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Building carbon wheel: what's my margin of error on ERD measurement?

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Building carbon wheel: what's my margin of error on ERD measurement?

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Old 08-09-18, 01:23 PM
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spectastic
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Building carbon wheel: what's my margin of error on ERD measurement?

I busted a rear rim, which has an ERD of 541 mm according to the supplier. I'm going to reuse the old hub and most of the old spokes that appear to still be good. A lot of the rims I'm seeing now are ~544 mm ERD, meaning my spokes are going to be ~1.5 mm short. That's no big deal right? I mean I remember working on the wheels before, and they appeared seized up, like I couldn't tighten them anymore without twisting the spokes altogether. so 1.5 mm shouldn't be a problem, right? at what point does it start to be a problem? 2-3 mm?

also, I'm reusing most of the spokes, but replacing some of the spokes that appear even slightly off. would it be a big deal if I use different spokes for the replacements? the young's modulus for the spokes might be different, despite having similar tensions..
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Old 08-09-18, 01:27 PM
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AFAIK spokes are sold in increments of 2mm length differences..
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Old 08-09-18, 01:33 PM
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Originally Posted by fietsbob View Post
AFAIK spokes are sold in increments of 2mm length differences..
..no more like 1 mm
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Old 08-09-18, 03:37 PM
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Plug the dimensions into a spoke calculator and see for yourself.
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Old 08-09-18, 04:02 PM
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Originally Posted by spectastic View Post
I busted a rear rim, which has an ERD of 541 mm according to the supplier. I'm going to reuse the old hub and most of the old spokes that appear to still be good. A lot of the rims I'm seeing now are ~544 mm ERD, meaning my spokes are going to be ~1.5 mm short. That's no big deal right? I mean I remember working on the wheels before, and they appeared seized up, like I couldn't tighten them anymore without twisting the spokes altogether. so 1.5 mm shouldn't be a problem, right? at what point does it start to be a problem? 2-3 mm?

also, I'm reusing most of the spokes, but replacing some of the spokes that appear even slightly off. would it be a big deal if I use different spokes for the replacements? the young's modulus for the spokes might be different, despite having similar tensions..
Carbon wheels can vary in ERD due to the way they're made. I would not order spokes until I had rims in hand that I could measure.
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Old 08-09-18, 04:06 PM
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i don't understand, you're saying the ERD is measured differently by different people?.. or simply that the ERD is different between different rims? the ERD measurements i received came from the sellers, so i'm inclined to trust their information.
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Old 08-09-18, 05:00 PM
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assuming i got the right measurements for the hub, provided that the hub is laced radially on the left and 2x on the right.. I went to 3 different calculators and got these recommendations




spoke calculator recommendations


I remember measuring both drive side and non drive side spokes, and they both measured 258.. Could it be possible that the manufacturer of the wheel tried to shave some cost by using the same spokes for the drive and non-drive sides? it could help explain why some of my spokes were bottomed out and contorted from tightening.

I'll have to go back to confirm that all of the contorted spokes came from the non-drive side. but 8 mm (??) that's f*k ton of extra spoke! I could not see this when i took the wheel apart, because they didn't put spoke holes on the outer rim, but I'd imagine with 8 mm of extra spoke, you wouldn't be able to get a centered wheel that's also properly tensioned..

so if this were the case, I could just reuse the 258 spokes i have and put them on the drive side, while I buy 251 mm spokes for the ND side. is this a sound plan? I'd try buying and measuring, but everything is coming from overseas, and I have an important race in 6 weeks. this needs to happen soon. Plus, I trust the diagrams and calculators more than my own measuring skills anyway.




PFH R13 diagram
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Old 08-09-18, 05:04 PM
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Originally Posted by spectastic View Post
i don't understand, you're saying the ERD is measured differently by different people?.. or simply that the ERD is different between different rims? the ERD measurements i received came from the sellers, so i'm inclined to trust their information.
Bad strategy. I never trust manufacturers data, and I have lots experience to justify that.
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Old 08-09-18, 08:05 PM
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Originally Posted by spectastic View Post
i don't understand, you're saying the ERD is measured differently by different people?.. or simply that the ERD is different between different rims? the ERD measurements i received came from the sellers, so i'm inclined to trust their information.
The latter. The ERD can vary in production, so it's always wise to measure your rims yourself.

If you're too impatient to do that, you risk having to buy another set of spokes if the rims you get are off-spec. Suit yourself.
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Old 08-10-18, 02:45 AM
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Measuring a spoke w/o knowing how it sat in the nipple means you dont know what your margins are. I wouldnt want to do that.
If I had to, Id ALSO mark off spoke penetration before disassembly to find out the unseen length of the spoke.
After unhooking a spoke from the hub Id then screw it onto a nipple to see how much further it could go.

There is also some ambiguity in what measurement ERD actually refers to. Rim manufacturers tend to use the surface that the nipple rests against as ERD, while other sources say to use the desired position of the tip of the nipple as ERD.

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Old 08-10-18, 05:26 AM
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Measure ERD and calculate the optimal spoke length. I use this spoke length calculator.

It gives results that require rounding up - to the nearest even, or odd size - whichever you can source. So 280.1 would go with 281 (odd) and 282 (even) spokes. Not with 280!

Se how long the old nipples are. DT Swiss nipples come in 12, 14 and 16 mm lengths.
14 mm ones have thread sitting 1 mm deeper.
16 mm ones have thread sitting 2 mm deeper.

The results given by the calculator I've linked give lengths for using 12 mm nipples. So, in case the new ERD requires spokes that are longer by one, or two mm, you could use matching nipples, in case the old ones aren't already the "long type".

Having spokes 1 mm too short makes lacing more difficult and, once a wheel is finished, will make the thread engagement be a bit further from perfect.

Here's my project - I couldn't find good quality spokes in the needed lengths (Serbian importers do ONE import per year, in the spring and that's it, unless you go with poor quality Chinese spokes - wrote to both Sapim and DT-swiss, which told me to look in Chech and Hungarian shops abroad ), so had to improvise, using 16 mm nipples for the NDS spokes. The wheel works fine.



I've seen wheel manufacturers use same spoke length for both sides of the rear wheel. Which is what I've been forced to do in the picture above. Putting durability over looks - customer had agreed, of course.
If that's the case, you might also want to use different nipple lengths.

Last edited by Slaninar; 08-10-18 at 05:29 AM.
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Old 08-11-18, 01:36 PM
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Oh good, I get to be "Analogue Man" again.

By your best judgement you think your existing spokes will be 1.5 mm short. What do you think your alternatives are?

The analogue solution is to try to lace the wheel with the spokes that you have. My bet is that, if your spokes really are 1.5 mm short, you won't even notice. If they turn out to be significantly shorter than that, you are going to have to acquire some different spokes but since you would have had to do that anyway, you're not really out anything. The analogue system can't cost you anything and it might save you some money.
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Old 08-11-18, 01:49 PM
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RG- How can you expect a young one to even contemplate doing a real life test these days? From what I read here every day the expectation is that answers are easy to be given and the correct one will be obvious, be the first one and work always. Andy (who get's tired of suggesting one's actually trying something themselves) (hopefully readers see the satire in my reply)
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