Where to Install New Pairs of Tires?

Most vehicles are equipped with the same size tire at every wheel position. Ideally all of these tires should also be of the same type and design, have the same tread depth and be inflated to the pressures specified by the vehicle placard or owner's manual. This combination best retains the handling balance engineered into the vehicle by its manufacturer.

However due to a front-wheel drive vehicle's front tires' responsibility for transmitting acceleration, steering and most of the braking forces, it's normal for them to wear faster than rear tires. Therefore if the tires aren't rotated on a regular basis, tires will typically wear out in pairs rather than in sets. And if the tires aren't rotated at all, it's likely that the rear tires will still have about 1/2 of their original tread depth remaining when the front tires are completely worn out.

Intuition suggests that since the front tires wore out first and because there is still about half of the tread remaining on the rear tires, the new tires should be installed on the front axle. This will provide more wet and wintry traction; and by the time the front tires have worn out for the second time, the rear tires will be worn out, too. However in this case, intuition isn't right...and following it can be downright dangerous.

"When tires are replaced in pairs...the new tires should always be installed on the rear axle and the partially worn tires moved to the front."

When tires are replaced in pairs in situations like these, the new tires should always be installed on the rear axle and the partially worn tires moved to the front. New tires on the rear axle help the driver more easily maintain control on wet roads since deeper treaded tires are better at resisting hydroplaning.

Hydroplaning occurs when the tire cannot process enough water through its tread design to maintain effective contact with the road. In moderate to heavy rain, water can pool up in road ruts, depressions and pockets adjacent to pavement expansion joints. At higher speeds, the standing water often found in these pools challenges a tire's ability to resist hydroplaning.

Exactly when hydroplaning occurs is the result of a combination of elements including water depth, vehicle weight and speed, as well as tire size, air pressure, tread design and tread depth. A lightweight vehicle with wide, worn, underinflated tires in a heavy downpour will hydroplane at lower speeds than a heavyweight vehicle equipped with new, narrow, properly inflated tires in drizzling rain.

If the front tires have significantly less tread depth than the rear tires, the front tires will begin to hydroplane and lose traction on wet roads before the rear tires. While this will cause the vehicle to understeer (the vehicle wants to continue driving straight ahead), understeer is relatively easy to control because releasing the gas pedal will slow the vehicle and help the driver maintain control.

However, if the front tires have significantly more tread depth than the rear tires, the rear tires will begin to hydroplane and lose traction on wet roads before the fronts. This will cause the vehicle to oversteer (the vehicle will want to spin). Oversteer is far more difficult to control and in addition to the initial distress felt when the rear of the car starts sliding, quickly releasing the gas pedal in an attempt to slow down may actually make it more difficult for the driver to regain control, possibly causing a complete spinout.

Experience

Members of Tire Rack team had the chance to experience this phenomenon at Michelin's Laurens Proving Grounds. Participants were allowed to drive around a large radius, wet curve in vehicles fitted with tires of different tread depths -- one vehicle with new tires on the rear and half-worn tires on the front and the other with the new tires in the front and half-worn tires on the rear.

It didn't take long for this hands-on experience to confirm that the "proving grounds" name for the facility was correct. The ability to sense and control predictable understeer with the new tires on the rear and the helplessness in trying to control the surprising oversteer with the new tires on the front was emphatically proven.

And even though our drivers had the advantage of knowing we were going to be challenged to maintain car control, spinouts became common during our laps in the car with the new tires on the front and the worn tires on the rear. Michelin advises us that almost every driver spins out at least once when participating in this demonstration!

Experiencing this phenomenon in the safe, controlled conditions of Michelin's Laurens Proving Grounds rather than in traffic on an Interstate ramp in a rainstorm is definitely preferred!

Recommendations

Tires should be serviced periodically following the rotation patterns provided in the vehicle's owner's manual or as established by the industry to help enhance wear quality and equalize front-to-rear and side-to-side wear rates. The minor differences in tread depth between tires that might be encountered immediately after periodic tire rotations at 5,000-7,500 mile intervals won't upset the vehicle's hydroplaning balance and should not preclude rotating tires. For that matter, any differences in wear rates actually indicate that tire rotations should be done more frequently. Ideally tires should be replaced in complete sets. However when tires are replaced in pairs, the new pair of tires (assuming the vehicle is equipped with the same size tires all of the way around) should always be installed on the rear axle and the existing partially worn tires moved to the front axle.

