reddog3

I'm crazy, old, cheap, and nothing better to do with my time. I'm also the consummate DIYer, so I build everything I can. I build my own frames, forks, and wheels. This provides many opportunities for tool building- frame jig, fork jig, rear triangle jig, truing stand, etc.

Here's my homebuilt ERD measurement device- hopefully others can get some inspiration from this.

The first image shows the measuring device mounted in the rim. It consists of two spokes fixed to a 6 inch machinists scale (metric scale on one side). The spokes are attached to the scale using fender strut bolts, so the spokes can be adjusted to accomodate different rim sizes.



Second image is closeup of scale and readings. Sorry about the crappy pics!

The scale is drilled for the fixing studs as close to the ends as possible.
The spokes are cut so each, with the nipples bottomed on the threads is 250mm.
This gives a total length of 500mm with the spokes fully inserted into rim and nipples bottomed out.
Read the measurement between the spoke ends on the scale and add to 500. That is your ERD.
As you can see- one spoke is fixed (left in pic) at 20mm on the scale. I couldn't fix it at "zero" due to the limitations in drilling the scale for the studs.
The other spoke (right) is "adjustable." Pull it taught in the rim and clamp the spoke to the scale with the stud. The measurement you seek is the difference between the two spoke ends.
In this case that is 48mm, plus the 500mm for an ERD of 548mm.



I did my guage this way because it was too cumbersome trying to hold the spokes tight in the rim and collecting the measurements using other methods.

It's a lot easier to use than it is to explain. It folds up for storage.

C'mon let's see some ideas from others. Maybe we need a thread primarily for home-made tools?

Dan Burkhart 

I like it.

mrrabbit 

Long as the spoke used are cut exactly to an intermediate length - it works.

Had to cut and thread about 10 spokes before I finally got what I wanted for my "rim rods".

=8-)

HillRider

+1 That's a real improvement on the previous homemede tool I've seen.

Dan Burkhart 

I think it's probably even an improvement on the Wheelsmith Rimrods that I use. I can achieve a high level of accuracy with them, but only after much fussing to ensure the heads are both firmly seated, I'm holding the rods in perfect alignment with each other, and I'm holding the end of the ruler exactly flush with one of the rod ends.

BCRider

I've never seen these "rimrods" in action but it sounds like we're short of one or two hands to hold the rods and the ruler and measure what needs to be measured. So this tool of reddog's is definetly better in that way.

Two small things that I can see which might make it better. First off I would align the "fixed" spoke with one of the lines that has the MM hack marks coming off it instead of in the middle of the numbers. This would require a clamping gizmo that's a bit fancier than the fender washers though. I'd make it up out of a block of steel or aluminium with a blind hole such that the spoke goes in and stops in the hole even with the end of the scale on the ruler. That would set things up for the actual measurement that way. A wing screw would lock it in place to take the measurement. The other spoke would not be clamped. Instead the ruler would be attached to one of the spokes with the clamp block. Then one hand would hold the ruler side and the other the free spoke. The free spoke could then be pretty easily held against the line on the ruler which has all the mm hack marks coming off it. A measurement could then be taken which is easier to do because the end of the spoke is on the mm hack marks. At the same time the alignment of the spokes would be easy to do using the mm mark baseline for alignment one way and the flat of the ruler for alignment the other.

The only glitch is that it would require a bit of machine work to make up a clamping block which would hold the fixed spoke in the right offset manner and allow you to clamp it. Reddog, your tool has the advantage of simplicity in that it can be put together out of spare bits and with the simple effort of drilling just the one hole.

corynardin

So why can't you just use a metric tape measure?

EDIT: Okay, I just looked into what ERD is and I see why a taper measure wouldn't work. For others with the same question ERD is the distance between the ends of two opposing spokes when they are in their maximal insertion in their respective nipples. It would require some estimation as to where that occurs and would introduce too large an uncertainty into the measurement.

Flying Merkel

A big hearty Socal fukkin'A dood to you. Looks elegantly simple.

mrrabbit 

Metric tape measure works fine - but also have to make a rim-thickness+nipple-thickness tool in order to do 2 addends to get a final number.

I use:

1. My own rimrods.
2. My own tape measure plus a marked nipple on a spoke "stick".

Both work fine - but the rods are much better when eyelets are involved, or when the holes are angled such as is the case with diamond profile rims. Same for dimpled rims and concave rims.

=8-)

Tunnelrat81

The 'fixed' spoke doesn't need to be exactly 250 mm long. All you'd have to do is cut a new 'fixed' spoke to be 270mm long, and clamp the end in the same location that you have it now (at the 2cm mark) and take your exact readings on the other end, no need to subtract out the 48mm, you'll just read 48mm directly off of the ruler this way, and you haven't had to machine a special block to terminate the spoke at the end of the ruler.

By the way, this looks great, and will certainly be easier than juggling two spokes and a ruler with only two hands.

-Jeremy

Dan Burkhart 

OK, I'll play. You got nothing on me in the crazy department chum.
When I was a kid, I used to envision and contrive contraptions that would have made Rube Goldberg retreat to his drawing board had he seen them. None of them ever served any useful purpose except to keep me occupied and off the streets.
Fast forward about a hundred years or so, and here I am still cooking up wacky creations, only now I try to apply them to some practical use.
This latest gizmo I created is sure to keep my membership in good standing in the Crackpot Screwball Society.
It was inspired by discussions on another group I frequent about indexed shifter interchangeability. I built this tool for measuring total cable pull and cable pull between indexed detents.
I should mention that the design is partially plagiarized. I originally built it without the digital caliper, using a pointer that mounted on the cable and pointed to a metric scale. Someone else showed a design that incorporated the digital caliper, so I adapted mine to follow suit.


