I am very grateful to member KH1, who alerted me to the 'Cleminson principle', in reply to my earlier blog entry about 'Milk Churns and Syphons'.
In another thread in the forums, http://www.rmweb.co.uk/community/index.php?/topic/6484-cleminson-chassis-drawing/, I read that there was an article in 'The Engineer' of Feb 15th 1878, describing this system. Back volumes of The Engineer can be downloaded free from http://www.gracesguide.co.uk/The_Engineer_%28Bound_Volumes%29 , so I show an illustration from p111 of the relevant issue.
There is a kit available from http://www.brassmasters.co.uk/cleminson_underframe.htm but it didn't quite meet my needs, since I wanted to retain the existing outside frames on my model. I thought it would be fairly easy, however, to build something along similar lines so, having some spare brass sheet handy, I sketched out a design to apply to my K's Low Siphon:
I printed this sketch to scale, pasted the parts onto my sheet of brass, drilled the necessary holes for axles and pivots, and then cut out the pieces with jewellers' snips. The hatched areas on the drawings were folded up, to make the axle supports and the tabs within which the centre axle mount slides. The linkage pins were made from unfolded paper clips.
The various pieces were then attached to the floor of my Syphon, by means of screws through the pivot points:
I had some spare Bachmann wheelsets, so I removed the pin-point ends with a cutting wheel and threaded the axles through my, rather crude, axle mounts. After straightening everything up, I found that the vehicle would now traverse my 15" radius curves successfully! Here is a pic of it running through some pointwork.
This was very much a 'quick test' form of construction but, having established that the method works, I think I'll have another go, probably using nickel-silver sheet, which should be much more rigid and able to hold everything in good alignment.
I'm very pleased, as this now opens up the possibility of running the ubiquitous six-wheel coaches on my 19th-century micro-layout