Spoke length calculation??

[RecordAceFromNew]

You had me at a disadvantage with that last post. I knew that the stainless steel rims I was using, although marked 26", were a little bit smaller, but was unsure how much so I just checked the ERD on an original 26" x
1 3/8 Raleigh rim and it came out at 478 which ( I think) means that the spokes would be 224.9 rather than 216.6 ie 8.3mm shorter.

I am sure it is my ignorance but I still don't get it. According to Sheldon e.g. a 26 1-3/8 rim should have a BSD of 590mm or thereabout. I appreciate BSD is not ERD, but I am unsure how they could be different by over 100mm unless the rims have a 2+ inch profile?

 

[PpPete]

..... I just checked the ERD on an original 26" x 1 3/8 Raleigh rim and it came out at 478

When you say you checked the ERD - how did you measure it ? None of the six different diameters of wheel that are described as 26" (per Sheldon's tables linked to above) can have an ERD as small as 478mm.

The correct method of measuring ERD is given on page 5 of this document.

 

[bobg]

I'm probably dng something really stupid but I think I'm following exactly the guidance in the DT wheel building course notes my LBS mechanic lent me, which is pretty much the same as Chapter 8 of the PDF file and pretty much every other guidance I can find on the internet. I'll describe it in steps in case I'm misunderstanding something
1. Insert a couple of long spokes in the rim, one a 3 o clock and one at 9 o clock - I join then by the hook end, tighten the nipples till its taut but not distorting the rim then measure across from where the spoke goes into the nipple = 453mm. Do this 3 times in differnt places across the rim and take an average
Add twice the length of the nipple 2x12 .5 = 25
Add the two together ( 478 ) and add this into the spoke calculator along with the flange hole diameters etc as per my first post , the result = 224.9mm spoke length

The spoke calculator finds the SA 3 speed hub measurements automatically but I've even cheched them manually

Please dont tell me I've made an arse of what is on the face of it a really simple process!!!

 

[RecordAceFromNew]

Bob it clearly is a mystery at the moment, since we are not talking about difference of a couple of mm, but 100mm.

As you can see from the ERD given for some 26 1-3/8 rims sold by Harris here e.g. (see just under the photo of the rim cross section), they are exactly 100mm larger than yours, and they are consistent with such rims' BSD (which is 590mm).

I can see a couple of possible reasons for the difference - one is if your rim is very deep (like over 50mm or 2 inches), the alternative is if they are not 26 1-3/8 - but the funny thing is that your ERD is not even close to any BSD in Sheldon's extensive database.

2" deep rims are common in relatively modern aero wheels, am just not sure about 1948 vintage steel rims.

 

[bobg]

Bob it clearly is a mystery at the moment, since we are not talking about difference of a couple of mm, but 100mm.

As you can see from the ERD given for some 26 1-3/8 rims sold by Harris here e.g. (see just under the photo of the rim cross section), they are exactly 100mm larger than yours, and they are consistent with such rims' BSD (which is 590mm).

I can see a couple of possible reasons for the difference - one is if your rim is very deep (like over 50mm or 2 inches), the alternative is if they are not 26 1-3/8 - but the funny thing is that your ERD is not even close to any BSD in Sheldon's extensive database.

2" deep rims are common in relatively modern aero wheels, am just not sure about 1948 vintage steel rims.

Indeed, total mystery. I'll pop round to the LBS with the original Raleigh steel rim on Monday when he's quiet and get him to calculate it If he has a rim with an ERD stamped on it I'll borrow it and see if I can arrive at the same figure. I did mention in the op that these were Westwood rims to take roller lever brakes and as such had a deepish profile to allow the brake blocks to bear on the rime either side of the spokes rather than the rim sides.
I WILL get to the bottom of this and you'll be the first to know. If its my silly fault I'll pm you
Thanks for all the responses to date
Bob

 

[bobg]

Bob it clearly is a mystery at the moment, since we are not talking about difference of a couple of mm, but 100mm.

As you can see from the ERD given for some 26 1-3/8 rims sold by Harris here e.g. (see just under the photo of the rim cross section), they are exactly 100mm larger than yours, and they are consistent with such rims' BSD (which is 590mm).

I can see a couple of possible reasons for the difference - one is if your rim is very deep (like over 50mm or 2 inches), the alternative is if they are not 26 1-3/8 - but the funny thing is that your ERD is not even close to any BSD in Sheldon's extensive database.

2" deep rims are common in relatively modern aero wheels, am just not sure about 1948 vintage steel rims.

Indeed, total mystery. I'll pop round to the LBS with the original Raleigh steel rim on Monday when he's quiet and get him to calculate it If he has a rim with an ERD stamped on it I'll borrow it and see if I can arrive at the same figure. I did mention in the op that these were Westwood rims to take roller lever brakes and as such had a deepish profile (but not 2"! )to allow the brake blocks to bear on the rims either side of the spokes rather than the rim sides.
I WILL get to the bottom of this and you'll be the first to know. If its my silly fault I'll pm you
Thanks for all the responses to date
Bob

 

[bobg]

Indeed, total mystery. I'll pop round to the LBS with the original Raleigh steel rim on Monday when he's quiet and get him to calculate it If he has a rim with an ERD stamped on it I'll borrow it and see if I can arrive at the same figure. I did mention in the op that these were Westwood rims to take roller lever brakes and as such had a deepish profile (but not 2"! )to allow the brake blocks to bear on the rims either side of the spokes rather than the rim sides.
I WILL get to the bottom of this and you'll be the first to know. If its my silly fault I'll pm you
Thanks for all the responses to date
Bob

It is my xxx,,,xxx fault, When you said exactly 100mm difference I twigged. I'm tempted not to divulge my stupidity and pm you but what the hell, I can always blame it on my myalgic encephalopathy...
I've measured with a fat boy tape measure, reading the mm alongside the pointer and failing to add on the 100mm to take account of the tape measure body.. I think I'll submit this to the "which idiot " thread in the cafe on the 3rd Sept
SOO SORRRYY!!

 

[RecordAceFromNew]

Nothing to be sorry about! If I were to list my cock-ups it will be a looooooooooooong list indeed.

Hopefully the required lengths can be recomputed, and cut just right!