You can see it in action here.

https://www.youtube.com/watch?v=vAd0uGzHzmQ

Rowan

Or just look up the ERDs on the manufacturers' websites.

But a nice, esoteric tool to have around if you are into building on old rims for which the ERD is not readily available.

mrrabbit 

When manufacturers don't note where they are aiming with what nipple...their numbers don't do you any good. They mean nothing...literally.

=8-)

...that's why most wheel builders end up measuring their own.

Sixty Fiver

Park can bite me... have been building and truing wheels with a variation of this set up forever.

Rowan

Personally, I haven't had any issues with Velocity and DT Swiss rims. And if those figures are matched with either Damon Rinnard's Excel calculator, or the DT Swiss calculator, most people won't go far wrong.

Honestly, I think there is way too much navel gazing with a lot of this stuff. As Sixty Fiver shows, you can build very good, very durable wheels with very simple equipment. I used to build them on the bike in the same way, complete with msaking tape or zip ties.

Things like spoke tensionometers might be all well and good, but they are an expense that isn't needed if you have anything even approaching a muscial ear.

And... a millimeter or two one way or the other on spoke length isn't going to make terribly much difference in most wheel builds. Where I am, spokes come in 2mm increments, so getting an absolute measure isn't going to be much use if it's an odd number with a .5 on the end...

Lawrence08648

I still don't quite understand exactly how to measure ERD and why you can't use a tape. I don't know where to measure from. It isn't the maximum inside rim diameter (directly across the center to get the maximum).

I don't understand screwing the spokes into the nipples and then measuring? Measuring where? What good does that do? Again going back to the tape. There is no hub. I don't understand. I'm missing something here. I do a very good job as a bike mechanic and overall mechanic on everything, I have several college degrees in the sciences and engineering, my father was a contractor and then fixed everything at home. Fixing things is in my DNA and I still don't quite understand.

I don't know if anyone can explain how to measure ERD on this board or I will need to go to a bike shop and have them show it to me.

Looigi

Sheldon Brown states it pretty succinctly:

"Effective Rim Diameter. This is the rim diameter measured at the nipple seats in the spoke holes, plus the thickness of the two nipple heads. The E.R.D. is needed for calculating the correct spoke length."

So, you need to measure the distance between the surfaces of where two opposing spoke heads sit, which are inside the rim.

reddog3

You're absolutely right. You could use any spoke lengths you want. All you need to know in the end is the distance between the two spoke ends and add that to the length of the two spokes. In my math challenged mind the total length of the spokes at 500mm seemed to make sense, and dividing that by 2 (250mm) just made sense. I only provided the numbers so it was easier to figure how I arrived at the "real" ERD.

Arriving at the correct length for the spokes did take a bit of jiggling and filing until I was satisfied. To test the accuracy I measured a few rim with published values that were known to be accurate.

The reason I did this in the first place was (my confusion) with Mavic's published numbers, and I could never remember the password to get to their tech info. The tool has some limitations in the range of rim sizes it will measure.

Dan Burkhart 

Mavic, being French, has their own way of expressing this data. They don't even call it ERD. Their term is Spoke Support Diameter. As a general rule of thumb, I've found adding 3mm to Mavic's published numbers gets you pretty close.

mrrabbit 

While Sheldon Brown provides a definition that most people can grasp - that's what he is good at - it is not the correct definition - it's a modified definition for laymen.


ERD = The distance from the end of one opposing spoke to the end of the other opposing spoke in an already properly built and functional bicycle wheel.

Note: FOR AN ALREADY BUILT FUNCTIONAL WHEEL!!!


Sheldon Brown's definition basically says the same thing - but from the standpoint of someone who is trying to figure out an ERD BEFORE building the wheel in question.

ERD is the most wickedly fun part of building a bicycle wheel. You are essentially doing things backwards at this stage - trying to find the ERD of an already built theoretical wheel that you are trying to build.


Where the real confusion exists is when people try to construe ERD as a standard. People run off to Damon Rinard's SpokeCalc, read "top of a 10mm nipple" and assume that's the standard.

There is no standard...because ERD is as literally noted in the technical definition that I gave earlier a "what works" number. If the number you used worked for the threads and nipple used without issues - that's your ERD for that rim in that wheel.

YOU as the wheel builder for a particular have to decide:

Given

1. The nipple you have.
2. The threads you have.
3. The nipple and thread combination minimum and maximum insertions.
4. The need for roughly 1mm overhead room to ensure desired tension is achieved.

Where are you going to aim with that nipple and thread. Below flat? Flat? Top of head? Slightly past head?

It's a judgement call...do what works for you.

=8-)

mrrabbit 

That's if you are aiming for the top of a 10mm nipple for example with an Open Pro.

In that case, ERD = @605.00

As is, ERD = @602.00 for a 12mm nipple.

ERD is not a standard Dan....

=8-)

thorsteno

Nice work Reddog.

Dan Burkhart 

Perhaps not, but it's close enough to a standard for those of us not yet at your advanced level.

Soil_Sampler

nice tool reddog3.



https://www.sutherlandsbicycle.com/rimdiametersystem/

mrrabbit 

If you mean a standard for the occasional tear down and rebuild with shorter spokes...sure...if that makes you happy.

=8-)