Minimum Radius

[Weibel]

How's it going everyone?

I would like to build a small oval in my back yard- there is only one problem that I am running into. The maximum radius that I could fit would probably be around eight to nine feet. I was thinking of running the smaller gauges (3.5" or 4.75") back there, but how would I determine what the minimum radius a locomotive could handle before I spring the money for one and find out that the turns are too sharp for it? I have seen on the Maxitrak pages that the Alice (the Hunslet) that they sell can handle such a radius- but I wanted to find out a hard and fast way to determine this sort of thing myself.

Thank you!

[James Powell]

You really can't. The judgement I would use is that anything <15' radius will limit you to 0-4-0, and possibly 0-6-0. As you go up, bigger engines can be run. The way to figure out if a loco will make it around is to use a triangle to figure it out. So, if the fixed wheelbase is say, 18", then you can use a triangle from front L- Centre R- Rear L, and the track gauge (vs the nominal gauge) to see if the wheels are going to be hard rubbing against the track. (this will tend to show up on the leading axle lifting). The leading truck can be sketched out as well, that if they are going to hit the cylinders then that will be the limit on radius. There is no easy answer to what the minimum radius will be...but that a 9' radius will be very limiting. If it was me, I would look at 4.75 or 5" gauge, 0-4-0, rather than 3.5" gauge, because a bigger engine is going to make it easier to drag individuals around. Any 0-6-0 in 3.5" will limit you to ~2 individuals at most behind i, and a 0-4-0 (Butch, or Juliette try and avoid Tich !) would limit to probably an engineer only. Mind, a 9' radius circuit isn't going to take much of a back yard up !.

[James Powell]

I'd add- I'm looking at 30' radius for Caribou (0-8-0) and Britannia (4-6-2). We used to run a Hoffman Hudson on 30', 3.5" nominal, 3.75" actual gauge track. (well, the gauge was a bit inexact...the curves were originally 3.5", but the spacers were all cut apart and welded back together...)

James

[Weibel]

James, I'm really not buying that- I've seen the math for stuff with trucks (diesels, passenger cars) but cannot find it for the life of me.

Also, imagine this one: back when steam was still around, how on earth would they know if a locomotive would handle a certain radius before it was built? Make a rolling chassis and have some horses pull it around the railroad? I seriously doubt that Kentucky windage was used by locomotive builders for this sort of thing.

[Mr Ron]

One way is to increase the track gauge in the tight curves. Another is to use flange less drivers on the center drivers on an 0-6-0. This was common practice on prototype railroads.

[PRR5406]

The track section or curve, was drawn out on paper. The frame of the locomotive in question would be considered a cord of the curve and measured out. New England Steam Corporation (rebuilding Maine Central 470) sent a bunch of historic documents from the old Bangor and Aroostook RR, to the Barringer Transportation Library in St. Louis, showing the results for "modern" Consolidations.
Everything was mathematics, and the RR's had scores of engineers and draftsmen working on this everyday.

[James Powell]

Gauge widening. And the answer is "it depends", because if you read about enough small enough/delapadated enough lines, you will find that it was done by thumb ! (quite literally, 2'3" and a thumb...). This was with an 0-4-2 loco. The easiest way at 9' radius, would be to sketch it out- draw out your 9' radius track on a piece of paper at gauge, then measure your lengths. If the flanges are going to be rubbing, they are going to be rising, which is a great way to encourage derailments. For an 0-6-0 the way is exactly as I said- for a x-4-x, the same thing applies but with the pivot points mattering as well. Generally, the leading/trailing truck lengths are shorter than the engine wheelbase, so should have little effect other than the angular displacement and then running afoul of the cylinders. It also is going to matter what wheel profile actually looks like, as a different one will act differently as regards centering and track handling. I wouldn't expect many visiting engines on a 9' radius railway !. (though to be fair, the P&Q near me was only 15' radius- it was mostly run with a Shay though, I understand. The trestles had deteriorated by the time I moved out here, so there was no longer operation. (it's not there any more, location was: https://www.google.ca/maps/@48.3578661, ... 312!8i6656 )

[Weibel]

Here's my situation in a nutshell:

I wanted to run either 2.5" gauge or 3.5" (I would prefer 3.5") in my backyard. It's really narrow, like 22 feet or so.
I'm not trying to build anything larger than an X-6-X of some sort, and if I do it's likely to be a little tank engine. In fact, more than likely anything that I do build will be just that- little tank engines. I really like the De Waal Decauville.