what is it with MTH a-b-a's trailing truck "bouncing"?

I have not noticed that.  I ran some RK Scale F3s and Premier Alcos just now and didn't see that. 

@Lee Willis posted:

I have not noticed that.  I ran some RK Scale F3s and Premier Alcos just now and didn't see that. 

Pulling a long passenger consist up hill is fine. Also fine on level track. Just when going downhill is there an issue. I have to raise the speed to about 9 or 10 SMPH. Wish I could post a video but cant seem to do so on this forum. I have tried in the past from time to time.

I have experienced that with all my Proto 2 A-B-A’s My Proto 3’s don’t seem to do it. It’s jumping because it is running at a different speed than the other trucks. The traction tire is grabbing the rail. Your explanation in your original post, I believe is the reason why. I just figured the trailing truck was getting erroneous information from the cruise control of the lead unit when it was being pushed from behind, as in going downhill. A DCS expert may be able to fully explain the issue, but I don’t believe the trailing unit has a tach reader, but gets all it’s information from the lead unit through the tether. A-B-A’s without the tether, as in later Proto 3 models, don’t have the issue. I could have this all wrong, but this has been my (non-technical) experience.

@jini5 posted:

Pulling a long passenger consist up hill is fine. Also fine on level track. Just when going downhill is there an issue. I have to raise the speed to about 9 or 10 SMPH. Wish I could post a video but cant seem to do so on this forum. I have tried in the past from time to time.

Try it without passenger cars.  You said it only happens downhill but can be alleviated by raising speed.  Sounds your passenger cars are pushing thus causing the issue.

Have fun

Ron

Is the rear-most unit of your A-B-A set powered, or a dummy?  I'm assuming you have a 4-motor set and the rear unit is powered.

Going downhill at slow speeds below 8 scale mph, what you might be experiencing is called "bucking."  This happens because the worm gear drive used in MTH diesels is self-locking.  The wheels can't turn the motor, and the weight of the train cannot "push" the loco downhill.  When it tries to push, the worm wheel in the last truck block locks up against the worm on the motor shaft.

At the same time, the speed control may detect the slight increase in speed, and begins to rein in the voltage to ALL the motors.  If the timing isn't exactly right, it creates a vicious cycle and you end up with a surging effect.  I've seen this happen, even on a very gentle downgrade, with my own locos.

Lionel's Legacy diesels have back-drivable gears (as did all Postwar diesels.)  So this kind of bucking can't happen.  In your case, you may not be able to fully resolve the problem.  There are some things you could try:

(1) To test my theory, repeat your scenario with speed control turned OFF (you can temporarily disable it with your DCS remote.)

(2) If possible, put the motor with the tach sensor at the rear of the lashup, adjacent to the first car of the train

(3) Take the hoods off your locos, and make sure there is no mechanical resistance or binding in any of your motors or power trucks.  You can check this by turning the flywheels slowly by hand.  You might have to turn each one several times for all of the gears to run through their cycle.  Any sticking or binding can cause the problem you described.

(4) And finally...  A radical motor-ectomy.  Remove ALL of the motors except the one with the tach sensor and stripes.  This may eliminate the bucking.  It will run smoother and slower on flat track because the other motor(s) are no longer fighting the one with the sensor.  Of course, a single unit with half its weight on unpowered axles may no longer have enough traction to pull the train UP your grade.

Please try the first three suggestions, and let us know what you find.

@Ron045 posted:

Try it without passenger cars.  You said it only happens downhill but can be alleviated by raising speed.  Sounds your passenger cars are pushing thus causing the issue.

Have fun

Ron

 

 

Ron, I believe you are right. This is what I have been saying. I am looking for a fix. I am wondering if it is just the way it is. Probably no fix. I can even see this with a single loco going down grade. Railking RS-1's do this, PRR Centipedes, E-8's whatever. Keep in mind I am pulling 6 premier passenger cars and 2 K-line 21" aluminum. Cars are definitely pushing the locos and causing the traction tire to grab the rail as well.

@Ted S posted:

Is the rear-most unit of your A-B-A set powered, or a dummy?  I'm assuming you have a 4-motor set and the rear unit is powered.

Going downhill at slow speeds below 8 scale mph, what you might be experiencing is called "bucking."  This happens because the worm gear drive used in MTH diesels is self-locking.  The wheels can't turn the motor, and the weight of the train cannot "push" the loco downhill.  When it tries to push, the worm wheel in the last truck block locks up against the worm on the motor shaft.

At the same time, the speed control may detect the slight increase in speed, and begins to rein in the voltage to ALL the motors.  If the timing isn't exactly right, it creates a vicious cycle and you end up with a surging effect.  I've seen this happen, even on a very gentle downgrade, with my own locos.

Lionel's Legacy diesels have back-drivable gears (as did all Postwar diesels.)  So this kind of bucking can't happen.  In your case, you may not be able to fully resolve the problem.  There are some things you could try:

(1) To test my theory, repeat your scenario with speed control turned OFF (you can temporarily disable it with your DCS remote.)

(2) If possible, put the motor with the tach sensor at the rear of the lashup, adjacent to the first car of the train

(3) Take the hoods off your locos, and make sure there is no mechanical resistance or binding in any of your motors or power trucks.  You can check this by turning the flywheels slowly by hand.  You might have to turn each one several times for all of the gears to run through their cycle.  Any sticking or binding can cause the problem you described.

(4) And finally...  A radical motor-ectomy.  Remove ALL of the motors except the one with the tach sensor and stripes.  This may eliminate the bucking.  It will run smoother and slower on flat track because the other motor(s) are no longer fighting the one with the sensor.  Of course, a single unit with half its weight on unpowered axles may no longer have enough traction to pull the train UP your grade.

Please try the first three suggestions, and let us know what you find.

Ted, how do you turn off cruise control from the handheld? One example of a loco is a Premier e-8 with proto 3.    2 motors in each A unit, 1 tach reader flywheel in both A's. I am not able to turn A unit around as the tether would prevent this. I have run these with shells removed in the past. Without a doubt there is no binding(verified by turning flywheel by hand). I have removed the motor from the rear truck and that smoothed everyting out. However, That is not really a fix in my opinion. I feel I am taking away value of the product by removing a motor. I even went as far as purchasing another motor from MTH. That motor made no difference. As a matter of fact, the new motor seemed to make things worse. I reinstalled the original motor. As I posted above, I can even get single locos to the "bucking". Thanks for your response. Any more ideas are appreciated.

Try running without traction tires, How steep is the downgrade ?

@jini5 posted:

I have removed the motor from the rear truck and that smoothed everyting out. However, That is not really a fix in my opinion. I feel I am taking away value of the product by removing a motor. I even went as far as purchasing another motor from MTH. That motor made no difference. As a matter of fact, the new motor seemed to make things worse. I reinstalled the original motor. As I posted above, I can even get single locos to the "bucking".

@jini5 I had exactly the same experience with my first and only MTH A-B-A set.  "Run smoothly at 2 scale mph" my butt!!  (At the time, MTH tech support told me that performance was "within specs." They eventually changed their catalog copy to "runs smoothly 5 scale mph.")  There really is no other solution.  I still have my FT's, but after that I switched to Lionel with the back-drivable gears.  In the early 2000s Lionel had its own struggles with the "Odyssey lurch."  But they learned from the issue and engineered an effective solution instead of milking a cash cow.

I'm not a DCS expert and my DCS set is packed away right now.  It's definitely possible to turn the cruise control off.  Either browse the menu options on your remote, or check the manual.  I'm sure it's mentioned in Barry Broskowitz's guide to DCS, if you have that.

By "turning around," what I meant was to put the loco with the tach reader motor end closest to the train.  I guess if the tach reader is on the rear motor, you could try pulling the train with only that unit, just to see if it still bucks.  I would like to know if the root cause is mechanical, or a timing "feedback loop" caused by the speed control.

@Ted S posted:

 

@jini5 I had exactly the same experience with my first and only MTH A-B-A set.  "Run smoothly at 2 scale mph" my butt!!  (At the time, MTH tech support told me that performance was "within specs." They eventually changed their catalog copy to "runs smoothly 5 scale mph.")  There really is no other solution.  I still have my FT's, but after that I switched to Lionel with the back-drivable gears.  In the early 2000s Lionel had its own struggles with the "Odyssey lurch."  But they learned from the issue and engineered an effective solution instead of milking a cash cow.

I'm not a DCS expert and my DCS set is packed away right now.  It's definitely possible to turn the cruise control off.  Either browse the menu options on your remote, or check the manual.  I'm sure it's mentioned in Barry Broskowitz's guide to DCS, if you have that.

By "turning around," what I meant was to put the loco with the tach reader motor end closest to the train.  I guess if the tach reader is on the rear motor, you could try pulling the train with only that unit, just to see if it still bucks.  I would like to know if the root cause is mechanical, or a timing "feedback loop" caused by the speed control.

Ted, thanks for more insight. Its funny you mention that Lionel has the back drivable gears. I had and sold a Visionline big boy that would not maintain cruise control going down a hill when it had no consist behind it. I sent that loco back to lionel because of the coal load depletion was not working. At the same time I mentioned to them that loco will not hold cruise control down my grade. As the loco started downhill, the weight of the loco was pushing it at a higher speed. When it got to the bottom, The cruise would reengage and it would run as expected.  I made them a DVD of the downhill run. They sent it back to me stating that since their grade was not as steep as mine, they had no way to test/diagnose the problem. They even stated they watched my DVD  but still had no solutions. I felt this was a big time cop out on their part. They could watch first hand what was going on. So..........in the end concerning the Lionel Vision line Big Boy, I am guessing since those gears are back drivable, thats probably why that loco could not maintain the cruise going down hill. Sounds simple to Me. But Lionel had no idea?

 Anyway.......Back to MTH.....I am beginning to see there may not be any solution......Just the "nature of the beast".....Maybe removing traction tires might help as postd above, but again I feel like I shouldn't have to start modifying stuff. The flywheel with the Tach reader is located in each A unit at the rear truck. Therefore the trailing A would have the non tach motor in the front of the A which would be up against the consist. Following motor leads back to the control board shows that both front and rear motor are under the same connector.That would tell me that both motors are getting the same amount of voltage. Again, no binding when turning flywheel manually.

 Do you think there might be a way to keep that truck on the rails by using a stronger spring or using spacers or.....................?????

Again, Nature of the beast here?       Hope not

How steep is your grade?  Honestly I prefer a little speed variation, as it confers a "natural" feel.  Sometimes if the speed control is too tight and reactive, it doesn't feel "train-like."  Instead it feels like you're controlling a robot!

Did you try it with cruise control turned off?  I don't believe that removing the tires, adding springs, spacers, etc., will resolve this issue.  If your grade is excessively steep and the trains are very heavy, it might be outside the range of scenarios the manufacturers envisioned when they designed the system.  Back in the conventional control days, the solution was to put a voltage-dropping resistor on the downgrade.  Maybe- just maybe- you could simulate this by sending a reduce speed command using Lionel's IR sensor track.  Not sure how you could do this with DCS.