CSB Help

[mtaylor]

I have recently returned to the hobby after a considerable lay-off and have been catching up on developments with a view to converting from EM to Scalefour.

After reading the very informative articles about the CSB system, on both the Forum (particularly those from Will L) and the CLAG website I am tempted to give it a go. I was considering starting with my C12 but after reading Will's articles I think an 0-6-0 is a better place to start.

I have been going through the construction process in my mind before I start but there are two matters which keep coming back to me which I would like get clarified before starting.

Q1:- How is the weight of the axle hung gearbox/motor taken into account. My understanding of the CSB calculations for an 0-6-0 is that the centre axle should have slightly less load to avoid porpoising. If the centre axle is used as the drive axle, doesn't the unsprung weight of the motor/gearbox increase the load on that axle and possibly lead to porpoising?

Q2:- Related to the above, which is the best axle to use as the drive axle.

My conclusion is that as there is no mention of the above in any article that I have read, the answer to both questions is that it doesn't matter but I would like someone to put my mind at rest before I start.

Mike Taylor

[grovenor-2685]

A1. The weight of the motor gearbox which is carried directly on the driven axle is not carried by the springs so does not have any effect at all on porpoising. If this is on the middle axle it will compensate for the springs being set to carry a bit less load on the middle axle and the end result should be close to equal axleloads.

A2. Symnetry is usually good, for an 0-6-0 centre axle is best, but other considerations of space, appearance etc may overule that, drive on one of the end axles still works perfectly well.

Regards
Keith

[Will L]

Keith's advice is good.

It can be easier to get a 0-6-0 chassis to run if you drive the middle axle and that may be a worth while consideration on your first chassis. That said I routinely drive the axle under the firebox as that way I can hide the motor/gearbox in the firebox and boiler. Works for me.

[Russ Elliott]

Mike Taylor contacted me offline with some specifics of his particular case, which is a J50 (7'3" + 9'0") of 300g total weight, of which 25g will be unsprung motor/gearbox weight on the centre axle. The following diagram is the 'approximately equal sprung' one as appearing on the CLAG page, but also gives (in brackets) the increased values for the centre axle span reflecting the sprung axle loadings of 100g + 75g + 100g, i.e. 25g less sprung weight on the centre axle. This then makes them all 100g when the 25g extra unsprung weight for the motor/gearbox is added on the centre axle. In the suggested plot, I've kept the outermost fulcrum points the same for the sake of comparison. Btw, as a chunky J50, the beam is a correspondingly chunky 0.014" for this plot.

Where a plotter wishes to finesse the CSB situation such that the sprung and unsprung axle weights are equalised, as in this case, the Wyatt-engine spreadsheets are probably the easier route to an answer. It can be done on the Alan Turner spreadsheet, but is rather more long-winded. (In this particular expanded centre axle span case, the sprung axle weights are approx 36% + 28% + 36%.)

I've taken the liberty of posting this here because the exercise has I hope been a useful illustration, and I was quite surprised at how much longer the centre axle span needed to be for such an 'unsprung extra'.

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