Tyre Guide
You will find
Tyre Sizes & Markings
In addition to the tire's brand and line names (tire model), there is a lot of information provided by the manufacturer on the sidewalls of the tires they produce. Some of the branded information provides the tire's basic dimensions and identifies the week it was produced. Other branding lists the types of materials used internally to reinforce the rubber, along with the tire's maximum inflation pressures and loads. And others confirm the manufacturer certifies the tire meets various industry standards and measures up to the government regulations of the nations in which it will be used.
While not all information is branded on every tire, the illustration includes the typical information found on many tires.
Air Pressure - Correct, Underinflated and 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.
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.
Tyre Pressure Monitoring Sensors [TPMS]
Have you ever seen a vehicle with one or more tires that appear noticeably low on tire pressure? Didn't you want to warn the driver of the situation before that slight inconvenience became a calamity? What if the vehicle with the low tire pressures is the one you're driving? Wouldn't you want to be warned?
The United States Department of Transportation (DOT) National Highway Traffic Safety Administration (NHTSA) has developed a Federal Motor Vehicle Safety Standard that requires the installation of tire pressure monitoring systems (TPMS) that warn the driver when a tire is significantly underinflated. The standard applies to passenger cars, trucks, multipurpose passenger vehicles and buses with a gross vehicle weight rating of 10,000 pounds or less, except those vehicles with dual wheels on an axle.
Maintaining the correct tire pressure for a vehicle is an important factor in how much load its tires can safely carry. The correct pressure will carry the weight without a problem. Too little tire pressure will eventually cause catastrophic tire failure.
Tires aren't invincible. They are made of individual layers of fabric and steel encased in rubber. If a tire is allowed to run low on air pressure, the rubber is forced to stretch beyond the elastic limits of the fabric and steel reinforcing cords. When this happens, the bond between the various materials can weaken. If this is allowed to continue, it will eventually break the bonds between the various materials and cause the tire to fail. And even if the tire doesn't fail immediately, once a tire is weakened it won't heal after being reinflated to the proper pressure. So if a tire has been allowed to run nearly flat for a period of time, the tire should be replaced, not simply repaired or reinflated.
Studies have shown that running tires with too little air pressure is not uncommon. It's been estimated that about one out of every four vehicles on the road is running on underinflated tires. This also means that one out of every four drivers is needlessly sacrificing their vehicle's fuel economy and handling, and reducing their tires' durability and tread life.
This has made tire pressure maintenance an important safety issue throughout the automotive industry and caused the U.S. government to pass legislation mandating tire pressure monitoring systems. The main purpose of these systems is to warn the driver if their tires are losing air pressure, leaving the tires underinflated and dangerous.
What types of systems are being used now? How do they work? Which works the best?
The National Highway Traffic Safety Administration (NHTSA) provides vehicle manufacturers options with which they can comply with the law. One option is to install a direct tire pressure monitoring system that uses pressure sensors located in each wheel to directly measure the pressure in each tire and warns drivers when the air pressure in any of their tires drops at least 25% below the recommended cold tire inflation pressure identified on the vehicle placard. Another option is to install an indirect tire pressure monitoring system that would warn the driver when a single tire has lost at least 25% of its inflation pressure compared to other tires on the vehicle. While direct systems could offer more precise warning thresholds, indirect systems cannot offer the same information or accuracy.
| What's the Difference? |
| DIRECT VS. INDIRECT |
| Direct Systems attach a pressure sensor/transmitter to the vehicle’s wheels. An in-vehicle receiver warns the driver if the pressure in any tire falls below a predetermined level. Direct systems are typically more accurate and reliable and most are able to indicate which tire is underinflated. |
| Indirect Systems use the vehicle’s antilock braking system’s wheel speed sensors to compare the rotational speed of one tire versus the others. If a tire is low on pressure, it will roll at a different number of revolutions per mile than the other three and alert the vehicle’s onboard computer. Indirect systems (except for the TPMS on several 2009+ Audi models and 2010+ Volkswagen models) are unable to generate accurate readings in cases where all four tires are losing pressure at the same rate, such as the effects of time and temperature. |
Direct Monitoring Systems
Direct tire pressure monitoring systems measure, identify and warn the driver of low pressure. Because direct systems have a sensor in each wheel, they generate accurate warnings and can alert the driver instantly if the pressure in any one tire falls below a predetermined level due to rapid air loss caused by a puncture. In addition, direct tire pressure monitoring systems can detect gradual air loss over time. Some direct systems use dashboard displays that provide the ability to check current tire pressures from the driver's seat.
