As you say, watch for breakage - spokes pulling out of the rim or hub, or spokes snapping at the elbow or threads. Breakage is extremely rare, though. Unless the parts are radically mismatched, corroded or badly fatigued they won’t just snap.
The failure mode to watch for is spontaneous tacoing.
When the spokes are slack there is no compression in the rim and it is as strong and stable as it was before it was laced. In other words, easy to bend and floppy.
As the spoke tension is increased it becomes harder to radially or laterally displace the rim. (The wider the hub with respect to the radius of the wheel the more resistant the rim is to lateral displacement. This is the “secret” to U-Turn’s awesome wheels.)
When the tension is too high the rim is compressed too much. If you take a slender rod and push in from both ends it will resist up to a point and then fail by bending to “escape” the pressure. The rim is doing something quite similar when it deforms into the familiar four-bend potato-chip shape. By bending that way it gets closer to the hub and “escapes” the spoke tension.
So why doesn’t a rim automatically taco at any spoke tension? Because the initial move off-center increases the spoke tension. That, combined with the inherant bending resistance of the rim, is why wheels are strong. As the spoke tension increases and the stresses go up the bending resistance of the rim becomes less effective. At some point increasing the tension weakens the wheel. This is the point to back off from.
So when do you know when this tension is too high? When the rim starts to do squirly things. When you tighten a spoke and the wheel does something unusual, like going symmetrically out of true with a slight increase in tension on one spoke, then you are at the point of impending failure and need to back off a quarter or half turn on every spoke.
Sorry about the lack of clarity in explaining this effect. I learned it by watching wheels implode when I was a novice mechanic in a bike shop. Different wheel styles go “squirrly” in different ways and I don’t have enough experience to be clear in explaining how. I’d know one that was too tight if I saw it, or felt it, but I can’t really describe why.
> Sofa wrote:
> > *OK, so I’m getting Cokey all prepped up for the season, and have the
> > wheel tensioned quite nicely. (or so it would seem)
> > How do you know when the spokes are reaching their limits? Is the
> > first sign that something is wrong is a spoke starting to pull
> > through…and then it’s too late? *
> As you say, watch for breakage - spokes pulling out of the rim or hub,
> or spokes snapping at the elbow or threads. Breakage is extremely rare,
> though. Unless the parts are radically mismatched, corroded or badly
> fatigued they won’t just snap.
> The failure mode to watch for is spontaneous tacoing.
There is another failure that can occur when spokes are too tight -
the rim can deform or crack at the the nipple holes. Still, I like my
spokes pretty tight.
Anyway, in answer to Sofa’s question, you can either go by feel or use
a spoke tensiometer. My friendly LBS has a tensiometer which they
measured my last wheel with.
When I was building my wheel, I took it to the LBS and asked them if the tension was correct. They guy just felt the spokes and said, “yep, just about.” Maybe not the best advice, but it didn’t cost anything either. Generally the guys there are pretty nice.
Oops! I was trying to make a lame joke. I didn’t know wheels really imploded (or that they call this potato chip effect imploding)! Do people get hurt when this happens? I would imagine the tech’s hands are right there turning the spoke when the thing pops out of shape. Scary! (but not as scary as a wheel actually imploding )
Naw, it’s pretty benign. I’ve seen two wheels implode. One was an old wheel that I was working on, and the other was an ultralight wheel being built by another mechanic.
The wheel I was working on got a little squirrly (sudden 1 cm latteral hop) so I started backing off the spokes. Apparently I did it in the wrong order because the third one I relaxed allowed the wheel to buckle sideways at a point where there must have been a hidden crack. Anyhow, it happened in a fraction of a second but nothing moved very far so it just bumped my hand. It was quite a conversation piece for a few hours!
The other one was a very lightweight rim with very few spokes that was tensioned too highly. That one looked fine on the truing stand but potato-chipped while being stress relieved. I learned that symmetry in wracking the spokes is important. The mechanic was using both hands to squeeze the spokes together and must have been putting a lateral load on the rim at the same time. Again, no one hurt and a nice conversation piece. It was a pretty sad momement, though. He was building the set for an advantage in a local hill climb race. This guy was 130 lbs, grew up in a very mountainous third-world country, had a nice wife and kid and virtually no money. This hill climb WAS his dream. We all pressured the shop owner to give him another rim, but it wasn’t neccessary. He was already planning on wearing the shop colors on the climb so he got his rim.
Now, I have heard of a special unicycle wheel that imploded somewhere in Washington state. It was apparently being impulsively tensioned by an excentric physicist with access to large electro-magnets. That one crumpled into a golf-ball sized sphere when it imploded, which was a shame because it destroyed a novel geared hub. No one was hurt, but the entire field of unicycling was set back 183 milliseconds by the event.