Road Force Procedure:

A MOUNTING AND BALANCING ACT: Today's larger wheels and stiffer tires require the right equipment and skill

By Mike Mavrigian

Once upon a time (not that long ago or far away), any Camaro that sported 16-inch diameter wheels was considered a trendsetter. A Honda with 17-inch wheels was viewed as a perverted but ultra-cool rebel. Aspect ratios of 50 were considered outlandish. Today, 17-inch to 19-inch wheels have become almost commonplace, with huge alloys of 20-inch up to a whopping 26-inch diameter representing the sizes du jour. This has given birth to tires with increasingly dropped aspect ratios of 45, 40, 35 and even 30 series profiles. In other words, wheels are getting bigger and tire sidewalls are getting shorter. This is what the customer wants.

This fad of filling up the wheelwell with shiny aluminum skinned with a wide rubber band of black rubber certainly fulfills anybody's wish to look cool, but dealing with these large wheels and short, stiff sidewalled tires can be a nightmare in the mounting and balancing bay.

While the immediate goal of the mounter may be to slip the beads over the rim lips, always be aware of the potential for harm. Scratching, gouging or chipping an expensive alloy wheel is simply not an option. Likewise, tearing the tire bead is to be avoided at all cost. The first step toward the solution is to ante-up and invest in a new tire mounting machine that is designed to accommodate these large wheels and short/stiff-walled performance tires. The chances of damaging a wheel or tire are simply too great to not use proper equipment and techniques.

PAY ATTENTION TO BASICS

Enthusiasts plunk down a considerable chunk of change for a set of custom wheels and hi-po tires, and they don't want them scratched or otherwise dirtied. Having the best and brightest changer is only an exercise in bragging rights if you don't pay attention to cleanliness as well.

Make sure that socket wrenches (used in removing/installing the wheels at the vehicle) are clean and free of burrs. Keep clean hands to prevent rubbing grit onto the wheel during handling. At the end of the day, take the time to clean the changer. Even when dealing with new wheels and tires, a changer can become messy after an eight- to 10-hour day of nonstop service.

Also, take the time to keep the floor area adjacent to the changer and balancer clean. Laying a wheel or tire on a gritty floor will simply result in that grit being trapped in a bead seat or scratching a finely finished polished aluminum wheel. When it comes to custom wheels, or even OE alloys, try to keep the entire tire/wheel environment as clean as possible.

It may sound basic, but be sure to apply an even coat of bead lubricant along both entire beads before attempting to mount a tire. Lubrication of the bead surface will reduce the chances of snagging and tearing. And make sure that the lube is clean and free of grit. Also, keep at least one spare lube brush handy for those times when a brush is dropped onto a dirty floor.

When attempting to seat the beads, always use a lock-on type air chuck and always stand to the side, away from the tire/wheel. Use the changer's foot pedal to apply compressed air. Standing next to, and leaning over, an inflating tire is simply not a good idea. Just because you've never witnessed a failure does not mean that it can't happen, and there's no reason to learn this the hard way.

Also, never exceed 40 psi during bead seating. If both rear and front beads do not seat at 40 psi, don't try forcing the issue with gobs of added air pressure. The safe way is to remove the air chuck, allow the internal air to bleed off, fully unseat the beads, re-apply bead lubricant and try again.

After the beads have seated, closely inspect the bead edges, checking for traces of air bubbles. It helps to squirt a bit of soapy water onto the bead area to more easily locate small leaks. If bubbles are found, mark the spot with a tire crayon, break the bead seat loose and examine for the cause of the leak. More than likely, small amounts of escaping air may result from debris trapped between the tire bead and the rim's bead seat, or a burr or nick on the rim's bead seat area.

WHEEL/TIRE MATCH-MOUNTING

Match-mounting allows you to locate the tire to the wheel in the best location to minimize vibration.

There are two methods of match-mounting, to either reduce the chance of runout vibration/thumping (uniformity method), or to optimize weight balance (weight method).

The basic idea with regard to match-mounting is to place the tire in the "best" clock position on the wheel to reduce the chances of vibration complaints.

1. Uniformity method (matching for runout/force): When match-mounting to minimize runout-associated vibration, align the "high" spot of the tire (the area where runout is greatest) to the wheel's "low" spot where runout is minimal. This allows matching the tire's high spot to the wheel's low spot to hopefully neutralize the mounted combination.

