Index Home About Blog
From: (Norman Yarvin)
Subject: Re: Question for Jobst
Date: 30 Jul 2000 01:42:24 -0400 (Jobst Brandt) wrote:

>David Green writes:
>> Please can you confirm something which I am not 100% clear about. I
>> understand (from studying 'the Bicycle Wheel') that the 'danger
>> sign' to look for is whether the wheel deforms into a 'taco' shape
>> when over-tight spokes are stress-relieved.
>I wouldn't call it a "danger sign" but rather an indication that the
>wheel is too tight... and its not a big wobble but an out of trueness
>that is smooth and has that shape.  This is easily seen if the wheel
>was true in the first place.  If the wheel wasn't straight, the effect
>gets lost in what was already there and only becomes apparent when the
>wheel is greatly out of true after stress relieving.

Some numbers might also be helpful.  When I built a couple of wheels "by
the book" (that is, following Jobst's procedures) recently, I measured
the runout of the wheel with a dial indicator.  After each stage of
tightening each spoke 1/4 turn, I trued the wheel to within about 0.010
inches (probably overkill, but I like to do things carefully when doing
them for the first time); after stress relieving, this changed only
slightly, perhaps to about 0.015 inches.  Then, all of a sudden, after
one stage of stress relieving, it went to about 0.080 inches, with the
shape being the "taco" as in the book.

Another observation which might be of interest is what happened to the
radial runout.  In the book, Jobst recommends tightening the spokes until
the wheel deforms (as above) when stress relieving, then backing off 1/2
turn.  In both wheels that I built, there seemed to be a better
indication of the point of ideal tension: at each spoke, the rim deformed
inwards slightly (by about 0.002 inches), so that instead of the radial
runout being smooth, it had 36 tiny peaks, one for each spoke.  (The
radial runout was measured along the inside of the rim.)  I say this was
a better indication because in both wheels, it appeared 1/2 turn before
the wheel deformed into a "taco" shape, that is, it appeared exactly when
it was time to stop tightening the wheel.  However, this would have to be
observed by more people and in more cases before anyone could rely on it,
especially for different types of rims.  (These were MA-2s.)

Norman Yarvin

Index Home About Blog