dabac

Hi all,
I've been thinking about a way to determine if one rim is a viable replacement for another simply by comparing rim height. But I haven't got enough parts at hand to measure to come to a reliable conclusion. And undoubtedly there will be some oddballs that it doesn't work for.

How much difference do you think it is in the height/thickness of the bead hook, and in the hubside wall of a rim?

It would be neat if all one needed to do to find a matching rim would be to measure its height from tireside edge of bead hook to hubside face of rim.

So I'm asking if any of you who have some unbuilt rims of different models available would be willing to measure their height and compare that to their respective ERD.

With luck, there'll be a reliable correlation between the two, which would make picking a replacement rim easier.

Homebrew01

Since ERD is already an established method, and more accurate, stick with that.

Looigi

If you are trying to estimate ERD via rim height, then lots of things come into play that wouldn't when measuring it directly. So though you may estimate ERD from rim height, the other variables probably will not allow sufficient accuracy for it to be of value in determining proper spoke length.

Andrew R Stewart 

There is an ERD measuring poster/chart which you could emulate. As you don't really need to have an actual number you don't need to measure but you do need to compare, you need a gage not a ruler in a manor of speech.

Place the rims on a flat surface (bare, no spokes or hubs) on their sides. Take some form of a square (could be a credit card on edge) and place it against the rim's inner surface. mark this point of your flat surface. Do the same at the opposite point along the rim. (Actually doing this at a few points is smart to average out the rim's roundness). Then place the other rim over these marks and see how your "square" lines up with the first rim's marks of inner diameter. This is the first step to see if the rims share this inner diameter. Next is to take a spoke nipple and place it in a rim hole and mark on it how far it sticks out. Repeat on a few holes. Then try this with the other rim.

If both rims share the same inner diameter and if the spoke sticks out the same amount for each rim then the rims share their ERDs.

The DT/BPP Spoke-Calc chart basically does this but places a pair of specific nipples in opposite holes at the same time then lines their ends up against a ruler which has had the diameter to circumference corrected for the nipple dimension already done. Andy.

Andrew R Stewart 

Actually the Phil ERD measuring kit uses this method very well. But like any measuring the person doing the measuring is the bigger variable. Andy.

Edit- I've used the three common measuring systems and find that the "across the rim with spokes or rods" method to be the hardest to work with. Yet it is what most will try first. Andy.

fietsbob

using 2 spokes, on opposite rim sides then measuring the gap between them, in the center, has a certain simple clarity..

You get the place where the spoke end is up to where it should be.... at the slot in the nipple head

dabac

Apparently I wasn't clear enough in the first post.
This wasn't about determining the ERD of a "naked" rim prior to a build.
This was about determining ERD on a built wheel prior to a rebuild.
Which makes the method of measuring between opposing spoke holes rather impractical, if not outright impossible.
It's in this scenario - when considering a rim replacement on an already built and still rideable wheel - when determining ERD by measuring rim height would be handy, if it would be accurate enough.
And if it is accurate enough, it would be faster than the opposing spokes method, even on a "naked" rim.

andr0id

I think I understand what you're saying. The diameter of a 700c/622 rim is constant. But the spoke seat is not, so I don't think you can just measure rim wall height. I think you could measure from outer rim edge to the spoke head from there, do some overall math and come up with an ERD estimate.
Yes, it would probably work, but I'd try a few before ordering some expensive spokes based on that.


Added later...

I have a Open Pro so I just tried it.