If the tire features a red dot on its sidewall, this dot indicates the tire's point of maximum runout or maximum force variation. Mount the tire onto the wheel to align this red dot to the wheel's low runout point (this may be marked with a color dot or with a small notch. If in doubt, check the wheel maker's instructions, or physically measure wheel runout to find the low spot).

If only the tire or wheel (or neither) is labeled, determine runout manually, using a dial indicator, or take advantage of the radial runout checking feature now found on some high-end tire changers. If this is checked manually, mount the tire/wheel on the changer and secure properly (make sure there is no runout at the changer spindle). Place a dial indicator on a secure stand adjacent to the wheel. Place the dial indicator probe (equipped with a roller tip) against the outer edge of the rim radius. Pre-load the probe plunger and zero the indicator dial. Slowly rotate the wheel until the lowest spot of the radius is found, and mark that spot with a crayon or other marker (be careful not to apply a permanent marker to an exposed surface of the wheel).

If the high spot of the tire is already marked, simply mount the tire to align the tire high spot to the discovered rim low spot. If the tire isn't pre-marked either, go ahead and mount the tire to that wheel. Chuck the assembly onto a changer spindle, set up the dial indicator at the center of the tread and slowly rotate the assembly.

When the tire's highest runout spot is found, mark this and record the tire's runout reading. If it's acceptable, go ahead and balance. If the runout is too great, or to try to minimize it, break the tire beads loose and rotate the tire on the rim to match the two marks.

After re-inflating the tire, make a runout check again to see if tire runout has increased or decreased, or if the runout spot has moved. Adjust the tire to the rim accordingly.

Even if match-mounting for runout doesn't eliminate vibration, in the process of match-mounting, at least a "stack-up" of tolerances will be avoided, which occurs if the tire high spot is aligned with the wheel high spot. This stack-up, depending on its severity, would result in a radial "thump" during operation.

2. Weight method (matching for weight): If the tiremaker has also marked the tire for balance (a second dot, likely in yellow, will indicate the tire's point of lightest weight), you can weight-match the mounting by aligning this dot to the wheel's air valve.

The valve location should be the heaviest spot on the wheel, so it makes sense to try to cancel the light spot of the tire against the heavy spot of the wheel to optimize the initial state of balance. Assuming that the valve area of the wheel will always be the heaviest area is debatable, since the valve might actually weigh the same, less or more than the wheel metal that was removed to create the valve hole. Nevertheless, the theory is acceptable in most cases, so if a yellow tire weight dot is found, go ahead and align it to the wheel's valve location.

Always match-mount if possible to either reduce the chance of runout thump, radial force variation or to optimize the balance. Regardless of which method is selected, once this is done, the tire-to-wheel position is optimized and the assembly can then be balanced.

RADIAL FORCE VARIATION

There's runout, and then there's radial force variation.

When mounted tire radial deviation is measured statically, it is referred to as runout. Radial force variation refers to "runout" that is actually caused by a variation of road force when the tire rolls under load and its uniformity changes as a result.

Static runout, as noted earlier, refers to a high spot of the tire, a physical characteristic that can be measured with no load placed on the tire. Radial force variation, however, refers to a runout condition that only occurs when the tire runs at speed, under load, due to variations in construction stiffness.

Radial force variation is so-named because a concentrated area of "hardness" of the tire's radius may cause a force-against-the-road variation according to vehicle speed and load, which would be perceived as a vibration.

A mere 0.001-inch of loaded radial runout is equal to approximately one pound of road force.

Of course, any tire, because of its compliance, will slightly vary in radius at the load spot during operation. Although a "radial force" might be the result of a runout area that is pronounced enough to affect the tire's impact on the road, a radial "force variation" may occur if the tire has appreciably different "soft" spots and "stiff" spots in the carcass and/or tread or in the sidewall construction. Given the high quality control processes used by today's tiremakers, it's rare that a force variation problem will occur, but when it does, it can be a tricky demon to chase.

It's important to understand that radial force variation is a dynamic factor that is not directly related to balance.

Even though no problems may be found as the tire rotates on the balancer, when the tire experiences a load, the transition of the harder and softer sections of the tire may create a series of harmonic vibrations as the tire contacts and leaves the road surface. Depending on conditions, this harmonic may occur once per revolution of the tire, or it may occur in a series of multiple vibrations per revolution. It's possible that this phenomenon may vary according to changes in tire pressure, vehicle speed, individual tire load and the road surface conditions, all of which may serve to reduce and/or amplify the vibration problem. In other words, the vibration that the driver feels may not occur with any one rate of speed, or on any road surface, because the problem may appear only under a specific combination of these variables.

