Some toughts on the turning radius

Via my Guestbook I got a most interesting question from a fellow hobbyist in South Africa:

 

The DJH model's turning radius is rather tall. Is it because there is no axial play on any of the main axles? Inserting some of the bushes just a little deeper into the frame may help on not? I have Märklin Big Boy turning on R1 curves. I want a Garratt but I have no big layout, SO radius problem. What do you think?

Excellent question. I answered that question to him personally. On second thought the answer might be of interest to the wider community and also to more loco classes than the GMA/M alone because the ideas and techniques I described can be used universally. So here are my two cents worth.

GMA/M and AD60

I have not built the GMA/M yet and if I do it will be in 12 mm gauge. I built the New South Wales Government Railways AD60 though and my guess is the GMA/M's construction will be very similar except for the rear bogie which is a pony truck with the GMA/M. The pictures here are from the AD60 frame, so you must imagine the GMA/M frame there.

Basic strategy

First I would recommend building one unit's chassis with wheels and both bogie and truck first in such a way you can still take the wheels and bogie and truck off.

The AD60 frame in this building stage

In this building stage you should be able to test the whole unit on your desired curves.

Options to reduce the turning radius

If it does not take the curve desired there are a few options I would consider doing. After each option reassemble the frame and try again on your test track.

1. Remove washers

In the AD60's case all driving axles where provided with washers between the wheels and the bushes. For those who might not know, the bush, usually in the shape of a top hat, is the bearing in which the axle will run and which needs to be soldered in the frame. In DJH manual of the GMA/M this part is numbered 32 on page 16. (the bush is indicated with the yellow arrow on the photo above).

The AD60 from below. You clearly see the rim of the bush and the washer between the bush and the wheel.

I assume this will also be the case with the GMA/M, but I could not find it in the instruction manual, but maybe that is only for the 12 mm version. Take a close read in the manual.

Leave out the washers on two axles. Which two depends on where the gearbox sits. I would recommend leaving the washers on the axle with the gearbox, keeping that axle relatively rigid in the frame. A quick look at the instructions of the GMA/M shows the gearbox will be on the second or third driving axle, I can't see it properly. Let's assume it is on the second driver. I would recommend leaving the washers on two axles in total, which would warrant the unit's stable riding .

 

Washers on axle one and three removed

Leaving out the washers on two axles allows for more side play, and therefore for more ability to take a curve. In my example above I gave more side play on axles one and three. This is the best option for riding stability (longest rigid wheelbase).You could also opt to take out the washers of axles one and four.

Washers on axle one and four removed

This may decrease riding stability because of its shorter wheelbase but it will also offer just a little bit more turning capacity.

2 Slimming the bushes

If taking out the washers does not suffice (or if not present at all) you could consider also filing down the protruding bushes a little bit. Just take care that the wheel rims do not touch the frame plates.

Some more thoughts

• File them in stages.
• Like with removing the washers only take material away from the bushes of two axles (1 and 3 or 1 and 4)
• Make sure you file off evenly on the entire bush surface.
• Also ensure that all bushes are slimmer by the same amount. Measure carefully, we are talking tenths of millimeters here!
• Try not to take away too much material on one go, say 0.5 mm at a time and then reassemble the chassis for testing. If not successful remove more material.
• In case you decide to retrace your steps and need untouched bushes, do not worry, these are available as spares.

3. Give the drive rods enough space ont the crankpins

Once you are putting the drive rods on their crank pins, see to it that the crank pins get a bit of extra length to allow for the play in the axles after you followed the above two options. If you don't translate this play in the crankpins the drive rod's stifness will eat up that play bringing you hard work to naught.

4. Side play in the bogie

The slot in the AD60 bogie, allowing for side play

Whatever you do to the washers and the bushes the bogies need side play. On the AD60 that was built-in: a slot in the bogies enables that. On the GMA/M kit however DJH indicated a hole, only allowing the bogie to turn, but not to move sidewards. Solution: make the slot yourself! See to it that the bogie has sufficient side play without interfering with other components, if need be leave away some details and maybe file away a bit of the underside of the cylinders.

5. Both power units should be able to articulate sufficiently

My AD60 in its maximum articulation. Note the difference the front and the hind unit

Take a look at my AD60 and you will see that the rear unit articulates further than the front unit. That is because the front unit snags on some details. For me it is unimportant as my AD60 sees no layout service. But if any of the units limit the turning radius you might want to address this. Consider leaving out details on which they snag or adapt them so that they do not interfere anymore.