Spoke length calculators; a mixed bag...

[Brucey]

When I first started to build wheels I asked at my LBS about the correct spoke lengths to use, and since I was building standard wheels and the spokes were available in 1/16" length increments, the lengths came out OK. As soon as I started to do anything more complicated, I found I needed to calculate spoke lengths. A bit of 3D trigonometry later, and with use of a (then new and fancy) programmable calculator, and I was able to accurately calculate spoke lengths in seconds, which (provided my measurements were OK) was a vast improvement over various charts and approximations which were then available.

That was over thirty years ago and things have moved on since then. In particular there are lots of online spoke length calculators. I've mostly used the same one for several years (the EDD one) and it works for me. But others give slightly different answers and have more or less features. This is my quick review of the various calculators (plus links and reviews thereof) that came up in a web search. The list is not exhaustive by any means; please add any further info/comments you might have.

Note that part of the reason for doing this review is that folk are routinely confused/alarmed by the fact that different spoke calculators give different answers to the same question; if it is understood how/why, perhaps there would be less confusion.

To test each calculator I entered (where possible) a rim of 600mm ERD, built 32x2 onto a hub with flanges of 60mm dia and flanges at 25mm and 35mm from the hub centreline. Because it is a feature that I regularly use, I checked if each calculator would support arbitrary drillings and fractional crossing values. The latter feature is often required if the spoking is non-standard. The hub drilling diameter was set to 2.4mm where allowed but if there was a similar default value (+/-0.2mm) then it was left as is.

[Aside; spokes are conventionally measured from the inside of the elbow bend to the other end. Spokes also settle in the hub (and/or need to 'set'), stretch elastically, and rims deform slightly under spoke tension too. In practice the first of these things may make the spokes be too short (since the calculation is sometimes from the drilling centre, i.e. with a ~1mm error built in, which you can test using a radial calculation with close-spaced flanges) and the latter points would make the spokes too long or too short. In reality these things usually all cancel one another out pretty well, and the only correction that is worthwhile (sort of) is to allow another 0.5mm stretch for the lightest gauge DB spokes on large diameter wheels. But if 0.5mm is going to make or break you, your measurements are duff or you are incredibly fussy IMHO.

Where necessary, I routinely improve the fit of spokes into the hub (eg by using spoke washers), and/or make allowances for hubs that have been built before, since these hubs already have deformations that are caused by settling in the flanges. The idea is that the clearance between the straight part of the inside spokes and the inside of the flange is as small as possible, after stress-relief. This greatly reduces the bending stresses in the spoke, to the benefit of fatigue life. The J-bends and heads of good quality spokes vary a little from one another (so DT usually fit similarly to Sapim, say) but if you use off-brand spokes and/or you have odd spoke flanges and don't correct for the fit, then the spokes (once the wheel is properly stress-relieved) may appear as if they are too long when they are really not ( e.g. a long J-bend can sit at an angle through a flange that is drilled slightly oversize, or yield to a new shape entirely during stress-relief), or alternatively if you overdo it slightly (which is the side that it is safe to err on, in terms of fatigue life IMHO), so that the outside spokes need an appreciable 'set' to bring them into line, spokes that are nominally the correct length may appear to be slightly too short. IME this can make a difference between spokes of a given length appearing to be up to ~1mm too short (if they need a 'set') or appearing to be over 1mm too long.

The ability of a given calculator to handle such things as fractional spoke crossings and arbitrary drillings is not essential for those building conventional wheels. But features such as these (or similar) are required if you are trying to build rims with non-standard spoking of some kind (eg clustered spoking). They are also pretty well essential if you are to successfully build quite a lot of hub motors, as are becoming popular. There is one calculator (that I have now added to the review, the 'e-bikes' one) that allows paired spokes at the hub, with this in mind. If it works, it saves an awkward calculation of a fractional spoke crossing, which is how I have managed thus far.