Direct systems attach a pressure sensor/transmitter to the vehicle's wheel inside the tire's air chamber. Most Original Equipment and some aftermarket systems attach their air pressure sensor/transmitter to special tire valves. While the presence of a metal clamp-in valve typically identifies the presence of a direct tire pressure monitoring system, special snap-in rubber valves have also been used to support direct system sensors. The transmitter's signal is broadcast to the in-car receiver and the information is displayed to the driver.
Some aftermarket and Original Equipment direct monitoring systems attach the sensor/transmitter to the wheel with an adjustable metal strap. These sensors/transmitters and their straps only weight a few ounces and allow virtually universal application on car and light truck wheels. Since standard snap-in rubber valves are still used for these applications, it is important that the owners of these systems let their tire installer know that the vehicle is equipped with a direct system banded to the wheel before they change the tires.
Tire Rack works with wheel manufacturers to develop aftermarket wheels that accommodate direct tire pressure monitoring sensors/transmitters. This results in our ability to offer a wider selection of aftermarket alloy wheel styles that accept Original Equipment direct system components. Additionally, the Tire Rack's fitment specialists have carefully determined which aftermarket wheels will be compatible with the vehicle and system installed for customers purchasing Tire & Wheel Packages or wheel upgrades. Search results include notes regarding TPMS sensors and sensors can confidently be purchased online with wheels.
Indirect Monitoring Systems
In the interest of providing a lower cost Original Equipment system, indirect tire pressure monitoring systems were developed by vehicle manufacturers wishing to comply with the law while minimizing development time and cost. Indirect systems use the vehicle's antilock braking system's wheel speed sensors to compare the rotational speed of one tire to that in another position on the vehicle. If one tire is low on pressure, its circumference changes enough to roll at a slightly different number of revolutions per mile than the other three tires. Reading the same signal used to support ABS systems, the vehicle manufacturers have programmed another function into the vehicle's onboard computer to warn the driver when a single tire is running at a reduced inflation pressure compared to the others.
Unfortunately, indirect tire pressure monitoring systems have several shortcomings. Indirect systems won't tell the drivers which tire is low on pressure, and won't warn the driver if all four tires are losing pressure at the same rate (as occurs during the fall and winter months when ambient temperatures get colder). Additionally, our current experience with indirect systems indicates that they can generate frequent false warnings. We have found that false warnings may occur when the tires spin on wet, icy and snow-covered roads. In these cases, the false alarms would train the driver to disregard the tire pressure monitoring system's warnings, negating its purpose completely.
Concerns
While the Tire Rack applauds the emphasis on maintaining appropriate tire pressure and requiring a system that will warn the driver if low pressure is detected, we are concerned about the percentage of underinflation that the law permits before warning the driver. The driver of a passenger car that calls for 35 psi may not be warned about tire pressure loss until it drops to 26 psi depending on the type of monitoring system used. Under the same circumstances, a driver of a light truck that calls for 80 psi won't be warned until just 60 psi remains. In both of these cases, significant load capacity has been sacrificed before the driver is warned.
The only way to overcome this obstacle would be to fit significantly oversized tires to every new vehicle that could compensate for a 25% loss in tire pressure before becoming overloaded. Unfortunately, these larger tires would add to gross vehicle weight, generate more rolling resistance and increase the vehicle's aerodynamic drag. This would result in a loss of fuel economy and increased gasoline consumption in direct contrast to the government's Corporate Average Fuel Economy (CAFE) requirements for cars and light trucks.
