The Significance of Tires
Why are tires so important? Well, look at your car, at tell me which part of it touches the tarmac. Right, just the tires. It's the tires that grip the road and this grip allows us to accelerate, slow down, turn sideways, dampen bumps, etc. Many people look after their brakes, but in action it's not the brakes that stop the car (just like it's not the steering that turns the car), it's the tires. Go improve your braking system: Fit a bigger master cylinder, sturdier pipes, bigger rotors, discs and more grippy pads. How much feet of stopping distance will you earn? Nothing! Why? Because your tires stop the car and they can only apply so much of a stopping force. Put better tires on the car, and watch the stopping distance shorten by dozens of precents!
So, driving without good tires is much like driving without good brakes. Would you drive without good brakes? No, right? Than don't drive with faulty or badly inflated tires! Tires are also quite cheap (in spite of a certain increase of costs due to the latest oil crysis) relative to their contribution to both performance and safety, so there's no room for compromise. To have a better understanding, take two sheets of paper, and push them under both sides of the tires, front and back, untill they meet resistance that stops you from pushing them further. Look at the gap that is left between the two sheets: This is the whole size of the tires' contact patch that actually touches the ground, and don't be surprised if it's no bigger than your shoe. The fate of your life sits on four size-9 shoes!
History of Tires
The invention of Pneumatic tires is attributed to one Robert William Thomson, but they were only applied in action by John Dunlp on bicycles in 1986. The Tire became removable not five years later, and tubless tires were introduced in 1903; five years later tires became treaded and in 1910 they turned into the black round tires we know today. Of course the tire industry has made a great progress in constant development of new technologies like regroovable truck tires, recycled tires (which are still not very efficient in terms of road grip) and other developments. A new tire is ten times as durable as those that were used thirty years past, although this rigidity made the tires less resistent to aging which is caused by the atmosphere.
The modern tire industy is also a succesfull economy. It is the main buyer of rubber, which is being purchased at Malaysia, Indonesia and even in parts of Africa (after world war two), where woods of Haveae trees were planted. These trees, once matured, are bleeded by a spiral cut that protrudes their outside layer, causing them to bleed their elastic sap into bowls. By heating it with a mixture of acids it turns into rubber of various qualities.
Synthetic rubber (made of crude oil, charcolan and Acetylene) has turned more and more popular, especially in light cars, first as an economic move of the great tire and rubber buyers of America and Europe, which sought to reduce their dependency on the far-eastern countries (along with growing heavae trees in the british colonies of west Africa). Today, both the synthetic and natural rubber are made in the far east. Modern tire industries include over 100 manufacturers and one billion tires sold per year, with over 20 billion dollars made annually. Be worn, though, that the amount of cheap and low-quality tires is about equall to that of good tires.
Tire factories are a highly occupied industy, with many professional secrets. The development of a certain tire brand, even a simple road tire, costs hundreds of thousands and millions of dollars. The industy is based on far-eastern rubber, natural and synthetic, which is being sewn into layers. The grey color of the rubber is turned black by use of soot which increases the tires' radiation and heat resistance in a low cost, and by applying many chemical agents, such as anti-oxidants and anti-ozonenats. These "Plies" are than extruded with the other plies of polyester and steel. The tire is than sulphated and packed with sylicon before being shifted to the market.
Old tires were made in a diagonal shape: One ply was set diagonally relative to the next. These "bias-ply" tires had many disadvatages: They worked as one unit in dampening road bumps and dispersing heat. They were very vulnrable to under-inflation, heat buildup at high speeds (at 60mph, an average road tires rotates almost 1000 times per minute), and to punctures: The tires had in internal tube with air, which, once punctured, would allow the air to leak quickely from the point of connection between the tire and rim.
Modern tires are Radial: They have a thin sidewall which is seperated from the thicker tread. The tires are also steel-belted (for structural rigidity) and "Tubeless" which means that the air is being fed directly into the tire. This technology reduces the speed in which air is bled during a puncture. Fitting a tube into such a tire is dangerous.
Tires are divided into groups:
1. Standard Road Tires: Including high-performance tires (marked as such).
2. All-Season Tires: Marked by M/S for "Mud and Snow." These tires give a good compromise between dry road grip and grip on thin, loose snow or mud in the winter.