While insufficient tire rotation intervals and/or out-of-adjustment wheel alignment angles often caused the differences in front-to-rear and side-to-side tire wear rates in the first place, the need to keep deeper treaded tires on the rear axle to resist oversteer conditions caused by wet road hydroplaning is important. Unfortunately this precludes the future possibility of ever rotating tires.

Once a pair of tires has been installed, the only way to escape being forced to drive on mismatched tires continually is to install a complete set of new tires (especially on front-wheel drive vehicles).

The Benefits Of Nitrogen Inflation for tire.

Tyres runs cooler, last longer and improve handling & braking. Quieter tyre - muting more road noise.

For some years, inflating tyres with nitrogen has been known to produce some substantial advantages over conventional compressed air. Its use has been limited to specialised applications such as aircraft tyres and motorsport, mainly on the grounds of cost of equipment and difficulty of generating this inert gas on the premises.

R-Tec are pleased to announce that the benefits of nitrogen inflation are now available at our workshop.

The main advantages of Nitrogen Inflation for tyres are as follows:

Tyre Inflation Pressure Remains More Stable
It is normally recommended that tyre pressures should be checked (including the spare tyre) monthly, and before any long trip. With nitrogen inflation, natural pressure loss is less and the interval for adjusting pressures is considerably extended. However, it is naturally still prudent to check your tyre pressures at the recommended intervals and particularly the pressure of any suspect tyre.

Condensation On The Inside Of Wheels Is Drastically Reduced
One of the most common everyday problems we face is corrosion on the inside of steel and alloy rims. This leads to leaking around the rim with corresponding suspicion of a slow puncture.The introduction of nitrogen inside the tyre prevents oxidization which causes a rapid ageing of the tyre and corrosion inside the rim.

The Possibility Of A Blow Out Or Explosion Is Reduced
Nitrogen doesn't heat up like oxygen, the tyre therefore runs cooler and thus reduces the possibility of explosion.

Better Grip On The Road
Nitrogen being far more stable in respect of temperature and pressure translates into better grip and improved vehicle handling. There are also proven reports of extended tyre life, making nitrogen inflation very cost effective. In addition to the benefits for most road users, nitrogen inflation has particular benfits for caravan, camper and trailer users. Better pressure retention and a reduction in the ageing process are real benefits for these vehicles which often spend a great deal of time standing in one place.

So what should drivers do?

Overall, inflating tires with nitrogen won't hurt them and may provide some minimal benefits.

Is it worth it? If you go someplace that provides free nitrogen with new tires, why not? Additionally we've seen some service providers offering reasonable prices of about $5 per tire (including periodic adjustments for the life of the tire) to a less reasonable $10 per tire (with additional costs for subsequent pressure adjustments) or more as part of a service contract, which we believe exceeds the value of nitrogen's benefit.

Rather than pay extra for nitrogen, most drivers would be better off buying an accurate tire pressure gauge and checking and adjusting their tire pressures regularly.

When Should you Replace your Tires？在什么状况下该换掉你的轮胎呢？？？

A driver's ability to control their vehicle depends on the traction between their tires and the road. Tires don't require tread designs or even much tread depth to deliver traction on dry roads. A practical example of this is the racing slicks used on stock cars and open-wheel racers that provide traction at over 200 mph. However, tires do require tread designs to generate traction on wet, slushy and snow-covered roads. Liquids can't be compressed and require time and energy to move them out of the way as our tires drive through them. Those same racing slicks would lose traction at amazingly slow speeds anytime something prevented them from maintaining contact with the road.

So a tread design is necessary to direct water and slush from between the tire and the road, as well as provide edges that bite into snow. But that's only half the equation; because we've seen that tread depth also contributes to how well the design does its job.

The air our tires encounter at highway speeds can easily be compressed and moved out of the way with relative ease. However the same isn't true of liquids. When water collects on the road surface during rainstorms, the water depth, vehicle speed and vehicle weight, as well as the tires' tread designs and tread depths collectively determine when and if the tires will be forced to hydroplane and how quickly they can stop a vehicle.

"Tire Rack's advice is that if rain and wet roads are a concern, you should consider replacing your tires when they reach approximately 4/32" of remaining tread depth."

