Model Forum / General / Railroads / September 2007

According to NMRA S-4.2: "Wheels, Standard Scale":

http://www.nmra.org/standards/S-4_2ScaleWheels.html

note 3: "HO fine wheels will work on HO standard track but HO standard
wheels will not work on HO fine track."

To my understanding this should mean that the H0 fine wheel is supported by
wing rail at turnout crossing (at double flangeway).

According to S 3.2 and S 4.1:

http://www.nmra.org/standards/S-4_1ProtoWheels.html
http://www.nmra.org/standards/S-3_2ScaleTrackwork.html

This should be true:

2x Fmax(norm) <= Nmin(fine)

   2 x 0.050" <= 0.086"

No go!

If the idea is that the flange of fine scale wheel is not touching the wing
rail then the wheel width need not be twice the flangeway. This leads to:

Gmax(norm) + Fmax (norm) > Tmin(fine) + Bmin(fine) + Nmin(fine)

Unfortunately there is no Tmin(fine) specified. From the above one could
however conclude that:

Tmin(fine) = Gmax(norm) + Fmax(norm) - Bmin(fine) - Nmin(fine).

Tmin(fine) =   0.661"   +    0.050"  -    0.581"  -   0.086"

This results to Tmin  = 0.044"

Now Tmin(fine) should be about twice the Tmax(fine) 0.025" -- i.e. no go
again!

If it is permissible to use RP25/86 or H0 (fine) wheels, why the standard
wheel is specified as PR25/110 and why the "Standards gage" recejts RP25/86
wheels?

I must really be missing something -- what? Please advice!

Pekka Siiskonen