SAR GMA/M (rebuild)

Powering the front unit

The kit in its original form has only one powered unit, the rear one. This is probably due to the confined space in the front unit water tank. Well, I am going to try and find a motor and gearbox combination that 1. will fit in the front unit and 2. will drive the loco at a reasonably scale speed.

Why?

First a Garratt is a Garratt for the simple reason it has two drive units, so I want my model to have two powered units. Second, a model locomotive develops very little tractive effort. A decent motor simply overpowers the eight wheels. The AD60 has been known to be barely able to drag its own weight with only one powered unit. So my theory is that a model can better have sixteen powered wheels with two motors half the size. This is all the more true as the unpowered front unit only creates drag on the entire model.

The consequence is that I will have to discard the rear units existing motor and gearbox in favour of a set identical to the one in the front unit. Oh well, that 's life. I guess a Portescap motor and gearbox will find their way into other kits I have lying about.

On the quicky I can see two variants with High Level Kits gearboxes.

The feasibility of these ideas depends strongly on the internal structure of the frames: where are spacers, can they be moved and where the motion bracket?

Thoughts about gearbox facing forward

A first sketch

This is a combination of a

all from High Level Kits.

Given the motor's 13,000 RPM and the 1:40 gearbox ratio the model's top speed would be 83 km/h. The GMA/M has been known to reach 75 km/h so I think that is an acceptable top speed. I could choose a 1:54 ratio which would limit the top speed to 62 km/h in favour of a better slow speed performance and possibly more tractive effort. Food for thought

There are some frame elements in the way but nothing that cannot be removed or relocated.

View from the top. In front of the motion bracket there is room to pass a gearbox through.

View from the bottom.

In all likeliness the round spacer and the flat spacer cum brake hanger are in the way. The latter can be dispensed with, I will hang the brakes directly from the frames. The former will need to find a new home. I even think I can do without if the motion bracket give sufficient support. The frame plates are of an old make and consequently a good fashionable 0.9 mm thick, so not every strainer is needed. On the other hand this spacer is the place where the leading bogie is attached so I need to find a solution for that. I also need to ensure that the bogie does not collide with the gearbox.

Maybe I should reverse the motor/gearbox anyway.

Thoughts about gearbox facing backward

When I reversed the image of the SlimLiner and motor it soon became apparent that the standard drive stretcher was too short and the longer D3 drive stretcher was too long. I opted for the HiFlier PLUS and this gave an immediate fit with the standard drive stretcher:

I even have a choice between driving the first or the second axle. The setback is that the HiFlier is marginally wider than the SlimLiner, 7.8 mm and 7.2 mm respectively. This does not seem much but as I intend to rebuild this loco in 12mm track gauge space between the frames space is at a premium. I will have to work with that. If necessary I will mill a 0.3 mm recess in the frameplates.

After a pause in the project I ordered two motors and gearboxes in Febrary 2024.