Re: Pilot and Trailing trucks - an engineering question

kcsivils Wrote:
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> How do pilot and trailing trucks perform the
> function they were designed for?
>
> What specific functions are they designed for?
>
> Why would a four wheel pilot or trailing truck be
> used instead of a two wheeled version?

Lead trucks help support the front end of the locomotive, lengthen the wheelbase which results in a smoother ride, and guide the rigid wheelbase of the drivers into curves.

The guiding function comes from the bolster of the truck being separate from the truck frame. The bolster stays in line with the centerline of the locomotive, while allowing the truck frame to be shifted to either side as the locomotive negotiates curves. Guidance comes from heart links that connect the bolster to the truck frame


Look at the top center of the drawing to see a heart link (the source of its name should be obvious). In the assembly view in the lower left you see where the heart links connect the truck frame to the bolster. As the truck fames moves to one side in a curve, the heart links pivot out of vertical to allow that, but the weight of the locomotive bearing down on the bolster and the heart links causes them to want to return to vertical, and that causes lateral resistance in the truck to being off center, increasing as the truck frame moves farther off center in a sharper curve. That resistance helps to guide the locomotive around the curve, and the heart links help the truck to stay somewhat rigidly centered on straight track for a smoother ride.


A four-wheel lead truck does the same thing only more so. Two axles allow for more weight to be carried, and more weight means that there is more centering effort in curves and on tangents for a better ride. As a result, high speed locomotives usually have four-wheel lead trucks, for better guidance.

Trailing trucks share some of the roles of lead trucks regarding guidance, etc., but their primary role most of the time is to support a larger firebox than an engine without a trailing truck has room for. Four (and six) wheel trailing trucks came about as fireboxes grew to sizes that were too heavy for a two-wheel trailing truck to support.