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How to measure the diameter of a rim from a complete wheel?

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How to measure the diameter of a rim from a complete wheel?

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Old 04-27-18, 06:56 AM
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le mans
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How to measure the diameter of a rim from a complete wheel?

Anyone know a trick way of doing that, rather than to strip the spokes then measure the rims the conventional way?

i want to fit the hub and spokes from a steelie to an alloy rim, they look the same [but] i'd hate to go through the rigmarole then discover the spokes won't do!
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Old 04-27-18, 07:06 AM
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Not sure what you're wanting to measure, the ERD or the bead seat diameter.

Assuming it's the ERD you can note a spoke end's location within it's nipple, remove that spoke and measure it, measure the hub and note the lacing pattern then do the spoke length calculation with a guess of ERD. Retry the calculation with a changed ERD till you end up with the same spoke length as the measured spoke, adjust for where in the nipple that spoke was originally.

If it's the bead seat diameter test fit tires and read off the tire when you have one that fits. Andy
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Old 04-27-18, 07:19 AM
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Yep, the ERD[s]

but, both wheels are complete

The alloy 27" front wheel that i want to use the rim, has a 3 cross lace pattern,
the steelie has a 4 pattern 3 spd internal hub.. not that it matters, but i'd have to use those spokes

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Old 04-27-18, 07:28 AM
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I'm trying to wrap my head around your suggestion, Andy

i think i understand what you mean, take a calculation from both wheels to see if the diameter of both wheels are the same?
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Old 04-27-18, 08:09 AM
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Outer rim diameter minus 2 x the drop to the top of the spokes?
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Old 04-27-18, 08:57 AM
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Originally Posted by dh024 View Post
Outer rim diameter minus 2 x the drop to the top of the spokes?
removed the tyres, measured that and so far so good
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Old 04-27-18, 09:00 AM
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2X radius.?

run a string around over the spoke heads , measure the string ... for circumference. ..



(basic math the new rocket science?) internet has the formulas, if you forgot rhem
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Old 04-27-18, 09:18 AM
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Don't you have a tape measure?
Butt one end against the nipple and read at the spoke head flange.
Remove rim strip and observe how deep the spokes are in relation to the screw driver slot to see if there is any difference between the wheels to "adjust".
That assumes the nipples are the same length.
Compare the 2.

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Old 04-27-18, 09:24 AM
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Originally Posted by fietsbob View Post
2X radius.?

run a string around over the spoke heads , measure the string ... for circumference. ..



(basic math the new rocket science?) internet has the formulas, if you forgot rhem
the difference is 2 mm, that wouldn't matter, would it?
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Old 04-27-18, 09:31 AM
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Originally Posted by Bill Kapaun View Post

Don't you have a tape measure?
Butt one end against the nipple and read at the spoke head flange.
Remove rim strip and observe how deep the spokes are in relation to the screw driver slot to see if there is any difference between the wheels to "adjust".
That assumes the nipples are the same length.
Compare the 2.
Unless I misunderstand the OP's intent, he needs to measure the ERD of two rims to see if they are the same. You told him how to measure spoke length only.
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Old 04-27-18, 09:38 AM
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Originally Posted by dh024 View Post
Unless I misunderstand the OP's intent, he needs to measure the ERD of two rims to see if they are the same. You told him how to measure spoke length only.
Then measure to the axle.
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Old 04-27-18, 09:48 AM
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Originally Posted by dh024 View Post
Unless I misunderstand the OP's intent, he needs to measure the ERD of two rims to see if they are the same. You told him how to measure spoke length only.
Yep, i'm hoping both rims are the same diameter so i don't have to buy spokes :-)
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Old 04-27-18, 10:05 AM
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Originally Posted by Bill Kapaun View Post

Don't you have a tape measure?
Butt one end against the nipple and read at the spoke head flange.
Remove rim strip and observe how deep the spokes are in relation to the screw driver slot to see if there is any difference between the wheels to "adjust".
That assumes the nipples are the same length.
Compare the 2.

well, the first thing i noticed was the hub sizes are very similar but the spokes have different lace patterns so that told me the spoke lengths are different [measured them anyway]


And i dont think measuring to the centre of the axle will give you an accurate reading coz on the two wheels they are vastly different widths

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Old 04-27-18, 10:26 PM
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There's the rim circumference divided by Pi and lessened by twice the rim depth method that dh024 kind of suggested. The Phil spoke machine comes with a measuring tape that is wrapped around the rim's outer circumference and calculates this diameter. Then from this outer edge the depth to the spoke bed is measured (a caliper with a depth probe helps). Twice this depth and subtract from the calculator rim outer diameter.

