Air Pressure - Correct, Underinflated & Overinflated

Advantages of Correct Tire Inflation 
Maintaining correct tire inflation pressure helps optimize tire performance and fuel economy. Correct tire inflation pressure allows drivers to experience tire comfort, durability and performance designed to match the needs of their vehicles. Tire deflection (the tread and sidewall flexing where the tread comes into contact with the road) will remain as originally designed and excessive sidewall flexing and tread squirm will be avoided. Heat buildup will be managed and rolling resistance will be appropriate. Proper tire inflation pressure also stabilizes the tire's structure, blending the tire's responsiveness, traction and handling. 

Correct tire inflation pressure allows drivers to experience tire comfort, durability and performance designed to match the needs of their vehicles.

Can you easily identify which tire is 30% underinflated? Here is what they would look like in the morning parked in your garage.

Disadvantages of Underinflation 
An underinflated tire can't maintain its shape and becomes flatter than intended while in contact with the road. If a vehicle's tires are underinflated by only 6 psi it could lead to tire failure. Additionally, the tire's tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would experience a significant loss of steering precision and cornering stability. While 6 psi doesn't seem excessively low, remember, it usually represents about 20% of the tire's recommended pressure. 

Disadvantages of Overinflation 
An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. Higher inflated tires cannot isolate road irregularities well, causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures. The pressure must be checked with a quality air gauge as the inflation pressure cannot be accurately estimated through visual inspection.

Tire Contact Patch

Picture yourself driving along the highway, (just slightly over the limit, well, more than a little) and the sports news comes on the radio. The announcer mentions Shaquille O'Neal. Now, that guy has pretty big feet, but does he put more rubber on the ground with his Reeboks than your tires put on the road? Hard to believe, but Shaq puts more rubber down than most cars do. The contact patch of most tires is about the size of your hand and has to handle a lot more weight and force than those big Reebok's do.

 The shape of a tire's contact patch or "footprint" greatly influences its performance and is dependent on its profile or "aspect ratio". Low profile tires (most performance tires) have a short and wide contact patch that is effective in converting the driver's input into very responsive handling, cornering stability and traction...especially on dry roads.

High profile tires (light truck and most passenger tires) have a long and narrow contact patch which helps to provide predictable handling, a smooth ride and especially good traction in snow.

Wheel Lug Torquing

Proper installation requires torquing wheel-attaching hardware (lug nuts or bolts) to the recommended specification for the vehicle make, model and year. Torque specifications can be found in the vehicle's owner's manual, shop repair manual, industry reference guides or obtained from the vehicle dealer.

 Proper torquing requires using the correct tools, procedures and patterns to prevent over-tightening hardware, stripping threads and stretching studs, as well as will reduce the possibility of warping brake drums, brake rotors or suspension hubs. 

Under- or over-tightening wheel-attaching hardware can be damaging and dangerous. 

Torque specifications are for threads free of dirt, grit, corrosion, etc., so the hardware turns freely. While lubricating hardware THREADS is recommended, it is important NOT to lubricate hardware SEATS. The friction which torque is measured against should come from the hardware seats. Lubricating hardware seats can result in inaccurate torque readings and/or over-torquing the hardware.

Professional  recommends starting hardware by hand, snugging them manually and then using a beam or click-type torque wrench to apply the final torque and confirm the recommended torque value has been reached. 

Professional does not recommend using impact guns or torque sticks when installing wheels.

Impact guns deliver torque as torsional impacts, made by internal hammers. Delivering torque as torsional impacts can damage hardware and wheel finish. Some vehicles, such as Porsches, require the use of special sockets to tighten the hardware without damaging their anodized coating. 

Torque sticks are designed to limit applied torque by absorbing input torque thru torsion deflection, or twist. However, there are many variables that can affect the torsional deflection (absorbed torque) and the applied (delivered) torque. These can lead to inaccurately applied torque or variations in applied torque.

