tmorris@convex.UUCP (09/05/85)
I've got to agree with the previous response . As a former
tech. for a GM dealer , I saw quite a few cars develop those symptoms
after a change of tires . Radial tires put on older model Chevy ,Buick,
and Olds wagons were the worst offenders , with large sedans coming in
second. GM later countered with RTS ,or Radial Tuned Suspension. When
the gas crunch caused vehicles to shrink , the problem all but disappeared
as the vehicle mass/tire sidewall stiffness ratio improved.
The radial ply counts heavily on the centrifugal force of the
spinning wheel to maintain stiffness . You can see this take effect
on a spin balancing machine , as the sidewalls pull in about 1/4"
when the tire attains the rotation equivelent to about 50 mph.
The effect is further discernible at rest , when the tires display
"Radial Squat" , making them appear like they need air.
Unfortunately ,at 3 mph , the force to stiffen the sidewalls
just isn't there. This is especially noticeable when braking , as the
vehicle is experiencing a sudden weight transfer , and at the same time
the centrifugal force on the sidewall is suddenly decreasing...
HAPPY MOTORING
.