hybridbkrdr

If you measure the inside of the rim, can you just add 6mm to get the ERD?

mpetry912 

every rim is different and it is pretty easy to measure. Get 2 spokes, thread the nips on and cut them to exactly 250 mm

then put them thru opposing spoke holes and measure the gap. Add 500 mm. done.

E-Z

/markp

Andrew R Stewart 

"inside of the rim" can mean a few different things to different people. Where does the "6mm" come from? I suspect we have missing context or not well described terms.

Spoke bed is the surface inside the rim that the nipples seat on. It is the diameter across the spoke bed that ERD is.

Marks method of measuring is a great one for the home builders, and used by many shops too in one form or another. I'll add that one should always measure at a few points about the rim and average the results. Andy

sweeks

This is how I do it. Simple.
It is probably a good idea to do this a second time at 90 degrees to the first, in case the rim is not perfectly round.

Russ Roth

Different materials and designs have different thicknesses, the carbon rims I just measured were 6mm between the two sides. I wouldn't expect an aluminum rim to measure the same and those with eyelets were be still different.

FBinNY 

Most spoke calculators produce spoke length that end just shy of the ERD entered.

So the number you want is the diamerered measured to the tops of th we nipples you plan to use.

The EASIEST way is to measure the.drop from the rim's OD, down to the top of a nipple. I do this by putting a nipple in place, holding it in place with a pencil or similar, then laying a knife across the rim and making a mark.

Double* the depth and.subtract from the rim's OD. Because unbuilt rim a can be oval, measure at least twice and average.

Note: all 700c rims have th we same OD, likewise other sizes, so if you keep notes, you'll only need to measure th ERD drop. Also, if you like buying special tools, use a tread depth gauge.


*a common error is to mix radius and diameter measurements, so don't forget to double the drop.

SurferRosa

Park Tool's guide for measuring erd is super easy to understand. And they illustrate it with pics!

https://www.parktool.com/en-us/blog/...e-rim-diameter

Garthr

I use two spokes also, but I don't cut off the j-bends. I use a caliper and squeeze the two J-bend ends together, do the proper math. This way I'm not interpreting what a ruler says and wondering if the two spoke ends are perfectly straight or not. Using the j-bends in tension I get a consistent hard number from the caliper.(as consistent as one can hold it that is !) The benefit of doing yourself is in you can use the exact spokes and nipples you're building with, and the proper threading end point of the spoke in the nipple. Do this in a couple of places on the rim for good measure.

The six mm thing sounds like "another one of those" wonky myths/misunderstandings that go around not in cycling but in Life in general.

fishboat

+6mm..maybe, but, as others have said, "it depends".

On older Matrix and current H-Plus Son Archetypes (aluminum) rims the material thickness is 2mm..so ID +4mm is the ERD. HPlus Son seems to quote a slightly different number..no idea why, but measuring the ERD and building wheels (spec-ing spokes) based on that measurement has worked well.

sweeks

This confuses me a little. Isn't the purpose of measuring the ERD to help calculate the length of the spokes needed to build the wheel?
But if you mean the same *brand* of spokes and nipples, that makes sense to me.

Leaving the bends on the spokes is a good idea; with cut ends it's a bit fiddly (but not impossible) to get the measurement.

Garthr

(Laughing with myself...) ..... come to think of it, for measuring purposes it doesn't matter what brand or even type, as long as you're measuring what you're using correctly. Sorry for sharing my confusion with you ! I do take many readings for multiple places though, as when you squeeze the calipers there's always very small fluctuations, at least with my hands and the not fancy digital caliper.

When I bought the digital caliper years ago I almost tossed it as it was eating batteries from even not using it. I finally wised up and now only put the battery in when I use it, then remove it when done. The battery has lasted many years now !

FBinNY 

It barely pays to obsess over ERD when using a spoke calculator you're not very familiar with. There are a variety of small fudge factors people build into their calculators, including the 1mm+ difference between hole center, and where the elbow sits, allowance for the longer d, and bistance due to interlace, the difference between the nipple seat and top, adjusting to target 1mm shy of the top.

Obviously the 3-4mm nipple head height is the biggest factor, and it's a problem because some programs use ERD, measured at the rim, and add the height, while others (most? expect you to use an ERD based on where you want the spokes to end.

I suggest that you pick a program, make & enter your measurements carefully, then build a wheel, preferably a front. When finished, note how the results differ from your expectations, then keep using the same program exclusively with that difference being your final results fudge factor.