Updated: Jan 28, 2022
Before the wheel was invented, people used sleighs and the load was dragged along the ground. There was a great deal of resistance to forward movement unless the sleigh was able to slide easily over snow, ice or wooden rollers. Friction was high because the movement over the ground was equal to the speed at which the mass travelled.
To overcome this came the revolutionary invention of ‘Wheels’. Wheels allowed rolling and thus made movements easier. In vehicles the wheel is attached to the Hub and Axle, whereas tyre is simply the part that is made out of primarily rubber and assists the car with more practical movement. It surrounds the wheel’s rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide traction on the surface over which the wheel travels. Spoked wooden wheels lasted until the modern era of coaches, and then usually with iron tyres. Even the first Benz motor car introduced in 1886, still had spoked wooden wheels, albeit with solid rubber tyres.
The figure below shows the cycloid curve (in yellow) traced by any general point on rim of a rolling wheel. Tyres provide a gripping surface for traction and serve as a cushion for the wheels of a moving vehicle.
Thus, the most vital thing is the tyre grip. If there were no such thing as grip, cars just would not be able to move at all. The wheels would just spin and therefore the driver wouldn't be ready to budge the vehicle. Even on a straight road and at a steady speed, there's no alternative to grip. This is because a moving vehicle has to deal with natural forces, such as the banking, the slope or the unevenness of the road, or rolling resistance, which are constantly trying to slow the vehicle down or push it off its path. Generating grip involves generating friction forces which counteract the vehicle’s skidding off the road. However, it must be borne in mind that it is slippage which produces the friction forces of grip. In fact, there are two forms of relative movement in the contact patch, micro-movement, commonly known as slippage, which counteracts macro-movement, commonly known as skidding.
Furthermore, even though the flattening of the contact area constantly produces
micro-movements between the tread blocks and the road surface, the contact area does not move– it changes, as one contact area continuously replaces the previous one.
It is only when the vehicle brakes, accelerates or corners that the contact area and the road begin to move relative to each other: this relative movement is known as slippage.
Slippage in the contact patch is produced when braking, acceleration, or cornering occurs. Here lies a paradox, which indeed is very surprising: “a tyre slips in order not to skid”!
WHAT is a tyre actually made of?
A tyre is primarily made up of rubber. Rubber is a viscoelastic material. A viscoelastic material is a deformable material with a behaviour which lies between that of a viscous liquid and an elastic solid. The harder the force applied, the greater the resistance to movement. The force F applied is not proportional to the travel, but proportional to the speed of the piston’s forward movement (Χ), i.e.; F = ηx˙. where η is the viscosity constant of the fluid.
On application of the force the, no immediate movement is observed. It is only after few moments that the movement becomes noticeable; thus, creating a lag between the force applied and the movement, this is called hysteresis.
On removal of the force, it may/may not return to its initial position (even if it does it is not always perceptible to naked eye). The energy supplied is not restored, but dissipated in the fluid: implying a energy loss (hysteresis).
Where does the viscoelasticity come from?
The constituent rubbers of the tyre are vulcanised elastomers. These elastomeric materials are made up of one or more polymers, long molecular chains which spontaneously take on the shape of a ball of wool and become entangled with each other.
To make the tyre, these materials are vulcanised, i.e., cured after incorporation of sulphur. Curing causes the creation of sulphur bridges between the polymer chains.
In moving, the segments of chains between the sulphur bridges rub against the other chains in their environment. It is this phenomenon which gives the material its viscous component.
Passenger car, lorry and off-the-road (“OTR”) tyres are products of complex engineering. They are made up of numerous different rubber compounds, many different types of carbon black, fillers like clay and silica, and chemicals & minerals added to allow or accelerate vulcanization. The tyres also have several types of fabric for reinforcement and several kinds and sizes of steel. Some of the steel is twisted or braided into strong cables.
A common-sized all-season passenger tyre, weighing about 10 KG (22 lbs) new, contains:-
30 kinds of synthetic rubber
8 kinds of natural rubber
8 kinds of carbon black
Steel cord for belts
Polyester and nylon fibre
Steel bead wire
40 different chemicals, waxes, oils, pigments, silicas & clays
Lorry & OTR tyres contain higher proportions of natural rubber than passenger car tyres.
Silica replaces part of the carbon black in certain types of tyres.
Some of the additives include clays, which may be replaced in part in some tyres with recycled rubber crumb from waste tyres.
These approximate totals would be slightly higher if clays were replaced by recycled crumb rubber from waste tyres.
