BMX Calculators

ERD is the effective rim diameter: the diameter of the rim measured at the spoke nipple seats, which is where the spoke ends finish. It is the single most important number in a spoke length calculation. Use the rim maker's published ERD wherever you can, because it is not the same as the outside diameter or the bead seat diameter.

To measure it yourself, measure straight across the rim through its centre from one nipple seat to the one opposite, in millimetres, and take the average of two or three measurements at different points. Most 20 inch BMX rims fall between about 385 and 395 mm, and most 24 inch cruiser rims between about 490 and 500 mm.

You need four things from the hub: the flange diameter on each side, the flange position on each side, the hub spacing and the spoke hole count. Spoke hole diameter is a fifth, but it is 2.5 mm on nearly every BMX hub and the calculator fills it in.

• Flange diameter (also called PCD): centre of one spoke hole to the centre of the hole directly opposite on the same flange, not the outside edge of the flange. Where the two flanges differ, the drive side is almost always the bigger one.
• Flange position: how far each flange sits from the middle of the hub, published either as flange to locknut or centre to flange.
• Hub spacing: outer face of one locknut to the other, not counting the axle. Most BMX front hubs are 100 mm and rears are 110 mm.
• Spoke count: 36 hole is the BMX standard, with 28 and 48 hole also in use.

They are two ways of describing the same thing, the position of each flange along the axle, and either one gives the same spoke length. Flange to locknut runs from the outer face of the locknut in to the middle of the flange. Centre to flange runs from the hub's centre line out to the middle of the flange.

The two are tied together by the hub spacing: centre to flange equals half the hub spacing minus flange to locknut. Brands publish whichever they prefer, so the calculator takes both and converts between them as you switch.

Three cross is the right choice for almost every BMX wheel, front and rear, on 36 hole hubs and rims. It is the strongest, most forgiving pattern for the loads BMX puts through a wheel, and it is what nearly every complete bike ships with.

Four cross is used on some 48 hole wheels. Radial lacing, where the spokes run straight out with no crossings, takes a shorter spoke and puts more strain on the flange, so it only belongs on a front hub whose maker rates it for radial. Each extra cross lengthens the spoke, which is why the calculator asks.

Buy the closest length you can get. Anything within about 1 mm of the calculated figure builds fine, because the nipple has enough thread to take up the difference. If the calculated length lands exactly between two sizes, go with the shorter one unless your nipples are unusually deep or you want more thread engagement.

LUXBMX stocks most spoke lengths in 2 mm steps, so the spoke calculator rounds to a length we actually carry and shows the exact calculated figure beside it. Cut-to-size spokes cover any length between 80 and 220 mm if you need something in between.

On most rear hubs the driver side flange sits closer to the centre of the hub than the non-drive flange, so the spokes on that side are shorter. The difference is usually 1 to 4 mm. Front hubs are normally symmetrical and take the same length both sides, so tick Same measurements both sides for those.

Drive side means the side the driver is on, whether the hub is left or right hand drive. The calculator works out each side separately. For a 36 hole wheel that means 18 spokes of one length and 18 of the other, and the results tell you exactly that.

No. The calculator works in drive side and non-drive side rather than left and right, so a left hand drive hub is entered exactly the same way as a right hand drive one: the driver side's flange figures go in as the drive side. The rim and hub lists do this for you.

If you are not sure which figures belong to which side, the flange diameters usually settle it. Where the two flanges differ, the drive side flange is almost always the bigger one, and the calculator warns you if the numbers look the other way round.

No. The calculator is built for J-bend spokes in a normally drilled flange, which is almost every BMX hub. Straight-pull hubs hold the spoke head in a different way and are measured differently, so their spoke lengths come from the hub maker's own chart. If you are not sure which you have, a J-bend spoke has a 90 degree elbow at the head and a straight-pull spoke does not.

With correct measurements it is accurate to well under 1 mm, which is closer than the spokes are made to. The calculator uses the standard spoke length geometry: the square root of the sum of the squares of half the ERD, half the flange diameter and the flange offset from centre, less twice the product of the two radii and the cosine of the spoke angle, minus half the spoke hole diameter.

The errors that matter come from the inputs. Entering the rim's outside diameter instead of its ERD, or measuring the flange to its outer edge instead of hole centre to hole centre, throws the result out by several millimetres. Measure twice, use the manufacturer's figures where they exist, and email help@luxbmx.com if a number looks odd.

There is no single length, because it depends on the rim's ERD, the hub's flange diameter and position, and the lacing pattern. Most 36 hole, three cross, 20 inch BMX wheels land somewhere between 180 and 192 mm, with rear driver side spokes a few millimetres shorter than the non-drive side.

The spoke calculator gives the exact answer for your rim and hub. Pick both from the lists if we sell them and the measurements fill in for you.

Almost always no. Leave the offset at 0 unless the rim is sold as asymmetric or offset, meaning the whole spoke bed is built to one side of the rim's centre line so that the two sides of the wheel end up more evenly tensioned. A rim like that says so in its specifications, as something like a 3 mm offset.

