Good vibrations in a Corvette? They only exist when the Beach Boys are coming through the speakers. Other vibrations are unwanted, and most experienced drivers have had to deal with such problems from tires and wheels. Vibrations from these sources are even more likely today because one of the hottest automotive trends is installing aftermarket wheels and tires. Vehicle vibration problems often start when rubber meets the road, and sometimes the solution proves so elusive that an owner gives up and sells the car. Although it can seem like solving tough balancing problems involves voodoo, the actual forces involved are even more intriguing. Several months ago, we ran the first part of this two-part series on tire balancing. We are running the second installment this month.
For drivers who are bothered by a vibration, or those who want to avoid one, here's an in-depth look at the forces that need to be balanced or minimized to provide a smooth, precise ride. In addition to the theory, practical tips are provided on how to choose a balancer and the common problems that are unknown to most owners (and many tire techs too)
ImbalanceThe simplest force to understand that can contribute to tire-induced vibrations is caused by a tire that is heavier in one spot. While nearly all rotating metal parts are machined to close tolerances by precision lathes, tires are assembled from 20 or more components that are basically molded together during the curing process. Advances in technology by tire manufacturers have made their products better than ever, but tires still have some slight variations. When a tire/wheel assembly rotates and it's heavier on one side, an up-and-down vibration occurs once-per-revolution. This is the type of imbalance known to most people, and its vibration can be reduced or eliminated by adding weights to the lighter side of the assembly.
Another common cause of imbalance and vibration comes from parts that are not centered on the axle. Push a pin through a cardboard coaster just off from its center and then spin the coaster to see what we're taking about. Such parts are not concentric, or you can say they don't have a common center. This condition has two different components. First, it causes the wheel/tire assembly to be heavier in one place. This can be balanced by adding weights to the other side. However, just like the out-of-whack wheels on the Keystone Cop cars, it also makes the axle go up and down each revolution. Adding weights cannot eliminate this second component. But there are techniques that can be used when this "hopping" is so pronounced that it needs to be reduced.
RunoutConcentricity of three components-the tire, the wheel and the spindle-is a critical factor in wheel and tire vibrations. It is measured by placing the indicator of a stationary gauge on the rotating surface and spinning the part to see how much that surface moves in and out. This runout can be measured on the parts individually or when they are assembled. The best modern tire balancers will measure the runout of both the wheel and the tire. However, balancers will not measure the runout of the spindle. Fortunately, spindle runout is seldom a problem, with the exception of a rear spindle that has been bent.
There are actually two runout measurements. Radial runout refers to the tire or wheel going up and down like the off-center coaster. Lateral (axial) runout refers to the side-to-side movement or wobble. Although lateral runout is a major concern for brake rotors (because it can move the pads and caliper pistons back and forth), it has less effect on tire vibrations than radial runout in part due to the side-to-side flexibility of tire sidewalls. Therefore, runout will refer specifically to radial runout for the rest of the article.
GuidelinesWhat is an acceptable limit for runout? Tire/wheel manufacturers seldom publish a maximum acceptable limit. For one thing, they don't want their products singled out and returned as unusable based on a technician's runout measurement. Another reason OEMs don't like to give Pass-Fail specifications is these would cause rejection of tires that are fine for most drivers. Also, some applications are more sensitive to runout than others. For example, a Jeep driven off-road is less sensitive to runout than a lightweight late-model sports car driven fast on a smooth highway.
There are always exceptions, but here are some guidelines that I've found useful. It's not uncommon to see a runout .025 to .030 inch in a performance tire in a Corvette size. By comparison, a machined alloy wheel will usually have runouts of as little as .005 to .010 inch. Stamped steel wheels, hopefully, will have a runout of .025 or less. If a wheel has a runout of .030 or over, my recommendation is to mark it, and then you can look at that wheel first if a vibration is experienced. The solution may be match-mounting a tire to that wheel.
Match-mountingThe runouts of a tire, wheel, and spindle all combine to produce the overall runout at the tread. Sometimes the combination is good; sometimes it's bad. Here's an example of how it can be bad: If the high spot on a .025 runout tire happens to line up with the high spot on a .030-runout wheel when it was mounted, the total effective runout would be a troublesome .055. However, if you knew the location of the high spots of both the tire and the wheel, and then mounted them 180 degrees apart, the total effective runout would be a much more desirable .005. That is the principle of match-mounting. It takes more time than the standard mount and balance, but it can eliminate a high runout when needed.
