I was reading the review of the Branchline NYC coach in the latest Model
Railroader, and Andy Sperandeo wrote: "As the instructions note, many of the
prototype cars rode on four-wheel trucks. Although Branchline isn't offering
these yet, the body bolsters have extra kingpin holes for four-wheel trucks."
I'm not a passenger car modeler so that comment left me confused. What's up
with that? Why would a car with four-wheel trucks be supported at a
different location than one with six-wheel trucks? Is that just specific to
the models (perhaps for truck swivel clearances), or was that the case for
the prototypes, too?
On most model 6-wheel trucks, the kingpin is offset from the center (i.e.
not in line with the middle axle) to allow accessfor a screwdriver or other
tool, whereas no offset is needed for 4-wheel trucks so the kingpin is
almost always centered.
All pictures of prototype 4 and 6 wheel passenger car trucks I have seen
have the kingpin centered with respect to the axles. Gary Q
6-wheel trucks usually had longer wheelbases than 4-wheel trucks, so
sometimes the bolster/kingpin could NOT be physically mounted as close
to the ends of the cars.
Normally the prototype bolster will be in the center of the truck as
others have stated. On models with 6-wheel trucks it's often off-center
on the truck (which likewise then affects the bolster location). This
could be to allow access to the kingpin for truck removal, or to alter
the 'swing' characteristics of the trucks for clearance reasons on sharp
curves. Usually the kingpin, if offset, will be offset toward the outer
ends of the car. This causes the outer ends of the truck to swing
sideways less when the truck pivots (and the inner ends to swing more).
This may reduce interference with end steps or other details, depending
on their location.
Mark Mathu wrote:
> > On most model 6-wheel trucks, the kingpin is offset from the center (i.e.
> > not in line with the middle axle) to allow accessfor a screwdriver or other
> > tool, whereas no offset is needed for 4-wheel trucks so the kingpin is
> > almost always centered.
> Thanks, that makes perfect sense.
> > All pictures of prototype 4 and 6 wheel passenger car trucks I have seen
> > have the kingpin centered with respect to the axles. Gary Q
> That makes sense, too. The prototypes would probably have their support
> points at the same location regardless of how many wheels it rose on.
6 wheel trucks are longer so the centres would be moved in towards the center of
the vehicle if you want to retain end steps.
The pivot pin of the 4 wheel truck can be (and should be) at the center of the
truck, but the 6 wheel truck is going to have an axle there.
In addition, getting the truck to the proper end of the car means that the 4
wheel truck, being shorter, needs the kingpin closer to the end of the car.
The 6 wheel truck should rotate about the center of the truck but many
manufacturers make the truck's kingpin offset for many reasons, not the
least of which is to be able to easily put the kingpin in as the center
wheelset is directly under the proper position of the kingpin.
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Mark Mathu wrote, "prototypes would probably have their support points
at the same location regardless of how many wheels it rode on".
Carbodies are supported by more than just the bogie centres. Radial
bearers also support the carbody. The location and number of radial
bearers can differ between four and six- wheel bogies.