Scale width draft gear?

railwayman <stevelucas3@...>


The point that I am raising by citing the Bronx Terminal RR. is that prototype cars with conventional draft gear, couplers, and kingpin to pulling face dimensions, could be handled, coupled, and uncoupled, on curves of 90' radius. We consider this curvature to be "toy train" like.

Yet, by all accounts, the prototype cars stayed on the rails.

An article on a similar property appears in MRP 2007 on the DL&W's Harlem Transfer Company.

I hold out these properties as proof that scale-width draft gear can work on a model run on even 12 3/4" radius in HO--provided that we pay attention to coupler swing and pivot location. It appears to me that the current defacto standard of 1/4" from the back of an HO coupler head is too long. While on the topic of cause and effect, this width is caused by having the pivot point almost twice as far back from the pulling face of the coupler than on prototype cars.

I blame this on manufacturing tolerances at the time that the 1/4" figure was introduced. Some modellers used wood rulers back then, where 1/16" or maybe 1/32" was a minimum increment of masurement.

I imagine that most, if not all, STMFC modellers on this list have far more accurate tools today--digital Vernier calipers that can measure increments of .001"/.001 mm are very inexpensive now.

Now if we could get some model couplers with scale set-back from pulling face to pivot...

Steve Lucas.

--- In STMFC@..., "soolinehistory" <destorzek@...> wrote:

--- In STMFC@..., "railwayman" <stevelucas3@> wrote:

I think that it's time that we had a look at prototype couplers, in keeping with the STMFC list's mandate.

My 1995 AAR Field Manual gives coupler dimensions for Type E couplers. Rule 16, paragraph 8 gives a distance of 21" (.241" in HO) from back of the head to the end of the shank on a BE60AHT coupler, as well as several other Type E's. My belief is that this is a standard coupler length found on STMFC's...
Think about how much swing a prototype coupler can achieve on an 11 1/4" radius for a moment. It becomes clear that a standard Type E coupler is very tolerant of sharp track radii. Such as here, on the Bronx Terminal--

Notice the boxcar on a less than 100' radius curve next to the curved freight shed in both photos.


Your study of the key slot dimensions of the AAR Type E coupler is interesting. In all my work designing couplers, I have never bothered to calculate the effective pivot location of a prototype coupler... Thanks.

However, your use of the Bronx Terminal as an example is misleading. The main thing that drives the need for wide swinging couplers on our models isn't the length of the coupler shank; that's the effect, not the cause. The main thing that forces us to need wider than prototype swing is our desire to run equipment with different king pin (truck pivot) to striker (or coupler pulling face, take your choice) dimensions, because the longer that dimension, the further from the track centerline the coupler is thrown on curves.

In the case of the Bronx terminal, all the cars had the almost universal standard of either 5'-0" or 5'-6" to striker distance. The only exception was the locomotive, which was somewhat longer, although I can't find a dimension at the moment. Even so, this is a very short locomotive; note there are no steps between the outer axles and the end sill. Even as short as this locomotive is, the CNJ must have found lack of coupler swing troublesome. Note that the locomotive was originally delivered with short swing knuckles, similar to those used on steam locomotive pilots:

Which the CNJ changed to a short knuckle pinned to a radial drawbar:

Your first photo above also shows this arrangement to good advantage.

Unfortunately, these early boxcabs are atypical of later diesels, which have kingpin to striker dimensions in excess of 12'. This throws the coupler so far off track centerline on sharp curves that the coupler on the coupled car actually swings OUTWARD, toward the outside of the curve, in order to remain coupled to the locomotive. If the coupler can't offset a sufficient amount, the car will be dragged off the rails. This is a situation that just doesn't exist on the prototype, for all practical purposes.

The solution to his problem, dating to toy train days, was to fit wide swing couplers to all the equipment, similar to what was done to the prototype CNJ 1000. The solution today, for prototype modelers, should be to simply not try to run trains on curves that the prototype equipment can't deal with, but that only applies to prototype modelers. The bulk of the hobby dollars are still spent by the guys who want to run their SD50MAC's or whatever they are on a 4' x 8' sheet of plywood, and that's the market the manufacturers need to keep foremost in mind.


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