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The Bachmann Compound – part 1

I found a relatively cheap Bachmann Compound recently and thoughts have turned to what to do with it.

Lets start with a little disclaimer. Alan Gibson supplies a set of wheels to convert this loco to P4 and I would have every confidence that just swapping the wheels would get a p4 steam loco up and running pretty quickly. After all a 4-4-0 has got to be about the best case scenario you could really ask for. I didn’t try it myself but we’ve had a wheel swapped GWR Grange (I think) running on Moor Street for years now.

Being relative new to RTR steam locos, this is actually my first RTR tender loco I’ve had since i was a kid, there’s always 2 areas that stand out to me as looking a little weak on pretty much all RTR steam locos. No, not the wheels although big, in your face, wheels do perhaps yield the greatest benefit of swapping to p4 visually. The areas I am talking about are bogies and tenders. More specifically in the case of the latter, tender underframes. They just always seem so, for want of a better description, flat!

The bogie

So to the bogie. There was nothing about the supplied RTR one that i wanted to keep so its a straight swap with a Comet example. As supplied they can be built with central springing for side control but no springing on the axles. Setting some simple springs up however couldn’t be easier.

The loco chassis

To the loco. I decided I wanted to use some of the Comet chassis bits but not exactly as intended. So the first process was deciding what of the RTR offering I wanted to keep and what I wanted to replace.

I wanted to use the sideframes in a sort of Brassmasters easychas inspired way and keep the original Bachmann drive. Initially I thought the crosshead was just an RTR bodge but they do actually look like that. So that and the cylinders were keepers. I also liked the brake gear so that stayed.

The Comet chassis is not designed for this model and is too long. The wheelbase between the driving wheels and hence the coupling rods are also too long. Comet do specify this is the case on their website. The Bachmann frames are actually the right width at the front of the loco but narrow from the cylinders back to accommodate the 00 wheels. The cylinders look, from underneath that they might fit on little pegs coming down from the footplate. They don’t, they slot sideways into the chassis. Its best to pop them off and keep them safe.

I decided to split the chassis behind the forward step to loose some of its extra length. The front part being a relatively easy fit. The rear part needed some trial and error to cut away little sections to get it to fit. The Bachmann model is driven on the front driver ( it looks like the chassis was designed for gears but to both axles but it doesn’t have them), so the Comet chassis was carefully titivated so that the rear axles position matched. I wasn’t too worried about the front driver as I has decided to keep it rigid.

By leaving the RTR style bearings off the rear driving axle you get a little room for vertical movement. A Brassmasters sprung bearing was modified with a bit of tube (the Bachmann and hence Alan Gibson axles are an odd size). The frames were glued in place using 60 thou plasticard to space them out to something more prototypical. The springs are part of the RTR keeper plate so they are too to far back but I decided to leave them as is.

The brake gear needs a bit of modification to fit over the new frames and it was here that I hit a little unexpected snag. Bachmann use bigger wheels than scale. I wonder if this is because its a development of the national railway museum model which being an earlier example had bigger wheels? Anyway the effect of this is the brake gear sits too low and would likely hit the rails when crossing pointwork. The solution is to take a mm out of the top of the keeper plate so that everything moves up a little.

Valve gear

Lets be honest RTR valve gear is generally a bit weird. Its often both too big and to thin at the same time. The Bachmann coupling rods are about scale height (ignoring the bosses which are huge!!) but being only 1 piece of metal aren’t thick enough. So these were discarded and the Comet ones used in their place. Suitability shortened by 2mm.

The connecting rods as supplied are quite good though. Much more meaty and they feature the big square bosses that the Comet ones don’t, so hybrid valve gear it is then! The Bachmann crank pins are 2mm wide (really!) so a bit of tube was soldered in to make them fit the Gibson crank pins. While I was at it I made another 2 collars for the trailing driver a the coupling rods on a compound are outside of the connecting rods.

On to the tender

Body great but underframe – ugh!

Luckily Lanarkshire models do a replacement chassis kit for a Fowler tender. This was assembled as per the instructions. For the outer frames I was kindly supplied a spare etch by Brassmasters and mated this with some Comet springs and axleboxes. I decided to keep the Bachmann steps as they are moulded as part of the tender body.

As is often the case with this sort of stuff, the most pleasing view is the one you wont ever see!

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