7mm Pencarrow - Cornish BR(S)

Discussion in 'Layout Progress' started by Pencarrow, 1 March 2016.

  1. simond

    simond Western Thunderer


    I've not seen the article, so setting myself up here, but my thoughts:

    Adhesion wise, it should make no difference. Classical friction theory indicates that the friction force is independent of the surface area of contact. All wheels driven, all weight on driving wheels, weight distribution irrelevant.

    In terms of electrical pick-up, I would expect the middle axles to provide a slightly higher resistance path than the more heavily loaded wheels, but my guess is that the difference will be negligible, and given that there will always be two wheels on each side with higher loadings, the difference will be undetectable too.

    The third aspect, that of "road holding", rather depends on your track. If you were modelling some of the more extreme bad track, where significant wheel deflections were required, it would probably help in ensuring the outer axles stay in good contact with the rails, up to a point, but on a reasonably constructed model of British main- or branch-line track, I doubt that it makes any difference at all.

    The only important difference that I can see is that the middle axle travel will be increased relative to the other axle in proportion to the lever arms. In the extreme bad track example, this may work against keeping the outer axle loaded, as the centre axle may run out of travel before the outer axle reaches the track (or the track reaches the wheel...). In this case, I think the only option would be independent springing, (with the springs operating prototypically in both directions) or what the prototype often did in such circumstances - equalisation between the spring hangers. But you're modelling a British branch line to 0FS standards, so this isn't really relevant.

    My conclusion is that I would tend to equalise... that is, attempt to have more or less equal weights on all wheels, but, I would not get worked up about small deviations from this. The point of the triangle is fixed in your loco as the centre of the front axle - and the base of the triangle is between the centre & rear axles. This is much better, IMO, than having one fixed axle and two rocking axles on a see-saw, as the rolling moment caused by a step is halved, and the support triangle is bigger. Ballasting the loco to have the CoG about one third of the length of the triangle from the base, and as low as possible, seems ideal to me.

    If it is convenient to offset the pivot rearwards, by all means do so, but I see no other advantage.

    Please see my thread in the other place for a Civil Engineering Question...

    Pencarrow likes this.
  2. simond

    simond Western Thunderer

    Picking up on Mickoo's point - I had the nodding donkey effect with my 1366 - it's fully sprung. You won't get it on a compensated loco, of course, but on a sprung loco, softer springs, or less preload on the centre axle does help. Initially, it was amusing, if unprototypical, to do an emergency stop with mine, and see the lifeguards hit the track...

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  3. Ian@StEnochs

    Ian@StEnochs Western Thunderer


    Interesting proposal but I haven't read the article as I am no longer a member. However I used to model in P4, before moving up to S7, and tried a number of differing suspension systems.

    By far the easiest that I found was the old Studiolith sprung horn blocks with the hollow adjusting screw, which contained the tiny spring, making it very easy to adjust the ride height. Chris Pendlenton was in our group and he was having considerable success with rubber pad suspension. We also tried compensation beams which also worked. The common thing to all was getting the axleboxes and guides to slide with minimal side play. As I recall all systems worked after the initial setting up period of adjustment and fiddling.

    With all systems the vertical movement of individual axles is much less than you think. If you need more than 1/2mm between the front and rear axles of your loco you need to sort your track! The position of the pivot on the compensation beam will have very little effect in axle movement but moving it too near one axle could have a detrimental effect on the weight on the other axle. In the ideal case each axle would carry the same weight but this is very difficult to achieve within a small model especially a 4-4-0.

    7mm scale models have considerable mass which makes springing so much easier to set up. Having said that I admit that it is very simple to set up compensation beams and I did use the Finnie type on the Duchess which I built for a colleague.

    However I still advocate full live springing with all axles carried on the springs for anything with more than 4 wheels. As Simon says about his 1366, my 0-4-0 Barclay pug went nose down alarmingly when it stopped until I disabled the springs on one side.

    Anyone who saw my Greenock Bogie running on the S7 test track at Telford would have seen all the wheels rise and fall as the loco took the rail joints in its stride. The body remained steady with, admittedly very slight, height variations at the board joints.

    I have not yet built a loco with the new Slaters horn guides but I have a set to try on my next 4-4-0. My intention is to set the guides with the axleboxes at 1/2mm above the axle centre line. Once the loco is built and weighted the springs will be adjusted by adding shim washers to the pin until the axles are on the centre line. The front driving axle will require more washers than others. On a bigger loco it might need stiffer springs. Fiddly, but once set up, effective.

    Incidentally I still have a few of the Studiolith horn blocks and hollow screws which I use on loco bogies. The springs are not stiff enough for 7mm so I add an extra soft spring outside the screw and adjust the ride with the screw!

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  4. mickoo

    mickoo Western Thunderer

    Probably not, your only reducing the pressure or mass on the middle wheel, not it's travel but it wouldn't do any harm if you opened it up on the top side by 0.5mm or so.

    Not so sure I agree with the X and tick in the diagrams, the pivot point in the middle is not wrong, just an alternative, as is the offset one, neither are wrong and neither are exactly right, just alternatives to location.

