Wheels and tires are the components between the driveline and the road, as well as the suspension system and the road. Tires sit around the rim or wheel and hold pressurised air. They provide traction to the road, support the weight of the vehicle and act as a cushion to reduce road shock. Tires must also generate as little noise as possible while rolling down the road. Excessive tire noise can cause the driver to think something is wrong with the car, and even a mis-diagnosis by a mechanic (usually a wheel bearing). Tire noise can also get annoying on long trips. A tires rolling resistance can also affect fuel economy. The different parts of the tire are held together by a rubber compound which can vary from tire to tire.
A tires tread pattern affects the tires ability to provide traction in different conditions, tire noise and rolling resistance/fuel economy. The tread is located on the area on the outside of the tire. It is the part of the tire that contacts the road surface. Street tires have ribs and grooves that make up the tread pattern. Many different tread designs are in use on the road today. They must provide as much traction as possible without causing excessive tire noise or rolling resistance. Different tire tread patterns are designed for different applications. “Sport” or performance tires are designed to provide lots of tire to road contact and direct any water on the road away from the tire. Snow tires are designed to grab and dig into snow and ice to get as much traction as possible in winter conditions. They also tend to be softer than other tires so they can conform to irregular surfaces and increase tire to road contact. Snow tires tend to be noisier than other street tires and will wear down quickly if constantly driven on dry pavement. All-season tires do a reasonable job in all conditions but are not particularly exceptional in any condition. Street tires will have wear indicators in the tread pattern, indicating the minimum safe tread. When the tread is worn down to the wear indicator the tire is worn out. Driving with tires worn below the wear indicator is dangerous because the tire will not be able to redirect water properly. If there is any water or debris on the road, the vehicle will not be able to provide proper traction. This can cause a vehicle to spin out around a corner and it will increase stopping distance (potentially causing a (most likely) at fault) collision). It can also cause the wheels to slip while accelerating, which can cause an already worn out tire to continue to wear much faster. Race car tires may not have a tread pattern. They are called “slicks” and they look like a bald street tire. They do this so they have the most tire to road contact possible. This works in dry, clean pavement only. If there is any water on the track, these tires are almost useless and are dangerous to drive on. Slicks usually have small holes which act as a wear indicator. When the tire wears down to the bottom of the holes, the tire is worn out.
The sidewalls of a tire is the area along the side of the tire. It must be strong because it may have to hold up a portion of the vehicles weight if the tire is underinflated. It also must resist lateral movement as a vehicle takes a hard corner. If the sidewall flexes too much through a turn, vehicle handling and stability can be greatly reduced. Low profile tires are tires with less of a sidewall (lower aspect ratio). These tires will resist lateral shift through turns much better than tires with a greater aspect ratio. Many drivers have low profile tires installed simply for looks, but they do have a practical application. However, these tires are usually installed on after-market wheels with different dimensions than factory, which can cause changes in suspension geometry. This can have a positive or negative affect on handling depending on the application. The sidewall is also where information about the tire can be found. Tire sidewall damage of any kind warrants tire replacement. Sidewalls cannot/should not be repaired. Any puncture, bubble, crack or gouge in a tire sidewall is unsafe to drive on and should be replaced as soon as possible. Consider installing the spare tire to get the vehicle to your mechanic or tire shop. Scroll down for a full explanation on how to read a tire sidewall.
The tire’s bead is the area that contacts the wheel/rim. When the tire is installed on the wheel, this area can’t be seen. It holds the tire in place using the air pressure in the tire. An underinflated tire runs the risk of the tire being knocked off the bead in a corner. This area is a common place for air pressure to leak out (even slowly), especially on aluminium wheels. Many tire shops will use a bead sealer to prevent this.
Tire belts add strength to the outside area of the tire, while allowing it to remain fairly flexible. They sit between the plies and the tread area. Belts are typically made out of steel but tire manufacturers have started to use other materials as of late. Belts allow a tire to compress to allow some absorption of road shock, but they will not allow the tire to dent. They also help the tire keep its shape if the vehicle is parked for a long period of time without moving.
