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Locomotives - Mike Ainsworth

model loco showing chassis and superstructure

"If there is one thing that, more than any other, puts people off P4, it's the prospect of producing a working locomotive. Most modellers immediately imagine that something like the photo at the top of this page is required. They start to worry about springs and compensation, wheel quartering and universal joints, flexible coupling rods and floating gearboxes - and think that nothing in P4 will work without them. We aim to show that it isn't true: a P4 loco does not have to involve more technology than it took to put a man on the moon".

The most straightforward way into P4 locomotives is through a simple ready-to-run (RTR) conversion. The operative word here is 'simple'; there are some RTR locos that can be converted very quickly and very simply. Others involve varying amounts of work and ingenuity and there are some where just about the only realistic option is to marry the body with a kit or scratchbuilt chassis.

So where to start?

The general view is that those modelling diesels and electrics are the lucky ones, because many proprietary diesels can be converted by simply removing a keeper plate, lifting out the OO wheels and dropping a set of ready made P4 wheels into their place. Replacement wheels, to allow you to do just this, are available from Alan Gibson and Ultrascale. Both sell the wheels in sets appropriate to the RTR loco you want to convert and the Ultrascale ones come complete with the wheels and gears already fixed to their axles.

Photo of diesel loco With no rods, quartering or valve gear to worry about, this is about as close to the instant P4 loco as we are ever likely to get; it is quite possible to convert the Bachmann Class 25, for example, in about half an hour flat. The Heljan 'Hymek' is another good choice for the newcomer to P4. The one in the photo (right) as well as a set of Ultrascale wheels, has had new screw couplings, buffer beam pipework and air tanks but is otherwise pretty much out of the box.

Steam locos are slightly more complicated. Even the simplest 0-6-0 will have coupling rods and so, as well as replacing the wheels, we need to think about quartering them and about how to deal with the rods themselves. We can cheat to some extent by choosing a loco for which a complete set of wheels is available in the same way as for diesels. A good example is the Dapol Pug for which Ultrascale sell a set of wheels which come ready quartered and, again, with the gearset already mounted on the axle. So here we have a steam outline prototype that can be converted to P4 in not much more time than it would take to deal with an RTR diesel.

Photo of steam loco underside The photo shows you just about everything you need to know. The crankpins unscrew, so that the coupling and connecting rods can be removed. The keeper plate is then released by undoing a screw at the front and unclipping the plate at the rear. That allows the OO wheels to be dropped out. The pickups will need to be bent out a little to ensure that they contact the more widely spaced P4 wheels which can then be dropped in. Replace the keeper plate, fix the rods back on to the crankpins and there you have it - pretty well instant steam outline P4.

With other RTR steam locos, we are beginning to move a bit further up the food chain. Most inside cylinder 0-6-0s, though, will take little more effort than the Pug. A set of replacement wheels is all you will need in most cases.

Make sure, though, that you check the axle diameter. Most RTR manufacturers use axles 3mm in diameter rather than the 1/8 inch ones which have become standard among the wheel suppliers. It isn't a problem because the manufacturers who produce replacement P4 wheel sets supply them with axles in both sizes. You simply need to remember to specify which size you need when you are ordering. Remember, too, that you will probably need a set of crankpins to suit the make of wheel you are using - it is unlikely that you will be able to reuse the ones fitted to your RTR loco.

This brings us on the evolutionary scale to the more complicated RTR locos - which for these purposes means those with Interesting Complications like outside valve gear, eight coupled chassis and the like. This Bachmann N Class provides a good example.

Photo model locomotive
This Bachmann N Class gives a good idea of the results that can be achieved by skilled hands, in this case those of Philip Hall.

It's a bit beyond the scope of this supplement to go into details of a conversion like this. But the same principles apply - a set of replacement wheels, maybe replacement coupling rods and re-use of all or most of the original chassis and valve gear. If you're tempted by something of this sort, I can do no better than refer you to the articles on this sort of P4 conversions that have appeared in the modelling press.

So here we are. Two thirds of the way through this treatise on P4 locos and neither compensation nor springing has yet had a mention. Does this mean that, in practice, neither is necessary? Actually, no. It is true that, with half-decent track, springing or compensation is a bit of a luxury on diesels and electrics. As for steam outline models, many of the more recent RTR offerings have some degree of springing built in. This is usually restricted to one axle only, but a straightforward conversion will preserve it and the extra bit of flexibility it provides is well worth having. But for P4 to work to its full potential, compensation or springing is, if not essential, then certainly highly desirable.

The critical benefit of a compensated or sprung loco over a rigid one is that it will keep all of its wheels on the track. That may sound an obvious point but it is not a simple question of avoiding derailments - important though that is. The two major additional benefits are that electrical contact is much more reliable and haulage capacity is increased.

The principle of springing is obvious enough. Each wheel is pressed to the rail by either a coil or leaf spring so the chassis can take track irregularities in its stride. Compensation is slightly different. In this case, there is usually one fixed axle and - in the case of an 0-6-0 - two which float. The floating is achieved by housing the axles in hornblocks which can move up and down but not forwards or backwards. The degree of movement is controlled by a compensating beam which usually bears on the centre point of the floating axle.

Photo of compensated chassis clambering
over a lolly stick This photo by Eddie Bourne demonstrates the principle. It shows a compensated chassis clambering over a lolly stick which represents a track irregularity. (If your track does have lumps in it on this scale, you may need to revisit your track building skills, but that's another story). As a wheel hits the obstruction, it lifts to climb over it. The axle, meanwhile, tilts to keep the other wheel on the track.

The other axles remain flat and level and, crucially, still in contact with the rail. The trick is to ensure that the loco is always suspended on three fixed points - in this case, the two ends of the fixed axle and the mid-point of the compensating beam. A compensated chassis is like a three-legged stool, all of whose legs will always be in contact with the ground, no matter how uneven the ground may be. A rigid chassis, on the other hand, is like a four-legged stool; unless the ground is absolutely flat, one or two legs will always be clear of the ground and the stool will wobble.

Transfer this principle to your loco. If wheels lose contact with the rails you might, if you are lucky, get away without a derailment. Even if you do, though, you will be supplying electricity to the motor, and getting traction from, at most, four wheels out of six. It may well be fewer still. Look carefully at the photo of the compensated chassis, though, and you will see that all six wheels are still making contact. Springing achieves the same result and also gives an undeniably silky ride but at the expense of a sometimes more tricky setting up process.

A long treatise on compensation and springing, and on how the three-legged stool compensation principle can be applied to far more complicated locos - like the A4 or 9F which you are itching to build - is more than we can accommodate here. But if your appetite has been whetted and you want to know more, have a look at some of the articles that have appeared in the modelling press. Better still, join the Scalefour Society and be taken through the principles step-by-step in its indispensable Digest Sheets.

Quartering Wheels

Quartering is often seen as very difficult. Space precludes a detailed description here, but the basic process is very simple and it is easier to do than to write about!

Take the first axle and quarter the wheels by eye, gently rotating one wheel on the axle until its crankpin leads the one on the other wheel by 90°. To be honest, getting it exact is not essential - you cannot see both sides at once! This axle should now be your reference one and not be adjusted again. Fit the next axle, quarter its wheels by eye, and then fit the coupling rods to the two wheelsets. Adjust the quartering on the second axle, by easing the wheels on it, until the chassis runs freely in both directions with no binding.

Then add the third axle, and adjust the quartering on that, (but do not adjust the previous two), until the chassis runs freely as an 0-6-0. Repeat, if necessary with further axles with coupling rods attached until you have a free-running chassis. It really is that simple.


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