Originally Posted by Road King
Doh! I forgot to say which motor mount it was that they replaced -- it is the dead-center front mount. Free play/engine movement was in excess of 2 inches according to the service writer -- should be less than 1 inch.
The vibration for the stiffer chassis is sometimes due to tires that roll with a different amount of crush on one side or the other. Even though they are balanced on a spin balancer, under weight they don't roll quite round. This slight up and down movement has an impact on the chassis. The lighter suspension parts (aluminum alloy?) react more than older parts that were heavier react to the slight up and down movement.
Balance the wheels on a Hunter 9700 Road Force balancer with a technician who knows what they're doing (someone'* dealer may have had them who posted earlier).
That resolves the out of roll rolling unless the amount of mass goes above 12 lbs or so. That'* what they easily got replacing the tire with Michelins. Other brands sometimes have a much greater amount of crush runout.
GM did not discontinue the chassis because of problems with the tire companies not making high enough quality tires. Someone earlier said they had discontinued the H bodies for that reason!!!