Wheelpro spoke length calculator
10 March 2020, the calculator has been updated and the changes are shown here.
To get the most out of this calculator, you need to read the Help guide.
This calculator is different to the one described in the wheel building book, and you should use the rounding method described in the help guide.
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Every heading in the data entry form is clickable, and shows detailed information for the particular data input. Make sure you read the help information for "Spoke dia" (the last input) because it's important.
Every heading in the spoke length calculation is clickable, and shows detailed information about the data value. Make sure you click "Spoke lengths" because it tells you how to round the spoke lengths (which is different from all the other calculators).
To close the help text, either click the close link, or click the same heading again, or click a new heading which will close the previous. Do not use your browsers forward and backward buttons, because they won't work as intended.
The calculator will catch quite a few errors, but you should still use your wheelbuilding experience when entering data, and selecting the appropriate number of crosses.
For use with stainless steel spokes, cannot be used for titanium spokes, or any other spoke material.
Email : Roger Musson.
For normal hubs, this is the diameter of the hub flange measured spoke hole centre to centre, sometimes referred to as the PCD (pitch circle diameter).
Straight pull hubs
For cross laced straight pull hubs, the measurement is taken at the extended crossing point of two spokes. If the spokes are close to tangent then the diameter can be measured to the centre of the spoke holes, if they are not tangent (as shown in this diagram) then it's the midpoint between the diameters of the spoke entry and exit holes. A small error when measuring the diameter on cross laced wheels will not affect the spoke lengths.
For radial straight pull hubs the diameter is measured at the position where the spoke seats.
The distance from the hub centre line to the centre of the left and right flanges.
The red line is the centre line between the ends of the hub (the faces that locate on your fork or frame).
Measuring the offsets
Measure x and y, then:
L = h - x
R = h - y
Where h = half the hub width.
Always check your calculation. Measure between the flanges (dimension W) and it should be the same as L + R
Spoke hole diameter
The diameter of the spoke holes in the hub.
Good quality hubs have a spoke hole diameter of around 2.5mm or 2.6mm. You don't measure this yourself, it's usually available on the hub manufacturers website. If you can't find it (or don't want to bother looking) then use 2.6mm. Let's say you used 2.6 and the value should have been 2.5 or 2.7, the spoke length error would only be 0.05mm.
This is only used on straightpull hubs.
The measurement is taken from the centre line to where the spoke head seats. It can be either positive, negative or zero depending on the hub design. For a negative offset enter the value with a - (minus) sign.
For a radially laced straight pull hub, the offset is zero.
It is important to use an accurate measurement for the spoke offset because it directly influences the spoke length, for example a measuring error of 1mm will result in 1mm error in the spoke length. The offset is difficult to measure yourself and the hub manufacturer should give you this value.
The Effective Rim Diameter. You are strongly advised to measure the ERD yourself, using an ERD from any other source can lead to inaccurate spoke lengths.
The correct position of the ERD
It's important to measure the ERD using the same type of nipple that you are building the wheel with.
Take two spokes and cut them down to exactly 200mm. Screw a nipple onto each measuring spoke using light finger pressure tighten it until it goes no further,* then back it off 2.5mm (for the 56tpi spoke thread, this is 5.5 turns). It's a good idea to glue the nipples in place to prevent accidental movement during the measuring process. That's the ideal position for the erd, and this calculator will calculate an accurate spoke length that reaches this position. With this calculator, the rule for rounding the spoke lengths is to round up by up to 2mm, which means you will not bottom out on the threads, and you will always find a choice of an odd or even spoke length.
Place in opposite holes in the rim, and hold taut with no tilting (up and down or sideways). Measure between the ends and add 400mm. Average several diameters.
If you are using rim washers, then measure the ERD as described without the washers, then measure the thickness of the washer and add twice this to the measurement. Do not place the washers in the rim whilst measuring the ERD, because with only light pressure applied to the measuring spokes, the washers may not sit completely flush in the rim, and may give a false (over sized) ERD measurement.
To speed up the measuring process use the custom ERD ruler available from the support pages.
