ONE OF MY PENDING PROJECTS is to build a CRT Stove R brake van which I picked up somewhere ages ago. This will be my first 6-wheel van and the first potential problem that comes to mind with this kit (and other similar kits), is the problem of side play on the centre axle.

As part of my research, I referred to the articles by Will Taylor (Building the LMS Stove R – Gazette May 2018) and Colin Hayward (Better Running for Six-Wheel Stock – Gazette Winter 20 06) . There are many other articles covering various ways of installing the wheels allowing movement of the centre axle, to ensure smooth running through curves and points.

One axle is fixed and the other two are mounted in a centrally pivoted bogie. I think this is quite an old solution and is rarely seen nowadays. The arrangement (as far as I know) is not prototypical and I think it looks very crude on the model. I’m not sure but this arrangement is possibly a throw-back to coarse scale models of old.

There is an example of this method in a recent GOG video: Coach Building with Robin Taylor. Robin is very happy with it!

The Cleminson system is, I believe, the generally accepted standard solution to the problem. However, except for a few examples, the arrangement was not prototypical. The system links the front and rear axles on pivots to the centre axle, which slides transversely. Movement of the centre axle results in movement in the opposite direction on the outer axles. It is the only method that tries to keep each axle at 90⁰ to the track.

One plus point with the Cleminson system is that the brakes are fitted onto the moving plates and therefore there is no complication with axles moving into fixed brake shoes. However, the whole underframe can look out of line on tight curves. I don’t know for certain, but this arrangement could also have been developed during the coarse scale period when tight curves were the norm?

I am not a lover of the Cleminson system: a dislike which led to this article. Although it allows for the traversing of almost any practical 7mm curve, my very short experience of this arrangement is on a colleague’s secondhand van. The units are clunky and prone to sticking resulting in derailments, although this could be due to errors by the person who built it. (See the photo below).

It is obvious even in this view that the wheels are substantially out of alignment with the axle boxes when the system is set for a curve, giving the model a toy like appearance underneath.

Fixed axles are exactly what they say. The outer axles are fixed, perhaps with some (vertical) compensation, and the centre axle, similarly fixed, is normally allowed to float sideways by either filing the axles short, setting the bearings further apart or mounting them in a suitable tube filed to length. There is no misalignment. This is the method that interests me most.

Utilising the fixed axle method, there is the problem of how much play to build into the wheelbase. When Colin Hayward addressed this, he guessed how much movement he needed. There are other articles online and in magazines, but no-one I believe has calculated precisely the actual side play required in more detail. I’m not suggesting that what follows is definitive … far from it, but it might focus minds by showing that play on the centre axle can be calculated quite easily and, by accurately knowing the distances required, a more realistic representation of the prototype can be modelled, particularly on larger radius curves.

It has always seemed logical to me that, with a relatively short wheelbase on larger radius curves, there cannot be much movement required of the centre axle. This becomes apparent if you draw a schematic for a particular example: in this case, the CRT Stove R on various curves. The total wheelbase is 21ft (147 mm). The curves on my layout and my garden railway are between 8ft (2438 mm) and 10ft (3048 mm).

The examples below are for 6ft, 8ft and 10ft curves (The drawings are reduced from actual size). The vertical lines are 147mm apart, as are the outer axles of the model. Visually, the gaps are quite small even for the 6ft radius curve. The figures on the drawings are the actual sizes of the gaps. (See the table below). Calculating this measurement is relatively straightforward as there is some help within the pages of British Railways Track – Design, Construction and Maintenance. The 1943 edition explains the relationship between curves and chords and describes, mathematically, the distance between them at the midpoint of the chord. The distance is called the versine.

For the example above, the radius of the curve is 10ft (3048 mm) and the chord which represents the wheelbase is 21ft (147mm). The formula in the book allows the calculation to be made. However, to simplify the process there is an excellent circle calculator on the web (http://www.1728.org/circsect.htm) which will do it for you. The table below gives the minimum versine (VMIN) for 3ft – 10ft radius curves, for both the manual formula calculation and the website circle calculator:

The distance at 10ft radius for the example is: 0.89/0.91 mm. (There is some very minor variance between the two methods). So 1mm is all the axle has to slide from its central position to get round a 10ft curve. Bearing in mind that PECO track and Slaters wheels have quite a lot of slop, and axles mounted under the chassis also have a small amount of movement, I believe that there is no need to provide side play at all. Certainly, such a small movement cannot justify the need to install the relatively complicated Cleminson arrangement.

The amount of play needed clearly increases as the track curve radius decreases until, by the time you get down to a 3ft curve, the versine is about 3mm which becomes a slightly different problem. However, if the brakes can be fitted in such a way that they do not foul the wheels when they are hard over, 3mm might be achievable using the fixed axle method.

The following (prototypical) formulae from the book take the flange depth into account, (I have allowed 2mm for Slaters wheels), and allows the user to tweak sizes and radii so that a range of distances can be calculated. Some clarification of the terms will be useful.

V = Versine, the distance between the arc (track radius) and chord (wheelbase) at the centre point of the arc.
(VMIN is the minimum clearance to which a small amount of extra play could be added for peace of mind).

V_MIN = Minimum clearance between wheel flange and track.

V_MAX = The total clearance which is VMIN + an arbitrary little bit extra.

T_W = Total length of wheelbase.

FL = Flange length (translated as flange depth).

R_A = Actual track radius.

R_MIN = Minimum track radius.

As an example, for a 10ft radius curve, the minimum play required on the axles would be 0·89 mm. This is achieved either by filing down the ends of the axle or mounting the axle within a tube which allows 0·89 mm play on both sides. In practice, 1mm should be easier to measure.

I don’t know what the average O Gauge layout curves are. PECO lefthand and righthand points are 6ft radius. The side play for this radius is 1.48 mm (or thereabouts). Some will have smaller curves and will have to set their wheels accordingly. It may be that the bogie and Cleminson Systems work better for those with smaller track radii.

Having said that, the natural slop with Slaters wheels and PECO track effectively extends the wheelbase and on a 10ft radius, I believe it is unlikely that any side play on the centre axle will be needed at all.

The next question is: Am I right? To test my theory, I made a crude but effective chassis mock-up with the axles 10ft 6in (73·5mm) apart, 21ft (147mm) of total wheelbase. The wheels are fitted into Jim McGeown suspension units. Jim makes units specifically for 6-wheel coaches, with a narrower centre axle mount so that adding side play is simplified. I deliberately left the centre axle unit off my chassis and used a normal one so that there would be no excessive sideways movement. Therefore, all the mounts are the same and are fixed. Here is what it looks like.

The first check is on a 10ft curve: Mission accomplished! The chassis rolls round without a problem. Followed by testing with an 8ft curve.Again, no problem.

Finally, a 6ft curve. Even with my slightly misaligned axles it went round a reverse curve without difficulty. The conclusion? Always depending on wheelbase length, excessive sideways movement on the centre axle for the 21ft example is not necessary for larger curves. In fact, no side play is needed at all for curves as small as 6ft, although it might be prudent to add a small amount as the bottom right hand wheel is not fully on the track.

No side play will almost guarantee that the brake gear (if fitted on the centre axle of the prototype), can be installed ‘normally’. I will build my model with all three axles fixed. I’ll probably fit Jim’s centre axle unit with a tube arrangement so that if there are any tight spots on the layout, I can make very minor adjustments to the tube length.