BEMO OUTSIDE FRAME CHASSIS


Vorsprung durch Technik and Manning Wardles

I am often asked why I keep trying to find an easy way to fit an RTR chassis under Backwoods Manning Wardles. The answer is simple.  I feel I am a reasonable modeller but I am not a miniature model engineer. Despite three attempts to build a viable Backwoods chassis for these locos, I have failed every time.

Equally, if anything subsequently happens to a Backwoods chassis, one almost has to build a new one.

I have had to rely on outside experts. This means costs of at least 250 a pop and usually a long wait. Having seen the lovely Leek and Manifold models of Martin Radcliffe which are based on the Bemo outside frame chassis, I set out to find one. They run as smoothly as can be.

My attempt to use a Grafar class 08 chassis has resulted in a successful locomotive which still runs well but the effort was considerable.

The Bemo chassis layout fits under the MW body although, yet again, the motor will require moving as it is 1.5mm too high but the job is considerably easier. It is also much more narrow than the Grafar chassis. With the plastic side frames cut back and thinned, the Backwoods chassis frame fits nicely over it. As with the Grafar 08 chassis, the wheelbase is less than prototype but I feel that this really does not show.

I am also going to use a simplified Joy valve gear, as this is another extremely difficult thing to get working. Bob Barnard, who has modelled these locos over the past thirty years, uses a simplified system himself and I have to say, the appearance is just as good.

The additional Manning Wardle is being built for the Marches Narrow Gauge Group who will be the operator of the Bratton Fleming diorama currently under construction.

I was lucky enough to recently find and outside frame Bemo diesel on Ebay. Not cheap at 142 but still much cheaper than from a shop.


as delivered


as delivered - the jackshaft despite having a gear, is not driven

Round here, nothing remains unmodified for long, so the body was removed, the jackshaft removed and the coupling rods cut short. The footplate was cut away and the outside frame refastened with epoxy. The outside frame holds the electrical pickups, so it is best to leave them where they are. Looking at the build quality, I have to say it is miles better than anything else around.


the basic modified chassis alongside a Backwoods Manning Wardle chassis. The wheelbase is shorter than an MW but frankly one can barely see the wheels anyway! - click on image to enlarge

As the strip down continued, I noticed that there was a split in all of the cranks. This is not in itself a disaster but measures have to be taken.


click on image to enlarge

The cast adhesion weight had to be cut down to clear inside of the loco. The underside of the chassis was sanded down at the front end to allow for the installation of a pony truck. In the photo below, the split cranks can be clearly seen.

The counter balance weights on the cranks were also removed with fine snips and a sharp knife. The crank pins are in plastic and the pins of the driving wheels are pinched at the rear to hold them in.


click on image to enlarge

The wheels sets and spur gears were then removed and the plastic side frames removed.


A Backwoods 'Russell' frame which is the same width as an MW frame. This is a good time to paint the wheels and cranks - click on image to enlarge

There is really not that much left of this expensive chassis. So what do we get for the investment?

1. Current collection. This is well made and fits on the underside of the plastic footplate unit. The phosphor bronze pickups act onto the wheels rims; the ideal position.

2. The axles are made in brass and are of 'agricultural' thickness. These slot into the inside frame which is very accurately machined. There is not one iota of sideplay. Brass is a far better material to rotate in the chassis slots.

3. The gears are also agricultural and all wheels are driven by spur gears. This means that the coupling rod is more or less cosmetic.  The hole at the rear axles is round while the other two are slotted, which compensates for any gear slack.

4. The keeper plate is a very well engineered part in metal, for a change and fits perfectly, preventing and extraneous movement of the axles.

5. All wheels make contact with the rail (not always the case with Grafar) and all wheels turn absolutely round without any trace whatsoever of wobble (another Grafar problem).

6. The cranks are sturdy but have all cracked with time (my unit was bought second hand but had not been tampered with or barely run). This is fixable with the use of Loctite but the problem should never have happened in the first place. The cranks fit onto splines, so on dismantling, good quartering is necessary. The Grafar cranks fit into holes, so they self quarter. However, Grafar wheels are very prone to come loose on the axles.


The chassis with wheels removed. Note how I have relieved the rear of the chassis to accommodate the pony truck
 - click on image to enlarge

The hole where the jack shaft was installed is filled with a thick piece of Plasticard which is glued in place. A retaining nut is let into the plastic for the pony truck fixing.

The only place to attach the front pony truck is using the screw that holds down the keeper plate. It is necessary to grind away to half thickness some of the metal around the hole to allow for the thickness of the pony truck arm which must have free movement side to side. Be careful to leave the shoulders intact to enable the keeper plate to still function correctly. Likewise, with the wheels and spur gears removed, the chassis must also be relieved to give movement to the pony truck arm.



