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Reply to "MTH Engine Gear Ratios"

Mark, I'm not sure that 0x09 has anything to do with the speed or odometer function.  The byte at 0x08 does a pretty good job by itself of describing the effective gear ratio and by extension, motor revolutions per inch, stripe count per inch, stripe count per second at a given speed, etc.

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.  The 0-6-0 has the highest 0x08 value and would probably be the smoothest runner of this group.

The formula for the approximate gear ratio is: Driving Wheel size in mm * the value of 0x08h as an 8-bit integer * 0.00552. 

I'm not sure why some diesels have 0x54 and others 0x55.  But the difference is only 1 in 84, or about 1.1 percent which is within tolerances for double-heading.   I also can't explain why the formula doesn't work out to the exact value in every case. 

Some theories are: the exaggerated curvature of a very small flywheel may impact the accuracy of the tach reader, leading to missed or double-counted stripes.  Also, there may be a fudge factor to account for expected skidding / slippage on curves.  On tight curves, the inside rail is significantly shorter than the outside, so one or both wheels have to skid, and this may translate to lost motion.

A long time ago MTH warned that operating scenarios recorded in the TIU wouldn't be suitable for repeated playback, without eventually resetting the loco to its exact starting position.  They predicted that position errors would accumulate over time, leading to fouled switches, running through the block, etc.  Lionel's LCS tries to get around this problem by using track sensors to establish the position of the loco, and to trigger events.

Last edited by Ted S
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