Trucks with two dia. wheels?

The trailing trucks on the NYC hudsons have different diameter wheels front to rear.

Why ? Don't know, don't care.

Ron

Some trolleys had what were known as "maximum traction trucks", in which the axle with the larger wheels had the motor, and the other axle with the smaller wheels was just there to steer the truck through curves and switches. Essentially these were two-axle cars with steerable drive wheels.

The trolley museum I volunteered at has a number of cars with these trucks--none are allowed to carry passengers, or otherwise venture out onto the main line without close supervision--since there is much less weight on the smaller wheels, they were prone to derailments on less-than-perfect track (i.e. anything that wasn't girder rail)

---PCJ

Streetcars frequently were built with drop platforms.  The maximum traction truck allowed the smaller wheels to be run outboard putting the smaller wheels under the drop platforms.  There was a lot of effort put into reducing the number of steps and getting the step as low as possible.  This sped up loading and unloading the car.  With only one motor in a truck, it was advantageous to shift as much weight as possible to the motor axle, this allowed smaller journals and wheels on the more lightly loaded axle.

With the development of the high speed traction motor, the motors were made smaller in diameter, so the wheels could be made smaller, and the car built closer to the street.  This improvement put an end to the drop platform car design and the need for maximum traction truck.

Interestng about the "maximum traction truck", and alleged derailment problem.  If

derailment was a problem, I would not think they would have been used, even in that

much less litigious time.  Would seem to counter the purpose of having small diameter

pilot trucks proceeding large drivers on steam engines.  Have not heard that McKeen's,

with the smaller wheels at the front of the power truck, were prone to derailing, in

spite of faults of primitive engines. I do know that double diameter truck makes scratch building a model McKeen that much more of a challenge.

When you're running on girder rail, it's not a problem since the raised inner lip of the flangeway keeps the smaller wheels in check. From what I heard, when the cars were in service, constant use kept the trucks flexible enough to deal with occasional stretches of T-rail. In our museum, we have switches equipped with spring frogs to accommodate the variety of flange widths (streetcar, interurban, rapid-transit, etc), and the 'pony wheels' would rather climb over the railheads than push their way through.

---PCJ