3. All-Terrain Tires: General purpose off-road tires, including a sub-division for sand-dune tires
4. Mud-Terrain Tires: Aggressive off-road tires for driving in mud.
5. Winter Tires: For heavy-duty winter, deep water and puddles, and light snow.
6. Snow tires: Used for snowy and icy conditions.
7. Ice Tires: Studded tires for hard ice.
9. DOT-Racing Tires: Racing tires (soft rubber compound and shallow tread) which are street legal.
10. Racing Slicks: Racing-purpose tires used in professional leagues, with no tread or with a shallow tread (semi-slick).
11. All-steel tires: Used in heavy vehicles like trucks.
12. Bike tires: including road tires, performance tires, terrain tires, etc.
Every category includes various brands and qualities. For the average road driver, the main criteria is to purcahse a tire from a known brand. Unknown tire manufacturers (Like the "Ling-Long" tires that were used in an experiment in Britian) usually make tires that are highly ungrippy and in a serious risk of blowout. The quality of the tire is just as crucial (if not more) than the wear, age or inflation of the tire. New but low-quality tires can be more dangerous than old tires from a good quality.
Tires are rated in what is called a QUTG rating. These specify the tires' temperature, traction and treadwear ratings, as tested by the US Department Of Transporation (DOT) at the St. Angelo test track at Teksas.Temperature specifies the tire's ability to disperse heat and cope with high temperatures (road tires can reach 120 degrees celsius!). Most tires have an A rating. B-rated tires can be a satisfactory to people who don't make long drives on highways or with load. C-rated tires should be avoided. These tires run at 130km/h for 30 minutes before being demolished, while A-rated tires would sustain a speed above 200 km/h!
The traction rating is decieving. It does not imply the tire's grip levels. It indicates how well it stops on a damp road. It effectivelly illustrates the ability of the tire to drain water and grip a wet road. An AA-rated tire can be worst, on the dry, than some B-rated tires. Still, it's advised not to go below B. In countries with wet weather, it's preferable to go for A. The AA rating is new and does not nessecarily indicate a higher quality relative to an A-rated tire.
The Treadwear rating is very important, as it indicates how soft the tread's rubber is. Softer rubber grips the road better, but wears sooner. Don't go for below 300. If you don't drive very much (less than 30,000km per year, as a rule of thumb), go for tires with a relativelly low treadwear, for extra grip. The theory is that at a rating of 100 (The DOT "Standard", just above DOT-approved racing tires) the tire should sustain 30,000 miles of driving, but this refers to smooth highway driving in ambinet temperatures and not over long durations of time. In practice, the ability to dictate tire tread life based on this data is unrealistic.
An average road tire of about 11kg (with a maximal sway of 0.5%) will be composed like this: 3kg of seven to eight natural rubber compounds, and another 3kg of five synthetic rubber compound; 2.5kg of black carbon; one half of a kilo of steel, and another half kilo for the tire's heel, and a similar weight for Synthetic fabrics (Rion, Polyester, etc...). 1.5kg of around 40 types (!) of resins, adhesives, pigments, anti-ozonants, anti-oxidants, softeners, Cobalt and other chemicals.
Tire wear and Aging
Tires and roads polish each other. As a result, the tire gets worn. As they wear out, the tread depth is reduced and the tire's ability to drain water on a wet road is reduced. The legal limit in most countries is 1.6mm, usually ellapsed as a safer 2mm. Most tires also have six to nine tread wear indicator bars that become levels with the face of the tread when it has worn to this level. The indicators are lined up with the shoulder indicator, which is marked by a triangle or by numbers.
In practice a tire with 3mm or less (and 4mm in countries with a hard winter) is dangerous and should be replaced. In 60mph, with 2mm of water, each tire drains one gallon of water per second. Reduce the tread depth (14mm) to 3mm and the tire will fail and give you an overall grip level like driving on snow!
The tread does not increase grip on the dry. A slick tire is more grippy because it places more rubber against the pavement. However, when a treaded tire gets worn, it means that the soft layer of rubber is shaved and the stiffer layers of rubber (used for structural rigidity) are exposed. The tread also helps to disperse heat, so bald tires are bad. However, it's important to understand that modern tires don't wear out so quickely, and they will have to be replaced long before the tread gets worn, because of other kinds of wear.
The first kind of wear is a result of "Heat Cycles," where the tire heats up and cools down under changes of weather or while driving. The rubber expands and contracts, untill it looses some of it's elastic nature and becomes dry and sometimes even visually cracky. Also, the different layers of rubber, and the polyster and steel, all expand at a different rate, so the layers end up seperated.