If a tire/wheel will not balance properly, or if a vibration exists after a successful balance job, don't automatically blame the tire. If the hub and wheel haven't been checked for runout, you may be jumping to conclusions. If, however, all balance, static runout and chassis parts variables have been exhausted, then it may be time to suspect a radial force variation problem.

In order to check for and attempt to actually verify a dynamic radial runout condition, a spin-balancer with a built-in load roller is recommended. This type of balance machine places a load (which attempts to simulate road load) onto the tire as it's spun, while monitoring and recording variances of runout. If load variation is found, this may be corrected to an acceptable state via balancing weights, or in extreme cases, it may verify that the tire should be replaced.

As a general rule-of-thumb, a minimum range of between .3 to .5 ounce (seven to 14 grams) of imbalance is usually enough for the average motorist to notice an imbalance-induced vibration. If a vehicle is sensitive enough to exhibit noticeable vibration at only .3 to .5 ounces of imbalance, that same amount of vibration may be present with as little as 10 to 15 pounds of radial force variation, which (although hard to believe) can be caused by as little as .010 inch to .015 inch of loaded radial runout.

Using this as an example, it's easy to see how loaded runout can dramatically affect vibration. In other words, a little bit of "loaded" tire runout variance can make a big difference in terms of operating smoothness or harshness.

'YOU DID A LOUSY BALANCE JOB'

Most vehicle owners don't understand how their rides work or what's involved in repairing or maintaining them. When it comes to vibrations, they may blindly blame either the tires or the balancing job. Yes, it is possible that they have a "bad" tire, or that your shop had a problem balancing the tire. However, the harsh reality is that a host of variables can cause or contribute to what the customer perceives as "poor wheel balance."

POSSIBLE CAUSES OF VIBRATION OTHER THAN WHEEL/TIRE

The perceived "vibration" that the driver senses could be caused by worn or loose hub bearings, several loose wheel fasteners, unevenly tightened wheel fasteners, an out-of-balance driveshaft, mud caked inside the wheel, a buildup of crud between the wheel and hub face that prevents complete wheel-to-hub mating, worn control arm bushings, flat-spotted tires caused by severe brake lockup, worn or failed shocks, an off-center mounted wheel that causes severe radial runout, warped brake discs, transmission problems, an out-of-balance or damaged driveshaft, engine issues such as faulty ignition systems or clogged fuel injectors, loose brake calipers, etc.

It's important to properly diagnose any vibration complaint before assuming that the problem lies with the tire, wheel or tire/wheel package, because it's always possible that what the customer believes is a wheel/tire balance problem may in fact be completely unrelated to the issue of balance.

IT'S TIME FOR NEW EQUIPMENT

If your tire changer is an older unit that was designed to accommodate a maximum wheel diameter of 16 inches, and features a bead seat boom arm that is prone to flexing when trying to muscle a stiff sidewall/bead, the unvarnished truth is that you need to update. Aside from the high degree of difficulty that you'll encounter when attempting to mount large diameter wheel sizes, the chances of damaging a rim lip or tearing a bead are high.

The changer should secure the wheel solidly without digging into the material with big, fat, sharp teeth.

The changer should also feature a built-in "bead-pusher" aid (this may be a separate boom arm equipped with a nylon roller or other non-harmful attachment that hydraulically applies downward pressure along the bead circumference as the wheel rotates on the machine. If an overhead boom arm deflects, this changes the position of the guide spoon, which can then dig into the rim. The guide spoon should also feature protection to guard against rim damage (plastic/urethane coating, etc.).

An old rim-clamp style changer, for example, might offer a bare steel guide that, over the years, has developed a few sharp edges and burrs. If this jumps into the wheel rim flange, something is going to be gouged, and chances are that will include both the customer's wheel and your wallet.

As the years roll by, there is a definite weight trend. Cars and SUVs are becoming lighter (sprung weight), while wheel and tire packages are becoming heavier (unsprung weight). Add to this the increased use of wide, low-profile performance tires and the fact that today's suspensions have become more sensitive, and it's easy to understand why wheel balancing has become an increasingly vital and challenging aspect of shop service.

Back in the "old days" of the 1970s or so, dynamic balance was the hot topic, largely due to the popular use of radial-type tires. Prior to that, static balance was the primary concern. Static balance refers to the correction for weight when considering radial force only (the force that occurs in a perpendicular line from the center of the axle outward). We could also call static imbalance "radial" imbalance because of the direction of the force.