Normal rims are drilled alternately, with one hole angled towards the left flange and the next towards the right. That is standard drilling, not offset, and it does not change the calculation.

Gear inches are the standard way to compare BMX gearing across different chainring, cog and wheel combinations. The figure is chainring teeth divided by cog teeth, multiplied by the tyre's outside diameter in inches. A bigger number means a harder gear that covers more ground per pedal stroke.

For example, 44 tooth chainring, 16 tooth cog and a nominal 20 inch wheel gives 44 divided by 16, times 20, which is 55 gear inches. On a real 20 by 1.75 tyre that measures about 19.6 inches, the same setup is about 53.9 gear inches.

Most freestyle BMX bikes run 25 tooth sprocket and 9 tooth driver, which is about 55.6 gear inches on a nominal 20 inch wheel. Race bikes are usually built around 44 tooth and 16 tooth, which is 55 gear inches, and riders move up or down a tooth or two to suit the track, their power and the class they race.

Because a 24 inch cruiser wheel is about 20 percent bigger, cruisers run a smaller chainring or a bigger cog to land on similar gear inches. The gear calculator's chart shows every nearby combination so you can compare before you buy.

Very nearly. 25 divided by 9 is a ratio of 2.78, and 44 divided by 16 is 2.75, so on the same tyre 25/9 is about half a gear inch harder. Both are around 55 gear inches on a 20 inch wheel.

The small sprocket and 9 tooth driver combination gives more ground clearance for grinds and pedal hits, which is why freestyle moved to it. The trade-off is that a 9 tooth driver wears faster than a 16 tooth cog, and the chain runs under more tension.

Rollout is how far the bike travels for one full turn of the cranks. It is the same information as gear inches expressed as a distance: gear inches multiplied by pi. A 55 gear inch setup rolls out about 172.8 inches, which is 4.39 metres, per crank revolution.

It is handy if you think in metres rather than inches, and it is the figure used where racing classes set a gearing limit. The gear calculator shows rollout in inches or metres alongside gear inches and the plain ratio.

Yes. Gear inches scale directly with the tyre's outside diameter, so a taller tyre makes the same chainring and cog a harder gear. A 20 by 1.75 tyre measures about 19.6 inches rather than 20, which is roughly 2 percent, or about a gear inch, easier than the nominal chart suggests. A 20 by 1 3/8 race tyre measures about 20.5 inches.

The gear calculator uses measured diameters for common tyre sizes, and you can choose Custom and enter your own measurement for the most accurate figure.

Measure your chainstay length, then enter it with your sprocket and driver tooth counts and the chain calculator gives the number of links to run. The maths uses the two sprocket radii and the distance between them to find the exact chain length, then rounds up to a whole link count you can actually fit.

As an example, a 25 tooth sprocket and 9 tooth driver on 13.5 inch chainstays comes to 72 links. The result also shows how far the axle ends up behind your measured position, so you know how much to pull the wheel back to tension the chain.

Measure from the centre of the bottom bracket spindle to the centre of the rear axle, with the wheel sitting where you want it in the dropouts. Slammed, meaning the axle as far forward as it goes, is the usual starting point. Measure in inches or millimetres; the calculator takes either.

Frame geometry charts list chainstay length as the slammed figure, so if you have the chart you can use that number directly. A typical freestyle frame sits around 13.25 to 13.5 inches.

A half-link chain is made entirely of half links, so its length can change one link at a time, in half inch steps, instead of two links at a time on a standard chain. That lets you put the rear wheel exactly where you want it in the dropouts, which matters on frames with short or slammed chainstays where a standard chain leaves the wheel either too far forward or too far back.

Standard chains are lighter and stiffer for the same strength. The chain calculator handles both: choose Half-link and it rounds to any whole link count, choose Standard and it rounds to an even one.

More than a 20 inch BMX needs. Most standard BMX chains ship with 112 links and most half-link chains with around 98 to 100, while a typical freestyle setup runs about 70 to 76. You break the chain down to the count the calculator gives with a chain breaker, and the calculator tells you how many links to remove from the chain length you pick.

Yes, in your own browser on this device only. Each calculator remembers the last numbers you entered so they are there when you come back, and nothing is sent to us. Clear and start again, or the Start fresh link on restored results, wipes them.

Yes. Add #spoke, #gear or #chain to the page address and that calculator opens and scrolls into view. Handy for sending a mate straight to the right tool.

Type the measurements in yourself. The rim and hub lists only cover the products we sell, and the calculator works exactly the same with figures from a manufacturer's spec sheet or your own measuring. If you would like a rim or hub added to the list, email the model to help@luxbmx.com.

Yes. Email help@luxbmx.com with your rim, hub and lacing pattern and we will check the spoke lengths before you order. Our Brisbane workshop also builds custom wheels, with a discount when the rim or hub comes from us. See the workshop page for prices.