Tire plies make up the main structure of the entire tire. They can be made out of nylon or rayon, among other materials and run under the belts and also through the sidewalls. Radial plies run from side to side on the tire while bias ply tires run diagonally in a cross pattern. Some tires have cap plies which are placed above the belts which allow the tire to to remain stable at high speeds. This will greatly increase a tires speed rating.
Tire Specifications/Reading a Tire Sidewall
The tire sidewall is where you can find information about the tire. Some tires may not display all of the information our example shows, but you should be able to look up all this information on the tire manufacturers website. A tire sidewall might read P 225/45ZR17 91. The P indicates that this tire is designed for passenger vehicle. Other possible values could be LT for light truck or T for trailer. The 225 is the tires width in mm. The 45 is the aspect ratio. The aspect ratio is the tires profile compared to the tires width expressed as a percent. A smaller number will indicate a lower profile tire while a higher number will indicate a higher profile tire. Since this is the sidewall profile compared to the tires width, a 225mm wide tire with a 45 aspect ratio will not have the same profile as a 295mm wide tire with the same aspect ratio. The Z is the tires speed rating, this value can also sometimes be found after the load rating (as pictured). This is the maximum speed that the tire was designed to spin at. Take this into consideration when selecting a high performance tire. The R indicates that it is a radial ply tire. The 17 is the diameter of rim/wheel that the tire is intended to be mounted on. The 91 is the tires load rating. Heavier vehicles require a higher load rating.
Proper tire inflation is important for maintaining traction, fuel economy and preventing tire wear. Proper tire inflation pressure can be found in the owners manual or on the tire placard which can be located on the door jam or on the inside of the fuel lid. Proper tire pressure is not the number located on the tire. If you see something like “44PSI” on the tire, this indicates maximum safe tire pressure for that tire. If a tire is underinflated, it will increase its rolling resistance which will reduce fuel efficiency. It will also overload the sidewalls and cause accelerated tire wear along the outer edges of the tread area. If a tire is overinflated it can cause the middle of the tread area to be the only part of the tire that contacts the road surface. This reduces a tires ability to provide traction and will lead to accelerated wear along the middle of the tread area.
Tire Pressure Monitoring Sensors (TPMS)
Tire pressure monitoring sensors monitor individual tire pressure and alert the driver if a tire is outside of a programmed pressure range. Each wheel has its own monitor located between the tire and the rim. The tire will usually have to be removed, or at least be taken of the bead to replace a TPMS. These sensors are wireless and need to be programmed to the TPMS module on the vehicle. Many systems will need to have all 4 wheels reprogrammed, even if only one sensor is replaced. Some systems even monitor the spare tire.
Stretched Tires and Oversized Tires
A stretched tire is a tire mounted on a rim that is wider than the tire was intended for. This is done for “looks” only. The only real positive affect is the possibility of running a wider wheel with out rubbing. I am not going to get in the middle of stretched tires debate. All I will say is that tires were not designed to be stretched which makes stretched tires potentially dangerous since the bead must seal and hold on an angle it was not designed for. Also, if the driver gets “a little too close to a curb,” it will be your lovely wheels taking the punishment rather than the soft tire. Many tire shops will not stretch tires because of the liability and also because of the potentially dangerous air pressure that they may have to take the tires up to, to get the bead to seat. Maybe in the near future, tire manufacturers will design a tire that is meant to look like that. (hint, hint, tire manufacturers) At the opposite end of the spectrum is oversized tires. An oversized tire is a tire that is mounted on a rim that is not as wide as the tire was designed for. This setup allows more tire to road contact compared to the standard size of tire, which will increase traction. This type of wheel and tire combination is safer than stretched tires, (assuming you don’t go too far) but the bead is still not sealing on the angle it was intended to. If the oversized tires fit without rubbing, you probably could have gone with a wider wheel instead of using oversized tires on the smaller rim. The advantages of using the oversized tires would be that extra tire most likely weighs less than extra rim/wheel, reducing unsprung weight. The other advantage is cost. Oversized tires cost less than new wheels and tires. Disadvantages could be altered suspension geometry (although that could be an advantage), looks (matter of opinion) and safety (although not as bad as stretched tires).
