Denny Anspach <danspach@...>
Like many other aspects of our hobby, this is just another instance where we are again faced with givens and druthers.
Measuring the inside diameter between the points of the conical bearing holes is an effort that is better in the thinking than in the doing. The thinking presumes that the bearing holes actually have conical points, and that such an accurate measurement between them would then accurately predict what the ideal axle length would likely be. It doesn't quite work out this way inasmuch as the truck bearing "cones" are never perfect, and in fact are commonly either actually flat, of flattened curvature, quite assymetric, or any combination of the three. So quite (most!) often, an axle whose length theoretically seems to be ideally fitted in any given instance, while in real time it does not roll worth a hoot.
The most objective means of selecting ideal axle length is with some type of measurable actual roll testing against a standard; which in turn has to be balanced against the thoroughly subjective judgement of axle end play, i.e. "enough" but " not too much".
As a personal note, I have found over the six or more years that I have been testing and installing axle/wheel sets routinely "to order", this has been one of my most rewarding efforts, resulting in my current fair sized fleet of very free rolling cars that, in addition, do not sway or swagger their way down the track. The latter good behavior in turn also enabled me to make practical the use of semi scale couplers as a standard.