Effective Rim Diameter of Dimpled Rim

spongebob

100 µW
Joined
Aug 30, 2021
Messages
7
Location
Ontario
Hi,

I am lacing a motor around a 18" motorcycle rim, and because the rim is dimpled (see pic) I'm unsure of what the ERD would be. From my research, ERD is the distance from where one nipple sits to the opposite nipple on the other side of the rim. Here is what I did:
20220620_140813[1].jpg
20220620_141025[1].jpg

1. I fed a string (non elastic) through opposite spoke holes in the rim (one end to other), and marked the distance between the holes (not including the rim thickness)
2. I then pulled a nipple through the spoke hole, marked where the nipple came through the rim, and measured that as the rim thickness.
3. I added the distance between opposite spoke holes with twice the rim thickness for my ERD.

I'm not sure if this is the correct way to measure the ERD for a DIMPLED rim. Can anyone confirm that what I measured is in fact the ERD?

Thanks for your help!
 
Here's how Rix does it
https://endless-sphere.com/forums/viewtopic.php?f=2&t=55458&p=1181426&hilit=erd+measure#p1181426
Rix said:
Got a few PMs on determining ERD in MMs, or effective rim diameter. Here's how I do it, take some spokes, preferably with the same length of nipples intended for the next lace job. Thread them through rim's holes number one, count 18 if a 36 hole rim, and thread into the 19th hole. Use a rubber band to keep the spokes in place. Then measure from the end of the nipple to the end of the nipple. This rim is a 18x1.40 rim. The measurement was about 16.15 inches. When I converted 16.15 inches to MM, I get 410mm, then add the length of the spoke nipples to that number. The nipples I used in this pic were 18mm long, so I added 18x2 for 36mm to the 410 and got 446mm for my ERD. When using Ebikes.ca a spoke calculator, this was my rim measurement. It called for 115mm and 116mm for the TC30xx. I had ordered some 117mm long spokes. Well, I am was 1mm too long on some of the spokes and did some grinding. Not a big deal. Its possible my rim isn't perfectly around, and some of the spokes were maybe a mm or 2 long.

ERD1[1].jpgERD[1].jpg
 
This is telling me something important
I have been having so many flats lately
I've been eyeballing the space between seat stays :lol:
Figuring what size, width tire.
Need a cheap source of 1.40 used rims

That is the erd method I failed when I did mine, was a couple mm off, spoke protruding the nipple. Nothing yards long of electrical tape wont fix, then the valve hole, what to do what to do. Drill bits or razor :lol: choices man, no issues, not with what feels like 15lbs of glass, what glass.
 
Spongebob, I do my measurements like you. I put the nipples in the rim. mark it where it comes thru the hole, and get the distance to the top of the nipple. Then I add twice that to the distance between the two holes. Have done maybe six or eight different size rims, I always seem to wind up 1-2 mm too long. With double wall rims, a washer saves the day. Wouldn't have worked too well with single wall rims.

I've also found I get different numbers between the Grin online calculator and others. I use Grin for hubmotors. and the others for bike hubs.
 
amberwolf said:
Here's how Rix does it
https://endless-sphere.com/forums/viewtopic.php?f=2&t=55458&p=1181426&hilit=erd+measure#p1181426
Rix said:
Got a few PMs on determining ERD in MMs, or effective rim diameter. Here's how I do it, take some spokes, preferably with the same length of nipples intended for the next lace job. Thread them through rim's holes number one, count 18 if a 36 hole rim, and thread into the 19th hole. Use a rubber band to keep the spokes in place. Then measure from the end of the nipple to the end of the nipple. This rim is a 18x1.40 rim. The measurement was about 16.15 inches. When I converted 16.15 inches to MM, I get 410mm, then add the length of the spoke nipples to that number. The nipples I used in this pic were 18mm long, so I added 18x2 for 36mm to the 410 and got 446mm for my ERD. When using Ebikes.ca a spoke calculator, this was my rim measurement. It called for 115mm and 116mm for the TC30xx. I had ordered some 117mm long spokes. Well, I am was 1mm too long on some of the spokes and did some grinding. Not a big deal. Its possible my rim isn't perfectly around, and some of the spokes were maybe a mm or 2 long.

ERD1[1].jpgERD[1].jpg
Over 6 years ago, where does the time go? Since then, I learned a couple of things. Measure twice for certainty, and trust the spoke calculator from Grin Cycles. It has yet to be wrong for me.
 
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