The tyres in the cars and commercial vehicles are subjected to the greatest stress. In order to get the extended life of the tyres the rolling resistance should be as low as possible. Low fuel consumption can be achieved once the rolling resistance is reduced. Sample economic feasibility study shows that the tyre cost accounted for 3% and the fuel cost accounted for 18% of the total operating cost of the commercial vehicles. A good tyre normally meets the following requirements.
- Good power transmission between tyres and road.
- Adequate fast running stability
- Low wheel load fluctuations
- High puncture safety
- High suspension comfort and absorption properties
- Running smoothness and
- Low noise development
- Low dead weight and High load bearing capacity with
- Low rolling resistance
- Long service life
- Low side wall abrasion
- Retreading capacity
The tyre consists of following three major layers.
- Belt and
- Running surface with tread.
Carcass consists of 1 to 2 radial layers steel cord. It flows in to the rim and lies at the rim flange with its thickened ends (Bead). The tubeless tyre has an air tight rubber layer Inside the carcass. There is a belt consisting of number of layer steel cord between the carcasses and running surface. The tread is cut in to the running surface.
Tyres are classified as Cross Ply, Radial and Belted tyres based on their basic structure. Most of the commercial vehicles and cars use Radial tyres due to their following properties.
- High mileage
- High load bearing capacity
- More contact surface with the road which enhances braking and driving forces.
- Better spring effect
- Good adhesion on cornering
- Low rolling resistance
- High mileage due to low abrasion
- Small rolling bead.
Cross Ply Tyres:
Diagonal tyres in their structure have diagonally crossed layers of fabric. The main tread direction of the carcass layers of fabric intersects the tyre circumference line at a thread angle of 35 to 38 degrees.
Due to high requirement regarding the strength and reserves of load bearing capacity carcasses made of steel cord are normally used in the commercial vehicles.
The tread angle in radial tyres is normally kept at 85 to 90 degrees in such a way that the main thread direction is radial to the tyres. The threads of the carcass layer run across the shortest path from bead to bead. The radial tyres are able to built up with sufficiently high cornering forces and the side wall also structured with number of fabric layers in order to have better stability.
To ensure sufficient stability against axial forces the sub structure of the tyre is enclosed by a belt in the direction of the circumference. The carcass and the belt form the structure of the radial tyres. Depending on the type of tyre the carcass consists of 1 or 2 layers, and the belt is composed of 2 to 5 layers of steel cord.
The tread ensures interfacing with the various road surfaces. The drive axles require the traction in the longitudinal direction, the maximum cornering forces is most important for the steer axles. The properties of tread surface are responsible for adhesion as well as abrasion irrespective of the circumstances. A tyre designer aims at good compromise between adhesion and abrasion. The tread channels must be shaped in such a way that they makes way for contamination that enters.
When the tyre swims on the wet road surface the contact to the road surface is lost because the tread channels are no longer able to repel the water quickly enough. The vehicle can no longer be steered or braked.
-Good adhesion on the road surface
-Low rolling resistance
-Good traction (force transmission on the direction of travel)
-Low abrasion and high mileage
-Good self cleaning
-Low tendency towards Aquaplaning.
As a standard practice all the steered axles will have individual wheels. A single tyre can carry the maximum load of 4000kg.
It may not be possible to achieve high load bearing capacity in heavy transport with single tyre. For this reason twin tyres are used in the drive axles. The advantages are :
-Increased Axle load bearing capacity.
-Safety advantage in case of puncture on one tyre.
And the disadvantages are
-High rolling resistance.
-High fuel consumption.
-Higher dead weight has negative effect on unsprung masses and total pay load.
The low section tyres with low height-width ratio being used in some of the trailers where high volume transported with low frame height vehicles. The advantages are low dead weight and low space requirement.
In order to get required axle load so called super wide tyres with very small cross section being used in Heavy commercial vehicles. The advantages are
- Considerably lower dead weight
- Significantly lower rolling resistance
- Better true running properties
- Lower noise level
- Simpler Assembly
- Suitable for retreading.
- Lower space requirement.
By replacing four twin tyres of dimension 315/70 R 22.5 on steel rims with super wide tyres of the variable 495/45 R22.5 a gain in pay load will be up to 130 kg and reduction in fuel consumption up to 3 to 5% can be achieved.
The advantage of using super wide tyres is its low space requirement in the axle. This facilitates usage of non-dropped vehicle frame with spring track of 330 mm wider. The wider spring track with reduced sensitivity to rolling would mean that smaller shock absorber and stabilizer could be used. This would even save more weight.
In correct tyre pressure on the super wide tyre has a negative effect on the consumption, wear and safety compared to standard tyres. In order to exploit the good rolling properties in full and compensate for the slightly lower level of puncture safety super wide tyres are being used with tyre pressure monitoring system.
Energy Saving Tyre:
Energy saving tyres is designed to achieve lower fuel consumption and a longer service life. The reduced consumption is achieved by reducing the rolling resistance. These are divided in to:
- Rolling friction between tyre and road surface
- Deformation of the tyre
- Air friction at the tyre
The rolling resistances of tyres of heavy commercial vehicles with average speeds of 80km/hr are approximately half of the total road resistance. In order to get extended life of the tyre, abrasion property to be improved with low susceptibility to damage. At the same time the retreading capability must be equal to that of conventional tyres to achieve longer and more economic product cycle.
Material properties and engineering design are major factors taken in to consideration for the development of energy saving tyres apart from the carcass structure and tread. In such cases the rolling resistance is reduced to the extent of 20% as compared to conventional tyres without compromising on other properties such as adhesion, service life or retreading capability. These improvements are achieved by means of:
- Grooved running surface and sub layers are designed with harmonised combination of various rubber mixes for less inner deformation with good adhesion.
- More stable carcass structure for lower rolling deformation with higher inflation pressure.
- Treads bound to the direction of travel for low-loss drive.
- For reduced flexing work, more stable tyre bead seat.
- Adapted running surfaces for lower abrasion and higher mileage.
- Specially shaped tread channels and tread base reinforced with cleats for reduced susceptibility to damage.
The entire vehicle combination fitted with energy saving tyres, low age and wear of the tyres and the right inflation pressure enable fuel saving of between 3% and 6%.
Tyre Identification :
The tyre size is specified in two dimensions: Nominal tyre section width and rim diameter. All dimensions apply to inflated, unburdened tyre. Tyre with a height / width ratio of less than 88% are called low section tyre, other wise they are called low pressure tyre. The load bearing capacity of the tyres used to be specified with the Ply Rating (PR). 16 Ply Rate means that the stability and load bearing capacity correspond to a tyre with 16 layer of cotton cord. Now a days the PR is no longer matches the number of layers in the tyre and it has been replaced by the load index. The letter behind the load index indicates the reference speed at which the tyre has its load bearing capacity.