Tyres contain so many different compounds and ingredients because they are engineering miracles, expected to handle the tortures of heat and cold, high speed, abrasive conditions, and often not enough air pressure. They are expected to perform for tens of thousands of miles and retain their essential properties despite horrendous driving habits and sometimes poorly maintained or built roads. Further, the addition of carbon black makes tyres tougher and more durable.
Crossply tyres consist of carcass layers made from nylon cord. They are placed diagonally across each other in the tread and the sidewalls, at an angle of 55 degrees. Multiple rubber plies overlap each other and they form a thick layer, resulting in less flexibility which can make it more sensitive to overheating. Therefore all high speed Tractor tyres are of Radial construction. Crossply tyres provide a strong and rigid sidewall which tries to follow the natural lines of the road and this can cause a tyre to overheat when it is used on a hard road surface and this in turn, causes the tyre to wear out more quickly. However, the sidewall of a crossply tyre is more rigid than that of a radial tyre so is more resilient at preventing sidewall damage. Crossply tyres are therefore sometimes used if sidewall damage is a problem.
Advantages of crossply tyres include:
Improved vehicle stability
Higher resistance against sidewall damages
Cheaper to produce
Disadvantages of crossply tyres include:
High rolling resistance, which causes tyres to quickly heat up
Reduced comfort due to the tyre's rigidity
Increased fuel consumption
These tyres are able to absorb shocks generated by road surfaces. The cords in a radial tyre casing run perpendicular to the direction of travel. Viewed from the side, the cords run radially - giving the tyre its name. The flexibility of a Radial tyre, together with its strength, are two combined factors which mean a radial tractor tyre absorbs impact shock and bumps more effectively than a crossply tyre. However, the cords cannot sufficiently absorb lateral forces when cornering or circumferential forces when accelerating.
The flexibility of the sidewall enhances vehicle stability and provides maximum contact of the tyre with the road surface. This, in turn, leads to a more comfortable ride and allows the driver to work longer. These tyres are also stronger, which means machines that use tyres like truck tyres or tractor tyres can be operated at higher load capacities. These are ones being used in the modern cars mostly.
Advantages of radial tyres include:
Good steering ad better road contact
Improved driving comfort thanks to flexible sidewalls
Less heat generated in the tyre at high speeds
Higher resistance against tread-related damage
Lower fuel consumption through better transfer of energy from machine to road
Disadvantages of radial tyres include:
The soft sidewalls are vulnerable when, for example, vehicles collide with curb stones.
Minor bumps in road are dealt with less effectively because radial tyres feature a steel belt.
Height to Width Ratio:
This is yet another important feature of the tyres. A height-width ratio (aspect ratio) of 65% is standard for many vehicles today and modern tyres are getting even wider – now having a height-width ratio as low as 25%. These ultra- low-profile tyres are, however, built for special high-performance cars.
Since 1975 the maximum speeds possible with Continental tyres have risen from 210 km/h to 350 km/h. At the same time the weight of an average-size tyre has actually been reduced from close to 12 to a good 8 kilograms.
The tread/belt assembly provides a minimal rolling resistance, optimal handling and a long service life. In the early days of tyre development, the casing was made of square woven linen fabric embedded in rubber. However, the crossed treads of the fabric cut away at each other, resulting in a relatively short tyre life.
In high end vehicles generally the Height to width ratio is kept low for better handling response, whereas the higher ratios are used in cases where more cushioning or riding comfort is desired.
How to read a Tyre?
Type: It precedes the Tyre Width Marking. P stands for passenger, LT stands for Light Truck, If there are no letters at the beginning, this indicates a Euro metric tyre.
Tyre Width: It is the width of the tyre measured in millimeters from sidewall to sidewall.
Aspect Ratio: Is the ratio of the height of the tyre's cross-section to its width.
Construction: The letter "R" in a tyre size stands for Radial, which means the layers run radially across the tyre.
Wheel Diameter: Is the size of the wheel measured from one end to the other. It tells us the size of the wheel that the tyre is intended to fit.
Load Index: It indicates the maximum load that the tyre can support when properly inflated.
Speed Rating: The maximum speed capability of a tyre.
Tyre Indication Number: The series of letters and numbers following the letters "DOT." The TIN consists of up to 12 numbers and letters to identify the factory location and the week and year the tyre was manufactured.
Tyres used on public roads must have a tread pattern by law. The main job of the tread pattern is to expel water which can affect the tyre’s contact with the road in wet conditions. In addition, the tread pattern, especially that of winter tyres, provides grip and adhesion.
