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Z  scale wheels, trucks & couplers

OK, so Z is going through the evolution from plastic wheels to metal wheels, and people are also paying attention to standards.

Wheelset Standards, an opinionated view:

So, I decided to look up what the NMRA has to say.

Contrary to what the NMRA preaches, the most important dimension on wheelsets is the back to back spacing. This is because that is what controls the wheels going through a switch, where 95% of all derailments occur.

Sure, the argument is that the most important dimension is the check gauge, which is the back to back plus one flange thickness. So, the popular (NMRA) thinking is that this measurement encompasses both the back to back, and the check gauge including the flange thickness.

What the NMRA seems to have trouble understanding or perceiving is that this dimension changes in relation to the thickness of the flanges and that is not consistent enough yet in this scale. (likewise in my other modeled scale, G).

This is further exacerbated by the fact that model flangeway tolerances are so large/sloppy, the flangeway widths are so gross, that any flange will fit, and thus manufacturers don't care that much to control or standardize flange thickness.

This combined with the reality that it's the back of the wheel running against the guard rail that controls the wheelset through the frog, makes the NMRA's recommendation not the best right now. Fine for HO, probably fine for N, not for Z (or G) yet.

When Z gets to the level of close tolerances and close to scale dimensions, this will work.

Reality and practicality:

So, I use the back to back spacing of 0.214 inches (NMRA spec), and I set to this as close to possible, even though the tolerances are plus 0.002 and minus 0.007 (notice that this is asymmetrical, it accommodates grossly thick flanges)

The plastic wheels on my Micro-Trains cars are 0.219, over spec by 0.001 and over target by 0.005.... interesting. The tread diameter is about 0.154" (about 33.88").

The Fox Valley metal wheels were almost all exactly "on" at 0.214" with a tread diameter of about 0.150" ( again spot on at 33.00")

The metal wheels on my Marklin caboose had a back to back of 0.204 to 0.207", under spec by 0.007 to 0.010". The tread diameter was 0.174, scaling out to 38.28", but these are on a caboose, I need to see what prototype is. I carefully re-gauged them to 0.214"

Anyway, I switched out my 24 car train from plastic to metal wheels. I have not done any scientific measurements, but the same loco seemed to struggle less with the cars switched out to metal wheels.

Be sure you get the axle tips all the way into the journals. Test each axle by spinning the axle and seeing that it spins for a while. If it does not, try checking/re-seating or taking it out, inspecting the journals and trying again.

The Kadee gauge: