jon price wrote:Will L wrote:In the spirit of processes improvmnet, I would be interested in what it was you found more complex than expected?
the mechanics has so far been easy. High Level hornblocks and CSB jig, ...
That is what I would have hoped for. Given you can produce a compensated chassis, and that would bother some, a CSB chassis shouldn't be a problem.
...but the number of variables is high (point location, weight of loco, weight distribution).
May I suggest that this is more lack of familiarity than any thing else. Once you become familiar with the process it seems to drop into place and makes you wonder why people seem to find it complicated. I'm fairly sure Flexi Chassis produces similar concerns in people who have only ever built rigid chassis.
Establishng the weight requires a full build of the body and in many cases it is recommended that the chassis comes first.
That's because from the chassis design point of view, the important thing about loco weight is not how much of it there is, but how that weight is balanced . I.e. where is the loco's Center of Gravity, so you make an assumption as to where the CofG will be, and ballast the built model to suit. Adjusting for the true weight of the model (so the buffer height is right) is the one thing we can do on the completed CSB fitted model. Traditionally we have always advised putting the CofG where it ensures an equal distribution of weight on the driving wheels, because that gives the best haulage performance and why would you want to design a chassis with compromised performance?
Then each point location is variable (so four for an 0-6-0 or five for an 0-8-0). without knowing the importance or significance of the relative positions in relation to the axles it is in effect a combination lock with four or five single or double digit tumblers. In the end I called for help on here and other people (yourself included I think) offered solutions, but I still don't know how to guage the likely best solutions for a given wheelbase, which means either a call for help for each loco or a long period of guessing numbers and trying them in the spread sheet..
It is certainly true that just guessing results and trying them isn't a productive way of designing a CSB chassis. The existence of spreadsheets that can do the sums for you ought to be a significant aid, but being mathematical models they do tend to give worryingly precise answers. I would like to reassure you that much of this precision is illusory as CSB would be useless if you could not build them successfully within the normal modelling tolerances ( typically to the nearest half a milimeter).
You are also right that to calculate the fulcrum points successfully you do need to have some understanding of the game you are trying to play, however a little knowledge/experience should make it rather less daunting. I think
this post from my back catalogue gives a fair understanding of what you are trying to achieve. For the totally spreedsheet phobic there is a paper and pencil method of deriving an answer (reproduced below), but ultimately the spreedsheets do a better job. Of the three
available from the CALG website, the original Rodger Wyatt version requires that you have a good understanding of what will work and what you are trying to achieve, and for this reason I would recommend new users should avoid that one. The other two provide a lot more help. Of these Alan's has by far the simplest interface so is probably the best for the inexperienced user, while mine certainly looks more complicated but it does come with quite extensive instructions on how to use it (look under the Notes tab). It also implements a version of the paper an pencil method given below to give you a working solution which you can then adjust to suit your loco. Once you get your eye in both Mine and Alan's versions will allow you vary the location of the CofG.
One further point I wanted to include above, but it overcomplicated what I was trying to say.
In designing a CSB chassis we are making you aware that the weight distribution is an issue which you need to take into account. Similar considerations also affect compensated chassis as do stability issues which are unknown on a CSB chassis, but these often go unconsidered, so that avoiding a compensated chassis that disappoints is to some degree a question of luck.
The Paper and Pencil methodThis is illustrated by this diagram
Simply put, this deals in what for the lack of a better name I will call a standard fulcrum distance (SFD). This is what you get if you divide the total wheel base by twice the number of fulcrums that will occur within it. I.e 2 for 4 axles, 4 for 3 axles, 6 for 4 and 8 for 5. The fulcrums within the wheel base are then set symmetrically at 1, 3, 5 etc SFD. The outer two fulcrums are set at 0.57*SFD which will make the centre wheel suspension softer than the outer two. Actual fulcrum points can be rounded to the nearest 0.5mm unless you feel confident that you have the where-with-all to mark them up more accurately than that!
This compromise method and the 0.57 constant I have derived from long hours of thumbing in different configurations and seeing what happens. For best results the constant needs to be varied from between 0.6 and 0.55 depending on the exact dimensions of the chassis. My revised spread sheet contains an implementation of this method, that calculates the fulcrum points for you, allows you to vary the constant to see what gives the best results, and gives guidance as to what the best result might look like. But 0.57 will produce a workable result. I find it ironic that, after all I’ve said, here I am playing with a figure accurate to two decimal places!
I’m sure it is possible to argue that as a method, it is unlikely to produce the optimum configuration for a given chassis, particularly if it isn’t symmetrical. I may be moving away from the P4 "get it all right" way here, but my position is that it gets close enough to produce a satisfying result.