Under driving, serious wear will occur at between 50 to 80,000km, depending on how hot the weather is, how agressive the driving style, how much driving is done in highways or with heavy loads, and how carefully are the tires inflated and periodically rotated. The common standard is 70,000km -- beyond this point, the tire loses about 50% of it's abilities! However, many people don't drive so much, and their tires have to be replaced due to aging.
The tires suffer from the changes of weather from night to day and over the year, they suffer from sun radiation (mainly UVB) and effects of Oxygen, moisture and salt in the air. The effect of aging is acute, even if no visible signs of it are seen on the tire. Having said that, this kind of wear can often be seen as cracks in the edges of the tread. Tires that remain unmoved will in fact deteriorate more notably that tires which are driven at some rate or another. A slight heat and hystersis created by using the tires, will create chemical reactions which can help increase aging-ressistance abilities.
Also, tires that remain mounted on a car which is standing still for a long period of tire, will form a "flat spot" because the weight of the car will sit permenantly on one point of rubber. In this case it's advised to inflate the tires to just below their full inflation rate. Even stacked tires can exhibit wear when they are placed one over the other. A standing tire often attracks insects too.
The tire should be replaced within a time spawn of three to four years. Older standards of six years relate only to the tire's strutcutral well-being. I.e. After six years the tire at risk of failing, but modern tires don't fail all that much. It far more critical to relate to the tire's ability to produce grip and to stop you when you need to stop in a hurry, at which case three years are more than enough. Even after two years there is a notable difference of 15 to 20%!
Three years is a good standard for countries with a hot weather in the summer. It's possible for the tire to last a few extra months if it's regularly parked in the shade. In cold contries, it's possible to keep tires for four years, and towards five years if carefully parked in the shade. Aging is the reason why I recommended for driver with a low annual milleage to choose soft tires, because they would have to replace those tires due to aging, and not the wear at the tread.
The age of the tire is the only data which is encrypted "into" the sidewall, within a round frame. It is seen as four figures, standing for a week in the year: "2011" stands for a tire made in the 20th week of 2011. "5208" -- a tire from the 52nd (last) week of 2008. This tire should have been replaced by now. Old tires from the 90s, have three figures with an additional mark of a greek "delta". "129^" -- the 12th week of '99.
Just as important as tire wear is tire pressure. Modern Radial tires are not as sensitive to under-inflation as bias-ply tires. Having said that, it's importance is still very large, and is just important as having new tires. Tires naturally lose air at an average rate of 2PSIG per month, but that's just an average, low quality tires will lose even more on some tires, and especially in hot summer conditions, and in the front wheels which are effected by the heat of the engine and it's load. So the recommendation to check air pressure once a month is not a very good one. Air pressure should be checked once every two weeks. This would also make the required "tweaks" for the pressure to be little, so it can be done faster than it takes to refuel the car. The pressure should be checked and inflated on cold tires, and with a reliable pump and gage.
A change of even two-three PSIG can facilitate a subtle but noticable difference, and make dramatic changes to tire wear on long highway drives. A seemingly small change of a reduced inflation by 20% can be classified as dangerous, because the car's stopping distance, wear and chance of failure increase by about the same amount. In wet surfaces there is less wear, but the grip levels and stopping distance are reduced far more drastically than on the dry. Under such under-inflation, tires can be seen deforming laterally even in slow cornering, where any attempt to approach the limit will make the tire distort extremlly, leaving not one or two milimeters of rubber between the rim and road. Think about it, a tire deforms between 20 to 200 million times!
When tires are driven or even exposed to hot sun, they heat up and the air inside them expands. A tire with 28PSIG might show a reading of 30PSIG, and you might think it's well-inflated, where in fact it is still under-inflated! It's important to estimate the heat in the tire. After ten minutes and/or two kilometers of driving in the sun at even 50km/h (30mph), your tires might be warm enough to require an extra 5% of pressure. If you drive for 15 minutes you will need an extra 10%. 20 minutes of high-speed driving can increase the air by 15%, and 20% after half a hour, and even more in a hot day. Usually, an 10% addition solves the dillemas. The heat in the tire can felt by putting the back of your hand on the sidewall of the tire, near the tread. Don't expect the front and rear tires to heat up symmetrically.
