Bachmann 4-8-4 Northern Tender and NMRA wheel gauge

[joe935]

  I decided to check the wheels of both the engine and tender with the gauge.
All of the wheel distances had to be adjusted outward acording to the gauge. After I finished adjusting and proceded to test the performance the tender would not stay on the tracks.
The Ez-track seems to be in adjustment acording to the nmra gauge. So i then decreased the wheel distances of the tender which has 8 wheels per truck and the tender stopped jumping the track. This was occuring anywhere on the layout. In fact the rerailer would cause the wheels to derail.
Any one have an explanation?
One other thing adjusting the wheels of the engine seems to have stopped the engine hunting problem.

[James in FL]

The wheels need to be slightly under gauge to all fit within the confines of the arc of the radius. This would include the drivers on your northern as well as the eight wheel trucks. If they were all in proper gauge, the lead wheels and the trailing wheels would want to climb the rails while the middle wheels would be within the arcs.

It is absolutely necessary for the Northern 8 drivers to be slightly under gauge as well as the eight wheel tender trucks. Your lead and trailing trucks should, however, be in gauge as well as all four wheel trucks.

I can explain it better with a crude picture.





The tighter the radius the further under gauge they will need to be.
I don't know the radius that a perfectly gauged set of  Northern drivers will handle, but I would guess it to be above 20-22 inches, taking into consideration tolerances and slop.

[joe935]

Thanks James. All makes sense now.