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January 30, 2014 12 Comments
[Note: this is #19 of a series of 20]
But how tight is right? In other words, what is optimal tension?
About Tension Levels
(1) There is no one correct tension for all wheels. Resist generalities, like "tighter is better." No more true of spoke tension than of tire pressure. Situations vary widely.
(2) Even with low spoke tension, wire wheels can efficiently support very large loads with minimal flex. Damon Rinard's famous stiffness test should have put misconceptions to rest. But they're out there like zombies (... ideas that should have died long ago in the face of evidence or logic, but just keep shambling forward, eating peoples’ brains).
Having tested spoke wire for years and made and sold millions of spokes, I will confirm the single greatest defensible argument for higher spoke tension is its contribution to spoke fatigue life by minimizing moments of zero tension during load cycles.
How Much is Too Much or Too Little?
(1) Don't build extra tight to keep a wheel from vibrating apart. Spoke tension is for supporting loads. Thread compounds (or nylock nuts) are for loosening prevention. The effects overlap but don't mix them up.
(2) Extra tension is not the best way to get components to settle in. Spokes that need to be bent straight, nipples that need to be forced down into a lasting position, these are best achieved with dedicated manipulations (like stress relieving).
(3) Rim deformation at the nipple can be a bad sign. In the old days (decades ago) designs existed that were not stable until deformed. Today, that is no longer the case. When a rim deforms from tension, that zone enters its plastic phase. A structure with a major component in, or close, to plastic deformation is vulnerable to fatigue and failure. A well-designed rim should accept spoke tension without deformation. There are exceptions. None of this is black and white.
(4) Excessive tension takes longer to achieve, like an acrobat balancing an extra heavy stack of props. The customer pays for this. Is it a waste of money? And if you the builder don't charge, then is it wasting yours?
(5) Inadequate tension is indicated by flex noticed and resented by the rider, noise as the wheel rotates (and spokes rub), and short spoke fatigue life (assuming quality spokes to begin with).
(6) An overly tight wire wheel is prone to sudden deformation with little warning. The well known result is often called a "taco." It's up there with crank or handlebar breakage for danger. Creeping up on this possibility is not worth it especially. Like recklessly unrolling your sleeping bag at night, on the Grand Canyon's rim. If you make a mistake, it may be too late! I am more curious about acceptable low tension points than where is the high tension flash point.
Today a huge number of rims, spokes, and hubs are designed to support 100kgf of tension in each spoke with very high reliability. It would be a generalization to say 100kgf is "right" but if you go higher, know why and how much. Using half, on the other hand, is rarely ugly.
How to Manage Tension
(1) Get a tensiometer. Riders depend on builders knowing tension. They deserve it. Check these cool Mavic tension tools, built for internal use.
(2) Use a good fitting spoke wrench. Some of the options are discussed here.
(3) Use an effective lubricant. Pedro's, Park, Phil, Chevron, all have good options. Lube the nipple-to-rim seat and, unless you employ a dedicated spoke thread compound (like Spoke Prep™), lube the threads as well. Spoke windup, even when resisted by a holder or plier, slows you down and weakens the spoke at the first thread.
(4) Keep records and observations. Data is the holy grail for engineering. Why? Because a scarcity leads to bad decisions. Listen to people with lots of data. It sometimes takes a thousand examples before a phenomenon is well understood. In cycling, we draw conclusions earlier, but more data = better solutions, especially when talking tension.
Please share comments about your experiences and opinions with various tensions, especially if you've done testing (like Damon). But all thoughts are welcome. We're in this together!
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