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Some thoughts on springing

31-bogie-2---Tysley-22-02-0No I haven’t put this in the wrong place and yes this is a bogie off a class 31 but it serves to show how the real thing goes about springing 3 axles. When I first wrote about building the chassis for my Jinty I didn’t go into too much detail on how it was sprung, mainly because I wanted to check that it worked properly before telling people how to do it (or leading them down the wrong path , possibly!). In truth I knew it would work as when my friend Simon built his fully spring class 31 (from a Bill Bedford kit) he sprung it using a similar principle. AS he’s an engineer and I’ve seen his 31 perform faultlessly on may occasions there was little to worry about other than I possibly didn’t get it or couldn’t do it!

Anyway after a few days of shuffling wagons around on Brettell Road I feel confident to tell you how it was done. I admit off the bat that CSB’s work and work well as I’ve seen many examples of them (Continuous Springy Beams). I also admit that all the maths, tables and discussion put me right off the idea from the start. It just seems so ‘faffy’ somehow. Sure they first appeared when there was an element of the finescale side of the hobby who likes to pretend they were actually Stephen Hawking and seemed to revel in making things look as difficult as possible but there was always the thought in the back of my mind that a lot of the clever theory, whilst fine on paper, didn’t actually translate to any effect in the real world. That and why don’t real 3 axle vehicles do it that way then? (Yes I know a Jinty isn’t sprung like a class 31 either!)

The principle of equalised springy beams is very simple. If you have a beam with a pivot in the middle the effects on either end will be the same. If you move the pivot to a 3rd of the way along the effects are more on one end than the other, By using 2 beams on 3 axles, with 2 of them acting on the center axle and the pivots towards the outer axles, the effects on all 3 should be about the same. It’s a mix of old-fashioned, very rigid compensation beams and springs to get a sprung result. I am sure that you can apply loads of complicated maths to this to refine the thinking further but it works for me, appeals to me KISS approach to things and all you need is 4 handrail knobs and 4 springs of 18 gauge guitar string. Nothing has to be pivoted and you can just change the gauge of the springs to adjust the effect. equalised-springing-drawing