Dynamic imbalance denotes a condition where unequal weight is present on either side of the tread centerline. While that unequal weight does affect static balance (in a radial direction), it also affects the axial direction (forcing the tire inboard and outboard). This creates a wobble or shimmy-type vibration because the weight difference exists off of the tread centerline.

Because of today's vastly improved tire design and construction and much tighter manufacturing tolerances, dynamic imbalance concerns have been greatly minimized.

Today's high-end computer balancing equipment offers the ability to monitor and measure all dynamic issues, including loaded and unloaded radial runout. They also provide quick and easy methods of capturing wheel runout (radially and axially) for faster problem diagnosis and match-mounting correction.

MOUNTING ERRORS

Since improper mounting of the wheel to the balancing machine accounts for the majority of vibration-related comebacks, here are some important things to remember.

Always back-cone (cone positioned at the rear of the wheel). This is especially important when dealing with steel stamped wheels, which can feature burrs on the front of the center hole lip. Some alloy wheels can be front-coned if they feature a machined center hole.

When mounting a hubcentric wheel onto the balancer, special hubcentric back cones are sometimes available that replicate the vehicle's hub center. An easy solution for both lugcentric and hubcentric wheel mounting is the use of "pin plates" that install onto the front, secured by the balancer shaft spindle nut. These pin plates feature adjustable dowel pins that replicate the wheel's bolt pattern.

When installing a lugcentric wheel, these pins center the wheel as it is mated against the cone. For hubcentric wheels, the pins provide a "mating aid" for the wheel to the cone, resulting in an evenly distributed clamping force when the spindle nut is tightened.

One of the problems facing balancing technicians today involves the enormous proliferation of custom wheels, some of which may feature widely varying runout and weight variances. Another problem is posed by an increasing number of lugcentric wheel designs, which can be much more prone to off-center mounting, causing runout vibration complaints.

Static mounting errors can easily be caused by nicked or worn cones and shafts, or by improper clamping. Dynamic mounting errors can be caused by dirt or other contaminants on the faceplate, nicks or burrs on the wheel or by a bent wheel disc.

Likewise, dynamic mounting errors can be caused when mounting the wheel onto the vehicle hub, resulting from dirt or rust buildup on the hub or drum face or rear of the wheel, by obtrusive drum or rotor retainers, a bent wheel, or improper lug nut or bolt tightening.

How much can static balance be affected by a bit of dirt or a nick or burr? Let's use a 16-inch wheel mounted to a 40-pound tire as an example. If the wheel is improperly centered onto the balancer (or on the vehicle) by as little as 0.010 inch, this can cause a static imbalance by approximately 0.85 oz.

Think about that. Even before checking for dynamic balance, the assembly is out of balance by almost a full ounce simply due to careless preparation or handling.

Ten-thousands of an inch doesn't seem like much of a tolerance, so just imagine how much balance is affected by mis-centering a wheel by 0.020 inch or more. When installed onto the vehicle, that wheel/tire will now be out of balance and is a likely comeback. By the same token, if the wheel is properly mounted on the balancer, but incorrectly installed on the vehicle, a vibration comeback may occur.

The point is that the mounting conditions on both the balancer and the vehicle must be correct to prevent both imbalance problems as well as runout problems.
Dynamic imbalance can only be monitored on a "spin" balancer that allows the varying weight differences to be revealed in both radial and axial planes, which is why spin balancers are often referred to as dual plane balancers.

In addition to checking for weight balance, runout-related deficiencies can be checked by measuring radial runout and lateral, or axial, runout.

While radial runout is measured at the circumference, perpendicular to the axle, lateral runout can be measured at the tire sidewall and wheel rim face. If excessive lateral runout is measured, suspect a bent wheel that likely is distorted at the wheel center area. The tire can be measured for lateral runout as well, at the sidewall, but running into a brand name tire with excessive lateral runout is rare.

The process of balancing offers its own chances to scratch the wheel finish. Make sure the balancer is clean, which includes the spindle, the spindle backing flange, the centering adapter and the spindle nut/arm. Make sure the backing flange and the centering adapter are free of burrs as well.

In terms of wheel weights, the choices include conventional "clip on" weights which feature a formable tang that slips onto the rim lip, adhesive-backed "stick on" weights and more intricate and less-common screw-adjusted weights which are OE on some Jaguars and other exotic imports. This type of weight gently positions onto the rim lip but is secured with an adjustable tang via a threaded adjuster to prevent wheel damage.