Some of the variables that can cause inaccurate torque delivery: 

Type of impact used (air, electric, cordless) 
Power (air pressure, air volume, length of hose, size of fittings, battery power, battery age) 
Impacts per second 
Size of internal hammers 
Possible use of additional adapters 
Socket size, weight, length 
Operator grip strength 
Weight of the impact
 Applied angle during use

Attempting to fully tighten hardware with impact guns/torque sticks also prevents using a click-type torque wrench to confirm the specified amount of torque is present. While a click-type torque wrench can identify when the selected torque has been reached, it cannot diagnose excessive torque. 

Once you have the right tools, use the appropriate crisscross sequence (patterns shown below) for the number of the vehicle's wheel-attaching hardware positions until all have reached their specified torque value.

New wheels should be re-torqued after the first 50 to 100 driving miles. 

This should be done in case the clamping loads have changed following the initial installation due to the metal compression/elongation or thermal stresses affecting the wheels as they are breaking in, as well as to verify the accuracy of the original installation. 

When rechecking torque value, wait for the wheels to cool to ambient temperature (never torque a hot wheel). Loosen and retighten to value, in sequence using the torque procedures listed above.

Tire Sidewall Separations / Bubbles

Pneumatic tires are made of specialized rubber compounds reinforced by plies of fabric cords and metal wires. While most rubber compounds can be stretched easily, the underlying fabric cords and steel wires actually define the tire's shape by limiting stretching. In order to bond these dissimilar materials, the cords and wires are coated with adhesives and/or rubber before the other components are bonded to them during curing. 

A strong bond between these various components is necessary to provide the desired durability. However the strength of the bond can be reduced if 1) any of the components are contaminated during manufacturing (resulting in incomplete bonding), or 2) components are damaged in service due to use while overloaded/underinflated, or by impact with potholes, curbs or other road hazards that pinch the tire between the rim and the road, or simply stretch the rubber beyond the elastic limit of the underlying cords and wires.

Sidewall Undulation Past experience indicates that a sidewall separation/bubble caused by component contamination or incomplete bonding during manufacturing will appear within the first six months of service. Fortunately these separations/bubbles typically appear when they are small in size and before the tire's strength is substantially reduced. However since typical tires roll about 800 times every mile and the air pressure inside the tire is greater than outside, tire separations/bubbles that are unseen or ignored will continue to grow in size, further reduce strength, often generate noise and vibration, and ultimately lead to tire failure as the tire stretches under load (similar to the way that continuously bending a paperclip back and forth will cause it to weaken and break). However there is one last thing to remember; while a separation/bubble early in a tire's life is usually associated with a manufacturing condition, even a single, significant impact with a deep pothole or sharp curb can cause a new tire and wheel to be damaged. 

If the sidewall separation/bubble appears after six months of on-vehicle service, prolonged driving on overloaded/underinflated tires or a road hazard are the most likely causes. However it may take weeks or months after an impact for a separation/bubble to appear as the damaged or bruised area continues to weaken. Unfortunately the time differential between the impact that caused the initial damage and the delayed appearance of visible evidence often means that the driver has forgotten about the impact that damaged the tire in the first place. 

The varieties of possible causes make it necessary to inspect the tire while mounted on the wheel (sometimes the wheel will show impact damage adjacent to the separation/bubble), as well as to dismount the tire and inspect the condition of its innerliner thoroughly. Sometimes it is necessary to return a tire to the manufacturer's inspection center where it can be dissected before the actual cause can be determined. 

While taller profile tires can be damaged by the more severe impacts with deeper potholes and sharper curbs, low profile tires mounted on large diameter wheels are the most susceptible to this type of damage. The driver of vehicles equipped with low profile tires should make special efforts to avoid potholes, curbs or other road hazards.

