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Straight spokes will have the same diameter through the whole length, double butted spoked are thinner in the middle than at the ends. Here's my current understanding of the benefits of each:

Double Butted Spokes

  1. Lighter than Straight Spokes
  2. More elasticity in the wheel (less rigid / will flex some before breaking)
  3. Spokes will bow out less at the crossings
  4. Easier to bend/lace
  5. When overtightened or stressed too hard, the spoke is more likely to snap than the rim eyelets or the hub's flange

Straight Spokes

  1. Stronger than butted spokes
  2. Less elasticity in the wheel (more rigid / may break before flexing)
  3. More available at bike shops
  4. Cheaper

What are other reasons to use one over the other, or more pros/cons that may influence one's decision to use one over the other when hand building wheels? ('Straight spokes are easier to machine lace' may be true, for example, but doesn't help me decide which to order)

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You pretty much said everything. I don't think elasticity would be significantly different on butted spokes: I think the main reason for butting is 90% weight saving and 10% aerodynamics, more or less. Also, the very neck on butted spokes might be itself a stress-raiser and become a weak point in the spoke. –  heltonbiker Apr 3 '12 at 17:04
    
Sort of covered by "more available/cheaper", but: butted spokes are available in a fixed range of sizes. If you have an obscure wheel size or lacing pattern, you[1] can always cut down a longer plain spoke and rethread it, but there are limits to doing that with a butted spoke. [1] Or at least a local bike shop with a spoke threading machine: billys.co.uk/english/group.php?prod=2CY07836 –  armb Mar 22 '13 at 10:25
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2 Answers 2

up vote 6 down vote accepted

In his book 'The bicycle wheel' Jobst Brandt, says that double butted spokes will be more resistant to fatigue failure when built into a wheel.

This is because spokes break because of the cyclic stress they suffer as the wheel rotates.

As the spoke rotates thru the bottom of the wheel it experiences a reduction in tension.

Butted spokes are more elastic so this reduction in tension is spread over more spokes, each experiencing a smaller reduction in tension.

Wheels are an order of magnitude stiffer than the tyres they roll on, so any stiffness differences will not be noticeable. A wheel built with butted spokes will be able to carry a heavier load before any of the spokes become slack. At this point the rim is no longer restrained and the wheel will be more likely to collapse.

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Yes, I've read several claims that double-butted spokes are superior because they concentrate the strain (vs stess) in the straight part of the spoke rather then the ends. –  Daniel R Hicks Apr 5 '12 at 15:44
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You have to be a lot more careful about windup in butted spokes. You have to detension each spoke more carefully when building by turning a bit extra and turning back. Otherwise they might re adjust themselves. I think 14g can carry a heavier load than DB.

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