Dr Zarkoff wrote:
> It's the interaction of the low-mass tire with the rigid track structure which produces
> the corrugations, and the aluminum center in and of itself introduces a degree resiliency
> in this respect.


The mechanical aspects of BART's reinvention of the wheel, are beyond my specific expertise. But I can offer a few observations from over the years. I agree with the good doctor; the less dense material in the hub will alter total resonances. But not only that, mechanical energy (vibration) is being generated in a more dense material and then transferred to a less dense material. Of necessity then, to accept all that energy, the vibrations must displace the less dense material further, which actually amplifies the sound as it is transferred into the air. If the BART wheels had been solid steel throughout, the hub would vibrate no more intensely than the tire, greatly lessening this effect.

Exacerbating this is the roughly parabolic shape of the hub, which tends to direct and concentrate this amplified sound to the sides of the train. This is why a BART train can be heard up to two mile away, laterally; but remains almost silent to the front, as it literally sneaks up on wayside workers. If they turn their back to an oncoming train, they will not hear it - hence a high death rate. In the tunnels, that sideways transmission of sound just echos and re-echos off the walls and can be absolutely deafening.

Exacerbating noise and vibration even further is the cylindrical wheel treads, which were applied to all cars since about the twelfth car to be delivered. A conical wheel tread on a solid wheel set (the industry standard since forever), will tend to steer itself around a curve, due to centrifugal force causing the outer wheel to ride on a slightly larger diameter than the inner wheel. This greatly reduces flange/rail contact and slip-sliding of the wheel treads on curves, making them much quieter.

Indeed, the first eleven or so BART cars were significantly quieter than those that followed. But all cars since have been delivered with cylindrical wheels; so they slip and slide and skip and jump back and forth. Even worse, a smooth ride only happens when both flanges are in simultaneous contact with the rails. Naturally, this causes significant wheel and rail wear, and lots o' noise; not to mention side to side buffeting of cars with worn-out flanges.

And did I mention the NTSB's findings on a series of BART derailments caused by worn out razor sharp flanges; picking points and climbing over rails worn to a bevel, at what should have been the gauge corner. And I have not yet even talked about the very negative effects on the reliability of BART's track circuits. All-in all, cylindrical wheels were a disaster for BART - still are. Let's hope that lessons were learned - and that the new car orders will correct this. Also learned should be that the technical history of the railway experience, should be respected.

OPB