A tool that can help you is a personal tire gage. The cheapest and most simple is the pencil gage. It is a simple device made of a one-way pressure valve leading into a vacuum-chamber connected to a spring. You can easily find and purchase one in a very cheap price, with a pressure reading range of up to 40 to 60 PSIG. This kind of gage will be accurate down to about 0.5 of a PSIG, and be resistant to blows and shocks, unlike digital gages and dial gages. The most effective gage is a 0 to 60, oil-filled, rubber-encased dial gage.
This kind of gage enables to measure the air pressure at home and follow the changes when the tire heats up. Also, it enables to confirm the readings of the gas station pump, especially an analog pump with a handlebar, although it's recommended to ensure that the pump is doing a good job if it's a brand new digital one. They are often neglected, knocked about, and suffer from bad filtering, misfire in the electric engine, bad lubrication, and other problems that make it "miss" the right pressure.
Effects of under-inflation
The underinflated tire becomes softer and generated a wider profile. However, the softness makes it fold in the center of the tread, lifting parts of the tread off of the pavement. The center of the tread is more durable, grippy and is better in draining water. It is also directly responsible for accelerating and slowing down the car, so under-inflation dramatically increases stopping distances. The effects become much more acute in the wet, where the folds catch water inside them and make the tire skim and "hydroplane" very easily. The tread blocks twist and close up and the tire recieves a concave shape that disables it's ability to penetrate the layer of water.
The car's cornering is also compromised, because the softer tires is pushed by side forces so the sidewall cramms towards the outside of the corner, and twists the tire. This increases wear and, under severe under-inflation, can be heared as lound screetches (under-inflated tires are more loud). Slight under-inflation improves ride comfort, but the effect is bearly felt. Beyond that point the tire becomes too thin and cannot dampen bumps, and the narrow tread creates little turbulances of air, that generate more tire noise.
The tires generate more heat, and are in a risk of blow-outs, and slide earlier, albeit more progressively, but with less control over the slides, due to the sideways distortion of the sidewall. They increase wear on other car parts like brakes, drivetrain and steering linkages. Underinflation is useless, save in very specific situations of off-roading.
Effects of over-inflation
Over-inflation is better than under-inflation, in any day! Over-inflation does not cause the tire to blow up, unless you fill it well beyond it's maximum capacity (which is well above the "maximum pressure" limit stated on the tire's sidewall). An overinflated tire becomes stiffer and hence distorts less and generates less heat. It makes the tire recieve a concave shape, so it grips the road more with it's center and not the shoulders.
However, modern radial tires won't suffer from excessive wear due to this kind of inflation (although there is more wear than in normal driving), and will maintain higher grip because the tire will not bend so much under cornering. Within a range of about 10% of over-inflation, the tire will generate more grip than with proper inflation! The tires will react more instantly to driver's inputs. They will also break traction more suddenly and with less feel, but will provide better control even during slides, and supply a more accurate feedback than under-inflated tires. If you drive the car hard, on race-tracks or while roaming winding roads, over-inflation can reduce wear.
While it is recommended to drive with the appropriate pressure recommended by the manufacturer, it's advised to prefer over-inflation over under-inflation when in doubt, and even choosing to drive at all times with an addition 2-3 PSIG is very acceptable. If your tires have a problem with noise or wet grip, you might choose to experiment with such slight over-inflation to try and solve those problems. Overinflation is also used in performance driving, on winding mountain roads as on the race track, where it in fact reduces the wear. Do know, however, that over-inflation will increase tire wear on rugged surfaces (unlike on tarmac roads). Always remember than superflous pressure can be manually bled out at home.
Load on tires
Situations where you might be in doubt, other than hot tires, can also be when the car is loaded with luggage and/or passengers. Manufacturers recommend a different pressure for a fully loaded car and sometimes for a semi-loaded car. Even three adult passengers can be considered semi or even fully loaded. Some tires (snow, mud and all-terrain tires) will even be sensitive to the change of load formed by one passenger.
