Working out ERD by spoke length and hub flang diameter?

Hi is the above possible? (ehren im looking at you ;) )

Of course it is. You've got two options, at least.

1. Ask a good wheel builder to have a look at your hub and tell them the spoke length. They'll have a hunch of which rim will fit - then google/sheldon brown/damon rinard the erd. Failing that,

2. Find a mathematician. It's really quite simple ;) well: Spoke length L is

L = √A2+B2+C2 –s

i.e. first take square root of A2+B2+C2

    then minus s

(After previewing: the exponents don't come up properly, so take it that A2 means AxA etc.)

A= flange diameter/2 x sin(720 x 3/32)
B= ERD/2 – flange diameter/2 x cos(720 x 3/32)
C= flance spacing/2
s= hole diameter/2

All you need to do is replace ERD by Y, insert values for all the other variables and work out Y.

s hole diameter is usually 2.4.
3/32 is cross pattern 3, spoke number 32 which is what I assume you are doing.

So,

Spoke length = √[Flange diameter/2 x sin(720x3/32)] 2 + [Y/2 – Flange diameter/2 x cos(720x3/32)] 2+(Flange spacing/2)2 [END OF SQUARE ROOT] -2.4/2.

E.g. if you were building Phil Woods onto Mavic Open Pros pretending you know the spoke length but not the ERD:

294.4 = √[48.2/2 x sin(720x3/32)] 2 + [Y/2 – 48.2/2 x cos(720x3/32)] 2+(60/2)2 [END OF SQUARE ROOT] -2.4/2.

294.4 = √[24.1 x 0.923879532511] 2 + [Y/2 – 24.1 x 0.382683432365] 2+(30)2 [END OF SQUARE ROOT] -1.2.

and so forth. Adapted from Jobst Brandt: The Bicycle Wheel.

Needless to say, you have my head wrecked. The problem with high school mathematics is that one tends to leave it behind oneself asap. I ought to thank you though for the chance to prove my ignorance to myself and to show it to everyone else. Nice one.

Good luck.

Hmmmph, which rims where they anyway – I’m sure you’ve googled and checked all rim databases?

It's just like a triangle, innit.

Of course it is. You've got two options, at least.

1. Ask a good wheel builder to have a look at your hub and tell them the spoke length. They'll have a hunch of which rim will fit - then google/sheldon brown/damon rinard the erd. Failing that,

2. Find a mathematician. It's really quite simple ;) well: Spoke length L is

L = √A2+B2+C2 –s

i.e. first take square root of A2+B2+C2

    then minus s

(After previewing: the exponents don't come up properly, so take it that A2 means AxA etc.)

A= flange diameter/2 x sin(720 x 3/32)
B= ERD/2 – flange diameter/2 x cos(720 x 3/32)
C= flance spacing/2
s= hole diameter/2

All you need to do is replace ERD by Y, insert values for all the other variables and work out Y.

s hole diameter is usually 2.4.
3/32 is cross pattern 3, spoke number 32 which is what I assume you are doing.

So,

Spoke length = √[Flange diameter/2 x sin(720x3/32)] 2 + [Y/2 – Flange diameter/2 x cos(720x3/32)] 2+(Flange spacing/2)2 [END OF SQUARE ROOT] -2.4/2.

E.g. if you were building Phil Woods onto Mavic Open Pros pretending you know the spoke length but not the ERD:

294.4 = √[48.2/2 x sin(720x3/32)] 2 + [Y/2 – 48.2/2 x cos(720x3/32)] 2+(60/2)2 [END OF SQUARE ROOT] -2.4/2.

294.4 = √[24.1 x 0.923879532511] 2 + [Y/2 – 24.1 x 0.382683432365] 2+(30)2 [END OF SQUARE ROOT] -1.2.

and so forth.

Needless to say, you have my head wrecked. The problem with high school mathematics is that one tends to leave it behind oneself asap. I ought to thank you though for the chance to prove my ignorance to myself and to show it to everyone else. Nice one.

Good luck.

Hmmmph, which rims where they anyway – I’m sure you’ve googled and checked all rim databases?
WOW! thanks, ok that getting printed off and taken to my maths teacher :) they were some ambrosio rims, i dont know which, i bought the hub of ebay and he said he will include bladed spokes for no extra cost so i thought why not! but he didnt know which rim it was :/ oh well it was only £4