# Wheelpro spoke length calculator

This spoke length calculator produces very accurate spoke lengths. It's the only online calculator that takes into account tension ratio and spoke stretch.

# Wheelbuilding book

Professional Guide to Wheel Building.

**Important**: All editions of the wheelbuilding book, up to and including Edition 7 describe how to round the spoke lengths using the OLD Wheelpro spoke length calculator. Those methods do not apply to this calculator. Please use the rounding method shown in the help menu.

Copyright ©2017 www.wheelpro.co.uk

# Help

Select the type of wheel you intend to build, and whether it's front or rear. The hub and rim names are only required if you print the spoke calculation.

Every heading in the calculator and the spoke length table is clickable and displays help information. You are advised to click the heading before entering data so you can see exactly what the calculator requires. When you calculate a spoke table, you are also advised to click the spoke table headings which will tell you something more about the particular data value.

## Rounding the spoke lengths

The advised lengths are the **recommended minimum**, so always round up. For more help, calculate a spoke table and click the heading "Spoke lengths". The often quoted rule for spoke rounding that says +/- 1mm **does not apply** to this calculator.

## Hub and rim database

You **must** measure the rim ERD yourself, because if you take rim measurements from any other source it **will** lead to inaccurate spoke lengths. Hub dimensions for current hubs can be taken from the hub manufacturers website (but not from any other source). However, for both rims and hubs you should measure them yourself. Measuring components is easy to do, and it means you will always obtain accurate spoke lengths for *any* hub and rim combination.

## Navigating the calculator

Open a help topic (or anything else) by clicking the heading, and to close it, either click the *close* link, or click the same heading again, or click a new heading which will close the previous. Don't use your browsers forward and backward buttons, and don't use your browsers built in print function, always use the calculator print option in the menu.

# Hub diameter

The diameter of the hub flange measured spoke hole centre to centre.

## Straight pull hubs

For radial straight pull hubs the diameter is measured at the position where the spoke seats.

For cross laced 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.

## Note

*** Measure to the **centre** of the hub flanges ***

The diagrams show measurements taken at the inside face. These diagrams need updating.

# Flange offset

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

**L** = h - x

**R** = h - y

Where h = half the hub width.

Always check your calculation. Measure between the flanges with a ruler (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.

# Spoke offset

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.

**Positive**

**Negative**

For a radially laced straight pull hub, the offset is zero.

# Rim ERD

The Effective Rim Diameter, commonly referred to as the ERD.

Rim manufacturers often provide the ERD but you should never use it because the ERD is not a strictly defined term, and I've seen far too many errors to trust ERD data from any source. You should always measure the ERD yourself since the ERD has the largest effect in the spoke length calculation.

## The correct position of the ERD

The ERD is the diameter measured to the ends of the spokes in the finished wheel. The ideal position for the spoke relative to the nipple is that the nipple should still be able to turn a further 2.5mm before it reaches the end of the thread.

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 diameter. For a standard 12mm nipple this position will be at the bottom of the nipple slot, 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.

## How to measure the rim ERD

This example is for standard 12mm nipples.

Take two spokes and cut them down to exactly 200mm. Glue on a nipple 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 you must also use them when measuring the ERD.

To speed up the measuring process use the custom ERD ruler available from the support pages.

# Asymmetric rims

If your rim is an Asym design 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

This is for fatbike rims that use a double row of spoke holes. You can measure it yourself, or find the value from the manufacturers website. Enter the width between the two rows.

If you have a fatbike rim with one set of centrally drilled holes for use in a non offset frame, then there is nothing fatbike specific about this wheel (it's just a wide rim and a wide hub), so switch to Normal in the wheel selection list.

# Fatbike frame offset

This entry is shown when you select a rear wheel.

Examples of offset frames are the Surly Pugsley (offset 17.5mm) and the Surly Moonlander (offset 28mm). An offset frame uses a standard 135mm hub

If you build a fatbike wheel for an offset frame you must use a rim with a double row of spoke holes and lace it to the right set of holes.

For 170mm and 190mm hubs that are used in a symmetrical frame, the frame offset is 0.

If your fatbike has an offset that is not 17.5 or 28 please let me know and I'll add it to the list.

