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There are many vehicle conditions that can cause your vehicle to shake. In short, anything that rotates can cause vibrations: Transmission, drive line, brakes, motor mounts (ok, the motor mounts don't rotate, but if they're worn, they can allow vibrations to be transmitted from the engine), and of course tires. Also, worn suspension parts can allow for vibrations. Granted, tires are the most common cause of vibrations and they're usually the easiest to diagnose and cheapest to fix, but they are by no means the only cause of vibrations. In this article, we will give some simple guidelines to "pre-diagnose" any shakes you have so you'll know where to turn for help.
In this article we'll cover:
Shakes That Are Not Caused By The Tires:
It shakes when you hit the brakes, particularly under hard braking:
This is very common, and is almost always caused by warped brake rotors (or drums, but usually rotors). The rotors are the part of the disc brake that rotates, and when you apply the brakes, the calipers squeeze the rotor to make the vehicle slow down. Over time, heat causes the rotors to warp and you get a pulsation in the pedal and often in the steering wheel, especially under hard braking. If this is the type of shake you have, you should have the rotors checked for warping. If they are warped, they can be machined (turned) to be straight again, but you can only turn them a couple of times before they get too thin, at which time they must be replaced.
The front end wobbles at a certain speed, but not every time you drive that speed:
Some vehicles can pick up even violent wobbles (oscillation) at a certain speed, but not every time you drive at that speed. This is usually caused by loose suspension parts or bearings. What happens here is that a loose suspension component can allow the front end to start wobbling, like when you hit a bump, and once the wobbling starts, you have to slow down, or even stop to correct it. Tires are very consistent, they will always do the same thing at the same speed. So if the vehicle only wobbles some of the time, it is usually a loose or worn out suspension component that is to blame. In this case, you should see your mechanic, or we can look for the cause if you want, but we don't do this type of repair.
The vehicle wobbles while accelerating hard, or climbing steep hills
This type of vibration is usually caused by motor mounts. If they become worn, vibrations can show up when accelerating hard, or pulling hills. Experienced mechanics can diagnose this easily, and again, this is the realm of the mechanic.
There are actually many more types of "non-tire" related shakes, but these are simply some of the most common. Transmissions and drive lines can cause shakes too, but they are not quite as simple to diagnose.
Shakes Caused By The Tires:
As stated earlier, tires are very consistent, they will wobble at the same speed every time, whether, you're driving at a constant speed, braking, or accelerating. So if you car shakes at the same speed (or speeds) every time, tires are the first thing to check.
First Things First: Check for Cupping and Flat Spots
The simple test is to run your hand along the face of the tire, and you can feel it. (You may want to make sure you don't have steel cords showing on the tire first, so you don't get cut!) If the tire is flat, then good, if it has small waves like on the sea, you have cupping. Even though the tire in the picture is worn out, it clearly shows what we mean by cupping. If you look along the edges of the tire, you can see that one tread block is raised up, and the next one in the row is almost completely smoothed out, then the next raised again. If you can imagine what this would feel like if you ran your hand along it, then you should be able to imagine what this would feel like if it was mounted on your car!
Cupping can be caused by alignment problems, or inherent vehicle design issues. Rear tire misalignment on front wheel drive cars is a textbook cause of cupping. In the middle picture you can clearly see the dished out spots on the left side of the tire, but if you look carefully you can also see a diagonal "valley" in the tire. It goes from the outer flat spot towards the middle of the tire and downward in the picture. Rear wheel misalignment will always show this diagonal type pattern. If this type of wear shows up on rear of your front wheel drive car, the only way to correct it is to have a four wheel alignment done. You can put new tires on it, but unless you have the rear of the vehicle aligned, it will just be a matter of time before the new tires are cupped also.
On the other hand, all kinds of vehicles, particularly four wheel drives, but even high end European cars, can have inherent design issues that can cause cupping. On many of these vehicles, the only thing you can do is keep the tires rotated, and if possible, choose tires with straight groove type treads. A straight (circumferential) tread tire will cup less than more aggressive tread tires. The tire pictured above is a good example of an aggressive tread tire used on a four wheel drive. The tire directly to the right is actually off a trailer, but is a good example of what we're talking about. Usually, having the car realigned and the tires balanced will not solve this type of problem but should be done anyway as a precautionary measure.
Flat spots are caused by locking up the brakes. This is not usually a problem if you have anti-lock brakes, but if you've had to stand on the brakes for any reason and had them lock up, you can have flat spots on your tires. If you've locked the brakes, and slid a long way, you will definitely have flat spots. Simply run you hand along the tire again, or visibly inspect it, making sure to turn the tire, (so that you don't miss it if it's on the bottom). A flat spot is simply what it sounds like, instead of the tire having a round profile, there will be one spot on the tire that has been shaved off, (or flattened). You can feel it with your hand, or if you spin the tire, there will be an obvious dip when it gets to that spot. If it's not too bad, you can sometimes rotate the tires, since the front tires will usually be worse than the back. If not, the flat spotted tires will have to be replaced.
Again, tires shakes are very consistent, they will always shake at the same speed(s). So if your tires have passed the first test, you can often tell what kind of a problem you have by the speed that the vehicle shakes (or wobbles) at.
Vehicle Wobbles At 15 to 20 Miles Per Hour (mph)
If your vehicle wobbles at 15 to 20, go directly to a tire shop, do not pass go, do not collect $200.00, definitely do not hit the freeway with your family in the car, go directly to a tire shop. Though this can be caused by a severely bent wheel, or severe cupping, the most likely cause is a separated tire. Separated tires are a very serious safety issue, and must be dealt with immediately, since it's just a matter of time before the tire blows. A tire failure of this type almost always causes damage to the vehicle, but that's nothing compared to the many fatalities that are cause by this. Do not put this off!
