Rim ERD, hub dimension, spoke length confusion

[Dave855]

I would like to build my own wheels. I've read Roger Musson's book and I've bought some 26", 36 hole Rigida Sputnik rims. Now in Roger's book he warns not to use effective rim diameter data found online but to measure yourself. So I have followed his instructions and measured the rim three times across, with the results being 541mm, 541mm and 540mm. However, all online sources I have seen suggest the ERD is 537mm.

That's quite a big difference! Has anyone else measured a 26" Sputnik rim themselves?

Thanks,

David

[the snail]

There are different ways of defining ERD, so I would look at whatever calculator you are using and follow their instructions - a different calculator, or manufacturer's web site might measure ERD slightly differently. Quite often you will find different sizes quoted for the same rim on the web.

[Mick F]

I've not read that book, or any book on wheelbuilding TBH.
I did read Sheldon Brown's excellent instructions though.

How did you measure the ERD?

What I do, is get two old spokes and cut them in half. Discard the hook ends. Screw on a nipple onto each spoke and screw them down so the spoke is level with the nipple head. Thread the spokes through opposite holes and pull them taught and measure the gap between the ends. Do this two or three times in different places.

Measure the total length of both spokes and add that figure to the gap.
That is the rim ERD.

[Brucey]

well that 537mm ERD is 'correct' in that the sputnik rims normally work out 'BSD size - 22mm = ERD', so the 700C sputnik is 622 - 22 = 600mm, and likewise 559 - 22= 537mm

But....

a) it depends where you like your spokes to finish, and
b) it depends on how tall your nipples are, and
b) it can vary from rim to rim, even with the same type....

....so measure carefully. If the rims really are +4mm (which is not unheard of) then I'd be worried about getting tyres on and off. If you think they really are that big then I'd suggest that you check with some tyres for ease of fit and compare with other rims of that nominal size.

cheers

[robc02]

What I do, is get two old spokes and cut them in half. Discard the hook ends. Screw on a nipple onto each spoke and screw them down so the spoke is level with the nipple head. Thread the spokes through opposite holes and pull them taught and measure the gap between the ends. Do this two or three times in different places.

Measure the total length of both spokes and add that figure to the gap.
That is the rim ERD.

+1. That's also the method described by Roger Musson. I've used the figures given by this method in his online calculator and the results were spot on.

[MikeF]

Or
Measure the outside rim diameter.
Put a nipple into a spoke hole and measure the depth from the rim edge to the top of the slot in the nipple.
Deduct twice this measurement from the rim diameter to give the ERD.

Repeat measurements in several places to check consistency.

[hubgearfreak]

now, i don't know the measurements of your hub, nor which method of arriving at spoke length you might choose to use.

however, i've always used this .. http://www.bikeschool.com/tools/spoke-length-calculator. it's never let me down for many wheels. putting some typical hub values in and ERDs of 541,540 or 537 it shows spoke length variation of 2mm max, assuming a 3X build. (my guess is that other patterns aren't for first builders with sputniks). try it for your hub, for both 541 and 537 and see what difference it makes.

if it's as little as my test results, then go for the mean, or as near to it as the spokes stocked in your preferred spoke shop will allow. 36 hole sputniks in 26" size will be very forgiving - i doubt that there's a stronger rim commonly/affordably available and there's a decent size gap inside the rim's box section to allow for a mm of extra spoke if that's the way it turns out.

good luck in the build.

[bikes4two]

What I do, is get two old spokes and cut them in half. Discard the hook ends. Screw on a nipple onto each spoke and screw them down so the spoke is level with the nipple head. Thread the spokes through opposite holes and pull them taught and measure the gap between the ends. Do this two or three times in different places.

Measure the total length of both spokes and add that figure to the gap.
That is the rim ERD.

+1. That's also the method described by Roger Musson. I've used the figures given by this method in his online calculator and the results were spot on.

+1 to both the above. I used Roger's book for my first ever wheel build and followed his methods and have had consistently good results.

[cromo]

I've built wheels using 26" sputnik rims with shimano small flange hubs. I used 262mm for the front and 262 / 260 for the rear, the lengths were fine. Using the DT Swiss spoke calculator or the EDD spoke calculator these lengths would correspond to an ERD of 541mm. An ERD of 537 would have you use spokes that are possibly too short?
As mentioned above ERD can be measured in different ways............

[Brucey]

worth clarifying if your wheels were 32h or 36h; it makes a fair difference.

cheers

[UpWrong]

For single wall rims i've had more success measuring the internal diameter of the rim and adding on twice the rim thickness eg 2 x 3mm. I've not had much success with the 2 installed shortened spoke method. I think you need a third hand to get that to work

[cromo]

I've built wheels using 26" sputnik rims with shimano small flange hubs. I used 262mm for the front and 262 / 260 for the rear, the lengths were fine. Using the DT Swiss spoke calculator or the EDD spoke calculator these lengths would correspond to an ERD of 541mm. An ERD of 537 would have you use spokes that are possibly too short?
As mentioned above ERD can be measured in different ways............

Sorry, should have mentioned wheels were 36 spokes!

[Brucey]

For single wall rims i've had more success measuring the internal diameter of the rim and adding on twice the rim thickness eg 2 x 3mm. I've not had much success with the 2 installed shortened spoke method. I think you need a third hand to get that to work

if you use short spokes and keep the elbows/heads, you can use an elastic band (made from an old inner tube, of course) to hold them in place. Then measure the distance between the elbows using vernier calipers; simples...

cheers

[Dave855]

Thanks all for your thoughts. The method I used for measuring ERD was as described by Mick F and using elastic between the spokes to keep them taught. The nipples were screwed down until the end of the spoke was flush with the exit hole in the nipple. The spoke length calculator I'm using is the one on Roger Musson's website and the hub I intend to build into the rear wheel is Shimano Deore XT FH-T780. I understand that hub dimensions are less open to interpretation than ERD so I am intending to use the measurements I found online as follows;

Left flange diameter 44.0 mm
Right flange diameter 45.0mm
Centre to left flange 33.5mm
Centre to right flange 20.0 mm

The front wheel will have a Son 28 hub built in to it. It looks like this has the following dimensions;

Flange diameter 54.0
Centre to flange 50mm

I'm heartened by the comments made by cromo and am hopeful that my ERD measurement of 541mm is safe to use. Anyone have any further thoughts?

Thanks,

Dave

[tatanab]

The front wheel will have a Son 28 hub built in to it. It looks like this has the following dimensions;

Flange diameter 54.0
Centre to flange 50mm

Not possible since the hub is 100mm OLN. Flanges are 62mm apart hence 31mm centre to flange http://www.nabendynamo.de/produkte/pdf/ ... e_2012.pdf I think you just misread the drawing.