Tire Rubber Cracking

Tires are subjected to one of the harshest environments experienced by any consumer product. In addition to being stretched millions of times as they roll through their life, tires are exposed to acid rain, brake dust, harsh chemicals and direct sunlight, as well as summer's heat and winter's cold. And while a tire's rubber compounds have anti-aging chemicals in their recipes, exposure to the elements will eventually cause rubber to lose some of its elasticity and allow surface cracks to appear. 

"The anti-aging chemicals used in the rubber compounds are more effective when the tire is exercised...The repeated stretching of the rubber compound actually helps deter cracks from forming."

The surface cracks that occasionally appear have been called many things: Weather Checking, Weather Cracking or Ozone Cracking. These small cracks typically develop in the sidewalls or at the base of the tread grooves. Depending on their severity, they may be cosmetic in nature if they don't extend past the rubber's outer surface, or may be a reason to replace the tire if they reach deep into the rubber.

 Because all tires are made of rubber, all tires will eventually exhibit some type of cracking condition, usually late in their life. However, this cracking can be accelerated by too much exposure to heat, vehicle exhaust, ozone and sunlight, as well as electric generators and motors (that have armature brushes). For example, a vehicle parked outside instead of in a garage will constantly expose its tires to the rays of the sun, increasing the likelihood of cracking. Additionally, some sidewall cracking has been linked to abrasion from parking against a curb, or the excessive use of tire cleaners/dressings that inadvertently remove some of the tire's anti-oxidants and anti-ozone protection during every cleaning procedure. Interestingly enough, when sun exposure or excessive cleaning is the cause of the small cracks, the sidewall of the tire facing outward will show damage, while the sidewall facing inward is rarely affected.

 The anti-aging chemicals used in the rubber compounds are more effective when the tire is "exercised" on a frequent basis. The repeated stretching of the rubber compound actually helps resist cracks forming. The tires used on vehicles that are driven infrequently, or accumulate low annual mileage are more likely to experience cracking because long periods of parking or storage interrupt "working" the rubber. In addition to being an annoyance to show car owners, this condition often frustrates motor home and recreational vehicle owners who only take occasional trips and cannot even park their vehicle in a garage or shaded area. Using tire covers at least minimizes direct exposure to sunlight. 

Tire manufacturers' warranties typically cover cracking for a period of 4 years from the date the tire was purchased (receipt for the new tires or in-service date of the vehicle required) or four years from the date the tire was manufactured. 

There are a few conditions that would possibly void the manufacture's coverage. The same types of cracks can also be caused by poor tire maintenance practices. Driving on a tire that was flat, or one that was underinflated or overloaded causes excessive stretching of the rubber compound, and may result in cracks that appear similar to the surface cracks mentioned above. The manufacturers' warranty might not apply if an interior inspection of the tire clearly indicates that the cracks were due to these conditions.

Tire Wheels Finishes & Care

Once your new wheels and tires are installed, step back and take a good look. The new, sharper appearance will accent your vehicle splendidly. They look great now; but unfortunately, your wheels are often the dirtiest part of your car because they are constantly exposed to the elements (corrosive brake dust, ocean or road salt, stones, cinders and sticky tar). 

Cleaning Wheels Damage caused by prolonged exposure to these elements will void the finish warranty on your wheels. It's important to clean them properly and often. 

Here are a few tips on how to maintain a wheel's original splendor.

 Before you install them, a coat of wax will help protect your wheels and make them easier to clean.

 Treat the finish of your wheels as you would the finish of your car. Most alloy wheels today feature a painted and/or a clearcoat finish. The best way to take care of wheels without damaging their protective finish is by frequently washing them with a mild soap and water solution. Using a tar and bug remover can prevent permanent tar staining. Periodic waxing will protect the wheel's finish from the elements. 

Never use abrasive cleansers, steel wool pads or polishing compounds.

 Beware of automatic car washes. Some washes use acid cleaners either before or during the wash to remove dirt and grime. Others use stiff brushes for cleaning wheels and tires. Both of these processes could harm your wheel's finish. Wheels with low profile tires can be damaged by the metal tracks used in most car washes. Ask the employees of the car wash about their equipment and procedures before entering the wash. 