Modern tires are made to carry weights which exceed their own weight between 40 to almost 300 times their own weight (the highest value in achieved in aircrafts and very heavy trucks). The increase of weight increases the downforce that gives the tire more grip (based on the formula mv²/r = mgμ). So, if you have three adult passengers in the back, your back tires should have more grip, right? No, because the increased mass also increases the lateral force experienced by the tires, so the two effects should cancel each other out. However, the coefficient of friction is also reduced (slightly, though) under extra load, so over all there is a reduction of grip. The extra air pressure allows this effect to be canceled out so the grip level is overall increased
It can be beneficial to inflate tires with nitrogen. Nitrogen molecules escape through the innerliner at a much slower pace, and they don't allow moist to enter the tire and improve heat desperse. However, nitrogen inflation is not crucial to your safety, and it's possible to yield good results with normal air, by periodically replacing the air inside the tire with fresh air, to get rid of moist, dust and chemicals/rubber dust inside the tire. This is best done when the car is lifted, but it is possible to bleed out about 70% of the pressure safely when the car is parked and leveld, and immediately reinflate. As a rule of thumb, by applying slight pressure against the valve, you will bleed out one PSIG for every ten seconds. This changes on each individual nuzzle, on the amount of pressure applied and the form of the object used to apply pressure.
Tire pressure can also be monitored via a RPSM system fitted unto the rims (highly recommended! ), and through a less efficient system where changes in the air pressure are detected by the ABS sensors. It's still important to periodically use a reliable gage for a manual check. To ensure proper inflation with the gas station digital pump, you might choose to inflate each tire twice. The system will make adjustments and reach a pressure closer to what is necessary.
It's important to keep the nuzzle caps on. If they get lost, the nuzzle will pick up dust and dirt, which will apply pressure against the valve to lead to a slow but constant lost of air pressure. It's also important to realise that these rules apply for all FIVE tires, reserve tire included. It losses air just as much as all other tires, and it is important for it to be properly inflated. It's actually better to inflate it with a few additional PSIG, due to the lost of air over time. Some cars have a thin fifth wheel (due to reason based on cost, size and weight) that is usually inflated in a very high pressure (often the maximal possible pressure). Also remember than reserve tires placed on the rear bumper or under the hood, wear out just as much as the tires on the wheels. Have it replaced periodically.
Another note: Tire pressure cannot be judged visualy in modern radial tires. They have a reinforced sidewall and they lose air pressure at a slow, unnoticed rate. The result is that even when the tire is under-inflated by 50% (which increases milleage by 10%, tire wear by 70%, pollution by 45%, dry grip by 30% and wet grip by 70%!), will be bearely noticable when not moving. Kicking, pushing and looking at the tire is not going to show any difference and the naked eye of a skilled individual will at best notice that the shoulders are slightly crammed unto the road. While moving, however, and especially while cornering at a conservative speed, the tire's sidewall wil distort extremlly.
Tire placement and rotation
While it is possible to purchase two pairs of tires that share similar treats, it's highly advised to purchase a foursome of identical tires. These tires should of course fit the rims of the car, and the index of load and speed relevant for the car. Once every 10,000 to 20,000 kilometers, swap the front pair with the rear pair, to cancel out the wear caused by the alignment of the front wheels and the application of forces through the front in a front-wheel driven car. The rotation is particularly relevant where the car is front-wheel driven, powerfull and where the driving style is not very smooth or gentle. Driving in towns and winding roads also accelerates wear and requires a replacement at 10,000 instead of 20,000.
When tires get worn, the popular advice is to place the good tires on the rear. The front wheels of the car tilt when you turn the wheel and they rotate you into corners. The rear wheels have to keep the car in line with the front wheels. Worn rear wheels, resulting in a slide (or blow-out) will make the car slide out and spin around (oversteer). If the front wheels are worn and slide (or blow-out) they would slide forward and out of the corner, known as understeer. This kind of slide is easily felt through the steering, and is more naturally recovered from by slowing down or even turning the wheel more tightly into the corner.
However, it's important to understand the front wheels carry most of the weight, sprung and unsprung, and deliver forces of braking, cornering and acceleration onto the road. 70 to 90% of the braking force comes from the front (based on the forward weight transfer), so having bad front tires is like having bad brakes. So, the only sane advice is that the good wheels go on all four wheels! Tires are relativelly cheap, and if one or two have been worn, now is the time to replace all four of them. You cannot compromise your braking or your stability.
Another kind of wear is physical wear such as buldges, cracks, scratches and alike. These are caused by various conditions:
- A blow-out due to overheating. Will be caused by driving with too much load and/or too little air pressure, while moving at a high speed (above 80km/h) for long periods of time (at least 30 minutes) in hot days. The sudden heat and movement of the layers will make them become apart and the tire will blow out. The blowout will cut the tread from the sidewalls due the massive heat buildup in the thick shoulders.
- Tread detachment can occur due to under-inflation, combined with stress (sudden steering) and other problems like excessive load or misaligned wheels.
