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INTRODUCTION
The mode of transport is one of the most important criterions these days. The vehicles safety is thus essential. Accidents are also increasing at a quick pace. There are several factors which causes these accidents. The improper inflation of tyres is one among them. Tyres lose air through normal driving (especially after hitting pot holes or curbs), permeation and seasonal changes in temperature. When tyres are under inflated, the tread wears more quickly. Under inflated tyres get damaged quickly due to overheating as compared to properly inflated tyres. The under inflation also causes a small depreciation in the mileage as well. Above all the vehicles running with under inflated tyres can cause accidents.
Thus to rectify all these defects we are using self inflating systems. The pressure monitoring systems in such systems helps in monitoring the tyre pressure constantly. The system which contains sensors feed the information to a display panel which the driver can operate manually. The electronic unit controls all the information. The source of air is taken from the vehicles air braking system or from the pneumatic systems. Thus it helps in re-inflation of the tyres to proper pressure conditions.
HOW TYRES WORK
If you're in the market for new tyres, all of the variables in tyre specifications and the confusing jargon you might hear from tyre salesmen or "experts" might make your purchase rather stressful. Or maybe you just want to fully understand the tyres you already have, the concepts at work, the significance of all of those sidewall markings. What does all this stuff mean in regular terms?
In this article, we will explore how tyres are built and see what's in a tyre. We'll find out what all the numbers and markings on the sidewall of a tyre mean, and we'll decipher some of that tyre jargon. By the end of this article, you'll understand how a tyre supports your car, and you'll know why heat can build up in your tyres, especially if the pressure is low. You'll also be able to adjust your tyre pressure correctly and diagnose some common tyre problems!
How Tyres are Made
As illustrated below, a tyre is made up of several different components.
The Bead Bundle
The bead is a loop of high-strength steel cable coated with rubber. It gives the tyre the strength it needs to stay seated on the wheel rim and to handle the forces applied by tyre mounting machines when the tyres are installed on rims.
The Body
The body is made up of several layers of different fabrics, called plies. The most common ply fabric is polyester cord. The cords in a radial tyre run perpendicular to the tread. Some older tyres used diagonal bias tyres, tyres in which the fabric ran at an angle to the tread. The plies are coated with rubber to help them bond with the other components and to seal in the air. A tyre's strength is often described by the number of plies it has. Most car tyres have two body plies. By comparison, large commercial jetliners often have tyres with 30 or more plies.
The Belts
In steel-belted radial tyres, belts made from steel are used to reinforce the area under the tread. These belts provide puncture resistance and help the tyre stay flat so that it makes the best contact with the road.
Cap Plies
Some tyres have cap plies, an extra layer or two of polyester fabric to help hold everything in place. These cap plies are not found on all tyres; they are mostly used on tyres with higher speed ratings to help all the components stay in place at high speeds.
The Sidewall
The sidewall provides lateral stability for the tyre, protects the body plies and helps keep the air from escaping. It may contain additional components to help increase the lateral stability.
The Tread
The tread is made from a mixture of many different kinds of natural and synthetic rubbers. The tread and the sidewalls are extruded and cut to length. The tread is just smooth rubber at this point; it does not have the tread patterns that give the tyre traction.
Assembly
All of these components are assembled in the tyre-building machine. This machine ensures that all of the components are in the correct location and then forms the tyre into a shape and size fairly close to its finished dimensions.
HOW TYRES SUPPORT A CAR
You may have wondered how a car tyre with 30 pounds per square inch ( psi ) of pressure can support a car. This is an interesting question, and it is related to several other issues, such as how much force it takes to push a tyre down the road and why tyres get hot when you drive (and how this can lead to problems).
The next time you get in your car, take a close look at the tyres. You will notice that they are not really round. There is a flat spot on the bottom where the tyre meets the road. This flat spot is called the contact patch.
