Re: when are Coker spokes too tight?



S

Sofa

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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?


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has an atomic mass of 16. The total atomic mass within a D2O molecule is
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times as heavy as water." - Harper's response to 'Hi Greg, how are
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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.

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.


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cyberbellum - Level 1.0 rider!

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"cyberbellum" <[email protected]> writes:

> 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.

Ken
 
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.


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cyberbellum wrote:
> *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.*


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 :D )

Andy


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quark soup

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quark soup wrote:
> *
> Do people get hurt when this happens?
> *

Naw, it's pretty benign. I've seen two wheels implode while on the
truing stand. 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.


--
cyberbellum - Level 1.0 rider!

Optimists think the glass is half full. Pesimists think the glass is
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