While the legislation is well intended, we feel that direct tire pressure monitoring systems are the better means to warn the driver of low tire pressure before inconvenience becomes calamity. Additionally, we are concerned that the drivers of vehicles equipped with any tire pressure monitoring system will become over confident in the capabilities of their system and will be even less likely to confirm their vehicle's cold tire pressure with a pressure gauge at least once a month and before long trips.
Increasing Wheel Size
Large diameter wheels and wide, low profile tires go together and show up everywhere from America's new car showrooms to its roads, tracks and trails. So whether the vehicle came from the assembly line or was upgraded after it was delivered, Plus Sizing (also called "Inch-Up" sizing in other parts of the world) probably played a role in its tire size choice. Plus Sizing allows tires and wheels to make a fashion statement while providing a functional improvement.
Plus Sizing dates back to the 1970s when Plus One and Plus Two fitments were the available choices. Drivers could upgrade their cars from relatively narrow 13-, 14- or 15-inch wheels and tires up to wider 14-, 15- or 16-inch combinations. While Plus One and Plus Two are still popular today, the starting point now typically begins with 15- or 16-inch wheel diameters and grows from there.
Plus Sizing supports the premise that it's important to maintain the same overall tire diameter whenever changing tires and wheel sizes to ensure sufficient ground clearance, appropriate driveline gearing and accurate speedometer readings. Large changes in overall tire diameter can alter the accuracy of the speedometer as well as the effectiveness of anti-lock braking system (ABS), traction control and vehicle stability system.
Plus Size Wheel and Tire Examples
Plus Sizing wheels and tires is one of the easiest ways to improve the image of a vehicle. The visual appeal is obvious since alloy wheels are more attractive than tire sidewalls, and bigger wheels combined with shorter tire sidewalls produce a powerful image.
Using tires with shorter sidewalls also quickens steering response and increases cornering stability. Combining them with larger diameter wheels makes it possible to properly maintain the overall wheel and tire diameter, keeping odometer and speedometer changes negligible.
Plus Sizing's biggest risks stem from accidental encounters with potholes, curbs and debris on the road. Low profile tire sidewalls can be pinched more easily between the road and the rim because shorter sidewalls cannot accommodate impact as well as taller sidewalls. Once a vehicle has been Plus Sized, the driver has to try to go around obstacles, rather than run over them.
Additionally, wide tires tend to float on loose surfaces and cannot process water as quickly as narrow tires. This reduces snow traction and hydroplaning resistance when driving on water-soaked highways.
And finally, the maximum Plus Size applications for pickup trucks and sport utility vehicles result in wheel and tire combinations that are significantly heavier than the vehicle's Original Equipment (O.E.) tires and wheels. This increase in weight can lead to longer stopping distances, as well as increased suspension and brake wear.
Here's How We Do It!
We select O.E. equivalent tire diameters and load capacities by matching wider, lower profile tires with wider, larger diameter wheels. This maintains the accuracy of the vehicle's speed dependent systems, while reducing braking distances, improving responsiveness and increasing stability.
A Plus Sizing rule of thumb is to increase tire width by 10 millimeters and decrease sidewall height by 5 to 10 percent for each 1-inch increase in wheel diameter.
You will often find only +/- a few tenths of an inch difference in the overall diameter of the tires, as shown. This results in a negligible +/- four tenths of a mph speedometer variance.
Run Flat Tyres – mostly fitted on BMW vehicles
Tires That Help Maintain Vehicle Mobility...Even After Being Punctured
If you've ever been late for a date, appointment, or meeting because of a flat tire, you already know how frustrating it can be. If you've ever changed a flat tire in the rain, after dark, or on the shoulder of a busy highway, you already know how frightening it can be. So while we enjoy the freedom our vehicles provide, it's amazing how quickly that freedom vanishes when a flat tire strands us.
Since the early development of the automobile, tires have played an important role in determining a vehicle's overall comfort and safety. However, there are few consumer products placed in harms way more often than our tires, which encounter extremes in temperature, exposure to the elements, and attacks by debris on the road during their life. And while the tire manufacturers' continuous research and development efforts have improved tire durability and longevity, only recently have they developed tires that can temporarily maintain vehicle mobility using standard Original Equipment and aftermarket wheels. These run-flat tires provide the driver more flexibility when deciding where to have tire repairs made.
