Spoke length question

[chuyeu123]

Hello everyone,

This will be my first post in the forum and my first time building my own wheel with an Internal Gear Hub so if I post it at the wrong place please help guide me in the right direction.

The Rim I just purchased is "RIM SUN 26X1.75 559X23.5 RHYNO LITE XL 36 ABT/MSW BLACK SIDE WALL WELD" (link is https://www.ebay.com/itm/RIM-SUN-26X1-7 ... 2749.l2648)
Specs I got from sheldonbrown.com
ERD 544mm and 0mmOBS

The IGH is "Sturmey Archer RXRK5 Hub 6-Bolt Disc 36H Silver w/ Ttrim Kit/Shifter/Cable" (Link is https://www.ebay.com/itm/Sturmey-Archer ... 2749.l2649)

The specs I got from the website are
Pitch circle diameter 95.5mm
Center to flange Left is 27mm
Right is 30.5mm

I used the spoke length calculator and it gave me 251mm for left and 252 mm for right spokes.

Would anyone please kind enough to help me if I did it correctly? I just want to double check before I purchase the pre-cut spokes.

Thank you for your time

[fausto99]

I recommend you measure the ERD for yourself. Last wheel I built the ERD in the spoke calculator spreadsheet was wrong.

[Brucey]

+1 on that.

http://www.starbike.com/manuals/sun-website-rim-chart-2012.pdf

and several other sources suggest ERD is 549mm. However that chart dates from 2012 and there is more than one (later) version of this rim so 544mm may be correct.

If you are building the wheel 36x3 then spoke lengths for 544mm ERD using hub dimensions as specified by SA are 251.8mm and 252.1mm respectively, i.e. 252mm all round. If the rim is the larger size you need to go up 2.5mm.

cheers

[LinusR]

I recommend you measure the ERD for yourself. Last wheel I built the ERD in the spoke calculator spreadsheet was wrong.

How would you measure the ERD? Piece of string with a nipple-sized knot on one end then mark the corresponding point (with some difficulty) as it emerges from the hole at the opposite edge of the rim?

[fausto99]

How would you measure the ERD? Piece of string with a nipple-sized knot on one end then mark the corresponding point (with some difficulty) as it emerges from the hole at the opposite edge of the rim?

As per the Musson book (a very good book - buy it!):
Glue nipples onto the threaded ends of two spare/old spokes. cut the J-bend off leaving 300mm length from spoke end to start of the nipple countersink. Put the spokes through the rim in a pair of opposite holes. Measure the spoke over- or under-lap and add or subtract it from 600mm.

[Brucey]

no need to cut the measuring spokes about; for the (insignificant, unless you are building wheels all day every day) length of time it takes to set the nipples on the ends of the spokes you may as well do that and leave the J bends in place. This also gives you the opportunity for varying the nipple setting if you are using a single wall rim, or you are using very thin. stretchy spokes, or you think that there may be more 'settling' in the hub than normal. Any of these things could cause you to set the nipples differently on the measuring spokes. As it if it isn't obvious, this always allows you to measure the ERD using the exact type of nipples that you will be building with, also.

Finally it is very easy to use Vernier calipers to measure between two J-bends, but rather fiddly to do the same thing once the J-bends are cut off.

cheers

[chuyeu123]

Hi everyone,

Thank you so much for helping me.

I did double check with the distributor and they said the EDR is 549mm

May I ask was I correct to assume the "Pitch circle diameter" is the distance between the hub's spoke holes?

I just want to purchase the spoke now so when it arrive I can build the wheel right away instead of measuring it and go buy it and wait for it to be deliver again.

Thank you again for helping me.

[alexnharvey]

Pitch circle diameter is the diameter of the circle the hole centres lie on. On a hub that is the same as the distance, centre to centre, of two holes on opposite sides, but if the there were an odd number of holes it would not be (eg 5 bolt chainrings).

It is common to want to get all the bits and get it built so I sympathise. You must balance haste against the risk that the erd is wrong and you are stuck with spokes that are no good to you. if you cannot wait, order a couple of extra spokes this time that you can keep them for measuring erd in future.

Order one more spoke (it can be a short plain gauge one) and an extra nipple to make yourself a nipple driver too.

[Brucey]

https://leonard.io/edd/howtomeasure

cheers