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SSP Slomo Wagon

by Chris Bird

The Small Steam Performance Slomo

The Slomo is a very clever inertia device created by Terry Robinson in Australia. It consists of a heavy flywheel, driven by a gear train wchich, in turn is driven by a heavy duty chain from a sprocket on an axle. The Slomo was intended for mounting on the Roundhouse 0-4-0 locos where there is room for it to fit between the frames and axles. It fundamentally changes the driving experience with slow, steady starts and gentle, coasting stops. It also has the effect of ironing out indulations in the track.

You can see it in a video of my Katie here:

As I have many other types of loco, I decided to see if it would work in a tender. In this video you can see the result of the tender mounted Slomo - after Terry had supplied me with an 18 tooth sprocket (instead of 21 on Katie). It is very nearly as good and demands some skill on the radio control to avoid wheel slip - which adds to the fun!

The problem was that this tender mount is again limited to one loco - and I wanted it to run with my 7/8ths scale stock as well. So that is where the idea of the wagon came from.

The 7/8ths Scale Slomo Coal Wagon

Although this wagon is for 7/8ths, the same principles apply for 16mm and G Scale. Indeed I think it would be pretty easy to fit one in an Accucraft W&LLR open wagon for example. The think is that it needs to be heavy - the tender on Edrig was 2 Kg.

Also, I must say at the start that I used what I had in my workshop - you could use wood with lead weights, or steel, or whatever comes to hand. The important parts are the Slomo and the wheel sets - in my case Accucraft 30mm wagon wheels (more about that later).

The overall size and design was to match my Summerlands GP Wagons, with a flat bed wagon that could have a variety of superstructures. It needed to be heavy, and luckily I had some 3.5mm brass sheet and 12mm square brass bar. I cut the sheet with a jigsaw and filed it to shape.

For simplicity, I decided to soft solder the buffer beams:

The sole bars were of 12mm x 2mm steel and these were drilled and bolted to the buffer beams. I had thought of using Binnie axle boxes, glued to these, but in the end decided that I needed a more 'engineered' solution. I made simple axle boxes from brass and bolted them to the sole bars. I had hoped that the single 8BA bolt would be enough as they were butted up against the flat bed, but that was not good enough so I needed a better solution. They also looked rather horrible!

I fitted the Slomo on some rubber sheet using 8BA screws into drilled and tapped holes. Murphey's Law dictated that the holes were exactly in line with the sole bars so could not go right through. The aluminium couplings - which will allow a rigid bar to be used - were modified from some in my scrap box. I also filed notches to represent the ends of the planks - though I left some roughness on the edge to represent the wood.

The Accucraft wheels can be easily removed from an axle by supporting the wheel in the vice jaws and tapping the axle. The sprocket is bored for 1/4" so I made a simple adapter with K&S tubing, using a short length of 1/4" and 7/32" tubing. Each was slit lengthways with a razor saw (one cut) and with the two in the axle, the sprocket was a good, tight fit.

The chain is like bicycle chain with the same sort of connector and it is necessary to ensure it is the right length to give a small element of slack.

Here you can see it next to a 2 plank GP Wagon - showing that a third plank was needed to cover the Slomo.

So I made a simple box using wood from B&Q (a DIY store). The edges were sanded a little on the bench belt sander so that they would show as distinct planks when glued together with PVA. The corner supports are to hold the top.

I needed to make a cut out in the ply top - so I marked it out and cut it with a craft knife. At this point I decided to fix the top so that the whole superstructure could be removed as one. With hindsight it might have been better to allow the top to be removed (as on the tender) to make packing the sound insulation easier.

As I have a good stock of 9mm wide by 1.5mm thick steel, I cut lengths of this for the corner strapping. I embossed these to suggest coach bolts with a tool in my drill press. I used a gel superglue to fix them - but epoxy would have been better.

I then filled the gaps with car body filler.  Meanwhile I had been busy improving (a little) the look of the axle boxes. I turned some brass discs from bar and then decided that I would soft solder it all together. After cleaning the parts with fine emery, I fluxed and screwed the axleboxes on with 8BA cheese head screws (instead of the small head 8BA bolts) which I nutted on the back for good measure. Then I soldered all together and filed screw heads down to just a plain disc to represent an oil cap.

I then reassembled it and did a quick test out on the track with a good, heavy brass weight. It was no good - the wheels just skidded - and it dawned on me that the weight was in the wrong place for adhesion. It is easy to see why!

