Back to Back (B) Tire Wi
Min Max Min
Prototype inches
0.611 0.611 0.063
millimeters
15.52 15.52 1.610
Proto:87.1 inches
0.610 0.613 0.064
millimeters
15.48 15.55 1.620
Standard HO inches
0.566
0.108
millimeters
14.40
2.740
The catch: NMRA S-3.2 calls for a minimum flangeway width
of .050”, and allows a track gauge from .649” to .672”
(16.5mm to 17.1mm). If the tolerances are toward the wide
side, a Code 88 wheel can be a problem. Ideally, the Code 88
wheel would be used with a flangeway a little less than half
the tread width. S-3.2 is written for best operation with .110”
wide wheels.
A lot of manufactured track switches and crossovers don’t even
match up well with the NMRA RP-25 .110” wheels. The switch
dimensions and clearances don’t match the basic NMRA stan-
dard. So, even if track is gauged correctly, you can still encoun-
ter flangeways that are the wrong width and guard rails in the
wrong places. People using Micro Engineering switches and
switches built from Central Valley, P87 Stores, and Fast Tracks
materials report few problems, if any.
People who widen their track gauge on tight curves to accom-
modate long-wheelbase locomotives may find that wheels with
treads narrower than .110” drop between the rails and hang up
because the wheel assembly is narrower overall. People model-
ing bumpy, twisting track also need to be a little more careful.
Code 88 wheels won’t run well on Proto:87 track. That has an
even closer to prototype set of standards.
Questions, Answers & Tips -
3
MRH-Feb 2013
