16mm Projects

An anti-gravity slate train

This was one of those “why am I doing this?” projects…    I’m sure you know what I mean.

The masochist in me seems to enjoy “tinkering” rather than running trains.  I discovered this quite by accident… well, several accidents actually.  As a compulsive buyer of second-hand locos and stock, I found that sooner or later they required a bit of TLC to get them to perform as I think they should.  Having a track mainly on a 30:1 gradient doesn’t help.  Anything other than “Top Link” performance from a loco becomes immediately noticeable.

I digress….     Some months back Graham Pearce appeared with a couple of motorised FR type slate trucks he had picked up.  I think that one had been “home made” by a previous owner and another was a kit fitted with a motorising unit.  Both performed quite well, and as is usually the case I was interested to see “what went on under the bonnet”.

Both systems were RC controlled with all the “gubbins” (motor, battery, RC system) in the one truck.  This made for convenient operation and they could pull a train of half a dozen or so trucks.  One approach used a reasonably sized motor with a crown and pinion gear system.  This produced a good turn of speed and its weight helped avoid wheel slip, but the motor stood proud of the “normal” slate load height which I thought ought to be avoided if possible.

The second system used a worm and gear drive and was well engineered, but didn’t have the speed or the “guts” that I like my trains to have.  I’m not really a speed merchant (honestly), but I do like to have enough power “in hand” for my electric locos to keep pace (or keep out of the way) of the steamers.

Fresh from my “success” 3d printing the electrically driven bogies for my Pechot loco, I wondered what I could do in the confined space of a 2 ton FR slate truck.  I could see it would be quite a challenge….  Compared to the Pechot loco the slate trucks looked very small.

I pondered for at least 5 seconds and decided that my approach would be to have several motorised trucks “up front” linked to an RC system in a following truck with another truck carrying the battery… probably a LiPo so that it would be small enough to be hidden but powerful enough to provide the haulage capacity (speed, duration and train length) that I wanted.  I’d made around 30 slate trucks for Graham’s double Fairlie to haul and this made for an impressive looking train, so I envisaged a complete gravity train of around 20 trucks or maybe more.

I measured up the space available and could see it was all going to be a tight fit and require very small gears / worms, etc.  It seemed that most of the motors that were small enough to fit were 3 or 6 volt, so maybe 2 motors wired in series would be a good way to go?  Using motors with a built-in gearbox was a maybe, but the overall length of motor and gear shaft looked to be too long.  I found some interesting websites for slot car enthusiasts (think “Scalextric”) with crown and pinion gear systems that would fit, but the ratio permutations were endless.  What speed reduction would I need? A few different ones were ordered from e-bay for evaluation purposes.

These thoughts were all very much at the back of my head when I went to the 2022 Peterborough show and happened to spot some very small worm and gear sets on the “Dean Goods” stand.  Not only that, he had some small 3 volt motors too.  I duly purchased two motors plus 40:1 worm gear sets and started to get more enthusiastic about the whole project.

My main concern was that this was all going to be very small engineering compared to the Pechot loco I’d just made.  In my mind “small” means “fraught with problems”, such as having to deal with 2mm dia shafts on the motors and gears mounted on 1/8” axles.  The worms and gears looked tiny, with grub screws hardly big enough to see, let alone secure a gear to a shaft.  When it comes to garden railways I like what I call the Roundhouse approach…. keep everything that’s mechanical reliably meaty!

Back at base I modified my 3d drawing of a 2 ton slate truck to take the motor and worm drive.  I could have gone for the extra space provided by a 3 ton slate truck, but that would have been cheating!

The first piece of luck was that the 3v motor would fit under a “normal height” slate load.  I decided that the motor position should be adjustable so that I could adjust the “mesh” between worm and gear so as to get the best running characteristics.

As usual, it took 3 designs to arrive at a set-up that worked well (that worked at all!!).  I only draw half of my rolling stock and the other half is just a mirror image that can be “copied and pasted” in place.  As a result, I had a truck design that could take two motors, with separate drive onto each axle.  The two 3v motors could be wired in series to make up a 6v system.  What could possibly go wrong?

Here’s the drawing:

And here’s the printed truck:

I’d made some basic decisions by this point…

The motors would be carried in a “housing” (fancy term for a box) which could be positioned laterally by adjusting two 8BA screws sitting in slots.  Apart from adjusting the gear “mesh”, this also allowed me to remove the motors (if need be) and replace them with different motors in new housings without throwing away the whole truck.

The axles would be carried in “inboard” brackets rather than the outboard bearings on the prototype.  This allowed me to easily remove the axle plus gear system and replace it with another if I wasn’t happy with the arrangement or ratios.  The complete “axle bracket” system was held in place with a “keep block” screwed (8 BA again) through the centre of the truck floor.


Here are motors in their adjustable blocks.  The motors are glued into the blocks.

The larger motor on the right is a 9 volt specimen wired for parallel electrical feed.  The motor(s) on the left are 3 volt ones that had been wired in series.

I found it better to use a “split” axle mounting.  This made it easier to assemble the wheels and gears onto the axle shaft and to remove them if need be later.  Two interlaced fingers keep the whole thing “square” when clamped in place on the truck bed / chassis.

The photo with the gears in place shows a Delrin gear also fitted to the axle…. more about this shortly.  Please ignore the wonky wheel!

The system was tested hauling 2 more trucks, one containing the RC gear and the second with the battery. The results were a bit of a mixed bag.

Here’s a video of the Mk1 system being bench tested:  https://youtu.be/1LR2eEHZCUk

And here’s a video of the same truck cruising up my 1:30 gradient:  https://youtu.be/MQDx7WBOSk0

You can see from the video, the system had plenty of grunt!  There were, however, some teething troubles that soon emerged and a re-think was required.

Here’s a video of another test run.  I think this was the Mk2:  https://youtu.be/2TcqucBkkAs

One problem was that the two motor drives did not share the same characteristics, in that one started up well before the other!  At full tilt they both pulled their weight, but at low speed only one motor actually started up.  This was probably due to different degrees of “mesh” between worm and gear, or was it due to different performances of low cost electric motors?  Was it due to the “first start” motor taking current away from the “lazy” motor?  These are questions for greater minds than mine!  The motorised truck was also on the “light” side and suffered from wheel slip.

The whole system was also very noisy, which rather detracted from its “gravity slate train” ambience.

Going back to basics, I decided that having two motors in one truck was a waste.  It would be better to go for 1 motor in 1 truck and obtain 4 wheel drive using a Delrin chain. At least the two axles would then run at the same rpm!!!

It was about this time that the Scalextric “crown and pinion” parts arrived that I’d already ordered from e-bay before I found the Dean Goods items.

Here’s the Scalextric crown wheel plus the delrin drive gear removed from the axle:

This approach was quite good.  The train ran at a good speed, but the motors kept failing (burning out / overheating) after 20 minutes running or so….  and the complete train was still only 3 trucks!

After several experiments with motors and gears, I decided that a brass worm and a steel gear was the best approach and finished up using a 27:1 ratio from Scale Link (ref SLGS02) combined with the reliable and cheap MFA 719RE 140/1motor.  With a rating of 3v-9v I find this motor does the job and hasn’t yet burned out with my RC transmitter set to give a max of 75% throttle.

Here’s a photo of the final (Mk3) arrangement, with a lump of steel added to give extra traction:

Keen eyed readers will notice that the motor had to be mounted slightly off-centre of the truck to give the required clearance to the Delrin chain drive system.

A major advantage of having the motor and drive axle systems as easily adjustable / removable parts was that trucks “rejected during development” (and there were a few) became unpowered trucks to add to the train…. so very little got thrown away!

Here’s a 3 ton truck devoid of motor unit:

And here’s the battery truck:

At present I’m using 4 powered trucks along with the RC and battery trucks, plus 11 unpowered trucks giving a complete train of 17 trucks.  The results are very satisfactory on the flat and good on my 1:30 uphill gradient.  As you’d expect from a gravity train, it also looks and runs very well down the gradient!  The whole drive system is also much quieter now than it was in the Mk1 variant.

The original rake of trucks I made to go behind Graham’s double Fairlie all had weight added to keep them on the track.  Up to now, all the trucks being dragged in the gravity train have no weight added to keep them as light as possible so as not to over-tax the electric drive system.  Running characteristics are fine unless you have spring-loaded points!

Here’s a video of the train in action:  https://vimeo.com/743699464/a878f71ed7

I’m currently making another two motorised trucks to add to the rake.  Being “modular”, there’s no reason why you can’t just keep adding more motorised trucks as required.  Cost isn’t a major issue with motors @ £1.66 and gear sets @ £7.60.   It will be interesting to see if the single ESC (speed controller) can deliver the current drawn by six 9 volt motors rather than four.  I’d best check the spec sheet! 

In action at Almondbury : Dave Webb

September 2022

and some FR pictures from Andy Wilkinson

running through Penrhyn station

on Gwyndy Bank



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