A typical passenger car tire has about twenty square inches of total footprint surface and begins with about 1/3" of tread depth. While the majority of the footprint surface is made up of the rubber that grips the road, the remainder is the void of the grooves that make up the tread design. (See Photo #2)

In order to confirm how much wet traction worn tires sacrifice, members of the Tire Rack team measured the stopping distances from 70 mph (the typical speed limit of U.S. Interstate highways) with vehicles equipped with sets of new tires and compared them to tires with about 4/32" (3mm) of remaining tread depth, followed by sets with the legal minimum of 2/32" (1.6mm) depth. The differences surprised us! Vehicles equipped with the 2/32" minimum tire tread depth took about 100 more feet to stop and were still traveling at about 45 mph at the same distance the vehicles equipped with the 4/32" deep tires had already come to a complete stop!Obviously the tread will wear away over the life of the tire and the volume of its tread grooves will be reduced. While this occurs so slowly that it may not be noticed day-to-day, ultimately there will be a time when the driver will notice the car slip in the snow, hydroplane in the rain or simply not stop in as short a distance on wet roads.

The Tire Rack's advice is that if rain and wet roads are a concern, you should consider replacing your tires when they reach approximately 4/32" of remaining tread depth. Since water can't be compressed, you need enough tread depth to allow the rain to escape through the tire's grooves. If the water can't escape fast enough, your vehicle's tires will be forced to hydroplane (float) on top of the water, losing traction and increasing stopping distances.

Additionally, if snow-covered roads are a concern, you should consider replacing your tires when they reach approximately 5/32" of remaining tread depth to maintain good mobility. You need more tread depth in snow because your tires need to compress the snow in their grooves and release it as they roll. If there isn't sufficient tread depth, the "bites" of snow your tires can take on each revolution will be reduced to "nibbles," and your vehicle's traction and mobility will be sacrificed.

While replacing your tires before they are legally worn out may not appear the most economical practice, it is far less expensive than repairing your car if it can't stop in an emergency situation in less distance than the vehicle ahead of you!

Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires

New, Full-Treaded Tires Generate More Rolling Resistance Than Shallow-Treaded, Worn Tires

Tire rolling resistance gradually drops by about 20% during the life of a tire as the tread wears from its original molded depth to worn out. This can be attributed to the reduction in tread mass and rubber squirm, as well as subtle hardening of the tread compound during years of service and exposure to the elements.

While this gradual reduction in tire rolling resistance and minor increase in fuel economy may be too subtle to register during the tire's life on a tank-by-tank basis, the virtually instantaneous switch from worn tires to new tires (even if they are the same brand, type and size) will typically result in an increase in rolling resistance of about 20%. Since the automotive industry estimates a 10% increase in tire rolling resistance will result in a 1% to 2% decrease in vehicle fuel economy, drivers should expect to experience a potential 2% to 4% decrease in mpg.

New, Full-Treaded Tires Travel Farther per Tire Revolution Than Shallow-Treaded, Worn Tires

Vehicles are programmed with their Original Equipment (O.E.) tire's revolutions-per-mile to allow their odometers to calculate the distances traveled. Unfortunately vehicle odometers aren't always 100% accurate and the tire revolutions per mile will change as its tread wears.

The diameter and circumference of a new, full-treaded tire is greater than that of an old, worn-out tire. Considering that many passenger car tires are molded with beginning tread depths of 10/32" to 12/32", the tires will be approximately 1/2" to 6/10" shorter in diameter when they've worn to the minimum tread depth of 2/32".

In order to determine how much odometer error this dimensional difference might cause, the Tire Rack team drove one of our 2008 BMW 328Ci test cars on a set of new, full-treaded 205/55R16-sized tires, as well as another set of the same tires shaved to worn-out (2/32") tread depth. We used a Global Positioning System (GPS) to measure the 100-mile test distance on a dry Indiana Toll Road and maintained an average speed of 70 mph. We then compared the number of miles traveled indicated by the vehicle's odometer to the GPS receiver and highway mile markers.

When the GPS indicated exactly 100 miles had been traveled while the vehicle was equipped with new 12/32" deep tires, the vehicle's odometer registered 99.4 miles. When the vehicle was then equipped with another set of the same tires shaved to 2/32" of remaining tread depth, the odometer indicated 101.0 miles. While the 100-mile test distance didn't change, essentially the vehicle's odometer overstated the distance traveled by about 1.5% when equipped with the worn out tires.

Since drivers traditionally monitor their vehicle's fuel economy by dividing the number of miles traveled as registered on the odometer by the number of gallons used to fill the tank (or by simply letting the vehicle's trip computer handle the task), the accuracy of the vehicle's fuel economy calculation is dependant on the number of miles indicated by the odometer.

This means that the vehicle's fuel economy (whether calculated by the driver after filling up or the trip computer) would instantaneously appear to drop by about 1.5% when fitted with new tires simply because the vehicle would actually have traveled 1.5% farther than it did when equipped with its recently removed worn-out tires.

