BVC-CHAT Tire/spoke Advice

[Jeff Ackerman]

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.
>
>-chris


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."

http://www.appliedthought.com/danny/Spoke/SpokeCalculator.html

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.




Jeff