Sounds like a balance problem or out of round tire. You can check the balance on these big wheels by raising the front end & backing off the brakes so the wheels turn freely. Then turn the wheel slowly to see if it will rock back & forth. If it does you can place weights on the opposite side of the heavy side of the wheel. Place a jack stand or block of wood close to the tire & rotate the wheel to see if it is out of round or has lumps on the tire.
George