* Light finger pressure will not work if the nipple is a self locking design using adhesive, such as the DT Squorx (with Proloc), because you won't be able to tighten it to the correct position using finger pressure alone. Instead, measure as described using standard 12mm nipples and use a correction factor to modify the resulting spoke lengths.
If your rim is an asymmetric design with an offset spoke bed, then specify the amount of offset here. The rim manufacturer will tell you the amount of offset which is typically around 4mm. Asymmetric rims are used to improve the tension balance on dished wheels.
If you enter left and right hub flange offsets that are equal (or they become equal as a result of an offset frame), then using an asym rim will make the tension balance worse. For equal left/right offsets, the positioning of the rim will be based on whether it's a front or rear wheel (to follow the loosely defined convention that asym offset is away from the front left (disc) and away from the rear right).
When you calculate a spoke table, there will be a note advising you which way to orientate the rim when building the wheel, and you must follow the advised orientation because the left/right spoke lengths are calculated based on the specified orientation.
Width between holes
If there is a measurable gap between the left and right spoke holes then enter it here.
Most rims are centrally drilled, or with a negligible left/right spoke hole stagger, and for these rims, the width between the holes is zero (or just leave the entry blank).
Fatbike rims tend to have a wide hole spacing, and often a double row of spoke holes. Enter the width between the holes, measured centre to centre.
If a spoke hole width is specified, then the spoke length calculation assumes normal lacing where the spokes from the hub flange do not cross over to the opposite side of the rim. The exception is Fatbike wheels for offset frames (eg. Surly Pugsley), where the spokes are laced to the right hand set of rim holes.
An offset frame shifts the position of the rear hub outwards towards the right, which affects the spoke lengths. The frame manufacturer will tell you the offset value. An example of an offset frame is the Cannondale AI (Asymmetric Integration) frame, that has an offset of 6mm.
The more common symmetric frame has an offset of zero.
Fatbike offset frames
For an offset Fatbike frame such as the Surly Pugsley (offset 17.5mm) and the Surly Moonlander (offset 28mm), select the appropriate offset.
An offset Fatbike frame uses a standard 135mm hub and you must use a Fatbike rim with a double row of spoke holes, and lace it to the right hand set of holes.
Fatbike offset forks
Offset frames can have a matching offset fork. For example, the Surly Pugsley has two fork options, a 100mm symmetrical fork, and a 135mm 17.5mm offset fork. With an offset fork you actually build a wheel that is identical to the rear wheel, so for offset Fatbike front wheels, select a rear wheel in the selection list.
The number of spokes in your wheel.
18 spoke radial wheels
Radial spoke lengths are the same regardless of spoke count, so for radial 18, choose any other spoke count.
18 spoke wheels cannot be cross laced using a standard cross lacing pattern, because the spoke count must be divisible by 4.
When choosing the number of crosses, you should take into account the following:
- Spoke entry angle. Ideally this should be no more than 8 degrees.
- Spoke head clearance. The spoke should not overlap the head of the adjacent spoke.
The values for spoke entry angle and spoke head clearance are shown in the spoke length calculation, click those headings in the calculation for more information.
For radial lacing select 0 cross.
Do not use radial lacing on the right side of the rear (drive side), or the left side (rotor side) on a disc brake hub, because a cross pattern is required to efficiently transmit the drive and braking torque.
When radial lacing, you should lace the spokes with the heads out (elbows in). This reduces the stress on the hub flange, and for dished wheels helps improve the left/right tension balance. Lacing heads out results in a slightly shorter spoke length, and the calculator will make the necessary adjustment.
Straight pull hub cross pattern
With a straight pull hub you have no choice with the cross pattern because it's dictated by the hub design.
The spoke diameter is used when calculating the elastic elongation of the spoke due to spoke tension, which is then taken into account when determining the spoke length.
For a butted (swaged) spoke, use the diameter of the central portion.