What total runout is too high? The value often referred to within the industry comes from the Rubber Manufacturer's Association, (RMA), which has long published .050 as a guideline for maximum runout of a tire/wheel assembly for use on cars and light trucks.
Loaded runoutThink that's all you should know about tire balancing? Well, there are two other forces of major importance. Fortunately, they are fairly easy to understand. The first force is detected in a loaded runout measurement. As a tire rolls down the road, the weight of the car compresses the rubber and flexes the sidewall between the wheel and the road. Variation in rubber flow during curing and in the construction around the tire can make a tire easier to compress in one spot than in another and, therefore, the axle will sink a little closer to the ground when that spot rolls over the road. This creates a "hop" similar in effect to the runouts discussed earlier, which are caused by parts that are not concentric.
Although loaded runout is a concept new to most car enthusiasts, tire manufacturers have made it a priority to measure this effect for many years using expensive machinery that presses a large rotating drum into a tire as it is spun. Recently Hunter has introduced this technology to dealerships and tire stores with their GSP 9700 balancer. Up to 1,500 pounds of force presses a 6-inch diameter rotating drum into the mounted tire as it is spun to measure the combined runout and loaded runout. Although the loaded runout measured by the Hunter machine is only one component of the force variation, it often identifies the force high point. As is the case with standard runout, the vibration caused by loaded runout has one component that can't be eliminated by addition of weights, but match-mounting can reduce it.
UniformityThe last major cause of tire/wheel vibration is perhaps the easiest to understand. As mentioned earlier, tires are not articles of manufacture that can be made perfectly. They are comprised of layers of flexible material bonded together. They will be heavier in one spot. Their mounting bead will not be perfectly concentric with the tread. Their sidewalls will vary in flexibility. Also, they will not be perfectly round. An out-of-round tire causes vibration even if it's perfectly balanced and perfectly concentric. Think of a square box mounted through its center. It is balanced and it is centered, but it would cause quite a vibration rolling on a road.
A tire or wheel that is not concentric will cause a motion (hop) from its high spot once per revolution, a first harmonic vibration. An egg-shaped tire will cause two hops from its two high spots per each revolution, a second harmonic vibration. A triangular-shaped tire, or one with three high spots, creates a third harmonic vibration. Our square box tire would create a fourth harmonic vibration.
The Hunter 9700 tire balancer measures up to fifth order harmonic vibrations and then uses an algorithm to determine the best method of weighting the tire assembly. Plus it calculates a single high spot for match-mounting if needed.
PullingVariation in a tire's weight distribution, concentricity, loaded runout, and uniformity can cause vibration and should be measured during balancing. A different performance parameter that is not a balance issue but that is important to drivers is pulling. Today, pulling can be detected, measured, and reduced by a new generation of tire balancing machinery-the Hunter GSP9700 with the StraightTrak option. When radial tires were first introduced, if a tire tended to pull the car to one side of the road, it was usually blamed on plies that slipped. Today plies rarely slip, and when a tire pulls to one side, it can be caused by a small difference in the stiffness of one sidewall compared to the other. This causes the tire to effectively roll more like a cone than a cylinder and, therefore, consistently turn to one side. Pulling is also caused when a belt is slightly off to one side. This usually occurs when the tire is made; belts seldom slip during the tire's lifetime unless the tire is coming apart.
Hunter balancing machines that have the StraightTrak feature measure the side thrust on the roller to determine the amount and direction of any pulling forces. A driver feels pulling when such lateral forces combine on the steering axle. Therefore, the tires that pull the most can be placed on the rear when front and rear tires are the same size. If not, front tires can be selected or mounted so any significant pulling forces go in opposite directions to cancel the effect (if the front tires are not directional and asymmetric)
Advances in TechnologyChanges in automotive design and manufacture have made vehicles more sensitive to tire irregularities and vibration than ever before. Today's cars are designed for greater road feel, often incorporating tighter steering along with increased frame and body structure rigidity. Lighter weight suspension components are common, and they don't absorb road force vibrations easily. Also, strut suspensions and rack-and-pinion steering systems transfer road force vibrations more directly to the passenger compartment. Higher spring rates and lighter vehicles can further increase sensitivity.
Auto manufacturers are specifying larger wheels with wider, lower profile tires and stiffer sidewalls. Wheels that are larger in diameter are more likely to be damaged by pot holes, road debris, curbs, and parking lot barriers. And the chance of vibration problems from wheel or axle damage is further increased because vehicles are being driven longer than ever. One source states the average age of a vehicle on the road today is 8.5 years with the average accumulated mileage over 100,000. In addition, tires are trending toward lower aspect ratios that are more sensitive to road forces and dynamic imbalance.
