I am enjoying this discussion and all the theoretical information. It reminds me of a memorable evening sitting alongside Derek Genzel one Scalefour Society dinner long, long ago and Derek having all the figures in his head which was impressive and all the conversation being one way, which was less so, however interesting just the same.
A few years later and as a member of the East of Scotland 4mm Group I found myself for several years at Scalefourum on the Scalefour Society test rig. Derek had quite a bit to do with the construction of the rig and the test track. Since it was up in Scotland in between times it did allow those who wanted, to test their own locos if they wished and one or two of us did during that time.
My goodness what an eye opener it turned out to be in so many ways and in particular the weighting on individual axles - one of the gauges allowed a reading of each axle in turn. Freedom of the bearings being crucial. It was also possible to give some read out of tractive effort. The test track had worst case scenarios built in and a fair bit of effort had been put in to its construction.
There was a competition each year, I think to encourage individuals to take part. It could be very disheartening for some, I remember that one year we had one individual who did not have a layout at home turn up with a range of several large locomotives (about 9 or 10 if I remember correctly) None passed the test track, but we were able to tell him just where the problems lay and he went off seemingly content. I would be very interested to know if he was able to correct all the faults as his engines would have formed a very impressive stud of locomotives.(Mainly Pacifics) Others would go off a bit crest fallen as their favourite engine failed to come up trumps - some locos looked wonderful, but ran poorly, but visa versa was also true.
I remember being very disappointed
with a new locomotive which I had just finished, a J37 which trundled about OK, but when it came to weight distribution seemed to have virtually none on the centre pair of drivers! (It was sprung as most of my locos had been since I started way back in the early 70's.)
Most of the bigger engines came to grief on the test track with bogies and pony trucks and trailing wheels giving most trouble.
A consequence of all this was that a simple 0-6-0T seemed to win the trophy each year, perhaps not too surprising.
I think we should consider the beginner reading a thread like this as at the beginning they simply want a locomotive which will will stay on the track and pull a reasonable length of train and hopefully run slowly as well as at speed with equal ease. Our Livingston Starters Group started with a run by of engines. Each engine had to run very slowly and also at speed and they had to guess just what suspension system had been used. The results were interesting as they all found it very difficult to tell. An experienced eye could probably tell, but they all would have been very happy to have any of the locos running past in terms of behaviour.
In the West Scotland 4mm Group's "Build a loco" thread you will notice that I did not run a thread on using springy beams. Will's thread is very good and I had no wish to double up on it, but equally none of the starters were wanting to start with the system - all the calculations being a bit off putting, when they could get a loco working fairly quickly just following the system required for the kit. A couple of the LIvingston Starters group have taken on springy beams for their first attempt at a loco.
It is possible to have too perfect a glide, a Y9 without a distinctive waddle is not correct in my book, a K3 at speed without its rear end giving the crew "laldy" seems equally wrong. The real locomotive was a dynamic thing in terms of weight distribution as water in boiler and tanks varied as well as coal carried in the fire box and ash building up in the smoke box, but even empty they had mass which would keep them on track, so whatever system we use to keep all the wheels on the road the important thing is that it works and that all comes down to how well you make it work and that is more to do with your construction skills rather than any particular system. Me, I've built locos using all the systems, but have not developed a preference for any single system, rather choosing the system according to the locomotive to be built.
The test rig and track have now been discredited by some - was this a consequence of the results with certain engines I wonder? Derek is no longer with us, but I am sure that he would have had a strong opinion as to suspension/compensation, etc.
It struck me that the track is an even more crucial part of the system than I had fully realised as you did not want your locos coming across the extremes of track found in the test track. I am covering track making on the Livingston starters thread soon. Our teach in for the West Group has paid dividends as all the members have had a go at making points and laying track over the last year and are turning into competent builders in this area, so the same course will be covered with the Starters group over the next few months. I am not suggesting we go on to discussing track at this point by the way.