Rim diameter is 630mm
Drop from diameter down to where the nipple sit = 16mm
height of nipple head below the slot = 2mm

so 630 - 32 + 4 = 602 which is in fact the published ERD of an Open Pro.

dabac

Well, is it?
And which diameter are you referring to?
Sure, the diameter where the bead of the tire meets up with the bead hook shouldn't vary much.
But the width/height of the bead hook AFAIK isn't standardized.
Could mean two millimeters or so to the ERD.
Can't tell exactly tell what you mean by "rim wall height".
My thought was to measure the height of the rim profile when seen from the side with a Vernier caliper.
... Which is why we have different ERDs ...
Tried that. Fiddly, but possible. Was hoping for something faster.
That was where I was hoping for some help, if people here could measure the height of some rims and then compare it to known ERDs.

cny-bikeman

Just work backward - using a built wheel you have the number of holes, cross pattern, hub diameter, flange distance - the only variable missing is ERD.

Andrew R Stewart 

To repeat andrOid and myself- measure the rim's circumference at the outer most part (the top of the side wall). Convert to diameter. Measure from this outer spot down to the nipple's slot, double this dimension and subtract from rim outer diameter. Result is ERD. This is the Phil method. Andy.

FBinNY 

I've been using rim outside (edge) diameter, and drop to the nipple seat (times 2) to compute the nipple seat diameter for 50 years. (I prefer measuring to the nipple seat then adding for the nipple head later to using ERD specs)

You don't need anything special to measure this way. I measure drop by using an improvised gauge, ie a pencil or a throwaway chopstick as a depth gauge, and a knife placed across the rim to make a mark. Then I can make an accurate measurement of the drop. If you want to use ERD, place a nipple into the hole and measure the drop to the top of the head.

BTW - some time back Sutherlands introduced a similar method for measuring rims. It includes a measuring tape which reads the diameter directly when wrapped around the circumference and a small direct reading depth gauge similar to a car tire tread depth gauge. It has the advantage of being unaffected by any ovalness of the unbuilt rim, but is too expensive for what it is.

Andrew R Stewart 

It is this Sutherlands kit that I got from Phil Wood along with their spoke cutter. My mistake to not call it by it's correct name. Andy.

FBinNY 

BITD Howard Sutherland, Phil Wood and a few others in the Bay area were friends and worked together. So who knows who came up with the idea and who commercialized it. I suspect it's older than either. I do know that I was measuring and averaging diameters 50 years ago, and I didn't invent that method either.

dabac

Do you assume a value for outside diameter, or do you measure?
Measuring O.D. and getting it millimeter-right on a built wheel isn't entirely easy with the hub in the way.
If bead hook height is near enough universal between rims, then O.D. should be the same too(within each size).
Which would make ERD of a built wheel easy to determine by Vernier caliper only.

dabac

Would need to pull a spoke and measure that too. And the whole point of this question was if there was an easier way to determine ERD on an already built wheel.

FBinNY 

I don't assume anything or trust anybody's specs for stuff like this. besides BIRD there were no published specs anyway.

So I measure, though I also kept notes and don't have to measure the same rim every time. Keep in mind that until fairly recently, we were building with a much smaller menu of options. I can't count the number of times I simply glanced at a rim and laced up a wheel using 298/296mm spokes. The advent of more spoke count options and deeper rims has changed all that, so these days I tend to check everything unless I know for sure I don't have to.

Andrew R Stewart 

Again, measure the rim's circumference to determine the diameter. This is the best for a few reasons. First you can do it with simple tools, a strip of stiff paper works better then a tape measure as it rides on both rim side wall tops. Second you can do it to a build or bare wheel/rim. Third with a 3.1415... factor any miss measurement of the circumference (and resulting diameter value) is reduced by by by by by by by Pi. Andy.

FBinNY 

Using a paper strip that's slightly more than 6' long may buy a degree of accuracy, but not that many people have 6' strips of paper sitting around waiting for them.

Accuracy beyond reasonable is uncalled for since there are already many fudge factors and errors introduced into the calculation. For example, regardless of the precision of the calculated result, we'll be using spokes that come in increments of 1mm or even 2mm and rounding to the nearest length anyway.

Measuring the diameter twice at right angles and averaging is more than accurate enough. If you can measure to 1mm, that's an error of only 1/2mm in spoke length, well within the working tolerance for the system.

