Well, I didn't give up. At the end of the last post, I had the chassis running nicely with the controller wired directly to the motor, but picking up from the rails was less impressive, despite the fitting of the 'Simpson spring pickups' rubbing on all the axles. The chassis was disassembled and re-assembled a few times, to no avail, and another crank pin came loose from a wheel and had to be re-soldered back in. Quartering the wheels was trickier as the motor was now solidly in place, but plenty of trial and error had the chassis running smoothly again each time.
At the same time, I was working on fitting the Dapol footplate to the chassis. As my version of the Terrier has sandboxes above the footplate, the sandboxes below the footplate were sawn, cut and filed away. If the under footplate sandboxes are to be retained, a little must be removed from behind them as the replacement chassis is slightly wider than the Dapol one. The photo shows the Dapol footplate with motor, the footplate with the motor and cage removed, and finally the footplate with some bits cut away to make room for the new motor to pass through it.
I also had to file a little plastic behind one of the front splashers to clear a gear, and a little from the front edge of the cab floor to clear the rear wheel flanges. Other than that, it fits the chassis, but the wheels will not go round because of the steps. I did not want to lose these nicely moulded features, and it seemed like there should be just enough clearance, but it did mean removing the bosses from the ends of the con rods and cutting the crank pins down to the very minimum. The centre cranks are fine, but the wire insulation was not enough to keep the con-rods in position on the short crank pins, so I had to solder washers onto the centre crank pins to keep the rods firmly in place. The loco body fits without any alteration, which was a bonus. There is even room in the tanks on each side of the motor to add some extra weight.
As I was getting no improvement in pick-up with each rebuild, I decided to resort to adding phosphor-bronze wipers bearing on the tops of the wheel treads. I used the remains of the Dapol pickups to produce these, and soldered them directly to the chassis. Again, there was no improvement. It seemed that the wheels just did not want to make good contact with the rails, and the pickups were removed again as they got in the way of fiting the footplate.
So, as a last resort, I followed Jerry Clifford's advice and made up a pair of rail skids. I had a sheet of .4mm phosphor-bronze which I cut and filed to shape, forming two long arms, with a kink near one end, bent to form the skids. I added another 180 degree bend at the other end in order to produce the most flexible springing possible. The ends were soldered to the bottom chassis stretcher plates.
Success. I finally had a chassis that ran slowly and smoothly on my short test track without stalling. I was so pleased, I threw caution to the wind and fitted the DCC decoder. This is a CT DCX75 which stands upright in the cab. It is hard to see it edge on, and will be completely masked when I eventually fit the crew.
The results I have captured on video. Hopefully the photos will explain everything. I will probably replace one of the skids, as the part that touches the rail is a little too short and seems to catch on some rail joints and on points.
Still to do, add couplings, touch up the chassis paintwork, think about adding the brakes, add the crew, then get on with my other two Terriers.