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Ask MR: Why do my tender’s wheels bottom out on turnouts?

By Steven Otte associate editor | September 22, 2021

Not all wheels are created equal, even those that conform to RP-25

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A diagram of a train wheel in cross section with various dimensions labeled

Q: I have an N scale Model Power 4-6-2 steam engine whose tender derails on my Atlas code 55 turnouts. Though the major problem was related to the drawbar, I also found that the tender’s wheel flanges were bottoming out through the frog’s shallow flangeways. I thought when I bought the engine many years ago that it had “low profile” wheels, but it appears the wheel flanges on this model are somewhere between the National Model Railroad Association Recommended Practice RP-25 and old-fashioned deep flanges. Are there standards for wheel profiles besides RP-25 that I should be aware of? Is “low profile” a standard or a manufacturer’s description? Also, as I look for replacement wheels for the tender, will 33-inch or 36-inch wheels most closely approximate the originals? – Richard Nesbitt, Minneapolis, Minn.

A: N scale models, especially those made many years ago and by certain manufacturers, tend to have proportionally larger wheel flanges than other scales. This is because flange depth, even on wheels that conform to NMRA RP-25, is independent of scale. Rather, it depends on the wheel thickness. Just like scale rails, wheels have codes that correspond to their thickness at the wheel tread, measured in thousandths of an inch. The width of the tread, flange thickness, flange depth, and other wheel dimensions are based on the wheel code — not its diameter or even its scale.

So if one N scale car has code 88 wheels and another has code 79, the first will have a larger flange depth, even if both supposedly follow RP-25 guidelines. And while the second car’s manufacturer might say it has low-profile wheels, that’s not a term with an accepted standard definition. There’s nothing keeping the first manufacturer from claiming the same thing.

Just because a car supposedly has RP-25-contour wheelsets doesn’t mean it won’t bump over high spike heads or shallow flangeways. Check the flange depth with an NMRA wheel gauge first. For more detail on the RP-25 profile, download the NMRA’s document.

If you replace your wheels, you should use ones of the same diameter as the originals, just with lower profile flanges. Since I don’t know what size wheels your model has, I suggest taking a caliper to them before buying replacements.

A diagram of a train wheel in cross section with various dimensions labeled
The National Model Railroad Association’s Recommended Practice RP-25 defines the shape and dimensions of a model’s wheels based on the thickness of the wheel, not the scale of the model it rides on. Models of the same scale can have wheels with different flange depths but still conform to RP-25. National Model Railroad Association

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