Bicycle Mechanics - ERD measurements gone haywire

I followed the instructions on http://www.wheelpro.co.uk/spokecalc/ and put a piece of tape at 200mm on two spokes so that makes 400mm. Then I measured the distance between the two pieces of tape which was 108mm. So, that makes 508mm. I measured the spokes on my broken wheel and it looked like 265mm. However, it's a single wall 26 inch rim so I figured maybe the ERD may be similar to other cheap rims. So I looked up the Alex X-202 rim and the ERD is 547mm. So, I entered that in the calculator and it gave me 263mm for spoke length. The spoke I measured may be 263mm depending on how I measure it. Like if I start measuring from the middle of the rounded end.

Then I looked at my 700c WTB Freedom Ryder 23 rim and I know it's 597mm. I can't even remember how I calculated that in the past. Anyway, I used the same method with the two spokes on the Ryder 23 rim and it was 260mm. So, I decided to add 37mm to 508mm and it gives me 245mm. OK, I don't know if there's really a difference of 2mm with the rim I wanted to buy but would that be a problem?

How/why is the two spoke method not giving me the real ERD?

EDIT: I might choose the Alex DM24 because it has an internal width of 24mm which I think I'd need for 1.95' tires.

I don't know how you set up your spokes, but try this if it's different that what you did.

Get 2 300mm or longer spokes and 2 nipples. Measure each spoke to exactly 300mm (you can use 250, or 200mm) and cut off a bit long and grind it back to 300mm. tip the spokes in glue and rhread each into a nipple until it ends between the bottom of the slot and top of the nipple (or 1/2mm short of the top). Let it dry.

Now you have gauge spokes that each measure 300mm from the ERD, and you can run them in from opposite holes, and add the gap, or subtract the overlap from 600mm (or double your gauge length) for a true ERD that includes the nipple head thickness.

I suspect that your problem is that you measured only the exposed spoke, and so your readings were short by the length of the nipples.

Oops, I measured from the wrong side of the tape. lol

Oh well. I'm hoping that if there is a significant difference with the rim I want to buy, that maybe 1mm or 2mm off may not be too bad.

Oops, I measured from the wrong side of the tape. lol

.

That's why I made permanent gauges and cut and ground them to length. Only one possible end to measure to. It's insurance in case I decide to calculate spoke length before my morning coffee.

2 mm is not that critical if you ask me, 5 mms could be an issue.

2 mm is not that critical if you ask me, 5 mms could be an issue.

Thanks, what I wanted to know. I'll found out later I guess. :)