Vibration Above 70 MPH in a 1968 Mustang

When dealing with a 70-mph high-speed vibration, the most common of all is a bent wheel or out-of-balance tire. A little better understanding of a tires construction will go a long way in preventing costly problems and instilling a greater insight into safety as related to tires.

Tire Construction And Grades

The construction of a tire is quite remarkable and complex. A typical tire may endure 40,000 or more miles over some of the hottest road, roughest surfaces, a wide range of loads as well as speeds. The circumference of the world is 24,901.55 miles, so your tires made it around the world twice or very close. No matter how expensive the vehicle or technologically advanced, without the tire its useless.

Tires have three main components. The bead that is the center of the tire and mounts to the wheel is made of multiple strands of wire encased in polyester or another compound. The carcass is the main part of the tire under the tread and attached to the bead. Depending on the quality of the tire, it has multiple layers of polyester, rayon, nylon or even Kevlar. It comes from a flat sheet and is formed to glue it to the bead. Finally, the tread comes off a roll and is glued on and cut to fit. At this point the tire is green. Carbon black is applied and the tire is placed in a clam-shell mold. It is heated to a high temperature to mold the components together and cure the tire. This is a slight oversimplification; however, it gives the correct insight to understand where the problem can arise.

Problems With Tires That Cause Vibration

Vibrations can be caused by a variety of tire-related problems. They are always heavier on one side of the tire due to their construction. Balance weights must be installed opposite the tire imbalance to counteract this. The wheel is usually heavy on one side as well, so they both come into play when balancing. Centrifugal force plays the biggest part in the vibration when coupled with an out of balance tire and wheel. At slow speeds up to 45 to 50 mph there will be no effect, because centrifugal force is insufficient to increase the weight factor. You could literally throw away the weights up to that point with no imperceptible symptom. Above 50 mph this force becomes formidable and will cause the tire to bounce and increase in intensity as the speed increases.

Unusual tread wear due to a weak shock, a loose wheel bearing, worn tie rod, poor alignment or worn ball joints can cause high-speed vibrations. However, some symptoms will be noticeable at all speeds, ever increasing with speed. Weak shocks can cause flat spots to appear around the circumference of the tread but must have an instigator to start the tire bouncing, such as poor balancing. Panic stops where the tire was locked up and skidding will flat spot a tire in one spot. This will barely be noticeable until the speed is increased.

If a tire is run underinflated, its temperature increases as the steel in its belts is flexed excessively. Remember, the tire was heated to mold the parts together. If the tire attains close to the same temperature the tread can easily separate from the carcass. By lifting the tire off the ground and rotating it while watching the tread, you can see this. If the tread is not even or straight it is separating. In a severe case, the tread will be virtually lifting or tearing off the carcass in the form of a big bump in the tread. This situation will be noticed primarily at very low speeds by the front of the car shifting laterally from the left to right. As speed increases, the shaking remains; however, its less in intensity. This is a dangerous situation and must be corrected immediately or the tire will blow out.

Drivetrain-Produced Vibrations At 70 Mph Or Above

The driveshaft is the only other possibility that will give no indication at all of a vibration until speed increases and the speed of rotation increases enough for centrifugal force to take effect. The rear axle can be supported on jack stands and the speed increased to locate this vibration. It can be corrected with two hose clamps. Install them on the driveshaft keeping the adjusters next to each other acting as a weight. Rotate and tighten the clamps until the imbalance is corrected.

Other Components Capable Of Creating Such A Vibration

There are many components that can cause a high-speed vibration; however, each will have a perceptible symptom at slower speeds. Warped rotors will cause a pulsating pedal. Ball joints will clunk and shake violently at times on a rough surface. Tie rods will induce loose steering and a wobble in the tires. A bent wheel will give the same symptom of a separated tire. A universal joint will make noise and low-frequency vibration at slow speeds. Worn ring and pinion gears will whine and have excessive backlash as well as different noises when loaded or coasting. Manual transmission input shaft will shake at slow speed, causing a slow vibration. Ripped flex plate on an automatic transmission will cause slow as well as high speed vibration.