In this post, I turn to the design of a chassis for my model Posting Coach. Fortunately, there is a detailed contemporary description and illustration, given in Whishaw’s book ‘The Railways of Great Britain and Ireland’, published in 1842.
According to Whishaw: “The under carriage is formed of two soles, 7 feet 8 inches apart, 9 inches deep, and 4 inches thick, and projects 10 inches at each end beyond the body ; and six cross pieces, one at each end without the line of the body, which are 15 inches deep and 4 inches wide; and the others arranged one before and the other behind each pair of wheels, and equidistant from the centres of axles. The two ends of each of these cross pieces are finished with ornamented scrolls, and project six inches beyond the outer side of each sole. The diagonal braces, which are of 1½-inch iron, run from each angle of the carriage towards the centre of frame-; the carriage is furnished with complete buffing and traction-apparatus (see Plate XI.), and is mounted on four of Losh's wheels, 4 feet in diameter, and 10 feet from centre to centre of axles. The bow-springs are each 5 feet 4 inches in extreme length, the bed of each being 6 inches below the centre line of wheels.”
Perhaps someone can enlighten me regarding the curious circular structures forming part of the draw gear? I assume it is a shock-absorbing system.
Rather than making my own drawing, I decided to import the Whishaw drawing into ‘Fusion 360’ as a ‘canvas’, on which I could build up the frames as shown below:
Building the Frame over a ‘canvas’ in 'Fusion 360'
I built the chassis ‘upside’ down and provided a thin floor, to ensure overall rigidity and to provided a firm mounting plate onto which the body can be fixed. I created the undergear, axleguards, springs, etc. by using exactly the same methods that I described in a previous post about my early cattle wagons. I did not print the headstocks at the same time, since these rise above the level of the floor and would not have allowed me to print from a level base. Instead, I drew these as separate ‘bodies’ in ‘Fusion 360’, aligned with the solebars.
3D model of Chassis in ‘Fusion 360’
I decided to take a risk by omitting any support structures in the openings above the springs, because I find it all too easy to damage fragile parts when trying to remove supports. In fact, the test print came out remarkably well and needed no significant cleaning-up before use. The following photo shows the model mounted on its wheels but, otherwise, exactly as it appeared immediately after removal from the printer bed, with no ‘cleaning-up’:
Chassis with wheels fitted
Once I had printed all the component parts, I could assemble them into my complete model of the Posting Carriage. I printed the headstocks separately and slid these into position over the ends of the protruding solebars, as shown below:
Model carriage as printed
This is my model exactly as it came off the printer. The remaining tasks are largely ‘painting and decorating’
It is really surprising to see how small a ‘luxury carriage’ of 1838 actually was! It is hardly surprising that Brunel’s idea of placing the wheels outside the body was swiftly abandoned. Unfortunately, this decision meant that very little space was left around the tracks, so Broad Gauge stock was never able to expand much beyond the confines of the British standard loading gauge.
Within a few years, railway vehicles started to outgrow their ‘road coach’ forbears and even the primitive 3rd-class carriage of 1844 was considerable larger than this small vehicle! I have placed my two models together to illustrate the comparison.
Posting Carriage model compared with closed 3rd-class Carriage of 1844