martin goodall wrote:All we do is to run EM wheels by manufacturers such as Ultrascale, KM and Alan Gibson set to a back-to-back of about 17.7mm (give or take a gnat's whisker) (i.e the back-to-back setting you get if you use one of the original Studiolith P4 BB gauges) on P4 track. It definitely does work.
The P4 check gauge is 18.15mm min. If you set wheels to 17.7mm back-to-back the maximum flange thickness which can be relied on to run reliably in all possible track formations is 0.45mm (i.e. 18.15 - 17.7 = 0.45). Flanges thicker than this risk hitting the nose of the vee when crossing the flangeway gap.
It is next to impossible to get definitive information about the flange thickness of manufacturers wheels, or agreement about how it should be measured, but none of them have ever published a flange thickness as thin as 0.45mm other than for P4 wheels. The Ultrascale web site currently says "What tyre profile comes closest to the RP25 standard? The nearest is our 00/EM profile, which is close to the RP25/88 specification."
For RP25/88 wheels the published flange thickness is 0.025" = 0.63mm, which is a lot more than 0.45mm. See: http://www.nmra.org/sites/default/files ... 009.07.pdf
Now I know you are going to say that it all works fine, and I believe you. But if so it is doing so by the skin of its teeth, and in my view can't be relied on always to work with future wheel production, or in all possible track formations. I wouldn't like to be the one who persuaded someone to build P4 track with the promise that they will be able to run EM wheels on it when it is finished.
Against that I have suggested some dimensions properly matched to EM wheels (and RP25/88 wheels) which I am confident will always give good results for those who want to use EM wheels on 18.83mm gauge track. I see no reason to apologize for doing so -- no-one is under the slightest obligation to use or take any notice of them. I have given them a name (EM4) which clearly isn't P4, to underline the difference and avoid confusion.