Frequently asked questions

I have a question about the spoke calculator. What should I do?

You should post the question in the comment box at the bottom of this page. People who know the answer, including myself, will answer there and you will receive an email notification if someone replies to you.

Also, a public discussion can happen should someone have a related question later.

Should I round the spoke length up or down?

Obviously, it's best if the spoke is completely flush with the head of the nipple, but the calculated length is rarely a multiple of 2mm, which is the increment that most manufacturers sell their spokes in.

Since it is better for the spoke to be slightly too short rather than poking out at the top, I recommend you round the spokes down.

What size of nipples does this calculator calculate for?


What should I do if I have nipples of a different length?

I once asked Damon Rinard (who wrote the formula used for calculating the spoke length) and this was his reply:

Personally, I don't adjust for nipple length differences between 12 and 14mm nipples, since the DT Swiss (and maybe other?) nipples I typically use are not actually threaded longer or shorter - the extra length is in the external flats.

If someone knows the specific nipples they are planning to use don't follow the same approach as DT Swiss, then a different philosophy for adjusting the ERD can be used. For example, internal hidden nipples (like Rolf, etc.) require a serious increase in ERD (several millimeters or more) depending on the details of those internal nipples.

Anyway, in the end ERD is defined as the diameter of the circle through the ends of the spokes. HIstorically that has assumed "standard" nipples, so adjustments can certainly be made based on known differences in specific nipple dimensions, such as thread length. For Spocalc, the basic question remains the same: at what diameter do you want your spokes to end? That is what you enter for ERD.

What does "ERD is Mavic's Nipple Seat Dia + 3mm for nipples" mean?

These measurements come from the original spocalc.xls spreadsheet maintained by Damon Rinard. I asked him about this phrase and he answered the following:

For wheel building, there are two important (and often confused) rim diameters: ERD, effecive rim diameter, and NSD, nipple seat diameter.

ERD is the diameter through the ends of ideal-length spokes. It's measured on the theoretical spoke end, not on the rim.

NSD, nipple seat diameter, is the diameter of the circle through all the nipple seats. It's measured on the rim.

Typically, with normal nipples the spoke extends radially outward a little bit beyond the nipple seat, and into the spoke head.

Spocalc calculates spoke length and wants the diameter of the ends of ideal spokes, so NSD isn't the dimension spocalc uses.

The comment in the database means I've added 3 mm to Mavic's nipple seat diameter to approximate typical (DTSwiss 14 mm) nipples. So for that rim there might be some adjustment needed if using other nipples.

What does OSB mean?

OSB means offset spoke bed. If this is something other than 0, the rim is asymetric. That means that the spoke holes are not perfectly in the middle of but are closer to one side of the rim.

There are very few rims that do that so most likely your OSB will be 0.

Measuring your OSB is described on the how to measure page under the Asymetric rims.

What is the tension ratio displayed under "Complete wheel data"?

Damon has the following to say:

Tension ratio works based on the fact that, to hold the rim centered in the frame or fork, the horizontal component of the tension in the left spokes must balance the horizontal component of the tension in the right spokes (equal and opposite). Here's how:

Spoke tension is a vector pointed along the length of the spoke. Because the spoke has a bracing angle, you can separate its tension into two components: a lateral component and a radial component. To center the rim, spoke tensions are adjusted until the left and right side lateral components of the spoke tensions are equal.

Two cases illustrate: In a wheel with equal bracing angles (common in front wheels), the rim is centered when the spokes are adjusted to equal tensions; in this example, tension balance is 100%.

However, in a wheel with unequal bracing angles (common for rear wheels and disc brake front wheels), the left and right side spokes are adjusted to different tensions until the lateral components are equal and opposite; in this example, the tension ratio is other than 100%.

It's the lateral components being equal which centers the rim, and spocalc, using this requirement and simple trig on the bracing angles, calculates the tension ratio and displays it for the builder.

The tension ratio, together with the expected maximum spoke tension (often dictated by the rim), can be used as an early warning to the wheel builder by predicting the tension on the slacker side. Too little tension means those spokes could go slack in use, making the wheel weaker.

What are the advanced options for the search?

You can filter the rims for size. Just type size:650 in the rim search box. This can be combined with other queries: size:650 zipp shows all 650s from Zipp.

You can filter hubs by front and rear by just using the the words, wait for it, front and rear.

What does the name "Edd" mean?

Edd stands for electronic diametre deductor... alright, that one is backcronym. Edd is friend of mine and we were living together when I wrote this calculator in college.

Is there a list of all components in the database?

Yes, it's here.

What is the relation to Damon Rinard's spokecalc.xls?

Edd is spokecalc.xls rewritten as a web application.

It uses the same formula and hence delivers exactly the same results.

I have also imported all the measurements from the original spreadsheet and additional hub measurements came from Arup. Over the years people have however submitted lots more components.

Who made this website?

Hi, I'm Leonard Ehrenfried. If you want to get in touch send an email to

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