Quote Originally Posted by Woody View Post
(this from Jon's post)
But lifting the tag when cornering is really not required because over the life of the tires, if they are inflated properly tag axle tires are not being scuffed sideways any more than the steer tires are during turns. I don't know this for sure, but I would suspect steer tires on average have a shorter lifespan than tag axle. When we turn the front wheels to alter the path of the bus those steer tires are resisting the forward motion of the bus and are being scrubbed sideways as the bus turns.

Just curious - is tire rotation ever considered?
The steer tires are rotating along the direction of the turn and they define the turning circle and have the easier time of it insofar as scrub, the drive axle wheels are the privoting point that describes the other point on the chord of the described turning circle. The tag axle wheels are outside the turning circle and are have a (sideways) component more at an angle to the tangent and 'dragged' (or forced to distort) more sideways than the wheels on the steer and drive axles. This is why they are 'scrubbed' significantly more. They are at the opposite end from the fulcrumm (the drive axle). It is sufficient enough to cause widening of the turning circle and the reason for 'lifting' or removing weight from the tag. It is also the reason for steerable tags that tighten the turn circle by removing this geometric issue, as on the recent Newells, and at various times M.A.N., Daimler-Benzes, Setras, Tatras, and others on some of their commercial bus offerings over the years. Interesting to some may be the reasoning for, and physical constraints against, that invalidated Ackerman steering (where the steer wheels were articulated to describle the two different turning radii, but failed to take into account the greater slip angle(s) of weighted outside tire (and is related to tuning based on specific tire characteristics). Other more distantly related topics we could discuss informally at POG9 might include the geometry behind what is/was known as zero (and plus or minus) point steering, the geometry behind unequal length wishbone suspension systems, camber, caster, toe-in etc.. Or, I can keep my big mouth shut (probably best).