On wet roads at high speeds, a wedge of water can build up between the tyre and the road surface. The tyre may then start to lose road contact or aquaplane, and the vehicle can no longer be steered.
Sufficient tread depth is vital not only in such extreme situations. Even at low speeds, there is a greater risk of having an accident in wet weather if the tyres are worn.
In order to ensure the tyres always offer best possible performance, summer tyres should be replaced when they reach a depth of 3 mm, and winter tyres when they reach a depth of 4 mm. Also, all four-wheel positions should be fitted with tyres of the same tread pattern design3, and each axle, at least, should have tyres with the same tread depth.
The tread pattern used on a winter tyre is particularly effective on snow and slush. In these conditions, the rotation of the wheel presses the snow into the wider grooves used on this type of tyre, thereby generating additional traction. When setting off, rows of fine lateral sipes enable the tread blocks to flex and bite deeper into the ice or snow for better traction.
It is vital that winter tyres are always kept inflated at the correct pressure since the volume of air contained in the tyre decreases at very low temperatures.
Summer rubber compounds begin to harden below 7 °C and no longer provide the levels of grip required and the tyre grip acts as if a glass. The special technology offered by winter tyres means they remain flexible and offer sufficient grip even at low temperatures.
The tread pattern used on a winter tyre is particularly effective on snow and slush. In these conditions, the rotation of the wheel presses the snow into the wider grooves used on this type of tyre, thereby generating additional traction. When setting off, rows of fine lateral sipes enable the tread blocks to flex and bite deeper into the ice or snow for better traction. They are designed to channel snow and slush and expel water.
The Graphs below show the variations in the tread rubbers with temperature-
Surface Friction Coefficient:
To have a better understanding we must accustom ourselves with some basic terminologies.
Microroughness: This is the name given to the road surface texture when the distance between two consecutive rough spots is between 1 and 100 microns. It is this degree of roughness which is mainly responsible for tyre grip.
Macroroughness: This is the name given to the road surface texture when the distance between two consecutive rough spots is between 100 microns and 10 millimeters.
Friction coefficient varies with molecular adhesion (due to Direct Contact) and Roughness Effects (Frequential Excitation caused by the surface due to slipping). If the depth of water increases (wet surface), Microroughness might become flooded. Macroroughness continues to indent, drain and store, but there is a risk of aquaplaning at high speed.
Water therefore interferes with grip and the tyres must be designed to disperse this water quickly and effectively by adjusting the shape of the contact patch, the tread pattern and the sipe arrangement.
Many vehicles are fitted with all season tyres when they leave the factory. Since they are built to provide a relatively quiet ride, good tread life and year-round performance, its no wonder why they are so popular. All season tyres offer versatile performance and are designed to perform in a variety of conditions including wet roads and light winter driving. All season tyres are designed to offer a combination of benefits from summer and winter tyres.
At temperatures between -5 and 0°C, the pressure of the tyre on the road causes slight surface melting of the ice, which is in turn covered by a thin film of water. The ice is then like a flooded microsmooth surface. Besides, the slopy nature of roads also help in draining snow from the path.
Snow and ice are cold surfaces which require the use of tyre compounds that retain a moderate modulus at low temperatures.
When the Brakes are applied, the angular speed of the wheels decreases and the rolling speed of the tyre drops below the vehicle speed. To compensate for this difference, the tyres begin to slip on the road at a slippage rate G.
During slippage, molecular adhesion and indentation induce a friction force, which opposes slippage, and the vehicle slows down
G = (ωR – V)/ V
G tends to infinity when ωR>>V, this happens when the wheels go round but do not move forward which is the case when we travel on snowy roads, the excessive slippage observed is just a consequence of this G equals -1 when the wheels get locked. Here the wheels continue to slide forward but do not roll.
This happens when the brakes are applied too sharply on icy surfaces or also when the vehicle has no ABS.
While cornering all the forces passes through the contact patch. The driver points the front tyres towards the inside of the bend and not actually along it as might be expected. This creates an angular difference called the slip angle. The slip angle in the rear wheels is developed in the wheels naturally due to this movement.
The ratios between the slip angles of the front and rear axles (a function of the slip angles of the front and rear tyres respectively) will determine the vehicle's behaviour in a given turn. If the ratio of front to rear slip angles is greater than 1:1, the vehicle will tend to understeer, while a ratio of less than 1:1 will produce oversteer. Understeer refers to the tendency to travel in a straight line during a bend. While oversteer is the tendency to take a tighter turn than intended.
How is a tyre manufactured?
