I have to be careful what tram number I use on my new model that these trucks are being built for as most of the droppies had the diamond frames removed and replaced with plate frames as they were converted to air brake operation. Not many droppies remained as handbrake cars right up to the end and it is the last few years that I am concentrating on for my modelling.
The first photo is of the museum's droppie No 231 which was built in 1925 and was never converted. The rather delicate nature of these trucks is clearly shown in the photo. Restoration was started on this tram some years ago and was stopped for one reason or other. Although drivable it remains in a partially disassembled state with all it's bits stacked inside.
The next photo is of our spare diamond frame truck which is tucked away in the back of a shed. The bearing that it pivots on is on the triangular hinged plate just in front of the motor. Only about 75mm in front, and this caused the pony wheels to swing outside the tram body on sharp curves. Hence the protective covers over the pony wheels.
The rubbing plates that the body sat on are much further forward, at about 45% - 55% spacing between the driving wheels and the pony wheels. I guess that with the one and a half ton weight of the motor hanging to the outside of the driving axle there would have been very little weight on the pony wheels with this light weight truck so the designers moved the location of the rubbing plates forward so that they got the correct weight distribution between the two axles.
On my model I'm hoping to set the pivot point at about the same place so I will be adding some lead weight over the pony wheels to insure good electrical pickup. But not too much. With a rigid chassis like this you can only ever have three wheels in contact with the rails at any one time. It doesn't matter how accurate the chassis is or how well the track is laid the tripod principal still applies and one wheel will always have marginal contact. I would prefer it was one of the pony wheels so there is always two wheels driving on each truck. A compensated chassis would solve the problem and I have built these before but I'm too lazy these days.
The last photo is the finished truck sideframes cast in polyurethane.
I hope to have the motors etc mounted in the next few days but Christmas is getting closer and closer and time seems to be becoming a premium item.
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