Re: 33 inch wheels

Earl Hackett <hacketet@...>

One of the things that most people don't think about is the energy required
to get bigger wheels moving. You have to accelerate the wheel rim to twice
the speed of the train - observe the speed of the top of the wheel vs the
speed of the train. As the center of mass moves away from the center of
rotation (the axle) the angular momentum goes up with the square of the
distance. So as a wheel gets larger you get hit with a double whammy. The
center of mass (most of it is in the rim of the wheel) moves away from the
center of rotation and the mass itself increases in proportion to the
circumference of the rim. So wheel diameter is a tradeoff between reduced
stress at the wheel/rail interface, and the ability of the locomotive to get
the train moving within a reasonable time frame. In the early days with
wimpy locomotives small wheels were a necessity. As locomotives became more
powerful, larger wheels became more practical to operate.

This was never more clearly demonstrated than with a Pinewood Derby car I
helped my son build. The thought was that reducing the body weight would
reduce axle friction and allow the car to roll faster. So I machined him
some weights that would fit inside the plastic wheels. When run on the
track with the other cars all of the same weight, by comparison it just sat
there. It took over twice as long for it to roll to the finish as the
slowest car in the bunch! OH YEAH, I forgot about angular momentum! Try to
explain angular momentum to a 5 year old who just got clobbered in a race.

Earl Hackett

Modeling the C&O in 1952

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