Changing the Aspect Ratio/Low Profile Tires
The most common way drivers change tire size is by aspect ratio. Many car owners want low profile or low aspect ratio tires. This was done primarily for looks but it can have an advantage for handling. Skinnier sidewalls tend to flex less while cornering, this improves the vehicles stability through a turn, although most drivers will never take a hard enough corner to notice a difference. Low-pros also decrease the tires ability to cushion road shock so they make the vehicle ride harsh. They can also cause the vehicles speedometer to be off because the outside diameter of the tire has changed. They provide less protection to the rim/wheel because there is less rubber between the road and the rim and tend to cost more than a standard size tire. Still, many car owners get “low-pros” and most tire shops will not have a problem installing them.
Directional Tires and Tire Rotation
Most performance tires are directional. This means that they must only rotate one-way and therefore stay on the same side of the vehicle. Keep this in mind if you choose to rotate or install you own wheels. Directional tires are usually marked on the sidewall with an arrow to indicate direction of rotation, writing to indicate left or right, or other markings. You can usually identify directional tires by the tread pattern. Directional tires tread pattern will contact the road differently if installed on the wrong side (symmetrical vs non-symmetrical). If there is an arrow shape to the tread pattern, the arrow should point in the direction of wheel rotation. Refer to your vehicles owners manual for tire rotation intervals and rotation pattern. If rotational tires have been installed, rotate tires from front to back only.
Schrader Valve/Tire Valve Stem
Schrader valves are used on almost all tires in automotive today. They allow the tire to be filled or topped up with air pressure or air can be let out through this valve. The schrader valve is threaded into the valve stem and can be changed without removing the tire from the rim (but this will allow air to escape from the tire). They have a needle which controls the opening and closing of the valve. When a tire has air pressure, pushing on this needle will allow air to escape from the tire, but it also must be pressed to allow air into the tire. Tire filling chucks are designed to press the needle and fill the tire at the same time. The valve stem houses the schrader valve and provides a way to fill or let air out of the tire. They come in different sizes to accommodate different wheels. Wheels will have a hole in them for the valve stem. A valve stem is simply pulled through the rim until it pops into position. If the valve stem needs to be replaced, the tire will need to be removed from the rim, or at least taken off its bead. Valve stems and schrader valves should be changed with tires.
Nitrogen does not leak out or loose pressure as quickly as regular air and it does not change pressure as much with changes in temperature. So, the vehicle owner will not have to adjust tire pressures as often. Another advantage of (pure) nitrogen is that no moisture will end up in the tire and cause rust or corrosion over the long term. Many race car teams will use nitrogen because their tires go through a lot of abuse which generates lots of heat. Since nitrogen does not change tire pressure (as much) with heat, this makes it an ideal choice in high performance applications. But, I would like to point out that the air around us is already about 78% nitrogen. Some nitrogen fill stations charge $30.00 for (at best) 22% more nitrogen than regular air. So do I recommend nitrogen filling? Sure, but I would not pay for it. Nitrogen filled tires are supposed to have green valve stem caps or caps that say N2 on them to indicate nitrogen filling. It will not cause any kind of damage to add regular air to a nitrogen filled tire but it will defeat the purpose of the nitrogen.
The wheel transfers rotational torque from the spindle to the tires. Wheels are unsprung weight and need to be made as light as possible for a vehicle to achieve the best handling performance possible. Most modern wheels are made out of aluminium or steel. Aluminium wheels are considered the better option because of their light weight and their physical appearance. Aluminium wheels will also dissipate heat better than steel wheels. This is partially because of aluminium’s heat transfer properties and partially because aluminium wheels tend to be more open than steel wheels. This allows more air to cool the wheels as well as the brakes.
