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I believe the MTH standard is 24 stripes on the flywheel.  However, I haven't actually counted them on every model.

Ted S posted:
I know from my own experience that the flywheel diameter varies by application, even among diesels.  @gunrunnerjohn, is it true that all locos have 24 stripes from the factory?  I know that you've researched the characteristics of the sensor... From your experience, as the motor RPMs increase, does the tach signal increase linearly, logarithmically, exponentially, or ??

The speed difference is simply the combination of gear ratio, driver diameter, and flywheel stripes.  It's a linear computation, as it's a digital relationship.  The diameter of the flywheel has no bearing on the speed computation, it's the number of stripes on the flywheel.

Ted S posted:
Bottom line, if I were to swap the file into a known loco, the error between the observed speed and the commanded speed could be due to a different gear ratio.  But it could also be due to a different flywheel diameter, stripe width, number of stripes, etc.  Why don't they just put the gear ratio on the darn box!?  

The PS/2 speed is determined by the sound file, the gear ratio, the driver diameter, and the number of flywheel stripes. 

I have a dc motor at home with painted on stripes.  i was told by the seller they were "from MTH".  Every time I count them I come up with something not 24 although I desperately want them to be a nice even number.  I mean the white ones btw, the white stripes.  Anyway it's close to 24.  I keep thinking "surely its not that # and its some nice and even number like 24."  Anyway that's not very helpful.   But one would imagine it has to be a constant universal to all engines.  I don't see why it would not be one, it just seems "fiddly" to have to adjust every flywheel constant in the "sound file" per engine...  eh, right?


I can add some insight into the "Scale Factor" value.

This value is read up as a part of the DTO (Trip Odometer) and DOD (Odometer) commands so I figure that this value is somehow connected to the conversion of the DTO and DOD values into Smiles. To check this, I marked out 1/10 and 1 Smile lengths on my layout. I noted the DTO and DOD values at the start and end of the lengths as I ran an engine over it. With trial and error, and some help from one of my RTC beta testers, I worked out two equations:

Scale Factor = bytes 0x08 (H.O.) and 0x09 (L.O.) taken as a 16 bit integer

DTO response taken as a 32 bit integer

DOD response taken as a 32 bit integer


Smiles on Trip Odometer = DTO Response / (Scale Factor * 10.0)

Smiles on Odometer = (480.0 * DOD response) / (Scale Factor * 10.0)


Thanks Mark!  What do "H.O." and "L.O." mean in your post above? 

One thing I noticed, is that the values of 0x08 by itself in the various sound files seems proportional to the motor revolutions per inch.  For example:

Any diesel:  Byte 0x08 is "54" (84 as an 8-bit integer.)  Observed travel, about 3.82 motor revolutions per inch.

Premier Hudson:  Byte 0x08 is "46" (70 as an 8-bit integer.)  Observed travel, about 3.13 motor revolutions per inch.

Taking a ratio of the two observed values: 3.13 / 3.82 = 0.82

Similarly, taking a ratio of the two converted byte values: 70 / 84 = 0.83. 

Almost the same!  So the first number by itself approximately explains the difference in gear inches.

I've been looking at pictures of MTH parts on a popular auction site.  It does seem that most have 24 stripes on the flywheel.

A curious finding: it appears that MTH actually changed the gear ratio of some of its locos during recent production.  For example, if the numbers in these files are to be believed, the gear ratio of a RailKing Mohawk went from 17:1 in the early days of PS2, to ~26:1 after 2008.  That should translate into MUCH better slow-speed performance!  I think the same is true of the RailKing Pacifics and 2015-issue  B6 switchers.  But only the very latest ones got the lower gear ratio.  Definitely worth knowing about!!!

Last edited by Ted S

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