# Fatbike fork offset

This entry is shown when you select a front wheel.

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 offset forks you build the wheel using a standard 135mm rear hub. For an offset fork select the required offset, and for symmetrical forks use 0.

If you build a fatbike wheel for an offset fork you must use a rim with a double row of spoke holes and lace it to the right set of holes.

# Frame offset

This is only used if you have an offset frame. Offset frames are not common. An example of an offset frame is the Cannondale AI (Asymmetric Integration) frame.

The frame manufacturer will tell you the offset value, which is typically 6mm.

**Do not** confuse this with Fatbike Offset frames. If you have a Fatbike then choose "Fatbike" in the wheel selection list.

# Spokes

The number of spokes in your wheel.

# Cross

This is the cross lacing pattern. For radial lacing, select 0 cross.

You must choose an appropriate cross pattern for your wheel. Cross pattern does not affect wheel stiffness or ride comfort. It's important that you choose a cross pattern that avoids the following conditions:

- Spoke entry angle. Ideally this should be no more than 8 degrees, so reduce the number of crosses if necessary.
- 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.

## Radial lacing

Do not use radial lacing on the rear cassette side (right 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 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 resuls 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.

# Spoke diameter

The spoke diameter is used when calculating the elongation of the spoke due to spoke tension. The elongation (or spoke stretch) is then taken into account when determining the spoke length. When a spoke stretches, it stretches *elastically*, meaning it returns to its original length once the tension is removed (this note is added for people who think spokes stretch permenantly - in a wheelbuild they don't).

For a butted (swaged) spoke, use the diameter of the central portion.

## Bladed spokes

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.

Spoke | Diameter |
---|---|

Sapim CX-Ray | 1.5 |

DT Aerolite | 1.5 |

DT Aerospeed | 1.8 |

DT Aerocomp | 1.8 |

If your bladed spoke is not listed, then refer to the spoke manufacturers website.

## 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.

# Wheel data

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.

Print help.

You need to calculate a spoke table first.

Give your hub and rim a name, both of these will appear on the print.

Any text you enter here 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 be incomplete.

# Print help

The print option provides a detailed reference document for your wheel.

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.

If you install a pdf print driver you can print to pdf, and then use Adobe Reader to add comments to it. If you are on Windows, then try www.tracker-software.com/product/pdf-xchange-lite. When you print the spoke table select the PDF-Xchange printer. If you have Adobe reader installed then the print will appear in it, and you can add text to it (select Comment in the right side list to open up the comment toolbar). The sticky note isn't much use, but the rest are.

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.

# The spoke lengths

The spoke length at 1 decimal point will reach the point where you measured the rim erd, and if you are using standard 12mm nipples this will be at the bottom of the nipple slot. The often quoted rule for rounding is to go +/- 1mm from this length, but this rule should **not** be used with this calculator.

For this calculator you should round up, but no more than 2mm.

Here's an example of how you would select an even or odd size based on the calculated length.

Calculated | Even | Odd | |
---|---|---|---|

260.0 | 260 | 261 | You can also use 262 for the even size. |

260.4 | 262 | 261 | Do not choose 260 for the even because it's a little too short. |

260.8 | 262 | 261 | |

261.0 | 262 | 261 | |

261.2 | 262 | 263 | You can also choose 261 for the odd length, but it will be marginally short (by 0.2mm). |

## Non standard nipples

If you are building the wheel with something other than standard 12mm nipples, then you should use those nipples when measuring the rim ERD.

# 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 120Kg 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/mm
^{2}. - The lengths and elongations can be seen by looking at the log.

## Radial lacing

Radial lacing with heads out (elbows in) should be always be used. Since the spokes lie on the inside, the hub flange distance is reduced. The calculator will do this automatically, and will subtract 2mm. 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.

## Other considerations

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.

**Touching**

**Overlap**

# 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.

# Tension ratio

This is the ratio between the left and right side spoke tensions.

The formula for the tension ratio uses the hub flange offsets and the spoke lengths.

Re calculate | Left | Right |
---|---|---|

Cross | ||

Spoke diameter | ||

Spoke lengths | ||

Initial selection | ||

Ideal length | ||

Alternative length | ||

Spoke head clearance | ||

Spoke entry angle | ||

Tension |