Ted S posted:
Your sample data only confirms what I already knew. All diesels are geared the same at 10.5:1 and typically have the same size wheels. With my understanding, and never having had any of your locos apart, I predict the gear ratios of your steamers as follows: 0-8-0 is 14:1, 2-8-0 is 18:1, Berk is 16:1 (I'm guessing yours is Premier- I have one), 0-6-0 is 18:1.
You are exactly right!
I have developed an equation which I think will let you calculate the gear ratio from the Scale Factor Value, the measured wheel diameter and the number of stripes per rev of the tach wheel (usually 24). The measured wheel diameter is the real variable in this equation. Its hard to measure with the traction tire in place. But its the traction tire that really determines how far the engine goes in one rev of the driving wheel.
Here are the values that I get which need to be rounded:
0-6-0 18.41 (I actually twirled the tach wheel and counted 18:1 on this engine)
2-8-0 17.86 (you predicted 18:1)
Berkshire 15.98 (you predicted 16:1)
0-8-0 14.14 (you predicted 14:1)
The equation can also be solved for the Scaling Factor given the gear ratio, the wheel diameter, and the number of stripes per revolution of the tach wheel.