Retrofitting CSBs: when do things get a bit too 'silly'?

[Russ Elliott]

Hello all. Long time, no see. (Health problems, essentially, and a consequent loss of mojo.)

Anyway, I've recently had a couple of enquiries from potential CSB-ers who are intent on replacing or altering chassis of existing locos. It's thrown up some issues I'm not sure I'm capable of answering.

The first enquiry seemed straightfoward initially. An LNWR 0-6-2T Coal Tank (LRM), bog-standard 7'3" + 8'3" coupled wheelbase. Lots of plots available, and ostensibly buckets of room fore and aft for fulcrum positioning, etc etc. However, in this particular case, there was a constraint in the back end of the frame, with a maximum spacing of 7mm from the rear axle to the rearmost fulcrum point. (No, I don't know why this constraint exists, either.) So I did a quick plot:

continuous-coal-tank01.gif

Let's be honest, it's a bit silly, isn't it? At the risk of harrumphing from Will Litchfield, those short 7mm distances are very silly in my opinion. (You all know I am fond of long-ish spans.) But here's the thing - in this case I didn't know how hard the radial truck would be sprung (if at all), or what that radial truck strength would be, and more crucially, I don't know where the actual CofG is. It was this lack of knowledge of the CofG position, and considering the affects of CofG shift as a result of drawbar pull, together with the 'errors' inherent in very short end-span lengths, that made me very reluctant to advise whether CSB-ing in this instance was a sensible course of action. Irrespective of the silliness of the plot, a bad CofG would bring no merit at all to such a CSB chassis.

The second enquiry was somewhat more complex. The loco is a conventional 0-6-0T. And ostensibly no problem with doing a conventional CSB plot for the replacement chassis. In this case however, the builder had measured the actual CofG of the built loco. That's the good news. The bad news was that the CofG is far to the rear (in excess of 10mm!) of where it should be if a normal plot were adopted. Not a great problem for the loco in its previous 3-point compensation incarnation, but the CofG outage was to such an extent that the only viable plot (via an Alan Turner sheet, and yes, with a absurdly short 5mm hornblock to rearmost fulcrum point) provided 52% of the weight on the rear axle, rising to a whopping 57% if fulcrum point width errors (+/- 0.5mm) were accounted for. By diving back and forth between Turner and Wyatt spreadsheets, the effects of the Turner axle weights could be seen on the axle deflections in the Wyatt sheet. It was not a pretty sight. My reaction has been to strongly advise whether some mass redistribution could be made to the existing body, to bring the resulting CofG into a more sensible position, but I think we all accept that there are pramatic contraints or impossibilities to this - motor positions, old lead weights glued in with Araldite that might be too difficult to shift etc etc.

Now, one could ask whether either of the above two types of problem amount to a significant hill of CSB beans. The truth is I don't know. I guess what I'm saying is that both of them put me into somewhat of a conundrum. Not a full-on moral quandary, but one that I felt was worth broaching here for others' views.

[Allan Goodwillie]

Hi Russ,
good to see you are back. I personally would only go down this route if the original chassis was giving very poor running, in which case I would build a new chassis, frankly.

When I started my local "Starters" group which has members from both the East and West Scotland groups we had an interesting afternoon where I had a pile of locomotives from small tanks to pacifics and 2-8-0s and we took each one and ran them both fast and slow and the gathered souls had to write down how they thought the locomotives performed and had to put down whether they thought the locos were compensated, sprung or had springy beams fitted.

The results were interesting in that the majority could not tell correctly what the system was underneath. I guess that if the system is made to work properly the resulting engine should ride well. I do have some reservations about what is best used for particular engines and try to pick the system which works best for certain wheel arrangements and other balancing difficulties, rather that expecting there to be a perfect system which will work better than all the rest in all circumstances.

There will be others who will treat this with almost religious zeal. I do have an engine running on a springy beam system built about 45 years ago - so nothing new, still has its Studiolith Mk1 wheels on.

There are fashions for all things including suspension systems, the important thing is to get what works and not worry about all that.

[Will L]

harrumph

Hi Russ nice to hear from you again, hope this means we be hearing more from again.

I've never been totally convinced that retro fitting CSB was a good idea unless you build a whole new chassis and have a reasonable amount of control over where the CofG end ups. That said it is the issues relating to the radial truck/CofG location on the first one that would concern me most. I agree that a 7mm gap between axle and fulcrum point is a bit small but I wouldn't describe it as silly, and I notice that this does seem to be quite a stable solution, which never seems to be the case if the outer fulcrums are to far from the axles. The only way to know for certain....

You haven't given details of the second one so its hard to comment on your findings. Without there being a constraint which you haven't indicated, I would have thought that as there is never just one viable solution, an end fulcrum so close to the axle was avoidable. Please note that, almost against my better judgement, my version of the Wyatt spreadsheet will now allow you to play the same games as with Alan's with weight distribution. While obviously a CofG that far from the chassis centre would impact the pulling performance it should still sit level and work. Unless the compensation was deliberately organised to avoid it, it is quite likely that the displaced CofG I would have had a similar effect on the weight distribution, but as nobody tells you to take notice, nobody does.