These days folks deceive themselves with notions of great precision because computers are there to help maintain the delusion. But before computers and even TI scientific calculators, nobody was going to pull out a trig chart to calculate a spoke length.

We measured the rim, subtracted the distance between the holes in the hub where opposite spokes would go and added some fudge factors for flange separation. Two measurements, a bit of simple math and a bit of "English" and we reliably had a spoke length. Even rim measurements were often eyeball whereby we used our X-ray vision to see through the side of the rim to where the nipple was inside. I still do this from time to time on non aero rims and it produces good results. Also I still use direct measurement, not a calculator, because what I see is what I get, vs. a black box where a data entry error can have me wasting time lacing a wheel.

Andrew R Stewart 

Francis- I agree with your statement of over concern with precision. I learned this when I got my flat surface and dial indicator. because I could now measure a frame's twist to a thou or two didn't mean that there was any real value to doing so. Other alignment relationships still were more important.

But back to my post- dabac mentioned the bane (second time I've used this word tonight) of measuring a built wheel. It's impossible to *** around the hub and have a direct diameter measurement. So I offer an alternative that is easy (making a 6.5+' long strip is kindergarten skills). And I offer a few reasons why this method is attractive.

I still have the first (and other) editions of Sutherlands manuals with the memorable spoke calc charts. I sometimes wish current newbies would go back to these. As one would work through the choices of dimensions, crosses and rim models one gets a better handle on how the resulting spoke length changes dependent on the factors. Which factors are important and which have more fudge to them.

But today's wannabe builders are not likely to go this route. They have grown up is a world of ask and someone will answer. So I fight what I call the "good fight" to try to show that some research/measuring/thinking can give an answer AND give some personal satisfaction too.

In the end as long as the readers get to the place they want it's all good. Andy.

Looigi

FWIW: This is truly measuring it, not estimating it.

dabac

So would you be willing to share your diameter values?

If you consider that proprietary, I don't need to know which rims they're from. But seeing the span would be helpful.

I'd be interested to know how much difference there is between diameters for rims with the same BSD.

It'd be a step on the way towards my idea of being able to judge ERD from rim height. Or maybe kill it off....

dabac

Sure, I could do the long paper strip. I have a set of measuring spokes set up. I've got a functional but ugly rig consisting of two right angles, an aluminium profile and a meter long ruler that can be used to measure both ID and OD of a rim, even on a built wheel with some care.
I can, and have used these methods.
But a vernier caliper is less clumsy, both to store and to use.
If accurate enough, I can still see a benefit from determining ERD by using a caliper only.

FBinNY 

My notes aren't in a form you could use. I never record the dimensions, just the specific build - ie. Campy Sigma tubular, Campy 32h front, 3x, the length of spoke and how close to ideal it came out (ideal is between slot and top of nipple). That info tells me exactly what I need to know to repeat a good build, or fine tune one for next time.

IME rim outer edge diameters are very close (within 2mm) for rims of the same nominal size. So if you accept my approach that measurements within 1-2mm in diameter are close enough, then once you've measured one 700c rim, you've measured them all. So all you need is a depth gauge and you're done.

Andy_K 

I was interested in this topic because I had just ordered some rims (Freedom Expanse) for which I could find no claims of ERD (and really not even any evidence apart from the Jenson USA site where I bought them that they exist at all) and I was anxious to starting playing with the numbers. The Jenson site listed rim height as 25mm and I found that Velocity Fusion rims have the same rim height and a listed 590 ERD. I wasn't going to order spokes based on that, but it gave me something to think about while I waited for the rims to arrive.

The rims arrived last night and I measured the ERD as 589 (and confirmed the rim height as 25mm). I'm rarely confident of the accuracy of my measurements, especially with something like measuring the drop to the nipple seat on a rim with recessed spoke holes, but I also measured using the two spoke method and got the same result. I could still be off by a millimeter either way. Close enough.