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I have a really odd issue with the 6-35482 R-30 Subway powered unit.

The symptom is jerky running, especially down grades.  Upon inspection, I saw that the encoder wheel was actually rubbing on the bottom of the sensor slot.  "Easy fix" I though, I just moved it up.  That seemed to cure it.

I got it back, same problem.  Sure enough, it still has the same issue, even though I ran briefly here looking good.  OK, maybe the end play of the motor is too much, so I ordered a new motor.

New motor installed, SAME PROBLEM!  OK, now there's no encoder rubbing, and as it turns out, the end play of the new motor is the same as the existing motors, both front and rear.  It's also less than 1/16", so it's probably not end play.

OK, I skywired the motor and encoder to my RCMC test set and used a totally different motor driver to drive the motors.  STILL THE SAME PROBLEM!  Tested the old motor on the test set, and ... same behavior!

OK, now I've eliminated the motor/encoder and everything else!  What do I do next?  I have run out of suspects!

I do notice that the motors in this set exhibit excessive "cogging" when you turn the flywheels manually.  Since a new motor and running with a whole new electronics package doesn't seem to fix it, the only thing I can come up with is the factory used a bad choice of motor.

Note that my testset uses the exact same encoder board on a different Mabuchi motor type, and it's butter smooth running there.  Also, the motor on the testset exhibits minimal to no cogging when manually turning the flywheel.

The back story is this issue was there from the time the customer got the set new, that was obviously in around 2009.


Question:  Does anyone else have this set, and have they had any issues with running?  It's hard for me to believe that this one is the only one with these issues.

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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:

Last edited by Ted S

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.

Last edited by gunrunnerjohn

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