The purpose of the lug nuts or wheel bolts is to hold the wheel on the hub. They are not designed to hold the weight of the vehicle. The hub will have a centre lip which centres the wheel on the hub and supports the weight of the car. This is important to realise when changing wheels. Most after-market wheels will come have a large centre hub which will need “hub-centric” adapters so they will fit on many different models. The problem is that these adapters can be plastic and only help centre the wheel on the hub for installation. Metal adapters are the only “proper” way to install after-market wheels. Also keep in mind that wheel bolts or nuts are tapered to bite into the wheel and increase contact surface area when tightened. The two main types of tapers are straight taper and ball taper or bubble taper. The type of taper that is needed is determined by the wheel so when changing wheels make sure that you are using the correct taper. If The wrong type of taper is used, the wheel may tighten up properly but it could come loose while driving down the road. Most after-market wheels use straight taper.
Lug Nuts/Studs vs Wheel Bolts
Some (mostly European) car manufacturers will use wheel bolts instead of lug nuts and studs. Many people think that this is only to make it more difficult to install the wheels. This may in fact be the case, but I believe there is a little more to it than that. When a wheel is placed up onto the studs, a small amount of damage is done to the studs thread. Wheels are removed and installed many times over the life of the vehicle. This small amount of damage can lead to a stud needing to be replaced over many wheel installations. Wheel bolts thread do not have to deal with this. Even if a wheel bolt does get damaged in some way, they are much easier to replace than a stud. Also, when the brake rotors are removed, it is much easier to clean the hub so the rotor and wheel will sit flush after installation. Yet, many European car owners still convert to wheel studs simply for the added convenience of wheel installation.
Wheel offset is the distance between the centreline of the wheel and the surface that mates with the hub. If this surface is right on the centreline of the wheel, offset is zero. If it is closer to the outside of the wheel, offset is positive and if it is closer to the inside of the wheel, offset is negative. Most factory wheels have a positive offset. When changing wheels, offset is important to take note of. Changing to wheels of a different offset can alter suspension geometry. Also make sure that your wheel studs or wheel bolts are an acceptable length for the new wheels. If wheel studs are too short, there may not be enough thread to properly hold the wheel on. Too long and they may stick out too far from the wheel. In the case of wheel bolts, too short and there may not be enough thread holding the wheel on. This can be hard to see without a hoist. If the wheel bolts are too long, they may interfere with other suspension or driveline components. If this happens with drum brakes, the long wheel bolts may damage brake hardware potentially making the brakes inoperative. My point is simply to make sure your wheel hardware works with your wheel choice.
Wheel spacers sit between the hub and the wheel’s hub mating surface and are used to alter a wheels offset. They can only decrease positive offsets or increase negative offsets. These can be used to bring an after-market wheels offset back to the factory intended offset or change the wheels offset to the vehicle owners liking. The same warnings apply to wheel spacers that apply to switching to wheels with a different offset. Make sure that your wheel hardware accommodates the change in offset and be aware of possible suspension geometry changes. Also, make sure the vehicles weight is still resting on the hub and not the wheel hardware. Thin spacers should not cause much of a problem, but thicker spacers will need different wheel hardware and should be “hub-centric” (the wheel rests on the spacer and the spacer rests on the hub). Spacers also are more susceptible to wobble compared to wheels designed with the intended offset. The hub and the spacer must be perfectly clean when installing the wheel to avoid this.
If a rim and tire assembly is not balanced, it can cause the wheel to wobble or hop as the vehicle drives down the road. This can cause the steering wheel to vibrate and can wear suspension components. An unbalanced wheel is usually more noticeable as speed in increased. Any time a tire is off of its rims bead, or new tires are installed on rims, the wheel assembly should be balanced on a tire balancer. A tire balancer spins the wheel up to a set speed and and figures out where weight needs to be added to make the wheel balanced again. Wheel weights can clip onto the edge of the rim or be stuck onto the inside of the rim with adhesive. The stick on type is used on most aluminium rims but the clip on type will work in some applications. The clip on type wheel weights are used on all steel rims.