the center pin of the conventional truck extended the
rigid wheelbase of the engine, and caused the truck to act much as
would a single set of leading wheels fitted rigidly to the engine frame
far ahead of the front driving wheels. Bissell proposed to correct the
faults of the conventional truck by fitting the locomotives with his
invention, the first practical safety truck to be patented. Since the
primary requirements were to keep the leading wheel axles at right
angles to the rails whether on a straight or curved track, and to allow
the driving axles to remain parallel, or nearly so, to the radial line
of the curve, he moved the center pin to a point behind the truck and
just in front of the forward driving axle. This shortened the wheelbase
of the engine and removed the danger of the pintle serving as a fulcrum
between the truck and the driving wheels, thus allowing them to assume a
comfortable position on a curve.

[Illustration: FIGURE 4.--A 4-wheel safety truck fitted with A. F.
Smith's swing-bolster centering device. Built by the Hinkley Locomotive
Works. From Gustavus Weissenborn, _American Locomotive Engineering and
Railway Mechanism_, New York, 1871, pl. 88.]

Since the truck could assume the correct angle when entering curves, it
was claimed in the patent specification that, unless all four wheels
were simultaneously lifted off the track, the truck could pass over
"quite a considerable obstruction."[3] Bissell further claimed:

In running on either a straight or curved track one of the truck
wheels often breaks off, and the truck swivels around on its center
pin in consequence, and throws the engine off the track, but with my
device one wheel, or even the two wheels on the opposite sides
diagonally of the truck might break off and still the truck would
not run off, because its position is set and it has no axis of
motion around which it could swing....

The other problem Bissell wished to correct was the oscillation and
chatter of the leading truck. This was accomplished by a simple
centering device in the form of a pair of V-shaped double incline planes
(D on fig. 3) situated at the center of the truck frame (A). The lower
planes of the pair were fastened to the truck frame and the upper, cast
in the form of a bridge, were attached to the locomotive frame (C) by a
center plate. But while the portion of the locomotive's weight assigned
to the leading wheels was borne at the ce