- Declamation of parts of tread caused by the tread being punctured (without reaching the tire's interal layers) in an old tire. The puncture developes into a large crack when the tire is driven.
- Cuts and Gouging caused due to contact with the chassis (wheel arches) due to misapplied spacers, wide rims, excessive steering angles or a twist of the tin.
- "Flat spots" Due to hard braking (without ABS), sliding or when a car is parked and not driven over a long time.
- Stains and swells due to chemicals around the tires, mainly oils.
- A pinch that occurs under low inflation when going over a bump or curb, resulting is swelling.
- A cut caused by a sharp object being projected by the car unto the sidewall.
- A scuff caused by contact with a curb
- Circumferential wear around the middle of the sidewall, indicating low air pressure and a strong side force in a high-profile tire
- Air pockets that occur due to a manufacturing defect or sudden blow from a curb, making air move from the innerliner and in between the other layers of the rubber.
- Sidewall rupture while driving on a tire that has been deflated or hit by something.
- Black powder inside the tire, as a result of constant driving on very low air pressure
- Diagonal wear: A series of flat spots running diagonally across the tire, indicating a misaligned rear suspension in both camber and toe angles. This wear creates a whooming road noise.
- Distortion of the tread: Where a soft, underinflated terrain tire has it's tread block distorted in what is called "Heel and Toe" or "Cupping" wear, which changes the shape of the tread blocks.
- Distortion of the tire carcas: Caused by infiltration of moist through cuts in the tire, making the steel nets rust and distort, changing the tire from it's round shape. Nowadays very rare.
- Peeling: Under extreme under-inflation and a sudden load of cornering and/or braking, the tire can peel off of the rim, causing it to dig into the road and potentially roll the car.
- Tread Chipping: Air bubbles and chipping of the tread when over-inflated tires are driven over rugged terrain.
- Tread Splice: A manufacturing fault where a lateral crack appears over the tread and develops into a rupture
- Split Cords: Where the cords inside the tire create a lateral bludge of the tread and both shoulders.
- Bad repairs where the tire's heel is damaged by the fitting machine, or where a small puncture is being repaired with molten rubber from the inside, or with a long cord. These repairs were once custom in bias-ply tires, but turned dangerous with radial tires, especially under loads. Punctures in the shoulders are irrecoverable.
There are also kinds of wear that are not significant: Darker lines that appear across the tread after a certain amount of wear. This is a result of the various kinds of rubber compounds fitted onto a single tires: The manufacturer might choose to fit softer rubber around the shoulders, for good cornering.
Others types of "alledged" wear are colored spots along the tire that indicate where shallow or thick spots of the rubber are placed. Sorts of bludging that occur as a result of where the rubber layers overlap, or used as tread depth indicators. All of these are natural products of the tire's construction.
Excessive wear in the center of the tread or shoulders is not likely to occur due to underinflation in modern steel belted radials, but more likely due to misalignment or bad springs. The effects can be felt withint three months or 4,000km.
Another kind of wear is caused by little stones that get stuck inside the tread. This stones don't usually cause damage, and they are thrown out of the tires while driving, but it is best to clear out whatever stones you might find, as an occasional bump can make them protrude deeper and harm the tread.
Reading the Tire
The outer circumference of the tire, below the shoulders, include an triangular indicator of wear. It's point marks the edge of the tread's shoulder, which should also mean the edge of the area that grips the road. Under very low inflation and/or faulty camber angle, wear might be exhibited beyond this mark, which is not wanted. Modern tires are somewhat adjusted to this possibility, so the external circumference of the sidewall includes a rugged rubber surface, like a rumble strip, which also helps to disperse heat.
Beyond this circumference are imprinted the first stats over the rubber.These include the name of the manufacturer and the name of the tire, as well as it's classification as "Radial", "Bias-Ply", "All steel", "stel-belted" and "Tubeless"/"Tubetype." It also specifies the tire sizes and destination (like snow tires, mud tires, etc...) and the required speed and load index and maybe a letter than means that the tire is designated for a certain car (Like N for Porsche).
At the small circumference, near the heel of the tire, are printed the data of maximum pressure, maximum load and American (DOT), Japanese (JIC) and/or European (Eu) stamps are found with the specification of country of origin and the tire's age code. On the heels themselves, covered by the lip of the rim, is the barcode of the tire.
