Steerable tag axles were introduced by Newell during the 2004 model year. They are provided by ZF. I know coach #699 and #701 both had a ZF steerable tag axle. I don't show any prior to that but there may be.
The ZF steerable tag used by Newell is active steering using hydraulics to steer the tag. At low speeds, as the steering wheel is turned, the tag axle turns in the opposite direction to the front wheels. There are two advantages to a steerable tag on a large, heavy coach: 1) the tag tires do not scrub when turning and there is no transfer of weight off the tag to the drive axle which would occur with a lifting tag or an air pressure release on the tag and 2) the turning circle is reduced significantly. ZF also makes a passive steering tag axle.
The degree of turn of the steering tag axle increases (at low speeds) as the steering wheel is turned sharper. At anything other than low speeds, the tag is locked in the center position.
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