sumpter250 said:
Aside from the aesthetics, there is a sound reason for outside frame usage. It is more stable.
While the stability of narrow gauge equipment is inherently questionable ( the narrower the base, the less stable the object, with respect to rolling over), using an outside frame does widen the base, with respect to the center of gravity, and decreases the tendancy of the equipment to roll/tip over.
I think what you mean is that outside frame produces a lower center of gravity, by giving additional weight lower down. This is more of a traction advantage rather than one of stability. It doesn't widen the base in an effective sense, because the wheels are still the same distance apart. The stability difference between O/F and I/F locomotives here is pretty negligable. In this sense, an inside frame locomotive is actually more stable because it's usually narrower. Outside frame however allows you to accomodate a larger boiler and larger cylinders, thus increasing the weight and therefore adhesion.
The thing with trains is that the physics aren't what the lay man usually expects. The instability of narrow gauge is more of a myth than a fact - people who believe this usually don't take into account that trains use weight to keep them pinned to the rails (adhesion, as I mentioned earlier). This is as opposed to (automobile) cars (with which people often have them confused), which use center of gravity to keep them from rolling over when cornering. So long as the weight above the wheels is greater than the weight either side of them, then it's stable, so long as it doesn't do anything crazy. The only reason a narrow gauge locomotive would roll is if it attempted a very sharp turn at high speed, and the weight shifted (water in the boiler, plus coal in tender). Generally when a locomotive attacks a corner when it's moving too fast, it will just either break the tracks and carry on going forward, or will jump off them and carry on going forward. Usually after doing that it will roll, because in most cases it had already started cornering by the time it had started putting excess stress on/started leaving the track.
The only other circumstance in which I've seen a narrow gauge locomotive roll onto it's side was due to poorly laid track, which is/was the most major failing of narrow gauge railroads worldwide. This is usually the result of underfunded permanent way departments, which is caused by the mentality among operators that narrow gauge produces significant savings in PW construction and maintenance costs, and therefore funding to that department can be cut without consequence.
The most important thing to remember, is that in terms of balance, the working surface is the part of the locomotive that is in contact with the ground. Making it wider or narrower doesn't change the fact that the wheels are only 2 to 3 foot apart. It does however mean that when it starts to tilt, it's more likely to roll in that direction. However, contrary to popular belief, it's not as easy to get a NG locomotive to tip as people might think. For a start, the suspension is trying to stop you from doing just that, and also the sheer weight of the thing would prefer it to be bolt upright at all times. Since it's more fuel effficient to drive like your grandmother, there's no need to worry about tilt during high speed cornering.
I'm sure you knew all this already, but I felt like stretching my brain a bit. :thumb: Also some of this may be of interest to people browsing the forum in general.