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Effective Rim Diameter Help
#1
Hello Folks


I have a set of old rims that I am going to build several wheels from. I am attempting to calculate erd (effective rim diameter) so I can then calculate the spoke lenth. The numbers that I keep coming up with are around 506.4 mm. Is it likely that these are 507mm rims considering the tires that were on there were 24 X 1.75 in size?


thanks,

jake
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#2
How do you calculate the ERD? I would measure it: take two spokes, shorten them to 20cm, screw them into nipples such that the end sits where the nipple would be seated in the rim and glue them to that position. Then, use the two spokes and a ruler to measure the ERD, at least twice around the wheel.
Oh, and if you can measure the ERD within 1/10 of a millimetre with any method, please tell me how (not that this is important, many shops only stock 2mm increments in spoke length).
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#3
(08-01-2013, 07:00 AM)Joe_W Wrote:  How do you calculate the ERD? I would measure it: take two spokes, shorten them to 20cm, screw them into nipples such that the end sits where the nipple would be seated in the rim and glue them to that position. Then, use the two spokes and a ruler to measure the ERD, at least twice around the wheel.
Oh, and if you can measure the ERD within 1/10 of a millimetre with any method, please tell me how (not that this is important, many shops only stock 2mm increments in spoke length).

First of, let me state that I am new at this. 2nd since I could not find a mm tape measure, I was using a standard (inches) tape measure and using google to convert to mm. So say that I had a measurement that was "3 13/16. I would first convert to a decimal, then I would convert to mm.
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#4
Hi Jake;

507 sounds like the B.S.D. (bead seat diameter) not the E.R.D.
BSD: http://sheldonbrown.com/tire-sizing.html

"E.R.D.
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. " from: http://sheldonbrown.com/gloss_e-f.html

Scroll down here on how to measure ERD:
http://sheldonbrown.com/rinard/spocalc.htm

I would expect something less than 500mm (19 5/8") for the ERD.
Nigel
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#5
(08-01-2013, 09:40 PM)nfmisso Wrote:  Hi Jake;

507 sounds like the B.S.D. (bead seat diameter) not the E.R.D.
BSD: http://sheldonbrown.com/tire-sizing.html

"E.R.D.
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. " from: http://sheldonbrown.com/gloss_e-f.html

Scroll down here on how to measure ERD:
http://sheldonbrown.com/rinard/spocalc.htm

I would expect something less than 500mm (19 5/8") for the ERD.

Hi Nigel,


I took a 2nd look at the link for erd and with the explanation you gave, it made sense to me now. I got my hands on an ace hardware standard/mm tape measure. I took several measurements. each time I came up with 501mm. I measured from nipple edge to nipple edge and got about 477mm. Each nipple is 12mm in length. So 477 + 2(12) = 501. Not entirely confident that this tape measure is very accurate. I ordered a better one that strictly does metric. So I will confirm.

Thanks,

Jake
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#6
(08-02-2013, 12:18 AM)jakelite Wrote:  I took a 2nd look at the link for erd and with the explanation you gave, it made sense to me now. I got my hands on an ace hardware standard/mm tape measure. I took several measurements. each time I came up with 501mm. I measured from nipple edge to nipple edge and got about 477mm. Each nipple is 12mm in length. So 477 + 2(12) = 501.

Possible, that is a very shallow rim - please post some pictures.

Double check the measurement from the Bead Seat to spoke nipple; which should be (507-501)/2=3mm or a fraction less than an 1/8"
Nigel
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#7
Careful: It is measured from where the nipples are seated (the... dunno... flanges?), so if the nipples are 12mm in length total you have to subtract (I guess) about 3-4mm. Good luck!

Take a look at Roger Musson's book
http://www.wheelpro.co.uk/wheelbuilding/book.php

Electronic only, free updates with purchase. It helped me a lot.

Edit: I did not mean to be derogatory / insulting, if I came across that way I am sorry. After all, English is a second language to me (just like for Dr King Schulz (if you saw that movie), even though I am not from Düsseldorf...).
Oh, and "standard" is mm Wink
http://en.wikipedia.org/wiki/International_System_of_Units
http://www.bipm.org/en/si/si_brochure/
I cannot get the whole thing with 12 inch = 1 yard, 3 yard = 1 foot, 22 feet = 1 chain, 10 chains= 1 furlong, 8 furlongs = 1 mile .... decimal makes your life easier.
How many pounds were one stone again Wink and how many quarts are there to one bushel?
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#8
(08-02-2013, 04:47 AM)Joe_W Wrote:  Careful: It is measured from where the nipples are seated (the... dunno... flanges?), so if the nipples are 12mm in length total you have to subtract (I guess) about 3-4mm. Good luck!

oh, ok, so I was measuring the entire nipple. If you are only suppose to measure the head (the part that sticks out through the spoke holes as shown in picture below), then the length is only 10mm so that would change the erd to 497mm .

[Image: 20130802_122850.jpg]
how to screen capture
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#9
How to find the bead seat for measurement.

I have been to some links where they talk about the bead seat and show illustrations. ( see images below) However, I am confused as to what exactly they are showing and I can't find it on my rim.

[Image: bead_seat_diameter.jpg]
image hosting youtube

[Image: 20130802_140835.jpg]
image shack

How would I find bead seat on this rim so I could measure bsd?

[Image: 20130802_122636.jpg]
image hosting

measuring across center of rim from nipple head edge to edge. (note: nipple on right side is covered by tape measure.) took this measurement in multiple locations around rim.

I am going to sandblast and repaint these rims.

Thanks again,


Jack
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#10
Jack/Jake: see arrows in the pic below:

[attachment=4339]

It is the surface that inner diameter of the tire seats on.
Nigel
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#11
(08-02-2013, 08:41 PM)nfmisso Wrote:  Jack/Jake: see arrows in the pic below:



It is the surface that inner diameter of the tire seats on.

oh, ok got it. bsd 507mm on these rims.
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#12
Jake;

Those rims look a bit scary. The original plating and the rust need to be removed, which may not leave much metal thickness on the rims. Oxalic acid, available from auto parts stores for rust removal will take the rust off; not sure about the plating. Alternately, you could take them to a plating shop to discuss your options.

For ERD; measure from the BSD; only need one spoke nipple. PM or e-mail me to remind me; and I'll send you a sketch on how.
Nigel
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#13
[attachment=4341]

Tools required: calipers (Harbor Freight; $9.99 with coupon) and small flat piece (metal, plastic, wood, etc) with flat and parallel sides.

Place flat across edge of rim, measure down the bead seat surface; record number (or zero the caliper), next measure to the end of the nipple dropped in a spoke hole. The difference between the two measurements is equal to (BSD-ERD)/2

Example: if your BSD is 507mm, and the difference between the two measurements above is 7mm; then your ERD is 493mm.

Repeat the measurement several times around the rim, and take an average.
Nigel
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