Model railroad trucks are not consistent precision assemblies, and if they were, we simply could not afford them. This makes a simple question like suitable wheel replacement not so simple. For a variety of inherent design, material choice, and molding reasons, truck frames vary considerably, even from the same manufacturer or from the same lot at the same time. This can significantly influence the choice of axle lengths and other wheel set parameters when choosing replacements. No two trucks are really the same.
Here are some of my observations and experiences with metal wheel replacements, presuming trucks with nominal NMRA cone shaped bearing holes, and axle-ends with nominal cone-shaped NMRA dimensions:
Wheelset axle length length, axle-end diameter, and axle end/truck bearing material dissimilarity will significantly influence rolling success in any given truck.
Alone, axles roll the best when axles and bearings are of dissimilar materials. i.e. plastic axles within metal bearings, and metal axles in plastic bearings. This is borne out in every day experience.
Alone, wheels roll the very best when the least end play is balanced to the optimal axle length, and is tailor-fitted and matched to each individual truck type (even better to each individual truck). It is extremely common for a variety of high quality trucks to optimally roll only with respective wheel sets of varying axle lengths from 0.900” to 1.020”. It is also common that the greatest rollability will not always coincide with the least end play, and a compromise must be reached.
Alone, wheel sets with two (2) millimeter axle ends seem to be consistently superior to those of fatter thickness.
Clean axle ends free of metal curls remaining from machining (usually not seen, but felt by finger pads) roll significantly better.
In the current quality wheel set replacement mass market (as writ in our small hobby), Kadee and Intermountain have dominated, and they have earned it with persistent high quality assurance (Kadee a bit higher than Intermountain in this regard). NWSL has had an important quality niche, not to ignore the late great Reboxx (my personal go-to for almost 15 years). I never ever personally discovered an out-of-gauge wheel set among any of them.
I love the Kadees for their appearance and for their stellar quality and predictable and precise 1.015” axle length. They work the very best in Kadee's own trucks, but can suffer variably because the plastic and slightly curved conical ends separately or alone will not always play well within plastic bearing holes, not to mention cones of different shapes trying to fit and roll well one within the other.
The Intermountain axle lengths have varied over the past few years, drifting from 1.012” down to 1.005 or 6 the last ones that I purchased and measured. I cannot comment on those produced by their current supply source (reportedly different from their previous failed contractor). The different lengths, small as they are in reality, can make a real difference in any individual application.
Their are other new metal wheel suppliers that I have not tried, primarily because I have yet had no need; or because (irritatingly) the axle lengths are unknown.
When doing mass replacements, a lot of the above may be far too fussy, but they can perhaps serve as a guide. Doug Harding’s suggestions as to Intermountain wheel sets would be a good place to start.
P.S. Lastly, although it has been heavily promoted over the years, and on occasion I have found it very useful, generally I have otherwise found cause for routinely using a reaming tool to improve observable rollability.
Denny S. Anspach, MD
Sacramento, CA 95864