Hi Garth,
Part of the art of building wheels is preserving the shape of the rim as it sits with no spokes connected to it. You can describe that shape in terms of flatness and concentricity. In my experience most rims come from the manufacturer in a very flat condition but they have slight irregularities with regards to concentricity. Rims are typically fabricated from straight lengths of extruded aluminum that are rolled into hoops and then the two ends are connected. The irregularities in concentricity are the result of that joining process... So what ever shape your rim has when it is sitting on your bench is the shape that the wheel should have when you have applied full spoke tension.
It is true that the relative spoke tensions have a dramatic effect on the trueness of a wheel and don't seem to effect the concentricity much. I'm not sure that I am capable of articulating the reason for this. I guess the best thing I can suggest is that you think about the direction that the spokes are pulling on the rim. Mostly they are pulling towards the hub and a very small amount of their tension is being applied sideways. When you are trying to move the rims sideways there are very few spokes that have much effect of the rim's position. When you try to pull in a hop or let out a flat spot every single spoke in the wheel has a big effect on the shape of the wheel.
The big issue is: are those hops and/or flat spots the true shape of the rim or did you put them there as you applied tension to the spokes? If those minor eccemtricites are the actual shape of the rim then you shouldn't try to change them. You should just support them with uniform spoke tension.
Jamie Swan Centerport Cycles Inc. Northport, New York, USA http://www.centerportcycles.com (mapped) http://www.limws.org http://www.liatca.org
On Sep 27, 2008, at 6:24 AM, Bianca Pratorius wrote:
> I just finished the first wheel of my track bike and am starting
> the second. Here's a question would appreciate feedback on. Why is
> out of round harder to correct than side to side wobble? And is
> slight out of round even as important to correct as slight side to
> side movement?
>
> Garth Libre in Miami Florida USA