[Humber-One-Ton Tom]

Interesting! - I had missed this entirely when I skim-read the text so did not pick up on it.
Funny you mention tyres....
I was intrigued so I went straight to an interesting book I have...Electrical and Mechanical Engineering Regulations, Wheeled Vehicles N252, Technical Handbook - Technical Description, Issue 3, 31 May 1966, Distribution - class 131 code 2 - Truck, 1 Ton, GS Cargo, 4x4, Humber MK1, etc. etc.
This describes every component of the vehicle in great detail down to type of iron used for castings etc.
Regarding secondary suspension:
The shock received by the road wheel is partially absorbed by the tyre but the greater part of the shock is absorbed by the torsional resilience of the torsion bar. Since, however, the torsion bar is not a perfect spring, it does not absorb the full shock and a certain upward movement of the chassis fram is caused at this point. (which means you do actually notice the bump !)When the shock is of such magnitude that the upper link meets the bump buffer, the action of the compressed buffer expanding in the steel cup progressively increases the resistance and absorbs part of the shock
So it is really the rubber cones in cups - (more than just bump-stops), but the tyres are stated as a factor too. This is not the same on all vehicles - recently changed the bump-stops on my Jeep Cherokee and they were hard blocks with no 'cup' - stop the axle smashing into the chassis but no more than that...
Re. the shock absorbers, the manual states that these are dual action (unlike lever-arm shocks) because the 'torsion bars do not have inherent damping effect of leaf springs'.
A lot of this stuff would be taken for granted on a more modern vehicle but on a truck from 1950 all this was a big deal, and very different to the cart springs still widely in use then and still used on Landrovers up to the '80s!
Regards,
Tom
Portsmouth, UK