Never allow your wheels and tires to be steam cleaned. Steam can dull the paint and clearcoat finish on your wheels.

 Don't clean hot wheels — wait until they cool. Cleaning wheels while they are hot may cause your mild soap solution to dry too quickly leaving spots or a film of soap on your wheels. 

Clean your tires and wheels first, one at a time. Tires and wheels tend to be the dirtiest parts of your vehicle and have a variety of surfaces to clean. So you will want to use the full strength of your hose to initially rinse away all loose dirt and to finally rinse off your soap solution. If you clean your tires and wheels first, you won't expose your washed car to the over spray as you rinse them. Cleaning one at a time focuses your attention and ensures that the soap doesn't dry on one wheel while you're cleaning another. Be sure to use a different sponge on the rest of the car's body to prevent scratching the paint from the particles that may have collected during the wheel cleaning process. 

Clean wheels on a regular basis. Your new tires and alloy wheels are like any other valuable investment. Care for them as you would care for your entire vehicle.

Flat Tire Repairs - After Driving on Flat Run-Flat Tire

Most run-flat tires make a promise of temporary extended mobility at up to 50 miles per hour speeds for up to 50-mile distances in the event a puncture allows complete air pressure loss. However, even if driven within their speed and distance limitations, driving on run-flat tires with low or no air pressure can permanently damage their internal structure, surrendering strength and durability. 

Therefore, Tire Rack recommends driving the slowest safe speed below 50 miles per hour and the shortest distance to a service provider's facility.

 Unfortunately, when it's time to consider repair, even a trained tire specialist may be unable to confirm internal structure damage resulting from a run-flat tire having been driven in a severely underinflated or zero pressure condition. Such damage may not be visible on the surfaces of the tire's exterior sidewalls or interior liner making it impossible to determine tire suitability for repair or reuse. Because of this, Tire Rack and many tire manufacturers recommend replacement of run-flat tires driven in a severely underinflated or zero pressure condition. 

Rubber Manufacturer Association (RMA) Policies 

The RMA is an industry association whose membership includes most major domestic and foreign manufacturers selling tires in the United States. The RMA collectively establishes the foundation of industry policies, from which the tire manufacturers can specify their company's exact position. 

The RMA makes available training presentations and workshop posters to assist tire technicians with the steps necessary to properly repair tires. Each tire manufacturer specifies RMA tire repair procedures must be followed precisely to have any tire repair deemed proper. 

While basic RMA tire repair procedures remain the same for run-flat tires as non-run-flat tires, many tire manufacturers had issued specific policies that supersede RMA's standard reparability policies. 

Selected Tire Manufacturer's Policies for Run-Flat Tire 

Repair The following list summarizes tire manufacturers' repair policies, however it should be noted that every tire manufacturer that consents to repair, does so only after the punctured tire has passed a thorough exterior and interior examination.

Manufacturer/Brand	Repair Policy	Additional Provisions
BFGoodrich	Permitted	1 repair maximum
Bridgestone	Permitted	Only if 15 psi maintained
Continental	Does not recommend	Mfg. Road Hazard Program*
Dunlop	Permitted
Firestone	Permitted	Only if 15 psi maintained
Goodyear	Permitted
Kumho	Permitted
Michelin	Permitted	1 repair maximum
Pirelli	Not endorsed	Mfg. Road Hazard Program**
Yokohama	Not to be repaired	Mfg. Road Hazard Program***

Replaced free of charge first year or first 2/32" of wear (whichever comes first). Continental advises that a repair to one of its tires invalidates all other manufacturer's warranties.

 Replaced free of charge first year or first 2/32" of wear (whichever comes first), then prorated until worn to 2/32" of remaining tread depth 

5-year prorated replacement until worn to 2/32" of remaining tread depth if punctured in RMA defined repairable area 

Use of run-flat tires deemed non-repairable may result in vehicle or property damage, as well as can cause personal injury or death.