Wheelpro spoke length calculator
To get the most out of this calculator, you need to read the Help guide.
Professional Guide to Wheel Building - This calculator is different to the one described in the book, and you should use the rounding method described in the help guide.
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Select the type of hub you are building, and whether it's front or rear. The hub and rim names are only required if you print the spoke calculation.
The data entry boxes are grouped logically, first you enter the dimensions of your hub, then your rim, then your lacing configuration.
Every heading in the data entry form and the spoke length calculation is clickable, and displays help information. Make sure you read the help information for Spoke dia because people just guess this and get it wrong. 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.
For use with stainless steel spokes, cannot be used for titanium spokes, or any other spoke material.
The calculator will catch a few errors, but you should still use your wheelbuilding experience when entering data, and selecting the appropriate number of crosses.
Do not use your browsers forward and backward buttons, because they won't work as intended, just click the previous link again, and do not use your browsers built in print function, always use the calculator print option in the menu.
Please also read :
- How to round the spoke lengths
- Setting the display type
If you have any questions, please email : Roger Musson.
If it's a question about how to use the calculator, please ensure you have first read through the help guide.
To change the type of display, click the text in the top right, and, if the hub allows*, it will toggle between:
This is a detailed calculation for the actual wheel being built (in terms of spoke count and lacing pattern). You are advised to print it, and use it whilst building the wheel, then keep it safe for future reference. The headings on the detailed calculation are clickable and will show information about the particular data value.
This allows you to explore all the lacing options for the particular hub. It displays spoke lengths for all cross patterns, and all spoke counts between the specified range. An additional list of spoke counts is provided for the 'to' entry.
If you hover over a spoke length it will provide additional information. Some lengths will be highlighted, meaning caution is required, the criteria for highlighting a spoke length is spoke head overlap, or adverse spoke entry angle (greater than 8 degrees). A non highlighted entry does not mean it's a valid option, you still have to use your wheelbuilding experience to select something suitable, for example you would not choose a radial pattern for the rear drive side.
* If the hub does not support multiple options then the heading is not clickable. For example straight pull hubs, because with a straight pull hub, the spoke count and the cross count are dictated by the hub design, hence no options to explore.
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.|
For information on how the spoke lengths are calculated, calculate a spoke table, then click the heading "Spoke lengths".
For normal hubs, this is the diameter of the hub flange measured spoke hole centre to centre.
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 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.
It's important to use an accurate measurement because a measuring error of 1mm will result in 1mm error in the spoke lengths.
For a radially laced straight pull hub, the offset is zero.
The Effective Rim Diameter. Rim manufacturers often provide the ERD but you should never use it because I've seen far too many errors to trust ERD data from any source, which means you should always measure the ERD yourself.
The correct position of the ERD
Screw the nipple you intend to use on to a spoke, and with 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). That's the ideal position for the erd, and this calculator will calculate an accurate spoke length that reaches this position, and because the rules for rounding the spoke lengths are to round up by up to 2mm, you will not bottom out on the threads, and you will always find a choice of an odd or even spoke length.
* This method will not work if the nipple is a self locking design, such as the DT Proloc, because you won't be able to tighten it to the correct position using finger pressure. My advice, don't bother with self locking nipples, they are not required in the first place.
How to measure the rim ERD
This example is for standard 12mm nipples. If you adjust the nipple position as advised above, then the spoke end will finish at the bottom of the nipple slot.
Take two spokes and cut them down to exactly 200mm. Glue on a nipple (or crimp with pliers) so that the top of the spoke end is flush with the bottom of the slot in the nipple.
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 without the washers as described, then measure the thickness of the washer and add twice this to the measurement. You must 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.
If your rim is an asymmetric design (often called Asym) then specify the amount of offset here. The rim manufacturer will tell you the amount of offset which is typically around 4mm.
When you calculate a spoke table, there will be a note advising you which way to orientate the rim when building the wheel.
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 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.
Non standard lacing patterns
This calculator is for standard cross laced wheels, you cannot use it for non standard patterns such as paired spokes, triplet (2:1) lacing, or missing out hub or rim holes.
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, the Sapim CX-Ray and DT Aerolight are made by flattening a 2.0mm/1.5mm butted spoke, the DT Aerocomp is made from a 2.0mm/1.8mm butted spoke, and the DT Aerospeed is made by flattening 1.8mm straight gauge spoke.
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).
Left and right spoke diameters
Wheels are normally built with the same diameter spokes either side of the wheel.
You have the option of specifying different diameter spokes for the left and right side. For a more durable wheel, put the thinner spokes on the lower tensioned side.
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.
If you change any of the information, you need to click the Calculate button again.
Print the spoke table
This will print a specially formatted spoke table for your wheelbuilding records.
You need to calculate a spoke table first.
Any text you enter here will appear on the print :
Give your hub and rim a name, both of these will appear on the print.
You must use the print button above. Do not use the print option within your browser because the print will not be formatted correctly.
The print option produces a reference document for your wheel.
If you are printing a detailed calculation, then on the print write down the spoke lengths you intend to use (initial length). When you build the wheel examine how good the selection 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 somewhere safe for future reference.
An alternative is to print to pdf. Browsers such as Chrome will give you this option, if not then install a separate print driver. A print driver is required because it will allow you to print the custom layout of the calculation (which is formatted behind the scenes), whereas add-ons tend to print the current view using a button installed in the browser. An example of a print driver is www.tracker-software.com/product/pdf-xchange-lite. If you have Adobe reader installed then you can then open the pdf, and add text to it such as spoke lengths used.
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 option in the calculator menu because it formats the print content before printing.
How the spoke lengths are calculated
The standard spoke length formula is used to calculate the left and right spoke lengths. This is pure mathematical geometry using the data you supply, and it calculates theoretical spoke lengths. These lengths need adjusting to account for spoke stretch.
- Determine the tension in the left and right spokes. The formula for tension ratio between the left and right uses the hub flange offsets and theoretical spoke lengths. A tension of 100Kg (average between 80 to 120) is used for the side that represents 100%, and the other side adjusted down according to the ratio.
- The spoke diameter is used to calculate the cross sectional area.
- The elongation can now be calculated using standard engineering stress-strain equations. I've used a stainless steel Young's Modulus of 210000 N/mm2. This calculator is for stainless steel spokes, other types of spoke material will have a different Young's Modulus which will affect the elongation.
- The lengths and elongations can be seen by looking at the log.
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. When radial is selected, the calculator will make an automatic adjustment and will subtract 2mm from the flange offset. You can see this is the calculation log.
For a cross laced wheel the hub flange distances do not require adjustment, because the crossing action in effect positions the inside and outside spokes central on the flange (where the flange offset is measured to), and explains why both inside and outside spoke lengths reach the same position in the finished wheel.
I have not noticed any rim compression when building a variety of wheels to test the calculator, the rim erd does not get smaller in the fully tensioned wheel.
The accuracy also assumes the spoke manufacturers make accurate spoke lengths. I don't know what tolerances on length they use, but I'd say it will be around +/- 0.25mm.
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.
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 angle should not be confused with the bracing angle. Bracing angle is not shown anywhere in the calculator, because it provides no useful information. It's tension ratio that is important (see the Tension help page).
This is the ratio between the left and right side spoke tensions.
This is for information only. When you build your wheel, this is how it will turn out.
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).
|Spoke head clearance|
|Spoke entry angle|