Let's look at an average tire of 195/60R15. What does it mean? "195" contributes the tire's tread width, from shoulder to shoulder, in milimeters, which changes in up to 4% (in this case 78mm!) depending on inflation. "60" Contributes a certain aspect ratio which means that the height of the tire's shoulder is equal to 60% of it's width. This state also changes slightly (but significantly) with changes of tire pressure. R contributes the tire's identity as a "Radial" and 15 is the width of the rim in inches.
This method is the "German" Method and it is widely used in Europe since the 70's in light vehicles and progressively in light trucks. The American method can be used in this example: "31x10.5R15." This tire has an external diameter of 31 inches, section width of 10.5 and an internal width (rim width) of 15 inches. Some tires have the same method without the section width data.
The aspect ratio is important because a bigger sidewall means more air and rubber used as a spring, but also more lateral motion of the shoulder under cornering. Low-profile tires are thus less comfortable and sharper, but also more grippy and compliant. Wider tires contribute by increasing grip levels, on the expense of a heavier tire, which increases gas milleage.
The speed index describes a speed where the tire, being fully loaded (based on the load index) and fully inflated (based on the maximum inflation state) will be critically damaged after ten minutes of driving. In practice, the actual speed a tire can sustain can be higher, in cold weather, normal loading and proper or slightly excessive inflation, or lower on hot roads (tarmac can heat to over 70 degrees celsius) with underinflated, misaligned, old or overloaded tires.
L = 120km/hM = 130N = 140P = 150Q = 160R = 170S = 180T = 190U = 200H = 210
The Load index is also determind at the maximum speed and with full inflation. It can therefore be theoretically possible to load the car beyond the stated amount, albeit being highly unrecommended. Bias-Ply tires either have a Ply Rating (specifying the amount of plies) and/or Load Range from A (two plies) to E (10 plies).
0 -- 450 Kilograms 81 -- 462 82 -- 475 83 -- 487 84 -- 500 85 -- 515 86 -- 530 87 -- 545 88 -- 560 89 -- 580 90 -- 600 91 -- 615 92 -- 630 93 -- 650 94 -- 670 95 -- 690 96 -- 710 97 -- 730 98 -- 750 99 -- 775 100 -- 800 101 -- 825 102 -- 850 103 -- 875 104 -- 900 105 -- 925 106 -- 950 107 -- 975 108 -- 1000 109 -- 1030 110 -- 1060 111 -- 1090 112 -- 1120 113 -- 1150 114 -- 1180 115 -- 1215
The "E" Approval is the European standard of all tires after 1997. It includes the "E" stamp (which can also be printed as a small "e" for tires in line only with directive 99/33/EEC), and a circulated (or rectangulated) number which is a code for the tire's place of origin: A country code, followed by the numerical approval for the given brand. For instance, a tire with the first two figures of "E10" stands for a tire from Yugoslavia; where a tire with the last two last figures, "0U" stands for Ling-Long Tires from Shandong, China (Tires that were found to be highly inefficient in a certain test in Britian).
The American DOT approval has taken it a step further to specify in twelve letters to describe the exact origin of the tire, plus the date of production, which is the date I described above.
Other tire markings include: An D or DA stamp (for "Defected" Tires), a Yellow dot which is a bit more dense and heavy than the rest of the tire. This is aligned with the heavy point of the rim, where the air nuzzle is placed. A red dot for a "protruding" spot on the tire, and lines and figures that are used in the manufacturing process to categorise the different layers, before they are extruded or mounted toghether. This process also creates the little "studs" of rubber than seem to cover the face of a new tire, formed by the escape of air bubbles from inside the layers when they are vulcanized.
The "dense" and "high" spots of the tire are discovered through the carefull inspection of the tire after it's assembled in the factory. After being extruded, wrapped and vulcanized, the tire is checked with X-ray, weight and Uniformity. New tires are also wrapped in an external sylicon layer than wears off after 5000km or so.
The tire also includes a specification for it's plies and, in radials, for the plies of the sidewall and the plies of the tread. The plies include: External rubber layer for grip around the tread. Internal rubber layers for structural rigidity of the tread and sidewall. Belts of dual steel nets that reinforce the tire's carcass, and than synthetic fabrics of one of these kinds:
1. Nylon: The first fabric applied into tires, nowadays replaced due to it's "morning sickness" which made it deform until heated by driving.
2. Polyester: The most popular replacement of nylon. Less rigid than nylon fabric, but does not distort. The Polyester can be produced in different densities for different qualities.
