Changing tires and balancing tire and wheel assemblies are routine jobs for most tire store technicians. But sometimes things don’t always go the way they should.
An expensive aftermarket rim may be accidentally scratched. A low profile tire may refuse to unseat from an alloy wheel, or be difficult to seat when it is mounted. A bead may be torn when mounting a tire on a rim. A customer may complain about a vibration problem that wasn’t there before you balanced his wheels. Rebalancing the tire and wheel may fail to eliminate the vibration.
Sometimes the underlying problem is simply carelessness or using the wrong procedures. But in other situations, it may be the equipment that’s at fault.
Yesterday’s tire changers can’t always handle some of today’s high tech tires and alloy rims. And most electronic spin balancers can’t detect certain kinds of problems that can cause vibrations.
More Wheels to Deal With
One of the spinoffs of a good economy is that vehicle manufacturers sell more "loaded" models. This means more and more vehicles today are factory-equipped with alloy wheels rather than standard steel wheels. Over 50 percent of new vehicles are now sold with some type of alloy wheels.
At the same time, larger disposable incomes have enabled a growing number of motorists to buy aftermarket custom wheels for their vehicles.
Aftermarket wheels run the gambit from relatively inexpensive (under $100 each) to extremely expensive ($1,000 or more per wheel). Needless to say, the owner of a set of exotic wheels doesn’t want them scratched or marred when tires are mounted.
Wheels are also getting bigger. Many passenger cars now come with 16- or 17-inch diameter alloy wheels, which seem almost puny in comparison to some of the exotic 19- and 20-inch aftermarket alloy wheels that are available.
Bottom line: the average tire dealer will likely never see many customers for the large aftermarket rims or ultra high performance low profile tires if they don’t have the equipment to handle them properly.
Root Cause of Problems
Run-flat tires like those on newer Corvettes and other vehicles have also made tire service more complicated. Though the number of vehicles equipped with run-flat tires is still relatively small, it is growing.
Special training and proper equipment can keep dealers from having to turn such customers away, and decrease the risk of damaging the low air pressure sensing system inside the rim or the tire when dismounting or mounting tires.
The import auto makers are also making tire service more of a challenge with asymmetric rims, centerless rims, and three-piece cylindrical rims with no drop center.
Safety beads and closer bead-to-rim clearances on many alloy wheels require higher air pressure to seat the beads. The old rule of using no more than 40 psi to seat a bead may not always apply on some wheels. For this reason, use of an inflation cage is necessary if 60 to 80 psi is required to seat a bead.
Ultra low profile tires can also be easily damaged if the sidewall is deflected too far when dismounting or mounting a tire on a rim.
According to one tire company, using a shovel style bead breaker is risky. Misplacing the bead breaker too high on the sidewall or shoving the bead too far into the rim risks damaging the carcass wires inside the tire.
This could permanently distort the bead and steel cords at the belt edges, and permanently deform the tire causing an out-of-round condition that can result in vibrations.
The company’s recommended practice is to use a tulip/rim style tire changer that uses a pair of rollers to push the bead loose.
Or, if such equipment is not available, to use a bead breaker that matches the curvature of the rim and to position it carefully between the edge of the rim and tire.
Breaking Technology
Shovel-style bead breakers were never intended for today’s low profile tires and "rim protection" tire profiles. Many low profile tires are designed so the sidewalls protrude from the rim flange to protect the rim from curb damage.
If the bead breaker is positioned close to the rim flange and slips when pressure is applied, the shovel can gouge the rim. And if the bead breaker is moved too far out on the tire sidewall, it may permanently deform and damage the tire.
Does this mean the traditional center post shovel-style tire changer is obsolete? No, because it’s still an easy-to-use piece of equipment for changing standard tires on steel rims. But it’s probably not the best choice for many of today’s low profile tires and alloy rims.
Tulip/rim style tire changers have become popular in recent years because they’re designed for today’s low profile tires and alloy rims. The bead breaker applies no force to the wheel itself, only the tire, so there is much less risk of damaging either the rim or tire.
These machines also use use a clamping system that grasps and holds the wheel from the inside out (or the outside in depending on the application), which eliminates the center post and cone. This approach also makes it possible to handle centerless wheels. Cushioned jaws are recommended to minimize the risk of scratching the finish on alloy wheels.
The mount/dismount head on tulip/rim style tire changers also does not make contact with the rim, which further reduces the risk of rim damage (even, so there is still some risk to low profile tires if the mount/dismount head is positioned too far from the rim). The table or tulip that holds the wheel rotates under the tower as the tool head slips the tire up and off the rim.
Mounting is just as easy, too. The tower mounted tool rolls the tire onto the rim as the table rotates. Some changers have plastic mount/dismount heads to reduce the possibility of damaging the tire bead even further. Once the tire is in place, high pressure air jets may be used to assist bead seating.
New Balancing Act
Balancing is a must anytime tires are mounted on wheels, and many would argue that it’s also a good idea to rebalance a tire and wheel assembly if a tire has been dismounted for repair.
Balancing provides a smoother ride by minimizing tire bounce. This helps improve traction, steering response and control and extends the life of the tires. But no matter how carefully the tires are balanced, they’ll eventually lose their balance. As the tread wears, the distribution of weight around the circumference of a tire changes altering the balance of the tire and wheel assembly. Eventually the tire may have to be rebalanced because only a quarter ounce of imbalance can create a noticeable vibration.
An out-of-balance tire and wheel will typically create a vibration or shake that become progressively worse as the vehicle’s speed increases. The speed at which the vibration first becomes apparent will vary depending on the size and weight of the tires and wheels, the size and weight of the vehicle, the sensitivity of the steering and suspension, and the amount of imbalance, but usually starts in at 40 to 50 mph and increases in intensity as the speed goes up.