Not All Tire Dimensions are Created Equal

Even though tires may be branded as the same size, their specifications may vary slightly by manufacturer and tire line. Tire Rack has seen the overall diameter of a single passenger car tire size vary by 2/10" from the smallest to the largest.

As an example, we'll compare the differences between an O.E. tire with two replacement tires in the 185/65R15 size used on the 2008 Toyota Prius,

	Diameter	Tread Depth	Tire RPM*
Goodyear Integrity (O.E.)	24.4"	10/32"	855
Yokohama AVID TRZ	24.4"	11/32"	850
General Altimax RT	24.5"	11/32"	843
*Tire revolutions per mile

As shown above, there are slight differences between the tires' published diameters, tread depths and tire revolutions per mile. However, if a tire rolls fewer times per mile than the tire it replaces, the vehicle will again actually be traveling farther than is indicated by the odometer. Calculating the influence of the different tire specifications on the vehicle's odometer would indicate the Yokohama AVID TRZ would travel .6% farther than the Goodyear Integrity, while the General Altimax RT would travel 1.4% farther.

Conclusion

While many of these individual differences may seem insignificant, it is easy to understand that when they are added together, the new tires may appear to reduce vehicle fuel economy. It also means that a Toyota Prius appearing to get 50.0 mpg just before replacing its worn-out tires with new tires of the same brand, type and size, might be reduced to registering just 47.25 mpg afterwards, even if all of the driving conditions were identical.

Remember, "your mileage may vary."

Tire Rolling Resistance Part 2: Defining Rolling Resistance

Tire rolling resistance is defined as the force required to maintain the forward movement of a loaded pneumatic tire in a straight line at a constant speed. And just like the laws of physics and forces of nature, it is an obstacle every vehicle has to overcome to transport passengers and cargo to their destinations.

Tire rolling resistance is caused by the natural viscoelastic properties of rubber along with the tire's internal components constantly bending, stretching and recovering as they cycle between their loaded (where the tread footprint flattens against the road) and unloaded states. The final contributor to tire rolling resistance is the tread's interaction with the road.

"Tire rolling resistance has an impact on vehicle fuel consumption estimated to range from about 4% during urban driving to 7% during highway driving."

The tread area represents a new tire's single largest and heaviest region and is the greatest contributor to tire rolling resistance. The tread and its underlying plies typically account for about 2/3 of a new tire's rolling resistance, while the sidewall and bead area represent the remaining 1/3.

Larger tires require more rubber and longer reinforcing cords than smaller tires. Therefore within a single tire model line, there is typically a relationship between tire size, weight and the resulting rolling resistance force where larger tires have more rolling resistance than smaller tires.

The most common laboratory test measures the force required to rotate a tire at 50 mph against a large diameter steel drum. Multiple samples of each tire size/model are tested to establish an average rolling resistance value. And since tire rolling resistance typically declines moderately as tire temperatures rise from cold to normal operating conditions during the first 30 minutes of driving every time the vehicle is used, values are recorded in the laboratory after operating temperatures and rolling resistance values stabilize.

Tire Rolling Resistance Force is measured in pounds or kilograms of resistance. Comparing Rolling Resistance Force provides a direct way to compare tires of the same size, as well as offers an accurate means of comparing differently sized tires to one another.

Tire Rolling Resistance Coefficient is calculated by dividing the measured rolling resistance force by the tire size's prescribed load during the test. Comparing Rolling Resistance Coefficients only allows comparing tires within a single size. Tire Rolling Resistance Coefficient does not allow comparing different sized tires.

As noted earlier, larger tires generate higher Rolling Resistance Forces than smaller tires. However the larger tire's greater overall diameter, circumference and rolling radius allows its tread area to bend, stretch and recover more easily as it cycles in and out of contact with the road. Larger diameter tires also revolve fewer times per mile and cycle at a slower rate than shorter tires for any given speed. While this essentially reduces Rolling Resistance Coefficient (larger tires will often have a lower Rolling Resistance Coefficient than smaller tires), it still does not change the fact that a larger tire actually generates more Rolling Resistance Force that the vehicle's engine has to overcome.

Unfortunately comparing tire Rolling Resistance Coefficients is somewhat like comparing the fuel efficiency of an 8 passenger, 15 mpg sport utility vehicle to a 4 passenger, 30 mpg car based on miles per gallon per passenger when fully occupied. While both vehicles offer the same miles per gallon per passenger fuel efficiency when fully occupied, the sport utility vehicle will always use more fuel than the car (as well as more fuel per passenger anytime the sport utility vehicle is driven below its maximum passenger capacity).

