An overview on innovation in tyre
By Arup Chandra & PK Mohamed (email@example.com; firstname.lastname@example.org)
Global R&D Centre-Asia, Apollo Tyres, India
Reinforcing materials / fabrics
There are several fibres that are currently used in carcass reinforcement substituting cotton; High modulus low shrinkage (HMLS) polyester is typically used in most radial passenger tyres and offers a good mixture of properties. Nylon is used extensively in bias tyre where strength and rapture energy is of paramount importance.
Rayon is the reinforcement cord of choice in Europe for high-performance tyres due to its ability to maintain mechanical properties at high temperature. Aramid fibre offers Ultra High Performance (UHP) properties but at a higher cost. This cost limits usage of aramid to tyres of high-end luxury and race cars. The performance properties of PEN present opportunities for replacement of rayon or polyamide in carcass construction.
In radial tyres, the carcass provides reinforcement along the radial direction. To provide reinforcement in the rolling direction, belts are necessary. Steel is the traditional material of choice for belt. The cap ply is wound over the shoulders or the entire body of the belt to provide a compressive force, which resists the centrifugal forces created in the belt at high speed. Nylon 6,6 has been typically used for cap ply applications due to its high retractive force at elevated temperatures.
Scientists are also trying to successfully develop a molecular architecture where crystallisation planes within the polymer structure would give the reinforcement to a flexible amorphous phase and can produce tyre without fabric reinforcement.
Reversion characteristic of natural rubber (NR) is of great concern. A lot of novel chemicals have been introduced to increase the reversion resistance of NR over the period. Examples of these are zinc soap activator (Structol-A73), silane coupling agents (Si-69, Si-266; Si-363; Nxt-Silane), anti-reversion agent (Perkalink 900), and post-vulcanization stabilizer (Duralink HTS and Vulcuren KA 91 88); These materials are enhancing the life of the tyre, enable the users for more retreading, and thereby reduce the demand of materials. Other types of low volatile silanes (eg. Si-363, Nxt- silanes) are also gaining popularity,
In the initial days, wooden carts with wooden wheels were used for the movement of goods and people. In order to protect the wheels from external damage, they used to cover the wheels with leather caps, which were later replaced with rubberised fabrics. These covers on the wheels have undergone changes and are known today as “tyres” (derived from the word attire or a protective cover).
The introduction of automobiles, as we know them today, hastened the process of development of the modern-day tyre. In 1888, a Scottish veterinary surgeon, John Boyd Dunlop, invented a more practical contraption, where he mounted a rubberised fabric on a metal rim holding it in position by stitching and bolting it. Later in 1890, CK Welch introduced a detachable tyre by incorporating high tensile steel wires in the area which touched the road or the bead area. It was further fine-tuned by American scientist, William Bartlett, who came up with improved beaded edge which was locked in position on the rim by internal air pressure.
Now it has been over a century from the time air filled “pneumatic” tyre was patented. Over the decades, the pneumatic tyre has undergone many changes in response to rising demands of modern transportation. Tyre technology is improving day by day with the efforts of tyre and vehicle manufacturers. Advancements in material use and technology have led to new products and even development of new market segments;. There are various exciting developments that have taken place in the technology area which will be discussed here.
As the tyre evolved along with automobiles, many changes have taken place in its structural design. A tyre is essentially an envelope which holds air, protected by different layers of compounds at different areas, depending upon local or functional requirement and a steel reinforcement to allow the tyre edge to grip the rim of the wheel. Over time, the design of this envelope has undergone a sea change in both materials and architecture. A key milestone in this is the evolution from cross ply to radial tyres.
The body of cross ply tyres, until around the 1970s, consisted of a number of rubberised cord plies with edges wrapped around the bead wire (the bead ensures that the tyre sits firmly on the rim). The casing strength determines the load capacity of the tyre. The individual cord plies of a cross ply tyre are arranged in a criss-cross pattern at a certain angle – known as the bias angle. This angle determines the tyre’s characteristics.
An obtuse bias angle, for example, gives better ride comfort but reduces lateral stability. An acute cord angle increases directional stability at the expense of ride comfort. Cords being at an angle, only a component of the cord strength will be available for load carrying and tractive functions and hence a cross ply tyre structure will have many numbers of plies for satisfying the given functional performance.
In the newer technology known as radials (since 1948), the load carrying cords run in the shortest possible path from bead to bead. As the tractive functions are addressed mostly by steel belts and tread, which work independently from sidewalls, an efficient utilisation of material strength is realised. This also makes radials very sensitive to variations, as any change from the optimal shape can have drastic difference in performance and hence calls for higher precision in the radial design and manufacturing process;
As the tyre evolved, along with the functional expectations, the shape of the tyre also has undergone considerable change. The height to width ratio of a tyre, known as aspect ratio, has constantly come down from the traditional circular cross sectional shape to the present possible aspect ratio of 25 (section height is only 25% of section width) ;
A higher aspect ratio tyre has broader sidewalls which allow for higher flexing and therefore give better cushioning for the ride, translating directly into higher comfort. But this also means a higher loss of energy. A lower aspect ratio tyre has shorter and stiffer sidewalls which are efficient in energy utilisation but poor in cushioning or comfort.
Low Aspect Ratio, or Low Profile tyres (lower section height / width ratio) is the concept that has been used to further enhance the performance of radial tyres. A low profile tyre is a tyre with a sidewall height of 70% or less of the tread width. Low profile tyre technology have advantages like high mileage, higher resistance to irregular wear, lesser weight, better stability, high volume capacity, etc. Many of the higher segments, new generation cars are fitted with high performance, low aspect ratio tyres which provide superior handling and tractive functions. The comfort issue is generally addressed here by sophisticated suspension systems in such vehicles.
Several theories have evolved about the efficient utilisation of structural stiffness to carry load along with meeting functional requirements. They are about orienting the load carrying plies in a tyre in an optimal fashion to better utilise their strength for performance. NIP (Natural Inflated Profile), NPL (Neutral Ply Line), RCOT (Rolling Contour Optimisation Theory) and TCOT (Tension Contour Optimisation Theory) are some of them.