Tires don't typically carry the weight of our vehicles, the air inside them does. There are three basic elements which determine the load capacity of a tire: the size of the air chamber formed between the tire and wheel, the strength provided by the tire's construction to hold air pressure, and the amount of air pressure actually in the tire.
Most flat tires (and tire "blowouts") are the result of slow leaks that go unnoticed and allow the tire's air pressure to escape over time. Therefore, monitoring tire air pressure in real-time gets us half way there. If we had tires that could maintain temporary vehicle mobility even after air loss, we'd be just about invincible.
Today there are three technologies used as Original Equipment on vehicles to help maintain vehicle mobility when a tire is punctured. They are self-sealing tires, self-supporting tires and tires supported by an auxiliary system.
Self-Sealing
Self-sealing tires are designed to fix most tread-area punctures instantly and permanently. These tires feature standard tire construction with the exception of an extra lining inside the tire under the tread area that's coated with a puncture sealant that can permanently seal most punctures from nails, bolts or screws up to 3/16 of an inch in diameter. These tires first provide a seal around the object when the tire is punctured and then fill in the hole in the tread when the object is removed. Because these tires are designed to seal the tire immediately upon being punctured, most drivers will never even know that they just had a puncture. Also because these tires feature standard tire constructions, the traditional loss-of-air symptoms that accompany a flat tire remain to warn the driver if the tire is damaged beyond repair. Therefore, self-sealing tires do not require a low air pressure warning system.
Example: Continental ContiSeal.
Self-Supporting
Self-supporting tires feature a stiffer internal construction, which is capable of temporarily carrying the weight of the vehicle, even after the tire has lost all air pressure. To provide "self-supporting" capability, these tires typically attach rubber inserts next to or between layers of heat-resistant cord in their sidewalls to help prevent breaking the reinforcing cords in the event of loss of air pressure. They also feature specialized beads that allow the tire to firmly grip current Original Equipment and aftermarket wheels even in the event of air loss. Because self-supporting tires are so good at masking the traditional loss-of-air symptoms that accompany a flat tire, they require a tire pressure monitoring system to alert the driver that they have lost air pressure. Without such a system, the driver may not notice underinflation and may inadvertently cause additional tire damage by failing to inflate or repair the tire at the first opportunity. Typically, self-supporting tires maintain vehicle mobility for 50 miles at speeds up to 55 mph.
Examples: Bridgestone RFT (Run-Flat Tire), Dunlop DSST (Dunlop Self-Supporting Technology) and ROF (Run-On-Flat), Firestone RFT (Run-Flat Tire), Goodyear EMT (Extended Mobility Technology) and ROF (Run-On-Flat), Kumho XRP, Michelin ZP (Zero Pressure), Pirelli RFT (Run-Flat Technology) and Yokohama Run-Flat and ZPS (Zero Pressure System).
The International Organization for Standardization (ISO), a worldwide federation of national standards bodies, has adopted a run-flat system symbol for extended mobility systems featuring self-supporting run-flat tires.
Auxiliary Supported Run-Flat Systems
Auxiliary supported systems combine unique wheels and tires used for Original Equipment vehicle applications. In these systems, the flat tire's tread rests on a support ring attached to the wheel when the tire loses pressure. The advantage to this type of system is that it will place most of the mechanical task of providing run-flat capability on the wheel (which typically doesn't wear out or need to be replaced), and minimizes the responsibility of the tire (which does periodically wear out and requires replacement). Additionally, auxiliary support systems promise better ride quality because their sidewall's stiffness can be equivalent to today's standard tires. The disadvantage to auxiliary supported systems is that their unique wheels will not accept standard tires and that their lower volume will make this type of system more expensive.
Example: Michelin's PAX System wheels and tires
The International Organization for Standardization (ISO), a worldwide federation of national standards bodies, has adopted a run-flat system symbol for extended mobility systems featuring an internal support ring.