Regardless of their calculated Rolling Resistance Coefficients, aunds of Rolling Resistance Force will require more energy to roll than a small tire generating 15 pounds of Rolling Resistance Force.

Tire rolling resistance has an impact on vehicle fuel consumption estimated to range from about 4% during urban driving to 7% during highway driving. The engine and driveline is estimated to consume 80% of the fuel, while the remainder is used to overcome inertia, wind resistance, converted into heat by the brakes or consumed when the vehicle is idling. The automotive industry estimates a 10% reduction in tire rolling resistance will result in a one to two percent improvement in vehicle fuel economy. While that might not seem like a lot, it can reduce fuel consumption by a couple of tanks per year and make the purchase of lower rolling resistance tires a better value over their lifetime.

Tire Rolling Resistance Part 1: Understanding Corporate Average Fuel Economy

In the United States, vehicle manufacturers are required to maintain an average fuel economy for the fleet of new vehicles they sell each year. In 2009, the government Corporate Average Fuel Economy (CAFE) mandate was 27.5 miles per gallon (mpg) for cars and 23.1 mpg for light trucks (including minivans, vans and most pickup trucks and sport utility vehicles). However because it's an average fuel economy, in order to sell large cars or trucks (that use more fuel), the vehicle manufacturer must also sell small cars and trucks (that are fuel-efficient). The vehicle manufacturer can be fined if their annual vehicle fleet uses too much fuel.

A tire's rolling resistance affects fuel economy. Most vehicle manufacturers demand their suppliers develop low rolling resistance tires to be used as Original Equipment on their new vehicles to help average out their CAFE-mandated mpg. In order to meet the manufacturer's demands, these tires are often designed with a priority on reducing weight and rolling resistance and are molded with slightly thinner sidewalls, shallower tread depths and use low rolling resistance constructions and tread compounds.

However, in order to understand CAFE tests and the role that tires play, it is important to recognize that CAFE tests are conducted in a laboratory and not on the highway. Many aspects that affect fuel economy in the real world are reduced to constants incorporated into the formulas specified.

A vehicle's fuel economy is the direct result of its total resistance to movement. This includes overcoming inertia (Newton's Law), driveline friction, road grades, tire rolling resistance and air drag. In order to offer the same level of performance, heavy vehicles require more power (and more fuel) than light vehicles. All-wheel and four-wheel drive vehicles require more power than two-wheel drive vehicles and boxy vehicles require more power than low drag aerodynamic vehicles.

But how much influence does each of these elements have and when are their influences felt? Once you eliminate the fuel converted into heat by the engine, the relative percent of influence that these other factors represent during stop-and-go city driving are very different then during steady-speed, highway driving.

During stop-and-go city driving, it's estimated that overcoming inertia is responsible for about 35% of the vehicle's resistance. Driveline friction is about 45%, air drag is about 5% and tire rolling resistance is about 15%.

Overcoming inertia no longer plays an appreciable role in the vehicle's resistance during steady speed highway driving. For those conditions it is estimated that driveline friction is about 15%; air drag is about 60% and tire rolling resistance represent about 25%.

Let's explore a scenario where a high performance replacement radial tire has a whopping 20% increase in rolling resistance over a low rolling resistance Original Equipment standard passenger radial. To calculate the potential change in mpg resulting from using the high performance tires in place of the Original Equipment tires, we would multiply the tire's percentage of influence on the vehicle's overall resistance (15% in the city and 25% on the highway) times the high performance tires' 20% increase in rolling resistance.

If the vehicle equipped with standard Original Equipment, low rolling resistance passenger tires normally provided 25 mpg in the city and 30 mpg on the highway, installing tires with 20% greater rolling resistance would only drop fuel mileage by a calculated 3% (to 24.25 mpg) in the city and a calculated 5% (to 28.5 mpg) on the highway. While this is a measurable difference, it probably isn't much more of an influence on real world fuel economy than being stuck in rush hour traffic a couple of times a week or being stopped at every red light instead of continuing through a string of green lights.

The easiest way to reduce rolling resistance and enhance fuel economy is to make certain that the tires are properly inflated. A vehicle that requires its tires to be inflated to 35 psi (based on the vehicle's tire placard) will have an increase in rolling resistance of approximately 12.5% if the tires are allowed to become under-inflated to just 28 psi. Therefore, maintaining the vehicle manufacturer's pressure recommended for light load and heavy load conditions may almost be as important as the tires being used.