Well, 6X6, I'll be glad to stand alongside you (well, just to the rear ever-so-slightly in case there's a load of incoming....:-D) in singing the praises of Scammell.
On the other hand, that rear axle drive is iffy.
I do not agree that 3 U/Js has much to do with it - the first one behind transfer box drives pretty much level with axis of box (is the slight angle towards the centre line of chassis compensated by angle of rearmost prop? I'll have a look some time, but they weren't daft in the drawing office).
But note the rear prop has U/J angles equalised by tilting the rear diff upwards, i.e. away from axis of gearbox (unlike other vehicles as in Landrover, Bedord etc where the axis of diff is the same as gearbox). This in theory is fine as each U/J still takes an equal angle and constant velocity ensues, but I believe that propshaft/joint balancing then becomes critical.
I do not know of any other vehicle where the U/J angles are equalised in this way - PTO shafts on agricultural tractor / trailed implements go like this when turning, as do the drive shafts to trailer on the Multidrive set-up. But these cases are only when turning, normal use is straight drive.
I reckon that any dynamic forces caused for example by unbalanced propshaft, would to some extent be compensated between U/Js at each end in a normal set-up (eg Bedford MK or Landrover)(i.e. when prop is pulling one U/J it will be pushing the other). However in this case the prop would be trying to "escape" in the same direction at each end, so U/Js would have their work cut out to contain the substantial forces.
Scammell's ploy of 'rotating the whole propshaft assy one bolt at a time until an acceptable state was reached' only serves to alter one set of out-of-balance forces in relation to another (the propshaft assy to second axle). If the drive to second axle was perfectly balanced then altering the relative angle of U/Js on rearmost drive would have no effect at all.
The reason I'm interested in this is that PGK888 came home with a thrown rear prop. It had even destroyed the substantial frame cross member carrying the centre bearing :shake:. Fortunately the sale included a full set of replacment parts (including cross member) so I have no excuse not to get on with the job. But I am anxious to understand WHY it happens, and will be getting all rotating parts fully balanced before installation.
Having said that, I cannot believe Scammell would not have done the same to try to resolve the problem:confused:
I'm in total agreement with you on the subject of gearboxes - but nothing is perfect and possibly the achilles heel was the lubrication system maybe?? so many oilways to get blocked, and as you say, it does need oil as well to work! But what a wonderful bit of engineering, says someting for the boys from Watford.
Also, CONSTRUCTOR was a purely commercial design, not a post war MV design complexity :cool2:
Mis-matched tyres? Well yes, be sensible. But keep it in context too - if you put a toally mis-matched set of tyres on a 6 wheel tipper and drove around all day empty I think you'd just wear some rubber out, but stick 16 tonnes on frieght on board and something might just 'give' :-D On the other hand, I would not like to run an old GMC 6x6 empty without a matched set, those props and U/Js are not over big!
My dad once put a high speed diff in the front of a Matador dumper (10T cap) by mistake, and the driver brought it round after a few loads complaining that the rear kept jumping up when empty and the front bucked when loaded. Just think of the wind-up on that! (I watched them driving it around the yard to check :shake:).