A large wheel increases the liability to bent axles in curving on
account of greater leverage unless the size and weight of the axle are
increased to correspond, and the wheel itself must be made stronger. A
four or six wheel truck will not retain its squareness and dependent
good riding qualities so well with 42 in. wheels as with 33 in. ones.
Besides the brakes, the pipes for air and steam under the cars
interfere with large wheels, and as a consequence of all this 42 in.
wheels have been replaced by 36 in. ones to some extent in some places
with satisfactory results. On one road in particular so strong is the
inclination away from large wheels that 30 in. is advocated as the
proper size for passenger cars.
On the other hand, there is no doubt a car wheel may be too small, for
the tires of small wheels probably do not get as much working up under
the rolls, and therefore are not as tough or homogeneous. Small wheels
are more destructive to frogs and rail joints. They revolve faster at
a given speed, and when below a certain size increase the liability to
hot journals if carrying the weight they can bear without detriment to
the rest of the wheel.
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