The changes in auto design have driven the tire industry to make tires that are more advanced and more uniform than ever before. Tires manufactured today generally are better than those made even 5 years ago because manufacturing technologies, processes and materials, have undergone continuous improvement. A closer look at some of the new technologies such as the continuous beads in Bridgestone/Firestone's UNI-T design is available at www.bridgestonetire.com/tech. Mark Kuykendall, engineering manager for Bridgestone/Firestone passenger and light-truck tires, reports 100-percent of Bridgestone/Firestone tires today go through an intensive machine inspection before leaving the factory.
OE versus Aftermarket TiresFor many years, car enthusiasts have thought the original equipment (OE) tires that came on new cars were relatively cheap. It was widely assumed that low-cost was a primary criteria when auto manufacturers selected the factory tires. Today, the competition between auto manufacturers for the new car buyer is intense. To make the most competitive vehicle, some cars are actually designed to maximize the advantage of a specific type of tire. And tire manufacturers design tires to complement a specific car platform. For example, some car platforms may be more sensitive to a third order harmonic vibration, while other platforms may be sensitive to lateral vibrations.
Today, auto manufacturers are careful to work with the tire companies to develop a specific tire that has characteristics to match the car. All this means that you should consider using the same tire that the manufacturer chose for the car. If you want to try a different brand, model, or size tire, it's worth the time to research what effect it has on the car. Tirerack.com is a good resource for learning customer feedback on specific tires.
Aging TiresTires change as they age. Heat cycling, ultraviolet light exposure, and time take their toll on tires. The rubber will harden, traction will decrease, and noise may increase. Although age will affect the performance of a tire, age usually won't affect the balance. If vibration was not an issue when the tire was new, then it won't become an issue unless there's a mechanical problem, such as those caused by road impacts or overheating. If you were given free balancing and tire rotation, it might be worthwhile to take advantage of this every 5,000 miles or so. Of course, rotation is no longer an option for Corvettes that have directional tires and use different sizes for front and rear. The good news is that rebalancing is not generally needed. it's only worth the price if you notice a vibration.
Tire MaintenanceWithout a doubt, maintaining the proper air pressure is the most important thing an owner can do to ensure maximum performance, safety, and longevity from their tires. The next most important thing is proper alignment. Particularly, when the toe-in is off, every mile you drive will rub the rubber right off your expensive tires.
Car storage is a maintenance issue for many Corvette owners. Tires can become temporarily flat-spotted after sitting in one spot for just a few weeks, but the vibration or thumping will generally go away after the tires have heated up once or twice. However, if the car is not moved for several months at a time, as is common with Corvettes that are not driven during the winter, tires can become permanently flat spotted. To prevent this, take the wheels off, or jack and block the car up to take the weight off the tires
Tire FailuresThe specter of catastrophic tire failures was raised in the public consciousness a few years ago by a rash of SUV accidents. Driving on a tire that has very low air pressure causes the vast majority of tire failures. When the air pressure is too low, the sidewalls flex much more than they were designed for, and the tire heats up. After a period of driving at highway speeds, the tire can come apart. Although this mode of tire failure is rare on Corvettes, here are a few simple things drivers can do to protect themselves regardless of the vehicle they're driving. First, check the air pressure monthly and before any extended highway trip. Second, be aware of what you've run-over. Do a careful visual inspection of the tires if you hit a nasty pothole or run over a piece of metal. Most tire shops will be glad to inspect the tires for you, too. Third, just listen. Turn the tunes down, lower the window, and listen for any howling or thumping. Tires will make noise and shake excessively when they are beginning to come apart.
SummaryModern car design has increased handling performance, and this in turn (pardon the pun) has increased sensitivity to tire balance and uniformity. Although most car enthusiasts are familiar with the concept of an out-of-balance tire, far fewer have been introduced to other factors that cause vibration, such as concentricity. Corvette Fever has taken you several stages beyond that by introducing the important factors of road force variation, lack of uniformity, and lateral forces. Knowledge of these can be essential in diagnosing a persistent vibration problem. Fortunately, a new generation of tire balancers has been designed to analyze and correct or minimize all these factors. With this knowledge and equipment, some owners will find out for the first time how smooth their Corvette can feel on the pavement.