Odd Legacy R-30 Subway Drive Issue

John, HO scalers have long referred to the phenomenon you describe as "bucking."  It's typically seen when a self-locking worm drive has to resist the train load going downgrade.  I've personally experienced it with my PS2 Premier Berkshire descending a slight grade.  I understand that it can develop with any self-locking worm drive, if the frequency of the mechanical system matches the passing tooth rate on the worm wheel just right.

I'm going to guess that the R-30 subway doesn't have the back-driveable LionDrive setup.  Please don't hate me for saying this!  But depending on the sampling interval, speed control could theoretically make it worse.  You could try turning speed control "off" (or just feed the motor DC for test purposes), let's see if the bucking / jerking improves or goes away.

If it is bucking, there might be a couple ways to "fix" it.  The first thing I might try is increasing the flywheel mass.  With more difficulty, you could switch to back-driveable gears, or change the gear pitch or gear mesh so that the passing tooth rate is outside the natural frequency range of the system.  With my Berk, I found that it woudn't buck with a heavier train load behind the tender.  You've helped so many on this board, I really hope this talented community comes through for you!

Edit:  This thread seems to imply that bucking is caused by excess thrust play:

https://cs.trains.com/mrr/f/88/t/206073.aspx

https://cs.trains.com/mrr/f/11/t/84094.aspx

Ted, it happens on the test bench on the rollers, so it's not the drive resisting the grade.  It also happens on the flat runs.  Interestingly, a load on the powered unit tends to lessen the effect, but not eliminate it.  Turning off cruise didn't fix it.

Increasing the flywheel mass is a non-starter, there's barely enough room for the existing tiny flywheels.  Similarly, changing the gear pitch would involve replacing the captive gears in the trucks as well as the worm on the motor, even if I knew what to replace them with.  That's something that would most likely exceed the price of a new subway set by the time the dust settled!

I considered the excessive end play on the armature, but the new motor and both of the motors in the subway have exactly the same movement, and it's maybe 1/16", pretty much the same as all the other similar Mabuchi RS-385 motors I have in captivity.

The only thing I can see different from other similar drives is the excessive strong cogging of this particular motor.

Still brainstorming this John... I have a couple of other ideas, but they might be irrelevant.  Any chance you can post a video of it running jerkily on the rollers?  Perhaps when we see, it it will inspire other ideas and suggestions.

It's back in the box right now, I'll consider that when I have a reason to unearth it again.