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Just trying to check if the ERD of my new rims tallies with what the manufacturer is quoting.
I've measured the diameter from outer edge to outer edge and subtracted 2x the distance from the outer edge to the spoke hole.
How much do you normally allow for the nipple heads - couple of mm each side? This seems sensible to me and would actually bring the measurement directly in line with the quoted one.
There seem to be some more accurate ways of doing it involving lacing a couple of dummy spokes across, don't want to get into that if I don't have to!
I just put nipples on two spokes and hook their jbends
Wind nipples down till the spokes are locked together - then unhook.
Hook em together and measure the resultant length from inside nipple face to inside nipple face.
I use the spoke method too. Also do it at 3 or 4 points around the rim and average the result.
At present, I don't have any spokes long enough to do this. Is that the only way - are the results from measuring diameter my way going to be unreliable as a check? I'm using the mighty Sheldon Brown's guide.
http://www.sheldonbrown.com/spoke-length.html
Just use two spokes you do have and measure the gap between the ends.
Not really if you use the correct length spokes as oppose to goinf 1-2mm either side as sometimes needed wen shops only stock certain size then you should absorbed any variance from the actual size that occurs with your method
I'd go with it if I had to. I've done worse........just blindly followed the manufacturers size -sat wondering if the manufacturer has ever looked at the rim before giving a size.
Althoufh ive seen some sites - for a while Stan's had a 26" erd on a 29er rim - was way back when 29ers were the exception.
Just use two spokes you do have and measure the gap between the ends.
To my hamfisted mind, that introduces a lot more opportunity to get the measurement wrong. I suppose I could tie them together with laggy bands, but still...
that introduces a lot more opportunity to get the measurement wrong
I don't see why, especially if you use a straight rule to do the measuring as you will see by eye that everything is in line
I just take the manufacturer's word for it, I reckon they're better at measuring than I am.
Althoufh ive seen some sites - for a while Stan's had a 26" erd on a 29er rim - was way back when 29ers were the exception.
...although [i]hopefully[/i] I'd spot it if it was that far out.
To my hamfisted mind, that introduces a lot more opportunity to get the measurement wrong. I suppose I could tie them together with laggy bands, but still...POSTED 16 MINUTES AGO # REPORT-POST
It works though. I have 2x 250mm spokes just for that task to make the maths easy.
I just take the manufacturer's word for it, I reckon they're better at measuring than I am.
They're often not, I've no idea why they're incapable of getting it right, but they are, they'll quote nipple seat diameter, rim diameter, anything except the sodding ERD.
At least nowadays you can look on google and someone on a forum somewhere will have measured the correct number.
That's how I spotted it ...I was buyih bits from afar for building a wheel and noticed the 29er rim erd was similar to my ex721 ....
Am I right in thinking that ERD is the distance between the slots in the two nipple heads once seated in the rim? So I'd need a couple of the nipples I was actually going to use, regardless of spoke length?
Yup
Am I right in thinking that ERD is the distance between the slots in the two nipple heads once seated in the rim? So I'd need a couple of the nipples I was actually going to use, regardless of spoke length?
Yes.
Although I've sometimes questioned that as I was told the spoke should end up flush with the bottom of the slot, otherwise the tension causes the nipple to close up on itself and crack. Which would give the same spokes regardless of nipple length. But calculators don't do that.
So to be on the safe side I always use the 12mm nipples.
I guess maybe machine built wheels use longer nipples/shorter spokes to be sure that the spoke can't push the driver out. And maybe modern nipples are stronger than when the advice I was given was relevant.
Hmmm... will have to see what nipples the shop in town has lying about. So I get two random spokes, lace them onto the rim, tension them across the j-bends, measure the the distance between the start of the nipples then add the distance from the end of the nipple to the bottom of the slot (x2)...
Thanks all, BTW!
I've built a few wheels now just usig the manufacturers ERD, didn't realise it might be wrong!
Right then, I've done the recommended thing and measured using this setup (twice)/
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Gives me, give or take for my poor measuring skills, an ERD of 569 vs 566 (quoted).
That translates to approx 1.5mm difference in spoke length. Would you be happy enough with that for ordering purposes, or splitting the difference?