear driver tooth counts.

Drive side – make sure your cranks and rear hub work with the LHD or RHD setup you want.

Chain – once assembled, check chain length, chainline and tension.

Shop BMX Drivetrain Parts

Waller BMX stocks BMX cranks, sprockets, bottom brackets, chains and drivetrain spares from leading BMX brands. Use the collections below to find the parts you need, or contact us if you're unsure whether something will work with your BMX.