Saturday, February 9, 2013
Scales And Gauges
The scale of the model is simply the ratio between the size of the model and the real life item. In my case I've been modelling at 4mm to 1 foot or as a ratio 1/76 scale. The gauge, on the other hand, is the distance between the inside surface of the rails, and in the case of OO gauge this is set to 16.5mm. Unfortunately as the real railways use a gauge of 4 foot 8 1/2 inches, a scale width of 16.5mm is at a scale of 1/87 or 3.5mm to the foot, i.e. the rails aren't at the same scale as the locomotives and rolling stock. Apparently (or at least according to Wikipedia) this combination came about as early clockwork mechanisms and electric motors were difficult to fit within HO scale models of British prototypes which are smaller than equivalent European and US locomotives. A quick and cheap solution was to enlarge the scale of the model to 4mm to the foot but keep the 3.5mm to the foot gauge track. What this means is that 16.5mm track at 4mm to the foot scale represents a gauge of 4 feet 1.5 inches which is 7 inches too small, and so the track should be approximately 2.33mm wider. What this means is that if you accurately model the width of a wagon at 1/76 scale then you have to narrow the underframe slightly so that the wheels will fit. Having to alter the underframe in this way means that you can't simply scale the model to fit a different scale unless the gauge is similarly modelled wrongly.
N scale (at least in the UK) is 1/148 scale with a 9mm track gauge. While the track gauge isn't completely accurate it's a lot closer than in OO gauge which explains why the wheels wouldn't fit on my naively scaled down wagon model. I think it should be easy to adjust the model so I might give it a whirl at some point.
Interestingly some people do model at 4mm to 1 foot scale with a more correct track gauge (these are EM and P4 scales). The problem with these scales is that they aren't supported by the main manufacturers and so nearly everything has to be built from scratch, and while I'm happy building wagons I don't (yet) have the skills to build a locomotive or track.
Of course there is another good reason to change the gauge of the track yet keep the same scale; to model narrow gauge lines. If you don't know what I mean by narrow gauge, or just want a laugh, then I'd recommend letting Thomas and Friends explain things to you.
In the UK at least most narrow gauge lines have a gauge between 2 foot and 2 foot 6 inches, and the common way of modelling these is called OO9. That is OO scale (i.e. 4mm to 1 foot) running on 9mm gauge track. At 4mm to 1 foot scale 9mm actually represents a gauge of 2 foot 3 inches, but the difference is small enough that 9mm gauge is used for most narrow gauge 4mm scale modelling. This of course means that N scale parts (track, wheels etc.) can often be re-used or at least used as a starting point for scratch building as there are very few ready-to-run OO9 items available.