Bicycle Mechanics - Measuring an ERD of 583mm for an ISO 590 rim -- can this be right?

I'm getting ready to buy spokes and lace up some wheels for a new build. The rims are 650A Sun AT18s, for which I can't find any info online. They're box-section rims that appear very similar to CR18s except that they're single-walled and lack eyelets.

Since I can't locate more than one extra spoke with a nipple to measure it (the method I see recommended), I'm using a brand-new brake cable, held taut while I mark the other end with a Sharpie. The outer edge of the Sharpie mark is about 2mm up from the rim bed, so I'm just adding 2mm to that measurement to include the other nipple head's height.

And that comes up to 583mm, which seems too high based on this logic:

1) that's not too far below the rim's bead-seat diameter
2) Sun's current ERD chart (http://www.sun-ringle.com/wp-content/uploads/2010/05/SUNRingle_rim_chart.pdf) doesn't include an AT18 in this size, but when I look at other AT18 rims, the ERD is 13mm less than the BSD, which I would expect to be consistent since it's the same extrusion. (CR18s are consistently 10mm less than the BSD, for example.) From this thinking, I get a predicted ERD of 577 for my rims.

Is it possible that Sun's specs exclude the height of the nipple head? At any rate, I know that the "real" ERD should trump online specs that may be out of date, but I thought I'd check here first.

Thanks.

I'm sorry, but I've never done it without two spokes, and the combination of the
spokes being a little pricey and my own OCD to get it right the first time would
lead me to advise that you do the same.......otherwise, no help here.

I'm getting ready to buy spokes and lace up some wheels for a new build. The rims are 650A Sun AT18s, for which I can't find any info online. They're box-section rims that appear very similar to CR18s except that they're single-walled and lack eyelets.

Since I can't locate more than one extra spoke with a nipple to measure it (the method I see recommended), I'm using a brand-new brake cable, held taut while I mark the other end with a Sharpie. The outer edge of the Sharpie mark is about 2mm up from the rim bed, so I'm just adding 2mm to that measurement to include the other nipple head's height.

And that comes up to 583mm, which seems too high based on this logic:

1) that's not too far below the rim's bead-seat diameter
2) Sun's current ERD chart (http://www.sun-ringle.com/wp-content/uploads/2010/05/SUNRingle_rim_chart.pdf) doesn't include an AT18 in this size, but when I look at other AT18 rims, the ERD is 13mm less than the BSD, which I would expect to be consistent since it's the same extrusion. (CR18s are consistently 10mm less than the BSD, for example.) From this thinking, I get a predicted ERD of 577 for my rims.

Is it possible that Sun's specs exclude the height of the nipple head? At any rate, I know that the "real" ERD should trump online specs that may be out of date, but I thought I'd check here first.

Thanks.

1. Sun has been all over the place with ERD reference wise. My suspicions are two fold:

- Right hand and left hand ain't communicating for the purpose of consistency. Had to call and literally give 'em hell once on the MTX's.
- Production changes - or outsource changes resulting in slight changes in rim dimensions. CR-18 seems to have been through this.

2. Come on Scott...don't believe for one second that you don't have two spokes and nipples laying around something...have another go looky!

=8-)

There's lots of info on this site about "ERD measuring" devices, and how to use them. Some of them are hoemade- including mine. All that I know of use two spokes. Yoiu have one spoke. One more couldn't cost more than 50 cents.

Don't be cheap; buy/borrow a spoke and do it right.

Simple soul-lu-shun.

Take your metal tape measure.. hold it firmly against the valve hole.. run it directly to the seam across from it. Execute that measurement with care.. divide by .03937.. that is your INSIDE ERD.

Now.. place a nipple into a hole as normally mounted... hold it firmly in place.. on the protruding end that would thread into the spoke.. take a BLACK magic marker and color the nipple closely where it comes from the rim. Removing that nipple.. the length ABOVE that black line is your 'nipple seat length'. Measure this closely.. can be done with that metal ruler.. if care is used. 1/8= 3mm [.125"] for example.

Inside ERD + 2times (nipple stack length -1mm) = ERD

Round up over .7 mm fraction.. round down .6mm and less. IE: 287.6 = 287's.. 287.7 = 288's.

Depending on the nipple style/length.. normally that means Inside ERD + 5.5-8mm =ERD

This is a perfect example why I NEVER rely on factory specs for ERD. Some forget the height of the nipple (which I wish were never included), others have eyeletted and non-eyeletted rims with the same profile, giving the same ERD for both versions, forgetting to include the thickness of the eyelet.

Measure the rim using 2 spokes, nipples and a ruler and get a measurement you can rely on. The few minutes it takes will be much loess than the time and effort of relacing a wheel because the spokes are the wrong length.

Looks like it's pretty unanimous! I looked again for the leftover spoke nipples from my last wheel rebuild and they're AWOL -- I was probably dumb enough to throw them away.

