Q: My car starts shaking when i accelerate over 50 mph. I have had my tires balance and rotated. I put air tires and new brakes.

Common causes of the vibration you are describing include wheel bearing faults, wheel/tire assembly imbalance, and defective tires. Other, less common possibilities include suspension faults (strut/shock failure), loose or worn out motor mounts, and even exhaust system faults. Certain tire defects, particularly internal belt damage or separations, as well as irregular tire wear will cause vibration. Defective tires will vibrate no matter how "well" balanced they are and indeed some brand new tires with irregularities in sidewall stiffness and excessive radial force variation will vibrate no matter how well balanced. Contributing factors in the circumstance you are describing could be worn struts in the suspension. If there are flat spots on the tire tread, that would be indicative of worn struts. If you request a noise/vibration diagnostic, a certified mechanic from YourMechanic would evaluate all the possibilities and resolve this for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.