Q: I’ve been a reader for several years, but don’t recall reading about the tightness of truck screws before your November issue. In “How to weather a covered cement hopper,” Cody Grivno writes, “One [truck] screw should be tight enough that the truck rotates freely but doesn’t wobble. The other should be loose enough that the truck has some lateral and longitudinal play.” Could you explain the rationale behind this? – Al Quosig, Elkhorn, Wis.
A: The reason is that track is not always smooth and level. If the screw on a model rail car’s truck is tightened down, the truck will be able to pivot right and left, but only in one level plane. If both trucks are tightened this way, then (in theory) all four axles will rotate on the same plane. But if that car encounters a spot where the track dips or rises, at least one side of one truck could lift from the rail, potentially causing a derailment. However, if one of the truck screws is left slightly loose so the truck can wobble, two of those wheelsets effectively become a single unit that pivots at the kingpin, forming what we call “three-point suspension.” When this car comes to an uneven section of track, the loose truck, or the entire car, can tilt slightly to keep all the wheels on the rails.
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