James
I'm happy with your design objectives, and yes, I've used sprung plunger pickups between bogie and loco myself in the past. Though the prime objective then was as a way to pick up current from the bogie without lots of training wires. I'm not sure they do anything spring wise.
The issue I have is back to a bit of basic sprung chassis philosophy. The real problem is knowing just how strong the spring over each wheel is (the spring rate), what that means in terms of how much of the locos weight is transmitted through each wheel, and hence what the overall weight distribution will be. I know of no reliable way to predict or measure this with individual springs over each wheel. It turns out this is not a significant modelling problem with four wheeled wagons, or bogies, but is increasingly problematic with 3 or more axles in a fixed chassis. There are plenty enough examples of people who have successfully sprung chassis like this, but as far as I can see this involves a large dollop of trial and error, and a significant amount of uncertainty as to whether the optimum even weight distribution has been achieved. Also, if you get it badly wrong, real running problems can happen.
The beauty of CSBs is that the way the single sprung wire is configured on each side means that, relative to each other, the spring rate over each wheel is set at the design stage. This is what the dreaded spread sheet does. You are guaranteed that the weight distribution will be as designed. The spread sheet will also tell you what size wire, and hence the absolute spring rate, you need for a given overall loco weight.
This happy state of affairs appears lost just a soon as you start supporting some of your locos weight on wheels which aren't supported on the CSB wire. A significant failing given how many locos have carrying wheels, particularly where those carrying wheels are important to the stability and running qualities of the loco. I'm thinking particularly of 4-4-0s but there are others. I did eventually sort out in my own mind
how to combine CSBs with a single load bearing bogie. As explained in this posting, I eventually realised that a CSB chassis, left to its own devices, will not sit level unless the weight being transmitted through the springs is as originally designed. Fortunately this remains true if one end of the chassis is sitting on a separate bogie. Meaning that, if we adjust the single bogie support (or the position of the locos CofG) so the chassis sits level under the loco weight, we have got the even weight distribution we designed for. Further, by comparing the actual loco CofG and where the CSB calculation said the CofG should be over the driving wheels, we can calculate how the loco weight is divided between bogie and driving wheels. This gave me back predictability on weight distribution.
I have no such solution for a chassis with load bearing carrying wheels at both ends. For this reason, I will always suggest that we don't try carrying body weight on most pony trucks and bogies where their use isn't dictated by long overhangs and/or a short driving wheelbase. This could bring on a whole discussion about why locos had non driven carrying wheels, what they really did for the running qualities of a loco and whether that has any relevance to a model. But this post is too long already.
Let’s just say that, In my view, the Fell loco’s long central rigid wheelbase means that, in our model world, it will run very satisfactorily without load bearing bogies. With a load bearing bogie at both ends of a CSB chassis such as yours, my fear would be that a level chassis was now merely an artefact of the support from the bogies. That it would be impossible to know what percentage of the weight was being carried buy what, and potentially that the centre of load being carried by the driving wheels was not where the CSB calculation said it should be. One of my learning points from putting CSBs into an eight coupled chassis is that it was relatively sensitive to errors in CogG placement.
You can read that story here..
I may be being oversensitive on this last point, which comes from being interested in knowing rather than guessing exactly what is going on. The fact that users who have adopted CSB chassis find then relatively trouble free suggest the method is pretty robust, and not, as a whole, subject to problems due to a failure to follow the rules to the second pace of decimals.
davebradwell wrote:...Will's content with the unsprung bogie is about equal to my hatred of it so it's up to you.
That’s fair comment. A lot depends on what your modelling objectives are. If you are trying to follow prototype practice as closely as possible, then Dave is on the money. However, in getting a model to run well you do eventually have to depart from the way the real thing did it.
Thinking about the Fell, if you decide to follow Dave's prescription you end up with a long chassis which is, in practice, rigid from the outer wheels on one bogie to the one at the other end. Dave and I would agree that theory says you should be able to deal with that by springing the axle boxes. Just remember you’re going to need a lot of quite softly sprung available axle box movement on those bogies or it will be markedly sensitive to track irregularities.
All you need to get both bogies carrying equal weight is to have the spring deflections the same.
My fair dollop of trial and error. We are dealing in quite small (second decimal place) differences in spring deflection here.
The actual weight carried is unlikely to matter much....within reason.
I would agree with that. Within reason.
In summary I think what ever way you go the Fell with load carrying bogies is going to be in interesting problem which may spring surprises. Just imagine trying to build it compensated! Without them it will be a strait forward and reliable CSB chassis.