    I'd also probably not go 2/3rds and opt more for 3/5ths as the middle wheel is often the one with side play to negotiate curves. As such it may well have too little pressure in turnouts and have a tendency to ride up on check rails due to it's lateral play. It depends on how long the wheelbase is and the tightness of the turnout really.

    Overall it's a clever idea and worth merit, whether it's has any real term effect is down to individual perception I suppose.

    Regarding ride or quality of ride, we all strive to keep our models smooth and lumpy, jarring free. However I had a ride on our pilot engine (Class 08) and it was the most jaw jarring ride I'd ever had on an engine, it nodded up and down and wiggled left and right furiously on our internal tracks and the banging and jarring from rail joints made anything loose....and fixed! bang and clatter like it was possessed.

    Short wheel base engines with only primary suspension are a lively ride, how you replicate that convincingly in a model is very hard.

    Last edited: 24 September 2016
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  5. Pencarrow

    Pencarrow Western Thunderer

    The real 1366s in action appear to exhibit the same rock and roll characteristics Mick! Problem is that is can look wrong in model form.

    I'm thinking i may stick with the pivot point spaced equally for the beams.

    Chapter 15 of this doc: Scalefour Digest 41.0 'The principles of model locomotive suspension' gives a mind blowing choice of compensation options.
  6. Ian@StEnochs

    Ian@StEnochs Western Thunderer

    Comprehensive coverage of suspension principles there. I cannot really disagree with any of them. However it can become too complicated in model form. Remember you only need a wee bit of movement in your axles. The shorter the wheelbase the less you need.

    Most locos only have sprung axleboxes which in 7mm scale are easy to incorporate into the frames without compromising prototype appearance. Where do you put compensation beams when you have inside valve gear?

    On a typical 4-6-0 an allowance of around 1/2mm between the front and rear axles is plenty. If that cannot cope with your track, sort the track!

    simond and Pencarrow like this.
  7. Overseer

    Overseer Western Thunderer

    Keep it simple. Central pivot points work perfectly well, I have several small 0-6-0s in Scale7 and HO on 18.3mm gauge which have managed not to fall off over quite a number of days exhibition running. Moving the pivot point can be useful in some situations, such as increasing adhesion on the driving axle on a 2-2-2T (reminds me I should finish my Highland 2-2-2T which has been in a box for far too long).
  8. simond

    simond Western Thunderer

    Ah, different kettle of fish, if it has carrying wheels, then the adhesion factor does matter and then asymmetric compensation beams may give advantages

  9. Pencarrow

    Pencarrow Western Thunderer

    Today I've been making and finishing off replacement parts.

    Front sandboxes (original casting at the rear):

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  10. Pencarrow

    Pencarrow Western Thunderer

    Those pesky brackets under construction:

    Last edited: 25 September 2016
    Rob Pulham, Peter Cross, AJC and 3 others like this.
  11. Steph Dale

    Steph Dale Western Thunderer

    I wouldn't unload the centre axle by more than about 10%....

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  12. Pencarrow

    Pencarrow Western Thunderer

    And my interpretation of the operating levers fitted:

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  13. Pencarrow

    Pencarrow Western Thunderer

    And now fitted to the body:


  14. Threadmark: 1366 body enhancements

    Pencarrow Western Thunderer

    The observant may note that I've also remade the springs and hangers. The old castings were removed and the short hangers cut off. I then added some new hangers from Laurie Griffin, which were the closest I could find on his stand, and shortened the top links.

    This has raised them about 2-3mm and places them in the correct location in relation to the other footplate items. The footplate was then drilled in the centre of the spring and some brass rod added to simulate the link to the hornblock.

    You don't see much of the springs on the drivers side due to the levers, but a better view on the fireman's side:

  15. Pencarrow

    Pencarrow Western Thunderer

    Oh, and to answer an earlier question, yes I've moved the top knob position of the cabside handrails (and the tankside ones too):

  16. Peter Cross

    Peter Cross Western Thunderer

    Getting on well now. Will soon be in the paint shop.
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  17. Pencarrow

    Pencarrow Western Thunderer

    Hi Pete, yes it's slowly getting there - the body at least. Don't forget it's getting a completely new chassis, but my plan is to finish the body and at least get that in primer first.

    Nice collection of shots here: 1366 0-6-0PT. There's a few of the undergubbins front and rear that should come in useful.
    Peter Cross likes this.
  18. Scale7JB

    Scale7JB Western Thunderer

    Eh up! That's a huge improvement and those sander linkages look brilliant! Well done..

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  19. Pencarrow

    Pencarrow Western Thunderer

    Thanks JB, these are probably mk4 designs and the workbench is littered with failed attempts at trying to fabricate the various components. Thankfully I'm starting to build up a useful selection of brass rod and section, plus a big box of scrap etch.

    Seeing the finished articles in place is quite satisfying.

    Today I'll be adding the L shaped fillet between the cab and top of the firebox/tanks. This is complicated by the kit parts being too big, not fitting and the GWR love of curves. I shall be delving into the box of scrap etch again.

    I've also noticed there's a lump on the top of the firebox on the driver's side. No idea what this is for but it's absent from the kit and needs fitting first.
  20. Pencarrow

    Pencarrow Western Thunderer

    Current state of play: