2 truck Shay jerks at slow speeds

[rperego]

I have a 2 year old lightly used Shay that jerks rhythmically at slow speeds.  Doesn't appear to make a difference whether it is on straight or curved track.  All track is flat.  It is DCC converted.  Track is clean and other locos with the same make/model decoder operate smoothly.

Ideas and/or diagnostic things to try?

Thanks, Bob

[rperego]

I just uncoupled the connection from the front truck to the fake piston mechanism to see if it is binding.  Twisting the piston mechanism by hand doesn't reveal any binding.

While waiting for suggestions, I may try running on DC (hopefully trusting the Tsunami folks that I won't hurt the sound board - didn't put in a switch to disable it). 

[altterrain]

Dirty power pick ups?

[rperego]

I thought about that but I doubt it is the problem for this reason:  With the Tsunami sound boards I use, if you mute them and the power is lost for just a split second it un-does the muting - which isn't happening.  Also, it does this all the way around the layout - not just in certain places. 

Last year (only drag this out for the holidays) I remember following TOGs advice on checking for no weak springs in the contact mechanisms.  Hard to know what weak is but their resistance was at least pretty consistent.  However, I'll clean them anyway.

[glennk28]

I assume you mean you disconnected the driveline somewhere between the two trucks. Did this cure the problem?  Sometimes the two motors are running at slightly different speeds and are  "fighting" each other. This is why the replacement drive shaft had to be made for the Climax.  Same thing happens on Shays.   gj

[rperego]

If we're on the same page, I don't see an issue.  I can envision a problem if the motors aren't running at the same speed, but I don't understand your comment about the drive line.  It appears to me that the drive line going to the forward truck is connected to the wheel, whereas the one going to the rear truck isn't connected to the wheel. 

E.g. when I disconnect the front drive line, I can twist the female part connected to the dummy pistons.  This only works because the one going to the rear truck isn't really connect to the wheel.  I shouldn't be able to do this if the rear drive line were connnected to a wheel - agree?

[rperego]

I got the answer from Dave G.   He says this is not uncommon with Shays because the motors seldom run at exactly the same speed accross the entire speed range, hence they're fighting each other.   I may have a worse than normal pair of motors but Dave says even with swapping motors around it's difficult to find a pair that run exactly the same.

Case closed.