About spoke lengths and rims

[zoslen]

Hi

Until now I haven't used any serious wheelset. Once my rims wear off, I just buy a new cheap wheel.

Now I am planing using a serious wheelset on my tourer like this
http://www.sjscycles.co.uk/26-(559)-wheelset-36h-rigida-andra-30-rims-with-shimano-deore-m590-hubs-in-black-prod24922/

As I like having with me a couple of extra spokes just in case, I contacted SJS Cycles to ask them about spoke lengths in this wheelset.

Their answer was  : "For the Deore/Andra wheels, you’ll need 260mm, 261mm and 262mm DT Champion spokes"

Here is my question: Why do I need 3 different spokes?
I know that normally wheelbuilders use shorter spokes on the rear drive-side wheel, but apparently I miss something.

So, where each of the 3 spokes is used?

And a second question, if in the future I wish to change just the rims, how am I going to be sure that the new rims will fit perfectly with my spokes? (I suppose it will be hard to find exactly the same rims.)
Sometimes ERD announced by companies isn't accurate.

Is it so important to be accurate to the last milimeter or can I diverge a little (and how)?

[Danneaux]

Hi Zoslen!

The reason for the three sizes of spokes is simple: The flange diameter differs between the front and rear hubs *and* the rear wheel uses two different-sized spokes -- shorter on the freehub side and longer on the other to facilitate the dished rear wheel necessary to allow room for the cassette.

The sizes sound about right, though my notes show the spare spokes included with my Sherpa (same wheel specs) actually measured:
255mm (front both sides)
259mm (right-rear)
260mm (left-rear)

You're absolutely correct -- published ERDs can vary from reality, and most rims vary from one another in this dimension.

When sizing my spokes for a wheel build, I like to get them so the spokes end no shorter than the bottom of the screwdriver slot atop the nipple and no more than even with the top -- when the wheels are fully tensioned. You can usually accommodate a 1-2mm difference in ERD if necessary and not compromise wheel strength, at least in my experience. Most threads are engineered for maximum strength when five threads are engaged...something to keep in mind. You can also purchase some nipples (DT for example) in long and short versions. This can allow a bit of leeway when changing rims as well.

One caution: If you end up doing a rim swap, be sure to check the spokes first to make sure you're not putting a lot of labor and effort into rebuilding a wheel that would fail relatively soon if the bends already show excessive wear; that would be false economy.

Hope this helps.

Best,

Dan.

[zoslen]

Thanks a lot Dan!

You are an infinite source of information and very helpfull!

Zoslen

[sdg_77]

Zoslen - my 10p worth having just completed my first set of self build wheels ... with excellent advice from Dan and others on this forum ....

Have a practice spoke replacing session at home in daylight - a cheap or salvaged wheel is ideal for this - it's not difficult but as with many things knowing what to expect is invaluable if you need to make repairs out in the sticks.

Possibly stating the obvious - I coloured our spare spokes with 'Sharpie' pens to make it easier to identify which length is which if/when we ever need to use them.

sdg.

[mickeg]

If replacing a rim and using the same brand and model rim but reusing the same hub and spokes, use tape to temporarily attach new rim to the old rim, taking care to put the valve holes next to each other.  Loosen all spoke nipples a few turns.  Then move one spoke at a time from the old rim to the new.  You will figure out immediately which spokes transfer from rim to rim easiest to start with, work with the easiest ones first.

A spoke that is too long can either run out of thread on the nipple, preventing adequate tension and/or creating an opportunity for puncture if the spoke protudes through the rim tape.  But, if your spokes are a few mm too short you probably still have enough thread within the nipple to provide adequate tension.  Thus, if your spokes are out of specified length by a mm or two, shorter may be better than longer.

I personally do not own the tools to measure ERD, I go to a local bike shop to buy my spokes.  They are kind enough to measure my rims and flanges for me.  Not all bike shops will take the measurements and perform the calculations, perhaps I am lucky that a local shop will do that for me when I buy spokes from them.