Now let’s discover the processes involved in the manufacturing of a tyre.
As many as 30 ingredients which include Natural rubber, Synthetic rubber, Carbon black, Sulphur and several other chemicals and oils go into the rubber blend of a tyre. All such ingredients are made to go through computer controlled gigantic machines called Banbury mixers. Here these are treated under tremendous heat and pressure to soften the rubber evenly distribute the chemicals. The output is a gummy black compound that is sent for milling.
Each such batch goes through rolling mills that squeezes them into thick sheets. These sheets are used for making different parts of the tyre. Further polyester sheets are unrolled onto a machine called calender. This machine is equipped with rollers which apply warm rubber to both sides of the fabric. This produces rubberized fabric that is used to reinforce the tyres and make the tyre sturdier.
This process is performed on a cylinder whose central flexible section can be inflated. The first element applied to the drum is a sheet of airtight synthetic rubber, this layer replaces the inner tube in today's tyres.
In a second stage a ply made of textile core sheeted rubber is added which forms a reinforcement radiating around the tyre it is the radial carcass. Two high resistance metal cable hoops are installed against strips of profiled rubber. Then the bead wires which will hold the tyre on the rim are installed. The casing ply is folded over the beads to anchor it. Other elements are then added. Sidewalls made of flexible resistant rubber are installed which will protect the tyre from lateral damage. The tyre is then shaped by inflating the central section of the drum. Two plies are applied to the crown of the tyre, they are reinforced with metal wire arranged completely and with the casing fly form a network of triangles to limit the deformations of the tyre.
The tyre blank is then installed in the curing mould. In the centre, a bladder filled with pressurized hot water pushes the still malleable material to the base of the patterns engraved in the mould the heat of the water and steam surrounding the mould causes curing to begin. The rise in temperature causes vulcanization of the rubber, the sulphur incorporated into the rubber components forms bonds between the polymer chains. At this point the rubber switches from a plastic state to an elastic state. The treads and sidewall markings are added inside the curing mould. The tyre is then removed from the mould with definitive form and properties.
Every tyre is carefully assessed by trained inspectors. Special machines are also used, designed to spot even slight imperfections. Quality control engineers randomly select tyres from the line and cut them open for closer inspection. In addition to this, some tyres are chosen off the line and x-rayed individually, to check for internal weaknesses.
Why different tyres for different applications?
The earlier discussion on slippage and surface friction coefficient makes it clear that a single tyre cannot be used all purposes. The possible terrains which a vehicle can be used for is too wide. There are all terrain type tyres as well. They may not deliver the optimum performance , but can easily give adequate performance on all types of terrain.
Snow tyres have metal or ceramic studs that protrude from the tyre to increase traction on hard-packed snow. Further they have the tread blocks set fairly far apart in order to ensure better grip in the snow. Ice tyres on the other hand are made up of blocks, set very close together, many tiny incisions in the blocks ensure a quieter performance. Further, the tread on ice tyres is usually manufactured from a mix of silica.
Off Road vehicles are those which are used for unpaved surfaces like loose dirt, mud, sand, or gravel. Compared to ice or snow tyres, they lack studs but contain deeper and wider grooves meant to assist the tread sink into mud or gravel surfaces. They use thick and deep treads that penetrate into the mud and dirt to provide more traction. Tough off roading conditions are also faced by ATS and rescue operations teams.
Besides these, tyres are used for space probe vehicles as well. Here the surface conditions where the vehicle might travel and the possible obstacles it might face is quite uncertain and needs to be estimated.
The surfaces have over pointed rocks and are usually very uneven, at times like a plateau and at other times like valley. Further the challenge is of exposure to UV radiations at these surfaces and the extreme temperatures here which can fall down to as much as -130°C, at which the rubberized material tends to lose its elastic property and turn brittle like glass, thus rendering it completely useless.
The challenge that further needs to be overcome is to produce the tyres with minimal weight capable of supporting heavy rovers so as to reduce the overall launch cost.
All these challenges have motivated scientists to discover new materials which can be used to make the tyres. One such material is Nitinol. It is an alloy of Titanium and Nickel. This is actually the same material used for teeth braces, it possesses shape memory and can easily revert to its original shape when adequate requirements are met.
Nitinol exists as Martensite below certain temperature. On the application of sufficient stress, it undergoes deformation, unlike other materials it uses the heat generated by moving over uneven surfaces to revert back to austenite (regularly ordered crystal structure) which on further cooling attains the original shape. Interestingly this transition temperature can be tailored for different applications.