I foresaw the possibility fo lacking power, so I designed the gearbox so that to fit a lower ratio, all I need is to cut one larger gear to replace an existing one. The motor can keep the same pinion, and moves upwards in its mounting slots to accomodate the new gear. To get an even lower ration, the pinion could be made smaller.
For a line that is less steep, the ratio appears near ideal. Here's my theory of the minimum drawbar pull needed by small locos:
On the level, with a ball bearing driving car, you probably need just over a pound of drawbar pull to keep moving, and no more than three pounds to get started.
The total drawbar pull needed from a small loco is therefore three pounds plus whatever is needed to pull the driver and truck up the steepest gradient.
I weigh 'about' 180lbs, driving trucks are heavier than I expected, lets say 70 lbs for a big one, so 250lbs total.
For different gradients the 'extra' pull required is:
1:100 … 2.5lbs
1:80 … 3.1 lbs
1:75 … 3.3lbs
My shunter weighs in at 26lbs, giving a drawbar pull at 25% AUW of 6.5lbs. Stall current for the motor is 7A and (with a 60% efficient drive train) at 6.5lbs pull the theoretical current requirement is 6.4 Amps.
In other words, I should have been able to predict that I would just manage a 1:75 gradient, but grind to a halt if there were any bits that were just a little steeper!
So three solutions:
- Stick to less steep tracks (it seems most are)
- Make my own, lighter driving truck
- Reduce the drive gear ratio by about 20% (and perhaps add some more adhesive weight)
A fourth, rather sneaky, option is supplementary drive in the driving truck; I have already thought of a way to make this proportional to drawbar pull to avoid overdriving the loco. Whilst not strictly cricket, this would make even th steepest of tracks an easy proposition.
Neil