One of my bike nerd friends will be bringing a couple spares to lunch, and I'll make the exact-length spokes on my grinder tonight. Thanks. :thumb:

I looked again for the leftover spoke nipples from my last wheel rebuild and they're AWOL -- I was probably dumb enough to throw them away.

One of my bike nerd friends will be bringing a couple spares to lunch, and I'll make the exact-length spokes on my grinder tonight. Thanks. :thumb:

You're making this too complicated. The spokes don't have to be any specific exact length. You just have to know how long they are. As for nipples, how can you not have 2 when you need much more than that to build the wheel?

You're making this too complicated. The spokes don't have to be any specific exact length. You just have to know how long they are.

That's true -- I was thinking I'd get them to an easy-to-remember length.


As for nipples, how can you not have 2 when you need much more than that to build the wheel?

I usually buy the nipples with the spokes, so I don't have them yet. If I had spares at one point, they're lost now -- lesson learned.

Update! My bike-nerd friend not only brought along a couple extra spoke nipples, but lent me his "measuring spokes" and Park Tool ruler. I used the latter to check the former before measuring the ERD.

I got 582mm.

Still, it's probably a good thing that I couldn't find published specs after all, because it got me to start measuring this stuff for myself. Thanks for the help, folks!

Update! My bike-nerd friend not only brought along a couple extra spoke nipples, but lent me his "measuring spokes" and Park Tool ruler. I used the latter to check the former before measuring the ERD.

I got 582mm.

Still, it's probably a good thing that I couldn't find published specs after all, because it got me to start measuring this stuff for myself. Thanks for the help, folks!

582 means nothing without a reference...

Was it 582 with spokes ends set at the screwdriver flats of 12mm nipples? Was it 582 rim seat to rim seat (SSD)?

And then if you don't mind, I'll add to my spreadsheet for Sun/Ringle 650A AT18...

=8-)

Update! My bike-nerd friend not only brought along a couple extra spoke nipples, but lent me his "measuring spokes" and Park Tool ruler.

I'm going to be as kind as I can in pointing this out, but the fact
that you are now assembling your own wheels puts you well advanced
on the linear scale of bike nerdiness.......just sayin':)

582 means nothing without a reference...

Was it 582 with spokes ends set at the screwdriver flats of 12mm nipples? Was it 582 rim seat to rim seat (SSD)?

And then if you don't mind, I'll add to my spreadsheet for Sun/Ringle 650A AT18...

=8-)

Very true! The nipple heads (~3mm tall, and the nipples are 12mm long) are included in my measure, and I word it that way because one of the spoke nipples wouldn't turn all the way on to get down to 200mm, so it stayed at 203mm. (I promptly let my friend know about that.) 200+179+203 gives 582mm for me, and I checked a couple spots around the rim.

I would be honored to contribute to the spreadsheet. :D


I'm going to be as kind as I can in pointing this out, but the fact
that you are now assembling your own wheels puts you well advanced
on the linear scale of bike nerdiness.......just sayin':)

Oh, I only use the term "bike-nerd" affectionately. This is my third wheelbuild, and the first where I measured the ERD and hubs and used an online tool to compute my spoke lengths. :geek:

Very true! The nipple heads (~3mm tall, and the nipples are 12mm long) are included in my measure, and I word it that way because one of the spoke nipples wouldn't turn all the way on to get down to 200mm, so it stayed at 203mm. (I promptly let my friend know about that.) 200+179+203 gives 582mm for me, and I checked a couple spots around the rim.

I would be honored to contribute to the spreadsheet. :D



Oh, I only use the term "bike-nerd" affectionately. This is my third wheelbuild, and the first where I measured the ERD and hubs and used an online tool to compute my spoke lengths. :geek:

...in other words, what were the positions of the spoke ends in the 12mm standard profile nipples when you snugged the elbows and did your readings?

=8-)

Mrrabbit-
I just built a pair using Sun Rims CR-18 in 27" 3X.
I'll basically confirm their website reading of having an ERD of 620MM using 12MM DT nipples to the bottom of the screwdriver slot using spokcalc.
I saw a couple vendor sites that listed 618MM.

...in other words, what were the positions of the spoke ends in the 12mm standard profile nipples when you snugged the elbows and did your readings?

=8-)

Sorry, more details: the "measuring spokes" my friend lent me are cut to 200mm, so that each would still measure 200mm when the spoke end is flush with the end of the nipple. Now, since one of the spokes has a damaged thread (or something), the nipple doesn't thread on all the way, so I had to use its actual length rather than the ideal 200mm.

Here's a pic -- try to ignore the crummy lighting, messy couch, and my hands not being in the shot to hold the spokes to the ruler more accurately:

http://s36.photobucket.com/albums/e26/sstulken80/measure800.jpg