Bladed spokes are made by flattening a round spoke. For a bladed spoke use the round spoke equivalent. For example:
If your bladed spoke is not listed, then ask the spoke manufacturer what the diameter of the spoke is before being forged (their website doesn't usually specify this, so you need to ask).
This table shows the information used by the calculation. It is the same data you entered, but presented in a clear and easy to read table. You are advised to check it and make sure it represents the wheel you intend to build.
Print the spoke table
You need to calculate a spoke table first.
Any text you enter here will appear on the print.
Warning, hub and rim names are incomplete.
This will print a reference document for use while you build your wheel.
On the print write down the spoke lengths you intend to use (my choice). When you build the wheel examine how good your spoke length choice was, and you may decide that something better is required if building the same wheel again (the ideal length). The alternative length is another length that would still build the wheel. The end result is that for this particular wheel you have two definitive spoke lengths. Keep the print for future reference.
If you want to adjust the margins on the print, then use your browsers Page Setup option, but do not use the browsers print function. When printing always use the Print button in the calculator because it formats the print content before printing.
How to round the spoke lengths
Always round the fractional spoke lengths up and never down.
The calculated spoke length will reach the point where you measured the rim erd, and you are advised to measure the erd using the technique shown in this guide. The calculated lengths are therefore the recommended minimum, so always round the fractional spoke lengths up. You can round up by up to 2mm, which means you can always find an even or odd size spoke length.
Here are some examples of how you would select an even or odd size based on the calculated length.
|261||260 on the even side is the absolute minimum, still okay, but 262 will be easier to build.|
|260.4||262||261||Do not choose 260 for the even size because it will be too short.|
|261 on the odd side is the absolute minimum, still okay, but 263 will be easier to build.|
|261.2||262||263||Do not choose 261 for the odd size because it will be too short.|
You should lace a radial wheel with the spoke heads out (elbows in), doing it the other way (heads in) puts more stress on the hub flange as the spoke is pulled over it. With all the spokes lying on the inside, the hub flange offset distance is reduced, and for radial lacing the calculator will subtract 2mm from your flange offset dimension. You can see this is the calculation log.
The biggest influence on spoke length accuracy is the accuracy of the hub and rim measurements, particularly the rim erd. If you use data found on the Internet, then there's a good chance it will be wrong, and so will the resulting spoke lengths. You should always measure your own components exactly as shown in the guide (click the headings in the data entry form).
Spoke head clearance
The distance between the spoke and the head of the adjacent spoke.
As the number of crosses increases, the spoke moves closer to the head of the adjacent spoke until it touches (clearance zero), then overlaps it (clearance negative). The overlap should be avoided because it puts an unnecessary bend in the spoke close to the elbow, and it also interferes when placing the spokes in the hub.
If the spoke overlaps a small amount (say 0.5mm), then it still should be okay. Anything more than this then be wary.
Spoke entry angle
The angle the spoke makes when entering the rim.
Standard nipples can swivel about 6 or 7 degrees. Nipples with a spherical design such as the Sapim Polyax and DT ProHead can swivel more (9 degrees for the Polyax).
For your hub, rim and spoke count, choose a cross pattern that results in a spoke entry angle of no more than 8 degrees, otherwise the stress on the spoke threads can result in fatigue failures (the spoke could snap at the threads).
Some rims are drilled at an angle to allow the spoke to follow the natural spoke line, in which case the spoke entry angle is not an issue. Examples of this are the WTB rims with 4D angled drilling.
This is the ratio between the left and right side spoke tensions. For example, if the measured spoke tension on the side that represents 100% was 120kg and the other side was 60%, then the tension in that side would be 72kg.
This is for information only. When you build your wheel, this is how it will turn out. There is nothing you can do during building that will affect this ratio.
Left and right is from the perspective of the rider.
Sometimes referred to as the non drive side. For disc brake wheels, the disc rotor is located on the left side.
Sometimes referred to as the drive side (where the chain and sprockets are).
There's no calculator help for this option. Triplet lacing will be described in the next update of the wheelbuilding book.
There's no calculator help for this option. Triplet lacing will be described in the next update of the wheelbuilding book.