So - onto the business of fitting Continuous Springy Beam (CSB) to a Parallel Boiler Royal Scot in 7mm scale. But before start, we ought to run over a few basics. That's all I'm qualified to do, being neither an engineer nor teacher, but it could be useful to explain a little of how I approach the subject. For in-depth detail you can't go wrong on the CLAG site at
Continuous springy beams
Some useful definitions/explanations (in Andrew-speak!):
CSB: A picture paints a thousand words...
The 'beam' itself would typically be made from steel guitar string or, more likely for the size and mass of larger 7mm models, thin piano or harpsichord wire. Steel would normally be used due to the predictability of its elastic modulus (whatever that means!) Frame fulcrum points can be provided in a number of ways - either designed into the frame from the outset or, more commonly, short handrail knobs from suppliers such as Markits are used. It might be useful to know that their short knobs measure 2mm from the hole centreline to the 'shoulder', meaning a consistent distance of CSB from the frame can be maintained. It's quite important to keep the CSB as straight as possible, longitudinally. Whatever is done to provide the fulcrum point on the axle blocks needs to account for this. In theory, its possible to simply lay the CSB over the top of the axles or a suitable rounded collar on the axle blocks, but this will limit exactly where
vertically the CSB must run. Most modellers create some kind of fixing which extends vertically from the block by a given distance - that's what I've done for this trial (see my penultimate photo above). It's worth noting that the CSB could run
under the block too, and this can often be useful in keeping it out of sight on more 'open top' frames.
Deflection: As the diagram above shows, once the mass of the loco (NB not the
unsprung mass, so don't include the wheels, axles, gearbox and motor) is resting on its wheels, the CSB will deflect between each frame fulcrum point. In the 4mm world, a deflection of 0.5mm has been settled on. I've not been able to find an equivalent convention for 7mm models. A balance needs to be struck: there's no absolute right or wrong figure and, in theory, one could have the
static deflection at any figure. The likely masses of 7mm models, the capacity of the handrail knob with regard to wire diameter, the limits of vertical movement of the axle blocks within the guides and the desirability of having suspension that is neither too soft or too hard suggest to me that a static deflection of 1.5mm is probably about right. This will allow me to use wire from 22-28 thou to maintain correct ride height from 850g to 2225g. As I anticipate adding additional mass to the model for traction purposes, I expect to use wires at the thicker end of the spectrum so it shouldn't end up too sloppy. To be honest, I just don't know, so this'll be my chance to experiment.
Why is deciding on a static deflection important? I have created fixings for the axle blocks that measure 7.5mm from axle c/l to its CSB mounting hole c/l. If I want to use a 1.5mm deflection I need to mount my frame fulcrums at 6mm above axle c/l (7.5-1.5mm). It's worth noting that if you are using CSB
below the axle blocks, you'd
add the deflection.
It's also important to note that we are not trying to mimic prototype spring deflection limits - static deflection is purely what puts the loco at the correct ride height. The actual amount by which the CSB can act as suspension is dependent on the physical limits of axle block movement.