Probably not ideal subjects for somebodies first attempt at a CSB, but then my first try was an 2-8-0 which I'm sure some would suggest wast an ideal subject either. My 2-8-0 worked beautifully.

[billbedford]

Coal tank.

On the original, the rear frames are inset by doubling from the rear driving axlebox. The main frame end about six inches behind the rear of the tanks. The drawing in Talbot comes from a kit of some sort and shows this rear frame extension as a pony truck with the from end close to the rear axle. Presumably, then, this truck is going to determine the maximum position of the trailing CSB anchors from the axle.

Weight distribution

You could point out to the enquirer that he would get a better haulage capacity if he redistributed the weight to even up the weight on each axle, or he could have the same haulage with less weight if it was more evenly distributed. But then modellers tend to fetishise weight per sae and not think too much about the consequences. On the other hand the fact that he wants to rebuild the loco with CSB suggests that it is not a particular runner in its present form.

[Russ Elliott]

Thanks Bill, that could explain it - I'm not familiar with LRM's actual frame implementation. I think I would be inclined to add some lengthening pieces on before launching into a CSB application.

[Jol Wilkinson]

Russ,

the LRM frames are one piece. The photo below shows the standard kit spacer location and the rear pony truck. There is a radial truck from the later LNWR 4' 6" 2-4-2T kit which is available as a separate etched part and can be used instead of the pony truck. This might be easier to use with CSBs.

CT frames 4 RMW.jpg

I don't know which Talbot drawing or book to which Bill is referring but I doubt if any drawings Talbot might have used would be based upon a kit. The earlier4mm Coal Tank kits (Proscale, Ks', Alan Gibson or Jidenco) didn't use the pivoted frames extension concept to the best of my knowledge. That was a design feature used on the Brassmaster LNWR 4-4-0 and 4-6-0 kits.

Whether the 7mm kit designers would have done that I don't know, but with the relatively short wheelbase between the rear coupled and trailing axles I would think it is not necessary. The "swing" of the trailing axle is limited by the cab steps anyway.

Jol

[Russ Elliott]

Thanks, Jol. I'm even more confused now. There would seem to be no impediment to a decent CSB length at the rear, but those frames seem to be truncated at the front? (I don't have a decent side view of the loco.) Is the lack of frame at the front end because of the footplate aperture exposing the cylinders?

Maybe my correspondent has done something a bit strange. Or I've misunderstood the request.

[billbedford]

I don't know which Talbot drawing or book to which Bill is referring but I doubt if any drawings Talbot might have used would be based upon a kit. The earlier4mm Coal Tank kits (Proscale, Ks', Alan Gibson or Jidenco) didn't use the pivoted frames extension concept to the best of my knowledge.

An Illustrated History of LNWR Engines, Oxford Publishing, First edition 1984? Fig 31 Page 59

[Jol Wilkinson]

Russ,

the front of the LRM Coal Tank frames are "cosmetic" as they have to fit outside the cylinder chest/ashpan (part of the smokebox/boiler resin casting) to provide the visible space behind the buffer beam. One of the design compromises caused by mixed 4mm gauges.

That's why, as you correctly point out, the front end of the main frames stop short of the buffer beam.

Bill,

I thought that would be the case and will take a look at my copy when I go up the workshop later (this morning is devoted to a trip out to a local beauty spot for coffee and cake, while the weather holds)

Jol

[Russ Elliott]

Jol, on the front of the Coal Tank frame, what would be the maximum length available from the front axle to a fulcrum point - 11mm? 12mm?

[Jol Wilkinson]

Jol, on the front of the Coal Tank frame, what would be the maximum length available from the front axle to a fulcrum point - 11mm? 12mm?

Russ,

I've just measured the loco in the photo and reckon 10.5mm would be about the maximum from the axle c/l.

The front spacer gets in the way but could possibly be turned around so that it's vertical face is at the front. That would then provide the front fulcrum points.

Jol

[Will L]

Weight distribution

You could point out to the enquirer that he would get a better haulage capacity if he redistributed the weight to even up the weight on each axle, or he could have the same haulage with less weight if it was more evenly distributed. But then modellers tend to fetishise weight per sae and not think too much about the consequences. On the other hand the fact that he wants to rebuild the loco with CSB suggests that it is not a particular runner in its present form.

We still haven't got that LIKE button have we. Will do?

[Jol Wilkinson]

I don't know which Talbot drawing or book to which Bill is referring but I doubt if any drawings Talbot might have used would be based upon a kit. The earlier4mm Coal Tank kits (Proscale, Ks', Alan Gibson or Jidenco) didn't use the pivoted frames extension concept to the best of my knowledge.

An Illustrated History of LNWR Engines, Oxford Publishing, First edition 1984? Fig 31 Page 59

Bill,

Talbot credits that drawing to J P (Jack) Richards, whose collection of superb 7mm LNWR locos and stock are in the NRM at York.

Jack was a renowned modeller of the LNWR and scratch built most of his locos. I expect that he made the drawing as an aid to building his own model of a Coal Tank.

From contact with Ted Talbot when I was a LNWR Society Trustee, I consider it is very unlikely he would have used any "model" information for his books unless from someone like JPR, who had done the research first hand.

Jol