It is too early to confirm which system, if any, will be widely accepted by vehicle manufacturers and consumers in the future.
Alignment
While it's often referred to simply as an "alignment" or "wheel alignment," it's really complex suspension angles that are being measured and a variety of suspension components that are being adjusted. This makes an alignment an important suspension-tuning tool that greatly influences the operation of the vehicle's tires.
Out-of-alignment conditions occur when the suspension and steering systems are not operating at their desired angles. Out-of-alignment conditions are most often caused by spring sag or suspension wear (ball joints, bushings, etc.) on an older vehicle. They can also be the result of an impact with a pothole or curb, or a change in vehicle ride height (lowered or raised) on any vehicle regardless of age.
Incorrect alignment settings will usually result in more rapid tire wear. Therefore, alignment should be checked whenever new tires or suspension components are installed, and any time unusual tire wear patterns appear. Alignment should also be checked after the vehicle has encountered a major road hazard or curb.
Front-End, Thrust Angle and Four-Wheel Alignment
The different types of alignments offered today are front-end, thrust angle, and four-wheel. During a front-end alignment, only the front axle's angles are measured and adjusted. Front-end alignments are fine for some vehicles featuring a solid rear axle, but confirming that the front tires are positioned directly in front of the rear tires is also important.
On a solid rear axle vehicle, this requires a thrust angle alignment that allows the technician to confirm that all four wheels are "square" with each other. Thrust angle alignments also identify vehicles that would "dog track" going down the road with the rear end offset from the front. If the thrust angle isn't zero on many solid rear axle vehicles, a trip to a frame straightening shop is required to return the rear axle to its original location.
On all vehicles with four-wheel independent suspensions, or front-wheel drive vehicles with adjustable rear suspensions, the appropriate alignment is a four-wheel alignment. This procedure "squares" the vehicle like a thrust angle alignment, and also includes measuring and adjusting the rear axle angles as well as the front.
Not all vehicles are easily adjustable or fully adjustable. Some vehicles require aftermarket kits to allow sufficient adjustment to compensate for accident damage or the change in alignment due to the installation of lowering springs.
When aligning a vehicle, it's appropriate for the vehicle to be carrying its "typical" load. This is important for drivers who continuously carry loads in their vehicles, such as sales representatives with samples or literature in the trunk. Additionally, when a vehicle is used for autocross or track events, some racers will sit in their car, or have the alignment shop "ballast" their vehicle to include the influence of the driver's weight on the suspension angles.
The primary static suspension angles that need to be measured and adjusted are caster, camber, toe and thrust angle. Here's a definition of each angle and its influence on a vehicle and its tires.
Camber
The camber angle identifies how far the tire slants away from vertical when viewed directly from the front or back of the vehicle. Camber is expressed in degrees, and is said to be negative when the top of the tire tilts inward toward the center of the vehicle and positive when the top leans away from the center of the vehicle.
Since street suspensions cannot completely compensate for the outer tire tipping towards the outside when the vehicle leans in a corner, there isn't a magical camber setting that will allow the tires to remain vertical when traveling straight down the road (for more even wear), and remain perpendicular to the road during hard cornering (for more generous grip).
Different driving styles can also influence the desired camber angle as well. An enthusiastic driver who corners faster than a reserved driver will receive more cornering grip and longer tire life from a tire aligned with more negative camber. However with the aggressive negative camber, a reserved driver's lower cornering speeds would cause the inside edges of the tires to wear faster than the outside edges.
What's the downside to negative camber? Negative camber leans both tires on the axle towards the center of the vehicle. Each tire develops an equal and offsetting "camber thrust" force (the same principle that causes a motorcycle to turn when it leans) even when the vehicle is driven straight ahead. If the vehicle encounters a bump that only causes one tire to lose some of its grip, the other tire's negative camber will push the vehicle in the direction of the tire that lost grip. The vehicle may feel more "nervous" and become more susceptible to tramlining. Excessive camber will also reduce the available straight-line grip required for rapid acceleration and hard stops.
