Center of gravity

[lionelsoni]

The "Tips-Tools-Techniques" article by Paul Ullrich on the last page of the March, 2014, CTT says, "If there are a lot of wires or lights on the floor of the car, mounting the hinges on the roof is the best way to go.  At 1:48 scale, they don't affect the center of gravity."  The roof may well be as effective for the purpose of resisting string-lining on curves, but not because the center of gravity (or more exactly, the center of mass) is not raised, but despite the fact that it is raised.

[fifedog]

I thought awhile on that too, Bob.  Although I thought the hinge under the hood was clever, I wondered how it would tax the motors.  And 'tis better to keep the center of mass low and over the trucks.

[lionelsoni]

I think that the situation, as far as string-lining is concerned, is that, in a curve, there is a lateral force on the coupler that is opposed by the vertical force of the weight of the car, to keep the truck from tipping inward toward the center of the curve.  When it does tip, the truck will rotate about the inner rail.  So the torque resulting from the lateral coupler force is that force multiplied by the coupler height, while the torque resulting from the weight is that weight multiplied by its lateral distance from the inner rail, which is going to be 5/8 inch for a symmetrical car, regardless of the height of the center of mass.  Once the car begins to tip, the center of mass moves inward toward the center of the curve, and a higher center of mass moves more quickly in that direction.  But by then the derailment is inevitable (unless the train is immediately stopped); so the height still doesn't matter.

Where the height of the center of mass would matter is in high-speed cornering, where the problem is again a lateral force, but now the outward force producing a torque about the outer rail that is proportional to the height of the center of mass.  Of course, if you've got a string-lining problem at low speed, then running fast around a curve, so that the two forces cancel to some degree, is a good thing.