Each truck consists of three parts, a bolster and two sideframes, all injection molded from acetal engineering plastic. When assembled, the sideframes are approximately 81.75 scale inches apart, or about six inches too wide. However, the bolsters are cast on a sprue separate from the sprue for the sideframes which suggests that a die for "scale length" Proto:87 bolsters could be fabricated without having to alter the sideframe die. The sideframe part has an integrally molded round pin located on the side facing the bolster.
Assembly is achieved by inserting the sideframe pin into the bolster and then pressing inwardly with finger pressure until the sideframe snaps into place. Disassembly is just as easy: pop the sideframe free by simultaneously lifting and pressing outwardly against the end of the pin with the blade of a small screwdriver. The overall design is such that the "rocking" action of each sideframe is held within a limited range and, although the equalizing action is quite smooth and free, the sideframes are held in a vertical plane with very little tendency to splay outward when loaded with the weight of the car body.
To gain a greater understanding of Red Caboose's design, refer to the Figure 1 which shows an elevation of the left sideframe and the bolster as viewed from the end of a car. A portion of the bolster sidewall is shown broken away in the area where the sideframe locks in place. The sideframe is poised for insertion into the bolster.
The bolster is basically a channel of square cross section with sidewalls that bulge outwardly on both sides at the kingpin area to clear a kingpin screw and screwdriver. An integrally cast circular kingpin bearing plate sits atop the channel. Within the channel, and descending vertically from the top inside surface, are two flanges or fins which extend downward only as far as the horizontal centerline of the channel. Below these fins, and with inner edges located slightly outboard of the fins, are two narrow crosswise straps which cause these portions of the channel to have the cross section of a square tube with side walls of uniform thickness. Not shown in the drawings are two other rectangular openings in the top surface of the channel. The only discernable purpose for these latter openings is to admit parts of the injection die to form the outboard vertical surface of the fins and the top horizontal surface of the straps.
The sideframe presents a viewable face which is very crisp and delicate. On the reverse side which faces the bolster is found the circular pin which has a staggered sawtooth profile at the end. The diameter of the pin provides a loose (but not sloppy) fit within the interior confines of the bolster. The extended centerline of the bolster in Figure 1 illustrates the height relationship between the sawtooth profile of the pin and the internal features of the bolster. The tip of the first sawtooth and the valley following are both blunted and each display a narrow horizontally disposed area which has a flat profile. A comparison of the centerlines shown in Figure 1 for the coned journal centers and that shown for the sideframe pin demonstrates that the center of the pin lies about 0.010 in. ( 0.254 mm) below the horizontal plane of the axle centers. This latter sideframe-axle relationship is noted because it may contribute to the equalizing action. Perhaps a SIG member trained in engineering can confirm whether this is so, and if so; why?
A complete assembly of bolster and one sideframe is shown in Figure 2. As it enters the bolster, the sideframe pin's sloped leading surface encounters the fin, thereby causing the pin to deflect downward enough to allow the fin to slide over the top of the first sawtooth and snap into place in the valley beyond. I believe the strap across the bottom of the bolster also responds to pressure applied by the bottom surface of the pin and flexes to some degree in aid of the above process. Although Fig. 2 doesn't appear to show clearances between the parts sufficient to allow the rocking action necessary for equalization; in reality, it is there. Note that the drawing shows the pin is concentric with the horizontal centerline of the bolster; however, due to the clearance around the pin, the pin will cock slightly with its centerline rotated clockwise from the position shown when the assembly is placed under load, thereby opening up the clearance between the fin and its opposing sawtooth valley. This slight cocking of the pin's centerline also tends to align the parts in a way which reduces the area of contact between the sideframe and the bolster.
As mentioned above, the Red Caboose design can be easily adapted to serve Proto:87 needs. A shorter bolster of commercial manufacture is one obvious answer, but that depends upon achieving consensus regarding a standard for axle length. Given the ingenuity of the SIG membership, a home-brew bolster also looks to be a reasonable proposition. Such a project is certainly worthwhile because, once we have the bolster, each and every sideframe that Red Caboose releases will simply plug right in with no modification. Red Caboose has, as this is being written, begun production of an Andrews sideframe to be included in their upcoming release of a Mather prototype car. Looking further, every sideframe currently on the market which will permit the home-brew attachment of a suitable pin (Eastern Car Work's range done in styrene, for instance) becomes a candidate for use.
Returning attention to Red Caboose's own trucks, and if you want to get something rolling right now, Alan Gibson's P8700/1 wheelsets with 26 mm axle length are almost a drop-in fit. All it took for me to make them fit was 3 to 4 gentle turns of a No. 78 drill spotted in the apex of each coned axle hole in each sideframe to provide the lateral clearance required. Before you do the same, I should mention that I later inspected the axle ends of the Gibson wheelsets I was using and discovered they did not come to a clean point; that they had a short cylindrical nib at the tip which I removed with a No 6 cut Grobet needle file. If I had discovered this earlier, the cleaned up wheelsets might have worked without modification to the sideframe.
