Le Corbusier wrote:...For Clarification purposes .... could someone define the difference between a 'pony' and a bogie truck ... and where each is used on a locomotive? and why the difference? - both I assume are there primarily for weight distribution.
A pony truck has 1 axle and a bogie (normally) 2. The use of either was primarily and originally driven by the need to stay within a given weight per axle. So as locos got bigger they needed extra carrying axles to carry the additional weight and to avoid the complexity of additional coupled driving axles. To start with these extra carrying axle(s) were in the same rigid frame as the driver(s) but the limitation in flexibility of a long fixed wheel base lead to the development of radial trucks (mounted in the frames but with additional side play and which turned slightly as they slid side to side), pony trucks and bogies. Only when they had such things did they also develop a role in providing more stability on the resulting long locos with relatively short fixed wheelbases travelling at speed.
A bit of an over simplification but gives you the general idea. The differences in stability issues at speed beteewn our models and the real thing (resulting from implications of physics on scaled models), and the fact that we don't have the axle weight constraint leads to us being able to do things differently, depending on how much we are being driven by a wish to build exact scale models.
Julian Roberts wrote:... I think the problem is, as always, with our less than prototypical curves, and (I think) the necessity to make a loco be able to cope with track irregularities that are more challenging than the real thing. The real Crab minimum radius I'll bet is more than the 5 chains that is our 4ft - and many of us want sharper than that. I wanted to preserve the real clearance between pony and frame which is 1mm or less and I think that in order for it never to have any chance of fouling there needs to be some means of preventing the wheel rising any closer. ..a backstop!!!
Which is the point about modelling in P4 isn't it, as it reduces, but does not eliminate, the need for modelling compromises. If we model scale flanges and the carrying wheels cleared the frames on the original then we should be able to do it on the model. If they didn't then either you don't use small curves or you start making the compromises. I'm not sure arraging for for your loco to lift its front drivers will prove operationally satisfying,