Prostar bouncing/shaking when empty

[Chamberpains]

Check your steer tire mounting, brake drum mounting and if you run equal or anything in the tire for dynamic balancing make sure its not clumping. I've had two occasions within the last year that my steer tires bounced and shook at those speeds and with the exact symptoms you describe.

The remedy that worked beautifully each time was take the steer tires off. Check runout, proper tire to rim mounting, spin balance and the most critical step... using proper stud centering pins to mount the drum and rim correctly.

Its a cheap easy check.