Well, having performed all the scaling, sourced your parts, etc etc. Why do you need to sit down and think about the design of your loco? Surely the prototype has done that all for you??? The simple fact is this -you are building a model -it will behave differently to the prototype. In my humble opinion it is better to have a well behaved model than a strictly meticulous museum piece that is a complete and utter -to run...

Very little thought is given to the Centre of Gravity of a locomotive -or rolling stock for that matter. Ideally they should all sit between the rails and be as low as possible -however whan you design an articulated model this can be somewhat fraught....

Let us take the example of the Price 16 wheeler -since this is the one where 'Design" really HAD to be thought out to produce a model -rather than a shelf piece.

This is the way that A.G.Price designed it, there are two bogies pivoted from a subchassis that is itself pivoted from the main chassis. So why did I feel it needed redesigning to work? Well on my layout there are curves of 2 feet 6inches (I use SM32 track). At one point there is a genuine piece of Derbyshire Millstone Grit that I needed to avoid. At the scale length of 54cm it would smash into the lump every time. Thus something had to be done. ..

The solution was to move the main pivot in line with the subchassis pivots. This meant that the main chassis had less of an overhang when it cornered. HOWEVER it did mean that the Centre of Gravity COG moved farther from the centre of the rail and there was now not an even load on all 16 wheels! The load problem was cured by having a Top Hat slider above the arms providing enough down force to keep the wheels in contact with the rails -and thus providing traction.

The way you support you locomotive on its chassis can depend on what type of locomotive it is. In the first example I have shown 'Pendulum suspension" which is very reliable and stable. The Battery sits low and well below the pivot points.The very bad part of this is that the system takes up quite a lot space within your model.

Unfortunately there are some locomotive designs where you cannot do this! The Meyer type tank locomotive is a classic example. Here the Pendulum pivot points would show through the cab. The solution I use is to have a flexible set of grommets blanking plates (these come from Maplins) thus everything moves by stretching rubber.

The problem now is that the COG sits higher thus the top of the model needs to be lightened or the bogies need to heve some weight added to them. The COG of any model should sit between to two pivots because this will improve the tractive effort of your model and also improve its cornering! The easiet way is to use a piece of dowel and see where the model balances.

Believe it or not the most oft encounted problem with designing an articulated locomotive model is : you have more than one bogie! This may seem an obvious point -but will your new loco be capable of cornering on your layout? The drawing above shows the exact limit of articulated length of a curve. The COG of the loco exactly sits on the inner rail. In a twin bogie loco like the one above this is an easy one to spot -but what happens when you have THREE bogies?

The next problem relates to the spacing between the axles. I have found that AS A LIMIT on 32mm track the maximum distance between two axles is 90mm. This will enable it to take the SM32 setcurve radius of 2 feet 6 inches.

Now we come to the design problems of a six wheel bogie.... The four wheel bogie sits comfortably on the curve while the six starts to ride up on the inner centre axle on the inner rail. On the outer wheels on the outer axles they are lifting as well. There is going to be a de-railment soon!

There are three solutions to this problem.

1. 1.You make the central axle and wheels move from side to side
   

2. 2.You make the central axle with flangeless wheels
   

3. 3.You make the outer axles pivot
   

Each of these has its pluses and minuses.

The shot below is an example of the first solution -except in this case ALL axles and wheels move from side to side!

Each axle has its own motor and worm+spur gear arrangement. the main problem I did not forsee in this model was that the tractive effort of the motors was so high that I suffered from total wheel slip if the track was at all greasy or wet.

The next shot is an example of the second solution. This shows the underside of the Hagans locomotive. The central axle of the 6 wheel chassis can be seen to be flangeless. This means that the outer two axles do all the steering for your loco and thus the radius of curvature that they can take is dependant on this. IP Eng themselves say that 3 feet radius would be considered the minimum for this.

The third solution is perhaps the most simplest of ideas but the hardest to practice. Only two types of articulation have successfully solved the problem of pivoting the axles. They are the Klose and the Klein Lindener methods. In the first the linkage changes length. In the second the linkage remains the same length -but power is transmitted to the outer pivoting axles via a central constant velocity joint to the wheels which have a tube axle. If this sounds mechanically very difficult -well it is!!!

As you can see from the above photo -the Klose linkage looks alarming. But, I have tested it at speeds up to 15cm per second and it rattles -but that is all. The linkage will pivot the axles to take the std PECO SM32 setcurve -even an S bend made of it.

It is not something that I would ever expect to do again, not because it was hard -I hope that the 'crib sheet' in Klose.html is easy to follow, but because the artist in me says "You have done it -why repeat it?"