HO Driving Wheel Diameter Questions

On some older ones with deeper flanges they used to make the driving wheels correspondingly smaller. So they kept the correct diameter over the flanges. This was critical for locomotives with drivers close to each other.

Reply to
Christopher A. Lee
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Hi,

I have a few questions about the diameter of the driving wheels on HO steam locomotives.

Is it true that on HO steam locomotives, the driving wheels are actually smaller in diameter than the prototype by a couple of prototype inches? I vaguely recall an article in MR on the Trix Big Boy that this was the case.

Is this also true of the expensive HO brass steam locomotives too? If so, why?

Thanks, Jim

Reply to
Jim

Yes, it's true for many model steam locos, especially the larger ones. The reason is the oversize flanges on model wheels. On a large steam loco, the wheels are as close together as they can be in order to make the coupled wheel-base as short as possible. So the flanges miss each other by a couple of inches or so. Scale size wheels with model (=oversize) flanges at scale spacing would cause the flanges to interfere with each other. So, you either space the drivers a little further apart, or make the wheels a little smaller or both.

On older 4-4-0s aand similar engines this needn't be done, since those drivers were spaced quite far apart.

There are other compromises. Eg, pilot wheels are sometimes mounted further forward than prototype so they don't bump into the cylinders when they swing to the side on our very, very tight curves. (See Footnote On Curves) below. Or they are made smalle. Or both.

It's possible to build a true to scale locomotive, but it won't run on our regular track, especially not through the turnouts. Proto:87 (or P:87) standards, however, are designed to take exact scale wheels, hence locomotives with exact scale driver sizes and spacing can be built to P:87 standards. Such locos must very carefully equalised, though. I've never seen one myself, but it would be nice to see one run... :-)

Footnote on Curves: In North America, real railroads measure curvature in degress: a 1 degree curve is such that a hundred foot long chord subtends an angle of 1 degree. This makes it very easy to lay out a curve in 100ft "stations" (points along the line where the track will go.) One of the tightest mainline curves in N. America for many years was a 23 degree curve on the CPR at Boston Bar in the Fraser River gorge in British Columbia. That corresponds to about a 36" radius. Speed around that curve was _very_ restricted. It was straightened in the

1920s IIRC, because the newer, bigger engines simply couldn't get around it.

HTH&HF

Reply to
Wolf Kirchmeir

Reply to
Jon Miller

The above answer is correct. Since NMRA standard flanges are much bigger than scale, the overall outside diameter if a wheel is also correspondingly larger than scale, assuming the diameter at the tire is correct.

Many prototype steam locos had the drivers VERY close together ... flanges almost touching. Thus an NMRA flanged driver of scale tire diameter cannot be placed there .. the flanges would overlap.

The only solutions are:

1) lengthen the wheelbase (often very noticeable) 2) reduce the driver diameter (often LESS noticeable) 3) use a non-standard smaller flange.

Most model manufacturers use #2, with sometimes a bit of #1 as well. This is true of even the best brass models, as there is NO other option if one wants to maintain accepted wheel standards.

... as an aside, our "standard" model tires are also way too wide, and our track gauges rarely match our scales precisely either. ALL these mean compromises with frame widths, wheel spacings, diameters, cylinder width, lead and trailing truck dimensions, etc. The whole model is thus compromised.

Dan Mitchell ============

Reply to
Daniel A. Mitchell

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