Wheel Building. Measuring Effective Rim Dia

[Ash28]

How do I do it.
I want to try wheel building. I have a rim and a hub and want to order some spokes. I need the ERD for a spoke calculator.
I have looked at this http://www.wheelpro.co.uk/spokecalc/ but I find it confusing.
What am I actualy measuring ? Can I just measure the inside rim dia with a meter rule ?

[stewartpratt]

No, that won't work. You'll need two spare spokes and nipples and a felt tip pen to actually measure it, but the easiest way is usually Google - what rim is it?

[JonMcD]

Always measure it yourself. There is no agreed definition for ERD and rims can vary. I think Mick F had a recent post where he found that old RIGIDA Chrina rims differed from new RIGIDA Chrina rims.

EDIT: Under the "RIMs" tab at http://www.wheelpro.co.uk/spokecalc/ the two spoke method is described in detail which will ensure the measurement you take will give the correct ERD to plug into the spoke length calculator on this site.

[Redvee]

I use the Wheelpro method for ERDand it has been spot on for me.

[Mick F]

Agree, measure it yourself AND your hubs.
By doing that, you are learning as you go and you are certain of you figures because rims vary!

Actually, the ERD isn't critical to the Nth degree as it's best to round up the spoke lengths, and remember, spokes tend to come in only even numbers of millimetres. If you round up, the threads will be inside the nipples but if you round down threads could be showing.

[Ash28]

Thanks for the replys. I have had another look at the Wheelpro method and it is clearer now, I will try that.
It is a 26" Mavic X517 rim and I have a Deore hub.

[[XAP]Bob]

This is why I said "I think " to my lbs when I took in a wheel to be built.

[stewartpratt]

FWIW I've built several 517s on Shimano hubs and the figures available on the Internet in conjunction with the DT Swiss calculator have always worked perfectly. That's probably one of the most common build combinations, too, so if the figures were wrong you'd find plenty of people posting up the right ones in various places

Not that there's owt wrong with measuring yourself. But it's worth checking your numbers against the quoted ones before buying the spokes...

[Phatman]

If you know your hub dimensions, the Mavic site can prove useful as a cross check here and there's a useful site here for cross checking too - but you are relying heavily on what others have loaded to the site. I have reliably built a few wheels with the aid of Roger Musson's book and it is very good.

[Ash28]

Thanks for the help folks. Used the Wheelpro method and cross referenced with the other sites.
Finaly got round to lacing it up and the spoke legnth was spot on. Yipee