Appropriate camber settings that take into account the vehicle and driver's aggressiveness will help balance treadwear with cornering performance. For street-driven vehicles, this means that tire wear and handling requirements must be balanced according to the driver's needs. The goal is to use enough negative camber to provide good cornering performance while not requiring the tire to put too much of its load on the inner edge while traveling in a straight line. Less negative camber (until the tire is perpendicular to the road at zero camber) typically will reduce the cornering ability, but results in more even wear.
Even though they have some of the most refined suspensions in the world, the next time you see a head-on photo of a Formula 1 car or CART Champ Car set up for a road course, notice how much negative camber is dialed into the front wheels. While this is certainly an example of wear not being as important as grip, negative camber even helps these sophisticated racing cars corner better.
Caster
The caster angle identifies the forward or backward slope of a line drawn through the upper and lower steering pivot points when viewed directly from the side of the vehicle. Caster is expressed in degrees and is measured by comparing a line running through the steering system's upper and lower pivot points (typically the upper and lower ball joints of an A-arm or wishbone suspension design, or the lower ball joint and the strut tower mount of a McPherson strut design) to a line drawn perpendicular to the ground. Caster is said to be positive if the line slopes towards the rear of the vehicle at the top, and negative if the line slopes towards the front.
A very visual example of positive caster is a motorcycle's front steering forks. The forks point forward at the bottom and slope backward at the top. This rearward slope causes the front tire to remain stable when riding straight ahead and tilt towards the inside of the corner when turned.
Caster angle settings allow the vehicle manufacturer to balance steering effort, high speed stability and front end cornering effectiveness.
Increasing the amount of positive caster will increase steering effort and straight line tracking, as well as improve high speed stability and cornering effectiveness. Positive caster also increases tire lean when cornering (almost like having more negative camber) as the steering angle is increased.
What's the downside to positive caster? If thevehicle doesn't have power steering, a noticeable increase in steering effort will be felt as positive caster is increased. Other than that, the effects of positive caster are pretty much "positive," especially increasing the lean of the tire when the vehicle is cornering while returning it to a more upright position when driving straight ahead.
Cross-Camber and Cross-Caster
Most street car alignments call for the front camber and caster settings to be adjusted to slightly different specifications on the right side of the vehicle compared to the left side. These slight side-to-side differences are called cross-camber and cross-caster.
For vehicles set up to drive on the "right" side of the road, the right side is aligned with a little more negative camber (about 1/4-degree) and a little more positive caster (again, about 1/4-degree) to help the vehicle resist the influence of crowned roads that would cause it to drift "downhill" to the right gutter. Since most roads are crowned, cross-camber and cross-caster are helpful the majority of the time, however they will cause a vehicle to drift to the left on a perfectly flat road or a road that leans to the left.
Using cross-camber and cross-caster is not necessary for track-only cars.
Toe
The toe angle identifies the exact direction the tires are pointed compared to the centerline of the vehicle when viewed from directly above. Toe is expressed in either degrees or fractions-of-an-inch, and an axle is said to have positive toe-in when imaginary lines running through the centerlines of the tires intersect in front of the vehicle and have negative toe-out when they diverge. The toe setting is typically used to help compensate for the suspension bushings compliance to enhance tire wear. Toe can also be used to adjust vehicle handling.
A rear-wheel drive vehicle "pushes" the front axle's tires as they roll along the road. Tire rolling resistance causes a little drag resulting in rearward movement of the suspension arms against their bushings. Because of this, most rear-wheel drive vehicles use some positive toe-in to compensate for the movement, enabling the tires to run parallel to each other at speed.
Conversely, a front-wheel drive vehicle "pulls" the vehicle through the front axle, resulting in forward movement of the suspension arms against their bushings. Therefore most front-wheel drive vehicles use some negative toe-out to compensate for the movement, again enabling the tires to run parallel to each other at speed.
Toe can also be used to alter a vehicle's handling traits. Increased toe-in will typically result in reduced oversteer, help steady the car and enhance high-speed stability. Increased toe-out will typically result in reduced understeer, helping free up the car, especially during initial turn-in while entering a corner.
