CRAB MODEL PONY TRUCK SUSPENSION DESIGN
Before I started making this bit I wrote the following to a pal.
The real locomotive pony truck may not go round such sharp bends that the flanges will encounter the frames. But if it did the springing would in any case make the frames rise with the pony wheel on any humps in the track, keeping the frames clear of the flanges.
On the model in all gauges there are various ways round the problem that, scaled down, the distance between wheel and frames is tiny. The normal way to get round it is to file away the frames so that there is no likelihood of any fouling. However, the following assumes that this drastic and visually naff approach, though not very very noticeable, is unacceptable. Alternatives are to narrow the frames, unnoticeable from an average side-on view, or to arrange the frames to be part of the truck, the subterfuge being invisible again from normal perspectives as it is hidden behind the valance. On the Crab neither of these approaches is possible because the framing is already on the Bachmann model, moreover it is very thick and strong and can’t be modified without a whole host of other problems, and as it is accurate it would be a shame for it to be modified.
The normal 00 gauge type basic design is for a free riding truck that contributes nothing to the suspension or direction of the loco. The weight of the wheels and truck is supposed to keep the assembly on the rails which is all that is required. Even in 00 this is not wholly reliable.
However this model cliché neglects the likelihood of the pony wheel flange catching on the frame cut-outs, which are only 0.6mm vertical distance clear in this case. They are 17.2mm wide over the outside. In 00 gauge the flanges will thus be inside this width but in P4 they will be 0.2mm outside.
This is the typical situation where problems will be blamed on the track being less than billiard table smooth and level.
I adopt the attitude that it is hopelessly unrealistic to expect a billiard table level layout and easier to design in suspension that can cope with probable humps and dips in the track.
In addition to such humps and dips the model will probably be required to go round sharper than prototypical curves. Thus the pony truck will definitely swing so that the flange comes directly under the frame cut-outs, and probably will even if restricted to prototypical radii bends.
So what is needed is for the pony wheel flange to be always clear vertically of the frames.
The usual P4 approach is to spring the pony downwards and horizontally to keep the wheels down and give some directional control. However it seems to me that this neglects the problem of the flange being trapped by the frame on a curve if one of the wheels rises just that half millimetre, and pony truck derailments are frequently a cause of trouble.
I propose to arrange for the front of the loco to actually rest on the pony truck. The front driving wheelset will be sprung sufficiently that there is not a great deal of weight on the pony truck, but not so much that the loco is lifted off it except momentarily.
I think this will be easier than attempting some means of springing the pony truck to work in the same way as the prototype
So then the question is how to design this? What is needed is a roller so that the truck can move from side to side completely freely while bearing some of the loco weight.
Now the question is, where should the roller(s) go?
Here is the Bachmann drawing of the loco, showing the 0.6mm gap between flange and frame cut out. And a sketch of the front view. (Unfortunately loads onto here upside down. I'll edit with another, clearer, drawing here tomorrow) The gap between the frames could be filled in to suit the physical size of the roller arrangement chosen. All will be hidden behind the front buffer beam, so there is no visual realism question.
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There are, it seems to me, two possible configurations, each with their problems. First, highlighted in yellow, are two rollers, one each side. Or second, highlighted in orange, a central roller.
The problem with the orange central roller is, would a rise of one wheel on a track hump of say 0.5mm translate into the frame lifting sufficiently? The frame would rise 0.25 - would that be enough?
If the answer is no, then two side rollers would not have that problem, but here would a seesaw effect be created, where if one wheel encountered a track hump, in lifting up the frame could that cause the opposite wheel to rise into the air and lift off the track?
After some thought, I have decided that the pair of rollers is the right approach, with springs to keep the wheels on the track whatever the pony truck does. In addition I will make the truck as heavy as possible to help keep the wheels down, and also hopefully to balance out the spring force, as I don’t really want the loco to be sprung upwards here and reduce the weight on the front driving wheel any more than necessary.
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Well I decided to go for a single roller. As that will tilt the springs aren't needed but they won't do any harm