Rim measurements are usually entered as ERD (effective rim diameter) which is the diameter at which you want the ends of the spokes to lie. Conventionally this is measured by using opposed test spokes in the rim with nipples screwed on to the desired length (usually mid-slot) and the calculation is made as ERD = 2x (spoke length) + j-bend separation. The latter measurement is taken using Vernier calipers. To allow for the possibility of rims that are not quite round, it is a good idea to measure the ERD at several locations.

Manufacturers often quote NSD (nipple seat diameter) and a few quote this as if it is really ERD, which causes a lot of confusion. In reality, NSD + 3mm = ERD, usually. Some folk abhor the term ERD (because it is misunderstood and misused) and would sooner have something else, but maybe if we had a term for that, it would soon get misused too and we would be back where we started.... Note that the instructions for different spoke calculators vary slightly in terms of how you measure ERD. The procedure outlined above works with most, but not all of them. ]

Some of these spoke length calculators contain rim ERD databases. I didn't test or check these; they are invariably riddled with errors, so you are best off entering your own values if you are able to measure carefully.

Several of the calculators indicate tension balances, the spoke bracing angle, and/or the rim entry angle. The rim entry angle is usually presented as an angle calculated in 2D (i.e. this result is not seen to vary with flange spacing or offset) which isn't strictly accurate but it is much better than nothing.

In no particular order (with the predicted lengths indicated)
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https://www.wheelpro.co.uk/spokecalc/

278.9, 280.1

Allows for different gauge spokes, eg. predicts that 1.6mm spokes ought to be 0.3mm shorter in the above case than 2.0mm ones used in calculation.
Does not support spoke gauges that are bigger than 2.0mm.


Comment; Roger Musson's calculator. Not sure if the prediction of spoke stretch is that useful, since it appears to assume a specific spoke tension, and it doesn't allow for all the variables associated with the way the spoke fits into the hub, which is at least as important. These spoke lengths are slightly shorter than many, (and he suggests you round up) so I'd suggest that you measure ERD with the spokes screwed flush with the top of the nipples before you enter values into this spoke calculator (edit- if you intend to round in the usual way, or ever want to round down at all).

Does not support fractional crossings.
Does not allow arbitrary drillings
[edit; but does allow different spokes and crossings each side of the same wheel, in the same calculation, which you may find useful. Also gives a 2D rim entry angle.]
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https://leonard.io/edd/

279.5, 280.6


Does support fractional crossings.
Does allow arbitary drillings.

Seems accurate and versatile enough to me.
[edit; is in fact an online version of Rinard's 'spocalc', including the rim and hub database, to which you can submit additional data]
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http://www.kstoerz.com/freespoke/

279.5, 280.6

lots of features that allow bracing angles, tension balances, spoke head clearances etc to be determined.
Supports offset hubs and offset rims.
Does support fractional crossings.
Does allow arbitrary drillings.

Appears to give the same lengths as the EDD calculator above.
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http://www.bikeschool.com/tools/spoke-length-calculator

279.5
280.5

Does not support fractional crossings.
Does not allow arbitrary drillings.

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http://www.ebikes.ca/tools/spoke-calc.html

279.3, 280.2 (elbows in, or 'inside' spokes)
279.8, 281.0 (elbows out, or 'outside' spokes)

Mostly for E-bikes with DD menu of hub motors, but allows any hub and rim measurements to be entered. Spoke lengths are slightly off vs many other calculators, (since it won't work out an average (elbow in/out) length value or allow both to be calculated on a dished wheel at the same time) but not by enough to be worried about. It must assume a flange thickness (which isn't stated or adjustable) to calculate the in/out spoke lengths.

Does not support fractional spoke crossings, but does allow paired spokes in the hub (as is commonly found in hub motors).