Before adjusting toe outside the vehicle manufacturer's recommended settings to manipulate handling, be aware that toe settings will influence wet weather handling and tire wear as well.
Excessive toe settings often bring with them drivability problems, especially during heavy rain. This is because the daily pounding of tractor trailers on many highways leave ruts that fill with water. Since excessive toe means that each tire is pointed in a direction other than straight ahead, when the vehicle encounters a puddle that causes only one tire to lose some of its grip, the other tire's toe setting will push (excessive toe-in) or pull (excessive toe-out) the vehicle to the side. This may make the vehicle feel unsettled and very "nervous."
Additionally the vehicle's toe is one of the most critical alignment settings relative to tire wear. A toe setting that is just a little off its appropriate setting can make a huge difference in their wear. Consider that if the toe setting is just 1/16-inch off of its appropriate setting, each tire on that axle will scrub almost seven feet sideways every mile! Extend it out and you'll discover that rather than running parallel to each other, the front tires will scrub over 1/4-mile sideways during every 100 miles of driving! Incorrect toe will rob you of tire life.
Thrust Angle
The thrust angle is an imaginary line drawn perpendicular to the rear axle's centerline. It compares the direction that the rear axle is aimed with the centerline of the vehicle. It also confirms if the rear axle is parallel to its front axle and that the wheelbase on both sides of the vehicle is the same.
If the thrust angle is not correct on a vehicle with a solid rear axle, it often requires a trip to the frame straightening shop to correctly reposition the rear axle.
A vehicle with independent rear axles may have incorrect toe-in or toe-out on both sides of the axle, or may have toe-in on one side and toe-out on the other. The suspension on each side of the vehicle must be adjusted individually until it has reached the appropriate toe setting for its side of the vehicle.
An incorrect thrust angle is often caused by an out-of-position axle or incorrect toe settings. So in addition to the handling quirks that are the result of incorrect toe settings, thrust angles can also cause the vehicle to handle differently when turning one direction vs. the other.
Alignment Ranges
The vehicle manufacturers' alignment specifications usually identify a "preferred" angle for camber, caster and toe (with preferred thrust angle always being zero). The manufacturers also provide the acceptable "minimum" and "maximum" angles for each specification. The minimum and maximum camber and caster specifications typically result in a range that remains within plus or minus 1-degree of the preferred angle.
If for whatever reason your vehicle can't reach within the acceptable range, replacing bent parts or an aftermarket alignment kit will be required. Fortunately there is a kit for almost every popular vehicle due to the needs of body and frame shops doing crash repairs and driving enthusiasts tuning the suspensions on their cars.
Recommendations
An accurate wheel alignment is critical to balance the treadwear and performance a vehicle's tires deliver. Regular wheel alignments will usually save you as much in tire wear as they cost, and should be considered routine, preventative maintenance. Since there are "acceptable" ranges provided in the manufacturer's recommendations, the technician should be encouraged to align the vehicle to the preferred settings and not just within the range.
If you are a reserved driver, aligning your vehicle to the vehicle manufacturer's preferred settings is appropriate.
If you are an assertive driver who enjoys driving hard through the corners and expressway ramps, a performance alignment is appropriate for your car. A performance alignment consists of using the vehicle manufacturer's range of alignment specifications to maximize the tires' performance. A performance alignment calls for the manufacturer's maximum negative camber, maximum positive caster, and preferred toe settings. While remaining within the vehicle manufacturer's recommendations, these alignment settings will maximize tire performance.
If you are a competition driver who frequently runs autocross, track or road race events, you'll typically want the maximum negative camber, maximum positive caster and most aggressive toe settings available from the car and permitted by the competition rules. If the rules permit, aftermarket camber plates and caster adjustments are good investments.
Many of today's alignment machines are equipped with printouts that compare the "before" and "after" alignment angles with the manufacturers' specifications. Requesting a post alignment printout can help you confirm the thoroughness of the alignment technician and preserve a record of your vehicle's intended settings in the case of an encounter with a suspension damaging road hazard.