Questions for wheelbuilders

[ultraman6970]

Ok guys i have a super good question...

Any of you knows of a spoke calculator for straight pull spoke hubs like the mavic hubs for example?

Any ways to do it by hand or something?

I want to change the rims in one set I have and the hubs have like a design similar to mavic, besides that to make the situation worse the rear wheel is a triplet 8/16 wheel, so the driver side is longer than the non driver side (for the ones asking why is because the non driver side uses no cross).

What I could do is estimate the new spoke sizes using a ratio based in the drive or non drive side spokes and the erd of the old versus the new rim erd but no idea if it will work or how far will be from the real numbers.

Any info or ideas?

Thanks.

Thanks.

[Brian Smith]

Universally:
distance = [(change along 1st axis)^2+(change along 2nd axis)^2+(change along 3rd axis)^2]^0.5

Casually, mathematically sloppily, and the way a normal LBS builder would do it: the number of crosses plugged into your usual spoke calculator is likely to be something other than a whole number, such as 2.5 or 3.5, depending on the hub manufacturer's intended flange to spoke angle.

Defying the way things are normally done sometimes requires above average understanding of the problem. Your challenge, should you accept this mission, is to do the groundwork.

Good luck, it's only spoke money. Nothing ventured...

[Peter P.]

I can't accurately answer your question on calculating spoke length.

A question I have for you which may influence your work is; you're trying to mate a replacement 24H rim to a 24H hub that has an 8/16 spoke split- because of the spoke hole offsets I'm not too sure you can perform a direct swap. This may be another problem to consider before you go ahead with the work.

[Black Dog]

Do it by comparing the ERD of the old rim with the new rim. Measure the existing spokes and adjust the length of the new spokes to match the difference in the ERD difference between the rims.

Example: If the new rim has an ERD that is 5mm less than the old the new spokes will need to be 5mm shorter.

[ultraman6970]

Thanks for the replies...

Probably my best shot is just compare the erd and wait for the best hehehe :) The wheels right now have pillar bladed spokes that are just super nice and hard to get in the US, actually only one guy has them at ebay, next option is asia and the price goes way too high.

Thanks guys :)

[monkeybanana86]

ultraman is asking a question? well that's something you don't see everyday

[ultraman6970]

Love you too :D

[cachagua]

I'm with the Black Dog. The fact that the ERD is measured radially, and the spokes come towards the rim at some other angle(s), will make negligible difference.

I used to choose spoke lengths as short as I could get away with -- save weight, right? -- but I read on Hjertberg's site recently that the very shortest a spoke should ever be is the top of the nipple, because not engaging the threads in the nipple's head induces a stress riser and tends to break nipples. This made a lot of sense to me.

And anyway, weight schmeight. Worrying about that is for younger men than I!

[Mark McM]

Do it by comparing the ERD of the old rim with the new rim. Measure the existing spokes and adjust the length of the new spokes to match the difference in the ERD difference between the rims.

Example: If the new rim has an ERD that is 5mm less than the old the new spokes will need to be 5mm shorter.

This method should work fine to give the correct spoke length within a millimeter (which the is the smallest spoke length gradiation anyway), but with one caveat:

ERD is Effective Rim Diameter, but each spoke only spans the wheel radius (half the diameter), so the spoke length difference should be half the difference in ERD. In the example, an ERD 5mm smaller would require spokes 2.5mm shorter.

[Black Dog]

This method should work fine to give the correct spoke length within a millimeter (which the is the smallest spoke length gradiation anyway), but with one caveat:

ERD is Effective Rim Diameter, but each spoke only spans the wheel radius (half the diameter), so the spoke length difference should be half the difference in ERD. In the example, an ERD 5mm smaller would require spokes 2.5mm shorter.

Opps! Good point. Spoke length difference will equal 1/2 of ERD difference. My bad.

[cachagua]

Ugh, I overlooked that too. Radius... diameter... they're almost as hard for me as left and right. Good catch.