BVC-CHAT Tire/spoke Advice
bk at jp.pair.com
Wed Sep 26 14:12:37 CDT 2007
At 12:14 PM 9/26/2007, you wrote:
>There's no such thing as a perfectly rigid body, Matt, as you well know.
>Every wheel has a certain amount of give in most any direction, and
>tensioning the spokes will certainly decrease the diameter *slightly*.
>It was mostly only this point, and the importance of proper spoke
>tension generally, that I was signalling agreement with in Jeff's post.
>As to whether the difference in wheel diameter between well- and
>poorly-tensioned spokes really makes more than a negligible difference
>to tire mounting, I have heard only anecdotal reports and have not seen
>any studies one way or the other.
Check the web site below. ...a standard spoke length calculator that
wheelbuilders use to determine proper spoke length prior to
building. That site states, "The calculator does not compensate for
rim shrinkage due to spoke tension. This can reduce the rim diameter
by as much as 2 mm, depending on the rim."
Chris and Jean can check my math here (what do us
pseudo-statisticians know about math anyway)...
Keep in mind that the circumference of a circle is pi*d or 3.1415
times the diameter. If you are able to decrease the diameter by 1
unit (the radius by .5 units) than you decrease the circumference by
3.14 units. Since you're pulling off the tire in only one place you
will be able to pull the rim up about 2/3.14 times higher for every
unit decrease in wheel diameter and not simply a single unit higher
(assuming that it comes straight up and then straight down - will be
a bit less given the angle of the tire.
As a methods instructor, I can relate to your skepticism, but I can
tell you that when I did this I was using the same tires before and
after on the same day with the same weather conditions, etc. and the
greater spoke tension did seem to have a legitimate effect.
...certainly possible that other things may have caused the decrease
in effort required, but I was pretty much convinced.
Although I have never built wheels professionally, I have made at
least 6 sets for myself and have trued a number for friends. I've
never worked on the expensive newer wheels that Chris mentioned and
would guess that they were made and tensioned better than most of the
factory stuff I've seen, but I can tell you that of the dozen or so
factory wheels that I've worked on for others and myself, the spokes
were terribly under-tensioned, some to the point of having almost no
tension at all - and yes, I do own a tool that measures spoke
tension. ...and most books and articles on the topic will tell you
the same about typical factory produced wheels. ...this is why it's a
big deal to get hand-built wheels if possible - at least for us
larger people who put a good bit of weight on wheels - for smaller
riders, it's probably not as big of a deal.
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