There are choices for weight location as well, providing the wheel/tire assembly isn't too out of balance. The weights may be attached to the rim lip using clip-on weights, but in some cases this isn't practical, either because the customer does not want the weights to show, or because of the close-gap fit of the rim design. Adhesive weights can be applied to outer or inner rim areas, but again in most cases the customer will want the weights to be hidden, so they'll go to the inside. On an airy, open-spoke wheel design, the adhesive weights may be visible between the spokes. In this case, the weights can be located behind the spokes.

Prior to installing adhesive weights, even on a new wheel, be sure to wipe the installation area with an approved evaporating solvent such as denatured alcohol or a paint-safe surface-prep solvent. If the adhesive weight involves something longer than one short section, pre-bend the weight strip to the contour of the area first by placing the weight onto the surface and bending it to conform to the rim curve. Then remove the backing to expose the adhesive and stick the weight onto the surface. Apply firm finger pressure to ensure that the weight adheres thoroughly.

With traditional clip-on weights, be extremely careful when dealing with a coated wheel. If the rim is clearcoated, the weight's clip might pierce the clearcoat layer, causing eventual moisture migration under the clearcoat. This will result in fogging and staining, essentially ruining the wheel's appearance. If the rim is coated or painted, and the owner cares about long-term appearance, stick with adhesive weights.

SAFETY FIRST

• When mounting a tire, do not stand directly over the wheel/tire during inflation.
   

• Never use more than 40 psi to seat the beads.
   

• Always wear eye protection when mounting or balancing.
   

• Never remove the safety shield on your balancer. It's there for a good reason. If a piece of dirt or a wheel weight releases from the tire or wheel when the assembly is spinning at full tilt, you can be seriously injured.
   

• Never wear loose items when operating a changer or balancer, and pay attention to hair and clothing. Watches, bracelets, long hair, untucked shirttails, etc., can get snagged in the equipment, resulting in injury.
   

• Keep the floor clean and dry. A slippery floor can result in slipping and back injury when loading/unloading a wheel/tire assembly.
   

• An electrical "kill switch" should be installed on the wall adjacent to the equipment, to allow quick shutdown in the case of an emergency.

UNDER PRESSURE: Proper tire inflation is a safety and cost factor

Tire inflation pressure must be correct for several reasons. An underinflated tire (by as little as 6 psi) can result in a 25% increase in tread wear rate, and can cause enough elevated operating heat and sidewall flex to pose a safety concern. Tire inflation must also be correct during service work, such as balancing and wheel alignment. Variations in tire pressure can alter tire outside diameter, which can lead to uneven ride height, which can affect alignment readings. Underinflated tires might also mask potential runout/vibrational issues. Always check and correct tire pressure before performing balance or alignment work.

Always check tire pressure when the tire is cool (at ambient temperature). Tire temperature rises when the vehicle is driven, and that results in increased air temperature within the tire, causing a rise in pressure.

Never exceed the maximum cold pressure that's indicated on the tire's sidewall. Always follow the carmaker's recommendations for tire pressure.

If the tire is underinflated, accelerated tread wear will result (usually at the outer shoulder areas), along with poor handling and damaging heat buildup due to excessive sidewall flex. Overinflation will also result in concentrated uneven tread wear (faster wear at the tread center area), poor handling and braking, especially in wet conditions, and because the excess pressure makes the tire slightly stiffer, its resistance to impact damage will lessen. The tire can be badly bruised (resulting in interior construction damage when driven over gravel, potholes, etc.).

ALL SHOOK UP: Top 10 reasons for bad vibes

Here are the top 10 reasons for vibration-related customer comebacks:

60% Improper wheel mounting on the balancer or clamping error

10% Wheel balancer out of calibration

7% Improper wheel weight application

6% Eccentricity problem

4% Improper wheel fastener torque

3% Used wheel weights

2% Bent wheel rim

.5% Radial force variation, lateral force, conicity and ply separation

.5% Bent wheel center disc

Source: Hennessy Industries

NON-STOP TRAINING IS ESSENTIAL

The procedures and guidelines that are presented in this article are, by design, general in nature. If there is a unique situation that arises, make sure to follow all industry standards.

We believe that there is no substitution for continual training. Training is available from the service equipment manufacturers, tiremakers, carmakers and industry associations. We recommend that you contact them for both your safety, and the safety of your customers. Be a professional.

KEEP THEM CALIBRATED

All mounting and balancing equipment requires occasional maintenance or calibration. Refer to the equipment manufacturer's maintenance manuals for details to make sure that your equipment is working properly.