[PRCo] PCC speeds

[Fred Schneider]

We have often seen these arguments about a PCC being clocked at 70 miles
per hour.  I've generally given up responding to people with inaccurate
speedometers in their automobiles (or the inability to detect that they
are rapidly gaining on the PCC).   I've pointed out before that the
speed/time charts for a PCC call for essentially linear acceleration at
4.75 mphps to about 15 miles per hour, a slightly tapering curve from 15
to 25 mph (obtainable in a city block), and then a curve that rapidly
flattens out until the car reaches balancing speed of 42 mph after
nearly a mile.  (Based of course on level, tangent track, 550 volts, and
a seated load.)  Of course acceleration will be slower up hill and
faster down hill.  It will also be faster at 600 volts and slower at
475.  If I remember the curves, balancing speed on a grade as steep as
Henderson St. (Fineview) in Pittsburgh was about 15 miles per hour.
Note here that up hill through South Hills Tunnel cars probably ran at
20 mph or less, and it was the noise that made you feel you were moving
faster.

Well, I had a long talk on the recent trip to the West Coast with a
friend who accompanied me for ten days on motor design parameters.  Jim
Holland should know that this discussion actually took place in a
Chinese restaurant in Dumbarton or Pleasanton, California.  Bruce was
explaining  motor failures owing to speed.  I learned something here.
Safe speed before the commutator starts to throw the brass bars owing to
centrifugal force is based not on rpm but in inches per second measured
on the surface of the commutator.  Thus a larger diameter commutator can
run faster (a higher rpm)  than a smaller one.  I think his number was
about 300 inches per second.  For a PCC car, the maximum safe speed
works out to about 50 miles per hour.  You guys can do the computations
if you wish.  I did it last night and confirmed the approximate 50 mph
speed that Bruce told me.   The basic limit here is how low you want
your car to be to the ground ... you could run your PCC faster but it
would require larger motor cases and/or larger wheels.   Shaker Heights
got a 2% increase in safe top speed simply by place one inch larger
wheels under their cars.  Unfortunately this raised the first step by
1/2 inch.

Because the commutator is not used on AC motors, this appearently is not
the same limiting facter on newer cars.