6 Coupled Chassis and Gradients

[John Donnelly]

Part of the track on my new SGW layout has a gradient on it and it occurred to me that this would cause problems with a rigid 6 coupled chassis (think Hornby 08 with an Ultrascale conversion) as, regardless of how slight the gradient is, at some point one of the sets of wheels is going to be in mid air so am I right in thinking, and please excuse my naivety here as I'm a diesel man so have never built a steam loco chassis, that some form of springing is going to be a must with gradients?

John

[grovenor-2685]

The issue is not with the gradient as such but with the vertical curves at top and bottom. The radius used for these curves should be much larger than you use for horizontal curves.
For a real life example, on the DLR we have a minimum horizontal curve radius of 40m, the minimum vertical curve radius is 1000m.
As you say a rigid 6 wheel chassis cannot keep all the wheels on the track when negotiating a vertical curve, generally the effect on the concave curve at the bottom of the hill is benign but on the convex curves at the top can be a problem particularly if the track is curved as well! On such a curve the chassis will rock about the centre axle with a risk of the leading or trailing axles having a flange climb derailment. Six wheel chassis are always susceptible to this, even on minor track irregularities if they rock on the centre axle.
So I would always try to have the centre axle sprung on an otherwise rigid chassis. My Bachmann 08 and Jinty both have this arrangement and hold the track well. On the other hand the bogies of my Bachmann 47 rocked on the centre axle even on level track and derailed all over the place until it got a Penbits conversion.
Things get worse as the chassis gets longer, I have an old Triang Brittania that still has a rigid chassis and stays on the track OK, I would not expect a problem with an 08 so long as you keep your vertical curve radius reasonable.
Regards

[John Donnelly]

Thanks Keith, much appreciated. It's a fairly slight gradient on a straight piece of track with what will be a shallow vertical curve so I'll see what happens - as the layout is to be set in the late 50s/early 60s I'm going to have to get around to building some chassis eventually...

John

[Will L]

...am I right in thinking, and please excuse my naivety here as I'm a diesel man so have never built a steam loco chassis, that some form of springing is going to be a must with gradients?

Frankly if you are using a P4 wheel on track which is in anyway less than perfectly flat, then some form of suspension is desirable on any chassis much longer than a 4 wheel bogie. Twist is more insidious and occurs less predictably than a strait forward change in g radiant. A degrees of twist which is barely perceptible to the eye can cause you trouble and will not be improved by letting the middle axle float. A bit of extra flange, á la Goodall, will help but the purists will call that cheating. A simple suspension system is preferable and compensation works, but dare I suggest that a CSB system is mechanically as simple as any.

But then I would say that wouldn't I.

[JFS]

Frankly if you are using a P4 wheel on track which is in anyway less than perfectly flat, then some form of suspension is desirable

Hello John,

I would agree with what Will is saying here BUT since we are talking about an RTR conversion here, then it is worth mentioning that the simplest form of "suspension" is the "sloppy 'ole". I have not actually converted a Hornby loco (yet) but I have done a number of Bachmann locos just using wheel swops and (though they will never be as good as a properly engineered chassis) they don't derail. Having said that, Hornby chassis have bushed holes in the frames and that might not give enough "play" by comparison with the slotted holes which Bachmann usually employ.
It would be good to hear from someone who has actually had a go at this particular conversion - it is not exactly an uncommon loco after all!

Best wishes,

[John Donnelly]

Howard,

I have converted an 08 with Ultrascale wheels and additional pickups and, whilst the conversion was straightforward (it was the first P4 conversion I'd done), it how has some running problems that I really must get to the bottom of - going forwards it runs perfectly at a very slow crawl but, going backwards (cab first), there is something very wrong as the coupling rods appear to bind causing it to lurch on every full rotation of the wheels...

John

[JFS]

Howard,
.. there is something very wrong as the coupling rods appear to bind causing it to lurch on every full rotation of the wheels...

Hello John,

It sounds like one of the cranks has slipped on the axle causing the quartering to to be "out". If you sit it on some level track with the cranks on the far side at the top of their throw, the nearest ones will be on their horizontal dead centre and you will be able see which one is wrong. It will most likely be the driving axle which is a bit "bad news" because if it has slipped once it might do so again.

Good luck and let us know how it goes!

Best wishes,

[John Donnelly]

Thanks Howard, I really do need to get it sorted out so at least now I know what to look for...

John