Gives a nice picture of how the wheel will actually look, if built with a CW stagger rim.
Gives a diagram of the spoke bracing angles, and gives a 2D value for rim entry angle.
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http://www.machinehead-software.co.uk/spoke_length_calculator.html

seems to be broken, could download but not install software

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https://www.alpinetrek.co.uk/spoke-length-calculator/

280, 281

appears to round up to the nearest whole mm

simple graphical interface

does not support flange diameters over 60mm or under 30mm
does not support drillings less than 24

no spoke drilling diameter correction
Does not support fractional crossings.
Does not allow arbitrary drillings.

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http://www.prowheelbuilder.com/spokelengthcalculator/

279.4, 280.5

Semi graphical interface

Lots of boxes to fill in, will run without all filled in

Does not support fractional crossings.
Does not allow arbitrary drillings.

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also see

http://www.bikeradar.com/forums/viewtopic.php?t=12845028

http://www.dalynchi.co.uk/wbpe.html

not downloaded or used it

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https://www.unicycle.uk.com/calculator

278.94, 280

for unicycles

only supports drillings that are a multiple of four

allows crossing values from 0.001 upwards. Will flag error message if crossings x 9 > drilling

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http://www.sapim.be/spoke-calculator

279, 280

Appears to round to the nearest mm.

does not use ERD, uses internal rim diameter and rim thickness. Used 591 and 3mm. (Gave the same answers using 590mm internal diameter BTW)
Presumably allows 3mm difference between NSD and ERD.
Uses hub OLN plus flange inset distances, and assumes that wheel is not offset.

Supports arbitrary values for drillings and crossings (even ones that don't make any sense)

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https://spokes-calculator.dtswiss.com/en/calculator

279.5, 280.6 (accurate)
279, 281 (recommended)

appears to round down or round up depending on exact value.

requires that you fill in all kinds of things (including spoke type and nipple type) before it will calculate. However
the calculated length does not appear to vary with spoke and nipple type used (at least in the few combinations I tried).

Will calculate complete wheel weight if you enter a choice of DT parts, but will calculate spoke length for input dimensions of rims
and hubs with selected DT spokes.

Will support fractional crossing values, won't support arbitrary drillings.

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https://bikefat.com/5-best-spoke-length-calculators/

2013 review of online calculators. Out of date

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https://www.pijin.co.uk/spokecalc.html

didn't work for me

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http://www.bikefix.co.uk/spoke-length-calculator

a version of the wheelpro calculator
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https://wallerbmx.co.uk/bmx-spoke-calculator

for BMX wheels. Only allows their chosen hubs and rims (from DD menus) to be used with the calculator,
so of zero interest to non-BMXers.
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http://spokelength.com/

279.6, 280.5

Simple graphical interface, no ads no BS, bare bones.

No offset flange function

Does not support fractional crossings.
Does not allow arbitrary drillings.

The simplest calculator to use; run twice to get the spoke lengths in an asymmetric wheel.
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https://sheldonbrown.com/rinard/spocalc.htm

contains a link to download a microsoft excel based 'spocalc' program by Damon Rinard.
If you use excel spreadsheets regularly it may be of interest to you.

also contains a review of various online spoke calculators, (including several that I have not trialled)

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https://tech.mavic.com/tech-mavic/uk/calcul.htm

only allows selection of (obsolete, not current) Mavic rim models from their drop down menu.
Page didn't display correctly on my PC.
Difficult to say for sure but appears to use Mavic's published NSD values as if they were ERD values
which means that predicted spoke lengths are about 2mm shorter than would be usual.

Avoid.
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https://biketechtools.com/spoke-length-calculator

279.4, 280.46

OK, but slightly annoying prompts.

Allows your results to be e-mailed to you.

Does not support fractional crossings.
Does not allow arbitrary drillings.

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Chris Juden posted this chart on the CTC website (as was) and it can still be accessed via the wayback machine.

http://web.archive.org/web/20120724225309/http://www.ctc.org.uk/resources/About_the_Bike/SpokeDiagram-622-559.pdf

an explanation of its use is here

http://web.archive.org/web/20120527030929/http://www.ctc.org.uk/DesktopDefault.aspx?TabID=3524

NB. It doesn't say so explicitly but the assumed ERDs appear to be ~614mm (for 700C) and ~551mm for 26" (559) rims in these charts. Use corrections to the chart to get different spoke lengths, i.e. if the actual ERD is 10mm smaller (604mm), your spoke lengths are 5mm shorter than the chart predicts. The chart also appears to assume a flange spacing of ~50mm which is OK for rear hubs but not so much for (wider) fronts or the left side of heavily dished rears. If the flange spacing is nearer 70mm (eg for some front hubs) then adding 1mm to the predicted spoke length would be a better estimate.


In addition the above page links to an outdated version of the Sheldon Brown/Damon Rinard page which contains links to other spoke calculators

http://web.archive.org/web/20120614064507/http://www.sheldonbrown.com/rinard/spocalc.htm

you may be able to access these by using the wayback machine even though many of the posted links are not current.

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[fastpedaller]

Since I gave up using my trig method from years ago, I've always used spocalc and it's never failed me.

[Brucey]

FWIW the EDD calculator is uses the same formula as spocalc, but is in online form. It contains all the rim and hub measurements of spocalc, to which others have been added. See

https://leonard.io/edd/faq

cheers

[tatanab]

I have the wall chart version of spocalc. 20 years old with measuring points to hold your hub and rim against to get the ERD and hub measurements. You then enter these into a notepad provided, add X, divide by Y or something, and there's the length. Mostly I use the modern spreadsheet, but since I still have the notepad with what for me are common combinations of rim and hub I sometimes look up the length used previously.

[Gattonero]

I usually reach for the DT Swiss spoke calculation chart, or use the Wheelbuild webpage, but mostly I go by intuition.
Given that I can lace one side of the wheel in 2-3 minutes, it takes more to think than to do it again if needs to

Usually the spoke length is correct if one side is complete and -with the nipples screwed about 2 turns, and the spokes not properly "seated" on the hub flange- one is able to push the hub to one side so that the opposite flange can go aligned with the rim. i.e. the Rh side of the wheel is laced, the Lh flange of the hub can be pushed to the Rh side and being inline with the rim.
This is not guarantee that the spoke length will be correct, but most often gives a good idea. Variables are the drilling angle in the rim, the hub flange thickness, the way the j-bend of the spokes would stay with the hub flange (still related to the thickness), etc.

[Brucey]

spoke length charts are all very well but everyone I know who uses one accepts that (quite often) they will make one approximation too many with that approach, and end up with the wrong length spokes for a build that they have not done before.

For whatever reason spoke chart errors appear to get larger and larger with small rims. It seems to me that you shouldn't really be consoling yourself with how long it takes to recover from such a screw-up; IMHO you should be aiming to avoid them in the first place.

Whilst most wheels are built by folk who have lots of spokes to hand etc, most wheelbuilders only build a few wheels a year (for themselves and friends, typically). They order spokes to go with a particular build and don't have boxes and boxes of spokes to play with, or access to a tool that allows them to cut and thread spokes to suit. For these people getting the right spoke length first time is more than a means of avoiding a slight inconvenience.

cheers

[jb]

Unless you can measure the ERD yourself on the actual rim your building up, then you'll most likely be out slightly. Unfortunately most people want, or have to, order the spokes and rim at the same time so don't get this opportunity.

[Keezx]

Exactly, an important point, and more important than which calculator (not much difference anyway).
Just ansered a phone call from a buddy who ordered Chinese rims and already found out after 5 minutes that his spokes were 20 mm too short.
The specified ERD was 20 mm shorter than the actual.....

[MikeF]

20mm!

[Brucey]

It goes without saying that you need accurate ERD values before you use any spoke calculator; as I mentioned the best thing is to measure the rims yourself, since anyone else's measurements (eg from the manufacturer/supplier) may be in error. Spoke calculator databases are riddled with errors, even in hub measurements. In one I was looking at the other day there are three sets of measurements for the same hubshell, two of which were obviously wrong because they said 'no dish' on an obviously dished hub. Such measurements are the product of a 'that will do' mentality and it is just lazy to make and use those measurements yourself as well as stupid to pass them on to others.

Does it matter which calculator you use? -Yes!

The wrong length is the wrong length! All the errors add up and you can (esp if you are using spokes that come in 2mm increments) easily be left with the wrong length spoke if your measurements and calculations are not accurate. 'Not much difference' becomes a substantial contribution to the net errors. This is why it is important to do accurate measurements and calculations, so that you know which way to go when ordering spokes.

If you are in any doubt about rim ERDs (and IME you should be, unless you have measured unknown rims yourself) you should order your spokes separately.

A lot of people are not fussy enough about measurements, calculations, spoke lengths and thus (quite needlessly) end up building substandard wheels as a result.

cheers

[tatanab]

I built a wheel last night, my first in a couple of years, using a Mavic Open Elite - one of their new offerings. Mavic give an ERD of 610 but there is discussion in some places that it should be 606.

I used my Spoke Calc wall chart to measure and came up with 610 and a spoke length of 294. Today I put the same figures into on line Spocalc spreadsheet which gave me a length of 294. It happened that I have 296 in the shed, so I used them instead. In theory the spokes are 2mm too long. In practice they protruded about 1mm beyond the nipple, so a 294 would have been 1mm inside the nipple, about the end of the screwdriver slot. Spokes used were DT, nipples were ACI Alpina.

I'm happy enough that this shows why we need not be bothered within a mm since some spoke manufacturers make even lengths and some make odd and it probably isn't worth hunting for a calculated ideal of say 295 when you have 294 or 296 available.

[Samuel D]

Mavic Open Elite - one of their new offerings. Mavic give an ERD of 610…

Where does Mavic say this? I ask because Mavic told me by email it was 606 mm. I asked Mavic because I couldn’t find a published ERD on their site. In fact, mine measured about 610 mm, as I noted here. What hub did you use?

[tatanab]

Where does Mavic say this? I ask because Mavic told me by email it was 606 mm. What hub did you use?

Sloppy writing on my part. Because I could not find anything on their website I did a general search and found 610 tucked away in Q & A on SJS and Chain Reaction websites, possibly more I don't remember. I also saw comments about 606 versus 610. Sorry for any misunderstanding, sloppy on my part. Hub is irrelevant, it is not something people would recognise.

[Samuel D]

Thanks. The hub matters inasmuch as its dimensions give context to your observations about how the 296 mm spokes fitted. But this is a bit off topic here, I suppose.

[Brucey]

....I'm happy enough that this shows why we need not be bothered within a mm since some spoke manufacturers make even lengths and some make odd and it probably isn't worth hunting for a calculated ideal of say 295 when you have 294 or 296 available.

if you had 292mm and 296mm spokes in the shed (vs 294 being allegedly 'perfect') you would certainly have valued accurate calculations and measurements in order to choose between them; one length would have been OK(ish) and the other one would have been horrible.

In many cases a 'perfect' spoke length might bring you in the depth of the slot, 1mm shorter means you don't reach the bottom of the slot (= 'fail') and 1mm longer leaves you protruding from the top of the nipple slightly (= a better choice in most cases). However.... if your calculation is 1mm in error to start with and you happen to make the wrong choice (50-50 chance there...) you can end up 1mm the wrong side of either condition (= 'pretty horrible').

Only an accurate measurement calculation gives you the opportunity to make a reasoned choice of spoke length vs (at best) an educated guess.

In the course of building hundreds of wheelsets I have occasionally been stupid, lazy, or believed someone else's ERD value that turned out to be cobblers. This has lead to me having to take apart about half a dozen wheels that were never going to be right otherwise. I think that spending a few moments measuring and calculating accurately in order to avoid this is time very well spent. If you think otherwise I guarantee it is only a matter of time until you (quite needlessly) come unstuck.

cheers