Back to Back (B) Tire Width (N) Flange Width (F)
Flange Depth(D)
Wheel
Check (K)
Min Max Min Max Min Max
Min Max
Max
Prototype inches
0.611 0.611 0.063 0.063 0.013 0.016 0.016 0.016
millimeters
15.52 15.52 1.610 1.610 0.340 0.400 0.400 0.400
Proto:87.1 inches
0.610 0.613 0.064 0.069 0.012 0.014 0.012 0.014 0.623
millimeters
15.48 15.55 1.620 1.750 0.310 0.350 0.310 0.350 15.84
Standard HO inches
0.566
0.108
0.030 0.028 0.028
millimeters
14.40
2.740
0.710 0.700 0.700
Proto:87 SIG
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 - 2
3
The NMRA categorizes Code 88 wheels as “Fine:HO”:
Fine: HO track tolerances are closer to Proto:87 than standard HO:
Recommendations and standards
RP-25 wheel contour:
S-3 track standards:
Proto87:
2
MRH-Feb 2013