The cure, of course, is to check the balance of all four wheels and tires, and rebalance as needed. But it’s also important to remember that speed sensitive vib-rations can also be caused by radial (vertical) or lateral (sideways) runout in a tire, wheel or hub.
Loose, worn or damaged wheel bearings as well as certain kinds of tread wear can also cause vibrations. So too can an out-of-balance or out-of-phase rear wheel drive driveshaft (FWD shafts usually don’t rotate fast enough to cause vibration problems).
Strive For Accuracy
To accurately balance tires and wheels, you need an up-to-date spin balancer that can achieve both static (at rest) and dynamic (in motion) balance. Old fashioned bubble balancers could do a decent job of achieving static balance, but dynamic balance can only be achieved with a spin balancer.
This is especially important with today’s larger, wider, heavier tire and rim packages, and absolutely essential for run-flat tires that have thicker, stiffer sidewalls.
Most balancers today have self-calibrating electronics with accuracy to hundredths of an ounce or tenths of a gram. Graphic displays also make information easier to read and understand, and reduce the chance of mistakes. Automatic data entry for wheel width and diameter on some balancers also saves time.
Most balancers today operate a lower speeds. This helps extend motor life and reduces cycle times, as well as risk to the operator.
Older balancers typically had to spin a wheel fairly fast (about 500 rpm, or the equivalent of 55 to 60 mph) to generate a usable signal. But the more sensitive electronics in today’s balancers are able to pick up vibrations at much lower speeds (only 100 rpm, or 10 to 15 mph).
Technique Counts
One of the limitations of balancing tire and wheel assemblies off a vehicle is that repeatability can be an issue. In other words, you may not get the same results when you attempt to rebalance a wheel that has already been balanced. What’s changed is the geometry of the tire and wheel on the balancer.
The way in which a wheel is mounted on a balancer will not only affect the accuracy of the balance job itself but also the repeatability of the balancing results. Worn mounting cones or shaft bearings may be the problem. Using the wrong type of cone can also give inaccurate results. So too can dirt on the wheel or nicks in the wheel center hole. But another often overlooked cause is using the wrong mounting technique for the type of wheel.
The basic idea is to mount the wheel on the balancer the same way it is mounted on the vehicle. A pilot hole centric wheel (one where the center hole positions the wheel on the hub and prevents it from wobbling sideways when the lug nuts are removed) can be mounted on a balancer with a cone from the backside.
But a lug centric wheel (one that does have some sideways movement when the lug nuts are removed) requires a different balancer-mounting procedure. A lug centric wheel must be mounted with a cone from the backside and an adapter flange plate against the front side. The fingers on the flange plate must be properly positioned so they line up with the lug holes in the wheel. This is necessary to center the wheel on the balancer shaft. If this is not done, you may just be going through the motions of balancing the wheel because the results won’t be accurate or repeatable.
Precision flange plate adapters are expensive and may only be offered as an extra cost option with a new balancer. A set of flange plate adapters that covers most vehicle applications may run from $1,300 up to almost $2,000 depending on what you buy. But the improvement in balancing accuracy and repeatability can be well worth the investment. They can also prevent unnecessary comebacks and dissatisfied customers.
Beyond Balance
How often have you had balanced tire/wheel assemblies come back because of a speed-sensitive vibration? Sometimes the problem is runout. Most tires should have less than .030- to .050-inch of runout. An out-of-round tire can produce harmonic vibrations that come and go at various speeds depending on how many "humps" are in the tire.
As a rule, most steel rims should have less than .050 inch of runout, or .040 inch of runout if the rims are aluminum alloy. Some trucks and SUVs can tolerate up to .060-inch of radial and lateral runout, but others can’t handle any more than .030-inch of runout before vibrations become noticeable.
Runout problems can often be corrected by "match mounting" the tire on the wheel - rotating the tire so the tire high spot is over the rim low spot.
Stiffness a Culprit
Sometimes the problem isn’t balance or runout. It’s "radial force variation" (RFV). This is the amount of change in stiffness of the sidewall and footprint when a load is placed against a tire. Subtle differences in the position of the cords and belts in a tire’s construction can create stiff spots that make the tire roll unevenly. The stiff spots act like runout to cause vibrations at various speeds.
Vibrations caused by RFV tend to appear at certain speeds, then disappear as the speed changes or in-creases, unlike vibrations caused by imbalance that usually get worse as the speed increases.
In one test, a perfectly round wheel that was properly balanced experienced a vibration at around 50 mph, but vanished at 70 mph. The vibration at 50 mph was caused by RFV in the tire, and created as much side force as if the tire were out-of-round by .030 inches or out of balance by an ounce and a half.
Until recently, there was no easy way to detect let along measure RFV. All a tire dealer could do was switch tires until the vibration was reduced - or the customer went away. Only the tire and vehicle manufacturers could afford the type of equipment that could detect and measure RFV.
But a couple of years ago, one aftermarket equipment manufacturer introduced a new balancer that checks RFV as well as balance and runout. It detects RFV by placing a load against the tire as it rotates to simulate loaded driving conditions.
If RFV is present, it shows you where it is and how to correct the problem - either by adding offsetting weights and/or rotating the tire on the rim. It also makes it easier to determine if a tire is responsible for a vibration problem. If there’s no runout, no RFV and the tire is accurately balanced, the vibration is in the driveline or powertrain, not the wheels and tires.
As you can see, modern technology and driver de-sires for more aesthetically pleasing vehicles have created potential problems for dealers. Proper mounting and balancing procedures can alleviate many customer concerns. But investment in newer high tech mounting and balancing equipment can make problems a thing of the past.