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 is 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 are caused due to contact with the chassis (wheel arches) due to misapplied spacers, wide rims, excessive steering angles or a twist of the tin.Likewise, infamous ”Flat spots” will happen due to hard braking (without ABS) on dry surface, even for relatively short intrevals, sliding or when a car is parked and not driven over a long time. Stains and swells will be normaly caused due to chemicals around the tires, mainly oils.
Other kinds of wear include:
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: The tire lugs recieve wear like “heel and toe” wear (when the front edges of the lugs are worn) which occurs a strong application of engine torque through the drive wheels when they are under-inflated. Other tires will exhibit a “feathering” wear on the lateral edges of the lugs, due to alignment problems, resistance of wind and other reasons.
Cupping: Caused due to wobble or worn dampers, the tires develop a “wave” like wear formation around the tread.
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 dangerous or important: 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, and they harm it’s ability to dispers heat. Truck tires have an angeled tread to prevent so-called “stone retention.”
Storing tires is a problematic subject. They are best stored in areas off with a constant temperature of less than 25 degrees celsius and above freezing temperatures, with no change of temperature of over 0.15 degrees celsius, in the shade (especially not around neon lighting), with dry air and low ozone-oxygen concentration (under 0.05 particles per million). The compartment should not be radiant-abosrbing (like black or bright surfaces) and clear of wetness, grease or dust.
If the tires are fitted on a rim, they should be inflated at one BAR (10KPa), stacked and the order of the stack should be reversed every four weeks and re-organised alltoghether once every six weeks. Free tires should be left standing, and rolled over once every four weeks over a machine. The tires should be covered with a sylicon spray. The use of such sprays and other substances during the tire’s actual use is less recommended, as most of them don’t work and many might do harm.
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: “31×10.5R15.” 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/h
M = 130
N = 140
P = 150
Q = 160
R = 170
S = 180
T = 190
U = 200