,
Let’s start the Engineer to English translation with the first part of that. If there are once-per-cycle imperfections, such as a mass (weight) imbalance, out-of-roundness or a variation in the stiffness in parts of the rotating wheel assembly (tire, wheel, hub and/or brake rotor), rotating forces (often called “Road Forces”) can be produced at the same frequency as rotation, with amplitudes relative to the imperfections. Eric continues, “At highway driving speeds the rotational frequency can coincide with the first natural frequency of the unsprung mass, causing resonance or amplification of the vertical component of the rotating force and potentially producing what is called a wheel hop.”
Small tire irregularities or bends in the wheel can cause a vibration at a specific speed—typically this is just under 70mph. That wheel hop is the force or shock that you will feel in the steering wheel or possibly even in the seat of your pants as it resonates through the chassis. The wheel hop resonant frequency (the speed or frequency where you’d feel it the most) is determined by tire spring rate, suspension spring rate and weight of the unsprung mass. Eric adds, “Due to the design constraints of normal passenger vehicles, this frequency typically occurs in the 10-15 Hz range, which unfortunately at some point will coincide with the rotational frequency of a vehicle’s wheel assemblies operating at highway speeds.”
Eric takes off his glasses, rubs his eyes and states, “In the case of the 2013 Ford Fusion driving at 68 mph, some quick calculations based on the tire dimensions suggest that the rotational frequency of the wheel is about 14.5 Hz, which is likely coinciding with the wheel hop resonant frequency.” He didn’t even need to grab his calculator for that one.
These HRE FF01 wheels for Project Mustang were mounted with extreme care.As for Doug and his Fusion, there could be a bend in one of the aluminum wheels (not that out of the ordinary, considering the roads around Cleveland) or a wheel weight might have come loose. One of the tires may also have a defect—something that OE tire manufacturers work very hard to eliminate. Tires can also become out-of-round after extended periods of sitting as the nylon, steel and rubber plies take a set. This latter issue can often be eliminated by taking the car on a longer drive so the tires can get up to temperature. There can also be damage to the sidewall—something that The Tire Rack's Woody Rogers sees quite a bit when there's enough of an impact to hurt the tire without actually damaging the rim.
The speed dependency of the vibration is simply due to how what could be a small issue with one wheel or tire is coinciding with a natural frequency of the chassis and suspension. Some conjecture based upon the fact that it’s felt most noticeably through the steering wheel is that the issue is with one of the front wheels or tires. The best bet is to take it into a dealer or a tire shop for a look-see. They can spin the wheel and tire on a balancer to check for bends in the wheels, tire irregularities or missing weights and get the situation smoothed out.
