On Mon, 01 Oct 2007 17:31:54 -0700, [email hidden] wrote:
[snip]
Quoted message said:Within the context of the whole book, put in the simplest terms, the
author recommends that a tensiometer be used to measure proper spoke
tension.
[snip]
Dear Spike,
Jobst does indeed suggest _measuring_ proper spoke tension with the
tensiometer.
But that's not the same as _finding_ the proper tension.
(Unfortunately, Jobst wrote "establishing proper tension" when it
turns out that he means "recording proper tension," which can give the
wrong impression, but he goes on to explain what he means so clearly
that there's no question how he thinks proper tension should be
determined.)
Jobst explicitly states that proper tension must _first_ be determined
by either raising tension until the wheel goes out of true when spoke
pairs are squeezed, or else until spoke windup becomes excessive.
That is closer to Jim Beam's description of raising tension as high as
the wheel can stand than anything else is ever going to be.
There is no practical way to raise a wheel's tension past where it
will go out of true if you squeeze spoke pairs or past where the spoke
windup becomes excessive.
Here's the passage again, with the clarifying sentence emphasized:
"The final tension of a wheel built by feel is affected by the mood of
its builder. By using a tension measuring tool a builder can establish
proper tension, then repeat it. Such a tool, a tensiometer, measures
the deflection of a spoke over a given span in response to a standard
load. The dial is calibrated in tension or displacement. Either
measure is equally useful for wheel building. It is not necessary to
know the actual tension but only that the tension has reached the
desired mark. The correct maximum tension for a specific wheel design
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
must still be determined first by stress relieving. For some deep
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
section rims or wheels with few spokes, the limit of tension is
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
reached when spokes can no longer be tightened further without
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
excessive windup. For subsequent wheels of the same type the
^^^^^^^^^^^^^^^^^
tensiometer can accurately and quickly determine whether spokes have
reached the predetermined tension."
--"The Bicycle Wheel," 3rd edition
It is a modern mistake, an anachronism, for us to think that "The
Bicycle Wheel" says to use a tensiometer to set the spoke tension to
some manufacturer's recommended value.
Many RBT posters do indeed use tensiometers to set tension to
recommended values (no explanation of how those values were derived
has appeared on RBT).
But Jobst did _not_ say anything at all about manufacturer's
recommended tensions. In fact, it's unlikely that there were any such
recommendations back then.
He wrote a book in 1981 that plainly tells how to find proper tension
by setting it as high as possible. He updated it in 1993 to include
deep rims and low-spoke-count rims.
He can't say it any plainer--raise the tension until a fairly hefty
36-spoke rim goes out of true when spoke pairs are squeezed, or else
until a deep or low-spoke-count rim's spokes show excessive windup.
No tensiometer is involved in finding the proper tension, which Jobst
wrote is as high as the wheel can bear without deforming either the
rim or the spokes.
Much of the debate about this is merely the conflict between the old
and the new.
In 1981, Jobst explained how we could set high tension on 36-spoke MA2
rims, which could generally stand such tension, despite a few photos
of MA2's cracking at the spoke holes.
It seems like a perfectly sound approach for sturdy rims in an era
where tensiometers were scarce and manufacturers' recommendations
probably even scarcer. (I'm curious to see what the earliest published
tension recommendation was from any major manufacturer.)
But nowadays rims are often lighter, have deep sections, and have low
spoke counts. We often hear of rim cracking. Jim Beam's point is
mostly that manufacturers recommend lower tensions for many rims than
what the sensible 1981 method provides.
In any case, we do Jobst no service by misunderstanding what he
plainly wrote about how to build a wheel to what was a proper tension
a quarter of a century ago.
And we do ourselves no service by insisting that Jim Beam is wrong
when he points out that Jobst's method involves the highest tension
that the wheel can stand.
Cheers,
Carl Fogel