3. Rion: Good for ride comfort and grip (better hysteresis), but is vulnerable for moist, should it protrude through even shallow cuts of the tire. Replacing it with polyester and improving the steel cords reduced the amount of carcass distortions in modern tires.
4. Kevlar: A replacment for the steel nets because it provides better rigidity for the same weight (about four times as much!), although being more expensive and less resistant to punctures.
5. Inner-liner: The internal layer supposed to hold the air inside the tire. It is made of special synthetic rubber (usually Bromo Isobutylene) and an internal fabric layer.
The tread depends on the tire's type. Normal road tires have a soft tread of about 8.5mm, while M&S tires will have a greater "void ratio" (i.e. more lugs). The tread ratio increases for mud tires, snow tires and special ice tires, and the rubber compound become more stiff. On the road, the greater "void ratio" means that a greater part of the rubber is not gripping the ground. However, it is required for dispersing heat and channeling water on wet roads.
However, on mud, snow and ice, the tread generates grip by biting into the road surface. Mud tires usually also "ridges" inside the tread that help push the chunks of mud (or snow) that get trapped in it, while a road tire will get "plugged." Likewise, the edges of the tread are placed in a sharper angle relative to the road, which helps disperse the chuncks. The tread is even present on the shoulders, to provide grip by biting into the sides of wheel ruts in the mud or snow. These tires don't have sips in the shoulders or on the tread. Those little "sips" are intended to give the tread some flexibility and to open up and sip water when it contacts the road, and than squirt them out as the tire continues to rotate. The sips are removed to improve the rigidity of the lugs in mud or snow tires.
These tread patterns are bad on dry tarmac roads as they will suffer from acute wear, produce a lot of vacuum and turbulances to create noise, and provide reduced grip. The more agressive or effective a mud tire is on mud, or a snow tire on the snow, the worst it is on the road. It's bigger "chunks" of tread will produce a louder noise and it will be highly effected load. On such tires, even one passenger will increase stopping distances in sudden braking. Ice tires with nails will also damage the road surface.
One interesting tire is a tire for dune-driving. These tires are round-shaped in all directions, including the section of the tread. It has no tread depth, and only little "flippers", all designed for the tire to drive over the sand and not "bite" into it and create steps that will make it sink.
The tread on most tires is also devised in such a shape so that the tread is not the same on any "cube." Each one is slightly different in shape and/or size. Even the most slim differences change the direction of air turbulence and make the different sounds that cancel each other out, to produce a noise below 75db (85 in DOT standards).
Got a puncture on the highway? Don't be so quick to pull over. Statistics show that vehicles standing alongside the highway, attract attention and get hit at speed, causing deadly collision. If there is no good stopping place, it might even be desirable to keep driving on the punctured tire at a crawling pace (About 10km/h) over the shoulder of the road, and seek a safe stopping place, like a side road, open dirt surface, gas station, parking lot, etc...
If there is no safe place within a reasonable range, you have to stop, but you don't need to replace the tire. Go over the guardrail and have a tow-truck drag the car. The risk in kneeling for a tire, regardless of whether the puncture is on the left or right, is too great. This is part of the reason why "Run-Flat" Tires (with a reinforced sidewall) are applied into new cars, allowing to run to the near tire shop (up to 80km and up to 80km/h) and replace the faulty wheel. Other solutions for flats include a foam that inflates and becomes stiff upon contact with air. This can be injected into a tire (used in cans sufficient for filling one empty tire in a normal car, or two semi empty tires), or be built into it (like in rally cars) and allow to drive for a few additional miles to a tire shop. The foam is not good for the tire and it's also toxic.
Tires contribute to safety in being more comfortable and silent (which means more concentration on driving), resistant for failures (which causes a lot of crashes), and provide good stopping distances in emergency braking, and good grip all around, especially on slippery surfaces or on tightening corners. Tires will carry a profound effect even at slow speeds and on dry surfaces, for slight defects (old age, excessive mileage, reduced inflation). They are the most important safety instrumentation of the car.
As a rule of thumb, all four tires should visualy check for wear before every drive (it only takes a few seconds), inlated with air once every two weeks (with considuration of tire heat and load in the car), rotated between 10 to 20,000 kilometers, and replaced once every three years and/or 70,000 kilometers -- which ever of the two comes earlier and preferably even sooner. Of course if the tire is damaged or worn to below 3mm in the tread is should be replaced regardless of age or milleage.