To test for this, simply drive the vehicle for a short distance at 15 to 20 mph. If the bad tire is in the front, the steering will rock back and forth, if it's in the rear, the rear of the car will wobble in a similar manner. If you can find out which tire is separating, it may be best to put the spare on right then and there, so that you can actually make to the tire shop. The tire with the problem will have a bump along the tread face which is visible, or you can feel it with you hand. If you use the hand test, the tire should have a uniform profile except for the place where the separation is, which will be distorted, usually with part of the tread pushing away from the tire, ie.; a bump on the tread face. The pictures below illustrate this: The picture on the left is the "normal" side of the tire and is pretty uniform, while in the picture on the right, you can see the "bump" where the tire is separating. This separation is pretty large, but they will always start out smaller, so you should look for even small distortions in the tread face.
Vehicle wobbles at 30 to 50 mph:
The most common cause of vehicle wobbles in this speed range is a bent wheel or mildly out of round tire. Transmission and drive line issues can also show up in this range, but tires are the first thing to check. Unlike the low speed wobble, this is usually not a safety issue. You can usually see the wobble in the tire/wheel assembly if you put it on the balancer. In many cases, the tire can be better matched to wheel (high spot to low sport, or vice versa) and the wobble can be eliminated. (This is where the "Road Force Balancer" comes in, and we'll explain that at the end of this article.) If that doesn't work, the problem tire or wheel should be replaced. If the tires and wheels spin true and you still have a problem, then you should have a mechanic look at the vehicle.
Vehicle Shakes at 50 mph or higher
The most common cause of vehicle shakes at 50 mph or higher is tire balance. Again, transmission or drive lines can cause this, but the tires should be the first thing to have checked. A tire or wheel that is slightly bent or out of round can also be a factor here, and this can be checked while the tire is on the balancer. Obviously, the first thing to check is the tire balance, and only after the tires have a clean bill of health should you take it to a mechanic.
Balancers: Static, Dynamic and Road Force
Over the years there have been various methods of balancing tires. Without going into the history, we will simply explain the types in common use today, and when you would use one method or another.
If you think of how your tires are mounted on the car, static balancing refers to balancing the tires only from up to down, or across the tire. In most cases, this works pretty well. Though not used very much these days, bubble balancers and "on the car" balancers can only do this type of balance. Also, if you don't want weights on the outside of your wheels, you will often get this type of balance. This is also called "single plane" balancing.
Dynamic balancing will not only balance the tire from up to down, but also from side to side, and is often called "dual plane" balancing. It is normal for tire/wheel assemblies to be out of balance in both of these ways, and this makes a dynamic balancing a much better choice than a static balancing. This can only be done by modern "computer" balancers, and requires weights to be put on both sides of the wheel. However, if you want a good balance, and either your wheels won't take weights on the outside, or you just don't want the see them, you can often still have a two plane balance. Many modern vehicles are designed with "positive offset" wheels, which means that the bolt face is pushed towards the outside, so most of the wheel is inboard, towards the vehicle. In this type of wheel, it is easy to put the outer weights just behind the face of the wheel and the inner weights on the inside edge. This allows for a proper dual plane balance and is the preferred method (apart from weights on the outside edge). However, if you have a standard offset (bolt face in center of the wheel) or a reverse offset wheel (bolt face toward the inside edge of the wheel), then you really can't get a good dual plane balance without putting weights on both sides of the wheel.
Road Force Balancing
Road force balancing is a relatively new technology. The idea is that a "force wheel" is mounted on the balancer, (this is the black roller wheel on the right side of the balancer in front of the hood) and as the tire turns, the wheel pushes against the tire and measures variations in pressure around it's circumference. Since this simulates how the tire will interact with the road, it is called "road force balancing". These balancers have prescribed "tolerances" that allow different levels of "road force variation" for different types of vehicles.
Finding out the "road force variation" of a tire is all well and good, but if a tire/wheel assembly has too much variation, then what would you do? This is where these balancers shine: Having recorded the road force variation around the tire, the balancer then directs the technician to take a reading of the wheel. The software in the balancer then calculates whether the tire or the wheel is the problem, and whether or not the two can be "force matched" to meet the tolerances. If they can be matched, the balancer specifies a spot on the wheel, and a spot on the tire, where if the two spots are matched together, (by turning the tire on the wheel) the tire/wheel assembly will then have the lowest possible road force variation. Granted, the machine doesn't always get it right the first time, and there are some tricks an experienced tire technician can use to expedite the process, but much of the time, problem tire/wheel can be made to balance without having to be replaced. This is important because no tire manufacturer will replace a tire for being "out of round" unless it is virtually brand new. (This is fair because tires can go out of round over time for various reasons other than poor workmanship.) So if your tires are even a little worn, this is often the only thing you can even do (short or replacement, which you would have to pay for). Without a road force balancer, a tire can be matched for roundness (not force variation) by trial and error, but that can mean turning each tire on it's wheel up to eight times(!) and visually checking it on the balancer every time. Not only does this type of balancer greatly expedite the process, but the specific tolerances create an objective standard.
Obviously, we at Souza's use this type of balancer. However, since this process is time consuming, and in the majority of cases not necessary, we don't road force every tire. We'd then have to raise our prices to pay for all the extra labor. However, if you have a problem that standard balancing won't fix, we will go to this next level and do everything we can to fix your problem.
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