The motor is held in position with 2 screws. This was removed and the backplate cut off. The motor was then re-attached as shown in the photo below. I used epoxy glue to fix it. Care is needed to ensure that the gears mesh correctly. In this position, all is well clear of the body except that the top of the motor, (a plastic block) needs to be sanded down a bit. I have marked this with a red line.


the repositioned motor - click on image to enlarge

Obviously, first time around, quite a bit of time was wasted just figuring it out and faffing around. The chassis is now ready for installation into the Backwoods kit and conversion takes around two hours of easy work. Time was lost rectifying the cracked cranks. The chassis runs extremely smoothly with no wobble at all and is actually very powerful indeed.

The Joys of valve gear

Joy valve gear is not very usual (except with the LNWR) and I rather suspect that even the Festiniog Railway, who have built a replica 'Lew' were a little scared of it. Walschaerts valve gear is excellent for a model as there are lots of bits flying around which is visually very stimulating. Joy valve gear is a much more demure affair and in reality there is very little motion. Because the gear tends to be over scale in 009, the amount of visible movement is much greater than in prototype.

Below is a photo of the full Joy gear as built up from a Backwoods kit. The problem is the tiny link between the connecting rod and eccentric rod. Apart from being extremely difficult to put together in the first place, experience has shown that this is where a failure is most likely to take place. The eccentric rod will then drop down to the bottom of the expansion link and virtually scrape on the ground, where it is further damaged until the whole lot fails. What is so annoying is that it is virtually invisible anyway!


overscale Backwoods valve gear

The simplified Joy gear dispenses with the 'link from hell' which is instead used as the eccentric crank. This will minimise the movement of the valve gear which will make it more accurate. The 'link from hell' could actually be solid soldered onto the connecting rod so the it looks as though it works!


The completed and tested chassis waits now for the delivery of the Backwoods kit

There was a small delay in receiving the Backwoods kit as Pete had run out of etchings.

The first job was to fill the main bearing holes and slots above them in the chassis for both outer drivers. New slots were then cut to clear the cranks of the new chassis. The chassis was then folded and the cylinders made up and fitted along with the front buffer beam assembly.

It is now the time to carefully offer up the Bemo chassis. We had to remove more from the front of the chassis but quite quickly it slip in just fine. Two fixing points were established, front and back so that the chassis is firmly held in the correct position and height.

The cranks were then removed, one side at a time. They were shortened and the axle holes drilled right through. They were then refitted leaving 0.5mm clearance between the outside of the frames and the back of the cranks. They were attached using Loctite 603 and the coupling rod also refitted, with the front and back crank pins fitted with Loctite.

The simplified Joy gear went together very easily. New centre driver crank pins were fabricated from brass and soldered to a return crank (there are many supplied in the kit). The brake rodding and remaining detail followed quickly and easily. Time taken to reach this stage was 14 hours. The motor was removed at this point and the chassis thoroughly cleaned.


the chassis is nearly complete - click on image to enlarge

The body went together with little changes. There is now room to include full cab detail, including the firebox. In order to minimise wiring, micro-connector plugs and sockets pass wires from the chassis to body.


the chassis is nearly complete - click on image to enlarge


chip installed which is wired up for rear light operation and red light in firebox reflecting onto the track
 - click on image to enlarge


The loco sprayed and weathered. It would be sent off for lining and panelling
 - click on image to enlarge

the verdict

The Bemo chassis is very well engineered although there were concerns that in our example, the cranks showed some cracking. The driven wheelbase is shorter than the correct Backwoods chassis and I was concerned that we would experience problems. This has not been the case although more careful balancing was needed when adding weight. We have previously added lead shot as weight. In this example, it was important to keep the ballast over the driven wheels and we have used 'Liquid lead' which is much more compact and therefore more weighty.

We did experience some meshing problems between the wheel drive train and the spur gears to the motor worm drive. These are held in a red plastic moulding which is clearly visible in the above photo. Careful paring with a Kraft knife allowed for good meshing.

The smaller wheels do mean that there are more 'chuffs' per mile but this really does not matter as our sound unit can easily be adjusted to suit.

The loco runs as smoothly and quietly as the best we have on County Gate although I suspect that it is a little less powerful as it is more prone to wheel slip. It does, however easily haul typical L&B consists with ease, even up the CG grades. On the Bratton Fleming layout, where it is destined to work, there will be no problems at all as the layout is quite flat.

All in all, a very successful build and far easier to achieve than struggling with the standard Backwoods chassis. Future maintenance will also be a breeze.


the new loco at work at Bratton Fleming