The body was tested in place and with a little filing inside a couple of the steel corners, it clips nicely into place. It was time for painting - so it all came apart to be rubbed down again, degreased three times with cellulose thinners and the etch primed with Upol. This was followed by grey primer and a bake in the oven.

When all was reassembled, I painted the corners and the inside of the top with water based matt black acrylic before making the load from crushed coal and dilute PVA - left to dry overnight.

I decides on some lettering to add a little interest. I used rub down letters from Back2Bay6 but didn't have enough for the name I fancied: "Slocoach & Cole" and so just settled for "Summerlands Loco Coal ". It appears that I need a lot more practice at this!
Then some weathering with a mixture of Revell rust and leather brown on the wheels and axle boxes, and airbrushed dilute leather brown and black on the body. I didn't do too much as I was in a hurry - just enough to take the new look off it..

And so it was ready for testing. I needed to get enough weight over the driven axle to give adhesion, but not to much to make the load just too great on the steep bits of my line. Here you can see an early trial.

When I came to try it out seriously and make a bit of film, I settled on this arrangement which gave a total weight of 2.3 Kg.

It was no good though - it was just too heavy for the loco on my track and it tended to stall on the tree root hump. It was very smooth everywhere else....

So this is where I ended up - at around 1.9Kg.

Now the Accucraft Quarry Hunslet would potter round without stalling - as you can see here:

The QH is a heavy loco at around 3.7Kg. The problem came when I tried to pull it with the lighter 7/8ths scale locos in my stud (around 2.8Kg). They tended to spin their wheels, and without radio control, there was no option to catch the spin by closing the regulator.

The Key Lessons

I have learned a lot from this - and the experimenting is far from over. The main factors appear to be:

1. Sprocket Gearing - the 21 tooth sprocket used on the loco mounted Slomos is just too high a gear and even my most powerful an heavy locos would just spin their wheels. The 18 tooth sprocket used here is fine for heavy locos but did not work in this set up for lighter locos. Currently there is no ready supply of 14 tooth sprockets which might be better.

2. Wheel size - after carrying out this trial, I realised that the Accucraft wheels I had used were 30mm diameter whereas the wheels on Edrig's tender were 32.5mm - thus increasing the effective gear ratio. When I came to try to swap them over, I found that the axles on the 32.5mm wheels were shorter (so they didn't reach my axle boxes) and thicker (so I couldn't swap the wheels over). To try them out I need to make new centres to replace the plastic ones.

3. Drive - my tests have been on two wheel drive, where serious weight is needed to get adhesion. I have not yet been able to test it with the axles coupled to give four wheel drive, but it should allow a lighter weight.

4. Adhesive Weight - there has to be enough weight over the driven axle, if 2wd, or in total if 4wd, to get enough adhesion to drive the gear train. It is a careful balance for the loco and track, but when it works it is wonderful!

5. Rolling resistance - Terry has suggested that the heavy wagon and the pull of the chain may give increased rolling resistance in plain bearings. He is supplying me with some miniature ball races which I can hopefully fit in my axleboxes without trashing them!

6. Sound insulation - a wagon like this is like a resonance chamber and amplifies the gear noise. It is important to design in some serious sound insulation and have this top of mind during the build. I have used rubber and solid foam, but this is really difficult to install in the wagon as I have made it. If the top came off I could pack the foam in the space round the Slomo as I did in Edrig's tender.

Further Work

The bearings for the axle boxes arrived from Terry in Australia and I managed to fit them without too much difficulty. They are flanged and have a 6mm OD and 3mm ID. I mounted the axle box in the vice of my drill press and, using a 3mm drill, located the box so it was exactly central under the chuck. Then I drilled 5.75mm and opened this to 6mm with a mill cutter. The bearings then just pressed in.

I have to say that they made a big difference to the smoothness of the running. It is clear the the chain drive creates extra drag in the axle box on acceleration and this eliminates that. I was getting good adhesion with 400 grams less (down to 1.5kg), but I could still not get the 2.7kg locos to pull it without slipping. These bearings (or at least cheaper versions of them) are available in the UK from Technobots.

Next I changed the wheels from 30mm to 32.5mm. That sounds easy - but it was not! The Accucraft 32.5mm wheels came on shorter axles and the these were also of larger diameter. I ended up turning new centres for them - and after a bit of a struggl, had them mounted on the axles.

Strangely, this reduction in gearing needed more weight for adhesion and made no noticeable difference - the 2.7kg locos still slipped their wheels. It might, of course, be possible to reduce this with a radio controlled loco.

Here is the latest development of the coal wagon running with my manual Quarry Hunslet on Roy Wood's line: