Will L wrote:The joy of Alan's spreadsheet is that it will allow you to specify a chassis with a CofG in any reasonable location, get it to sit level and and know what the weight distribution would be.
That's good news, Will, and when I can load Alan's spreadsheet properly, I'll want to do a few correlation checks, as you have done already, thanks.
Now, with Alan's spread sheet, if anybody badly wants a loco with the CofG well away from the chassis centre, we can now do a plot for that too.
Even though this thread title includes the words 'abstruse' and 'theory', let's try to stand back from this and question this stance from a working modeller's point of view.
Whilst it's great to have a range of spreadsheet CSB tools, I can't help wondering about the validity of promoting a constructional starting point that assumes a modeller has intentionally chosen to use a CofG well away from the chassis centre. I accept that there will be cases where constructed steam bodies might be a bit front or rear heavy, but the only obvious nasty examples that come to mind are 4-6-0s with heavy cast boilers, which are front heavy, or some 0-6-2Ts, which can be too heavy on the rear. Normally one would address this by adjusting the bogie or pony springing and support forces, a process which can be considered as quite separate from the mainframe (driving wheel) CSB. (See your previous posts on this matter, or 41.0, on applying a fairly simple principle of moments scenario.)
Let's take the example of an 0-4-2 tender loco with a CSB over the whole lot, and where the model loco body is perceived to be 'rear heavy'. The temptation of Alan's spreadsheet would (I think, not having played around with it yet!) be to intentionally slacken off the front drivers. Statically, or when running light engine, that won't matter much, but under the effect of drawbar pull, the resulting (effective) CofG will be moved back still further because of the effect of drawbar pull, and that will further unload the front driver and load up the rear driver. The situation is reversed for a 2-4-0, where the drawbar effect on CofG is likely to have a beneficial effect on the driver force for a 'front-overheavy' body, the downside being of course that the front carrying wheel could get a bit 'lively' (nodding donkey syndrome, not unknown on the prototype!). Such benefits or drawbacks are compounded, and may be exacerbated or diminished, in 0-4-2Ts and 2-4-0Ts, depending on their direction of travel. I do wonder whether all of these wildly different scenarios are likely to be thought about by a modeller who "badly wants a loco with the CofG well away from the chassis centre". The problem is with the word 'want' - are we talking about desire or necessity?
(This thorny CofG positioning and drawbar pull effect is why I personally would steer clear of a CSB that covers a non-driver. The simple springy equaliser is immune to all CofG movement, whether intentionally set in the 'wrong' place or by virtue of drawbar pull effects, and is immune to chassis horizontality. But let's leave aside my personal inclinations about the 2-axle case, and go back to the 3-driven-axle case, where the CSB is the killer technology.)
So let me ask a basic question relating to that difference between desire and necessity: why would anyone start building an 0-6-0T with the intention of putting or accepting a CofG in the 'wrong place'? (I can see the reason why the design CofG could/should be shifted to cope with drawbar pull on a tender 0-6-0 optimised for forward running.) Unlike those who can probably add symmetrical traction weighting to a near-finished loco with a 'conventional' CofG, a modeller choosing or accepting an enforced 'wrong place' CofG is unlikely to have any room for manoeuvre, because he probably won't be able to add symmetrical weight. He's burnt his boats. I think we need to reflect on this, sorry to use the jargon, 'audience-facing' question. I want to be able to sell Alan's spreadsheet (on the CLAG web page, for starters, with Alan's permission of course), but I'm kinda struggling to find the rationale, at least one that is predicated on good (best?) practice.
Please note that I am not knocking Alan's spreadsheet per se. On the contrary. It highlights the CofG issue, which is of course pertinent to any fulcrum plot. But I'm tending to think that Alan's spreadsheet might promote 'extreme CSB-ing', for want of a better phrase. Is this where we want to go?
P.S. For numbers context, typical drawbar pull CofG shift for a heavy train will be about 3mm to 4mm (on a conventional 14mm drawbar height). The CofG shift is proportional to the pull.