Hardly anything can make a car more miserable to drive than a persistent shake. And it's not just annoying; over time the oscillations generated by imbalanced or worn parts can literally tear a car apart.
It's easy to exorcise demons for most cars with the obvious remedies but others shake despite seemingly all attempts to cure them. But that doesn't make them incurable. Vehicles are nothing but a collection of parts, each of which can run true on its own. Often an incurable shimmy boils down to an easily overlooked thing, like the way those parts meet or the condition or tune of the parts they bolt to.
What follows are some familiar and uncommon things that can make the best car develop the worst shake. Most are universal but a few apply specifically to our cars, at least to the combinations of parts that we use to build them. We won't address every potential shake source in a car, but if your car still shakes despite these solutions it might need a minister more than a mechanic.
With few exceptions every shimmy diagnosis should start with a good wheel balance. There are two methods: Static and dynamic.
Static balancing equalizes the wheel's mass parallel to its axis (the point around which the wheel spins, say an axle). A classic example of an extreme static imbalance is a bucket swung on a rope. Even if you stop swinging the bucket its stored energy will drag your hand in a circle until it stops. Now think of your car's suspension as your hand and you'll see why that energy causes the car to shake.
An equal amount of weight (a counterweight) applied at the inverse angle of the imbalance will statically balance a wheel. In fact it will achieve static balance whether the weight mounts to the wheel's face or backside. For reasons explained later, narrow wheels and tires respond best to static balancing.
Now to dynamic balancing. Increasing a wheel's width creates a balancing conundrum. It potentially moves the imbalance and counterbalance points further away from the wheel's centerline. A counterbalance added to the wheel's backside to counter an imbalance at the wheel's face would statically balance the wheel; however, that likely won't prevent a shake at high speeds. In fact, doing that might actually make the wheel wobble as if it were mounted to a bent shaft.
Imagine a driveshaft to understand why. If the tailshaft end were imbalanced at 12 o'clock then a counterweight added to the yoke end at 6 o'clock would statically balance it. However, as momentum increased each end would follow its respective imbalance point. In fact that's how the earth spins: it wobbles. Now imagine if your wheels spun that way.
Summary: Static balances almost always work adequately for narrow wheels; wider wheels almost always need a dynamic balance.
Alternative Balancing Methods
On-car balancing addresses imbalances throughout the entire rotating assembly, tire, wheel, brake, hub, and all. In fact, it'll eliminate the imbalance created by removing a counterweight from a Buick drum.
This type of balancing has shortcomings: it balances only statically so it's most suitable for narrow wheels. And it requires that all parts index so they go back together the exact way every time. But the worst shortcoming is availability; the machines and their operators got rare a long time ago.
But balancing beads eliminate all but one of those shortcomings. They're just tiny balls (usually ceramic) that freely roll around inside the tire or, if equipped, tube. As the tire spins it generates centripetal force that holds the beads along the inside of the tire or tube. Ultimately the beads gather at the point opposite the imbalance until they amass enough weight to offset it. At that point the wheel rolls true and the spare beads just distribute themselves equally along the inside of the tire. When the vehicle comes to a stop they all come crashing back down to the bottom.
Like on-car balancers they affect only a static balance; however, they're available everywhere (mail order), don't cost much, transfer to new tires, and constantly correct imbalances over the life of the tire.
1. Imbalance that can be cured by static balancing. Vibration of the wheel while driving is shown at the right. At low speeds a wheel rotates around its principle axis of inertia, its axle (blue line). But at speed a static imbalance can force the suspension to deflect so the wheel can follow another axis called the axis of rotation (red line). Swing a bucket to experience the force a static imbalance can generate.
2. Imbalance that can be cured only by dynamic balancing. Vibration of the spinning wheel shown at the right. Imbalances away from the wheel centerline can generate an axis of rotation (red) irrespective to the axle itself (blue). When the axis of rotation is close to a wheel's centerline (left) the wheel won't wobble. But moving the imbalances apart (right) moves the axis of rotation further away from the centerline and a wobble is likely to follow.
3. A narrow wheel balanced statically can have all of the counterweights on one side (usually backside). But a wide wheel balanced dynamically requires weights on each plane with an imbalance, the face if one exists there.
4. Imbalanced hubs, drums, and rotors nullify a wheel's balance. Buick drums' weights may not be handsome but they're not as ugly as the shake that results from removing them. Most Buick-style drum covers aren't balanced. Personal experience indicates that some aren't even round.