This question is answered with two methods. Direct measurements with the only math being C= 2 times pi(r) and a simple subtraction. Or using a spoke calculator and running trials of ERD to get the directly measured, and known, spoke length. Pick your poison.

In the end some tolerance will be expected. Andy (who built a wheel this week and trusted a published ERD, only to find that the spokes were 2-3mm too long...)
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Old 04-27-18, 10:54 PM
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Remove the tires from both rims, measure it like you did a child growing up, pencil & yardstick style ? If the rims are labeled, a 27" is 630mm diameter.
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Old 04-28-18, 01:19 AM
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I was thinking too, Andy, if the circumference or the diameter of the steel rim is slightly smaller than the alloy rim by 2mm, the spokes aren't going to stick through the nibble heads

Fuji, both rims are marked 27" etc, my concerns was the rim designs are different [not where the tyre/tube goes] as you look at the rim where the spokes go through which might make it a different size.

I think i can go ahead and build this wheel.
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Old 04-28-18, 01:47 AM
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For rim transplant purposes I use (the functional equivalent) of two straight angles.
I lay a wheel flat on a table, insert two straight angles through the spokes until they are resting well on the table top. Then I slide them apart and measure the diameter of the hubwards face of the rim.
Repeat and compare for the other rim.
For build purposes, I do a guesstimate about how much to add for rim wall thickness and nipple protrusion to get an ERD.
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Old 04-28-18, 07:22 AM
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Frustrating

Originally Posted by Andrew R Stewart View Post
Andy (who built a wheel this week and trusted a published ERD, only to find that the spokes were 2-3mm too long...)
I have done this and think it is one of the most needlessly frustrating things I have run across in BICYCLE MECHANICS. I am reminded of it every time I come across the bundle of spokes on my shelves. Any more if my measurement is off by more than a mm I assume this is just sloppy customer support and cross the rims off my acceptable list.
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Old 04-28-18, 07:31 AM
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Originally Posted by le mans View Post
well, the first thing i noticed was the hub sizes are very similar but the spokes have different lace patterns so that told me the spoke lengths are different [measured them anyway]


And i dont think measuring to the centre of the axle will give you an accurate reading coz on the two wheels they are vastly different widths
Remove the tire, and roll the wheel, starting with the valve straight up and down for reference, along a tape measure. After you go 360, you will have the circumference. Divide by π to get the diameter.
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Old 04-28-18, 09:36 AM
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Originally Posted by wgscott View Post
Remove the tire, and roll the wheel, starting with the valve straight up and down for reference, along a tape measure. After you go 360, you will have the circumference. Divide by π to get the diameter.

that's given me an accurate reading. the difference in the two rims isn't 2mm.. the dimensions are exactly the same!

so this gives me more confidence to strip the wheels ..swap everything over, and once stripped i could do an online calculation just for the hell of it

the steel chrome rim can be used on a vintage indi 500 to flip, it's in such good condition, just needs a polish up to rid a bit of rust
i have a rear wheel for that bike too, a good matching set

i just don't want steelies on my personal bikes :-)
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Old 04-28-18, 09:52 AM
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Originally Posted by dabac View Post
For rim transplant purposes I use (the functional equivalent) of two straight angles.
I lay a wheel flat on a table, insert two straight angles through the spokes until they are resting well on the table top. Then I slide them apart and measure the diameter of the hubwards face of the rim.
Repeat and compare for the other rim.
For build purposes, I do a guesstimate about how much to add for rim wall thickness and nipple protrusion to get an ERD.

Good idea, i thought about your suggestion but it sounded like a bit of jigging, i must be slack
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