checked so that they are just at the right size and weight. Simultanously, the textiles that are designated to be used in that tire are rubberized by putting them into a caulderon of hot rubber. The fybers are organised by code so that they can be used for the right tire.
The internal layers are now fitted unto a drum (in a radial tire, two drums) to form the rough shape of a tire. This “raw” tire is not put into a vulcanization chamber where it is being heated with sulphur at a temperature of 85 to 150 degrees celsius, for a duration that depends mainly on the depth of the rubber layer, where each milimeter needs about seven minutes of vulcanization. This also frees trapped air between the layers, and the airosion of that trapped air forms the little “studs” on the face of the tire. Big truck tires can be vulcanized for five hours!
The tires contacts the road via a small contact patch which is divided into the small rubber fybers that make up the tire. As the tire is rotated, a given rubber fyber is loaded with the car’s weight as it comes against the road surface. The downforce generated by the car’s mass multiplied the force of gravity (the perpendicular euclidian vector of constant downward acceleration of 9.8m/s) creates an equall reaction known as the “normal force” (N) which, multiplied by the coefficient of friction of both the fyber and the tarmac, creates a gripping force (adhesion) which is professionaly known as static friction. Therefore, Fs=Nμ=μmg.
The reason is simple: The weight (vertical loading) deforms the tread element so that it is crammed into the small undulations of the abrasive road surface. However, the equation does not create a constant graph. The rubber has limitations that, if exceeded, would make it rupture and slide. When the rubber elastic properties are being fully untilized or under-exploited, Tmas>F, the result is for the tire to grip the road surface. If Tmax<F than the tire would slide and start to interact with the road with kinetic friction.
Heat is another important factor: When the fyber heats up, it will become softer and the coefficient of friction would increase. However, beyond a certain temperature, it would melt down and the tire will roll over a layer of molten rubber rather than on tarmac, reducing the grip levels. When the road is hot, it also makes the loose tar and the greasy materials (semi-burnt fuel, dripped oil, greasy car dirt, dust and sand, tire rubber residue) defuse towards the surface and reduce grip. Most people believe that the significance of tires is increased in the winter, where in action the tire is equally important over the whole year.
When we add forces of acceleration in any direction, we force the tire tread elements to distort in another direction too. The tire itself will begin to roll faster or slower than the car and this will distort the contact patch forward or back accordingly and create forces of acceleration or deceleration (negative acceleration). The formula dictates that the amount of relative slippage (s) by precent at this time would be S=100(1-[rw/V]), where R is the tire’s radius and W is the tire’s velocity, while V is the car’s velocity. Through empiric research, it is known that tires perform best (on tarmac) when the slippage is about 30% or slightly lower. Beyond that point the performance drops suddenly.
When a side force is introduced, the tire rubber deforms sideways so that the tread keeps on pointing forward while the wheel itself is turned aside. The rubber eventually turns, but always in a smaller angle than the wheel. This slip angle gives us a series of phenomenons that determind our cornering performance and safety: The lateral force (Fy) created by the lateral distortion (Pneumatic trail) of the slip angle is greatest in the rear-inside corner, creating torque that resists the turning of the wheel. This is known as Aligning Torque (Mz) and it is the origin of steering feel and the thing that makes the steering wheel return to straight if you let go of it.
The tire’s slip angle also limits the car’s critical cornering speed. In theory, the maximum safe cornering speed will be defined by calculating the equation mv²/r = mgμ (which adds up to v=√g μr). However, looking at this equaton by itself might be misleading, since it might give the false assumption that the parameters of friction and cornering radius are constant where in fact they are not. When you increase the slip angle you decrease the cornering radius. The reason is that the cornering radius is defined as the radian between the the center of the corner and the car’s center of gravity. While the weight transfer puts the center of gravity away from the center of the corner, this center (defined as the meeting of the perpendiculars of the four wheels) is moved inside, making the center of the corner smaller and the radius — tighter, reducing the speed in which the corner can be safely negotiated.
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.
The mechanism of Tire Aging
Various elements cause tires to age:
1. Sunlight: The effect of the photons and the UV-B radiation cause dryness and lost of the elastic properties of the rubber over the outside sidewall. The effects are greatly reduced (but not