If you were looking up at a car through a glass road, you could measure the size of the contact patch. You could also make a pretty good estimate of the weight of your car, if you measured the area of the contact patches of each tyre, added them together and then multiplied the sum by the tyre pressure. Since there is a certain amount of pressure per square inch in the tyre, say 30 psi, then you need quite a few square inches of contact patch to carry the weight of the car. If you add more weight or decrease the pressure, then you need even more square inches of contact patch, so the flat spot gets bigger.
You can see that the under inflated/overloaded tyre is less round than the properly inflated, properly loaded tyre. When the tyre is spinning, the contact patch must move around the tyre to stay in contact with the road. At the spot where the tyre meets the road, the rubber is bent out. It takes force to bend that tyre, and the more it has to bend, the more force it takes. The tyre is not perfectly elastic, so when it returns to its original shape, it does not return all of the force that it took to bend it. Some of that force is converted to heat in the tyre by the friction and work of bending all of the rubber and steel in the tyre. Since an under inflated or overloaded tyre needs to bend more, it takes more force to push it down the road, so it generates more heat.
Tyre manufacturers sometimes publish a coefficient of rolling friction (CRF) for their tyres. You can use this number to calculate how much force it takes to push a tyre down the road. The CRF has nothing to do with how much traction the tyre has; it is used to calculate the amount of drag or rolling resistance caused by the tyres. The CRF is just like any other coefficient of rolling friction: The force required to overcome the friction is equal to the CRF multiplied by the weight on the tyre. This table lists typical CRF for several different types of wheels.
TYRE-INFLATION BASICS
About 80 percent of the cars on the road are driving with one or more tyres underinflated. Tyres lose air through normal driving (especially after hitting pot holes or curbs), permeation and seasonal changes in temperature. They can lose one or two psi (pounds per square inch) each month in the winter and even more in the summer. And, you can't tell if they're properly inflated just by looking at them. You have to use a tyre-pressure guage. Not only is underinflation bad for your tyres, but it's also bad for your gas mileage, affects the way your car handles and is generally unsafe.
PROBLEMS WITH TYRES
When tyres are under-inflated, the tread wears more quickly. According to Goodyear, this equates to 15 percent fewer miles you can drive on them for every 20 percent that they're underinflated. Underinflated tyres also overheat more quickly than properly inflated tyres, which cause more tyre damage. The faded areas below indicate areas of excessive tread wear.
Because tyres are flexible, they flatten at the bottom when they roll. This contact patch rebounds to its original shape once it is no longer in contact with the ground. This rebound creates a wave of motion along with some friction. When there is less air in the tyre, that wave is larger and the friction created is greater and friction creates heat. If enough heat is generated, the rubber that holds the tyre's cords together begin to melt and the tyre fails. Because of the extra resistance an underinflated tyre has when it rolls, your car's engine has to work harder. A statistics show that tyres that are underinflated by as little as 2 psi reduce fuel efficiency by 10 percent. Over a year of driving, that can amount to several hundred dollars in extra gas purchases.
HOW TYRE PRESSURE GAUGES WORK
Underinflation can cause tyres to wear more on the outside than the inside. It also causes reduced fuel efficiency and increased heat buildup in the tyres. It is important to check the tyre pressure with a guage at least once a month. so it is essential to have a tyre pressure monitoring system in our vehicles.
There are three simple steps involved in measuring a tyre's pressure with a pressure gauge: Get in a steady position to apply the pressure gauge to the valve stem.
• Apply the gauge, forming a good seal between the gauge and the stem and releasing air from the tyre into the gauge. Note how the pin inside the gauge presses against the valve pin inside the valve stem to release air from the tyre.
• Read the pressure from the gauge.
SELF-INFLATING SYSTEMS
Tyre-inflation systems have three general goals:
• Detect when the air pressure in a particular tyre has dropped - This means they have to constantly (or intermittently) monitor the air pressure in each tyre.
• Notify the driver of the problem
• Inflate that tyre back to the proper level - This means there has to be an air supply as well as a check valve that opens only when needed.
Parts of Any Self-inflating System
While the available tyre inflation systems vary in design, they share some common elements.
• They all use some type of valve to isolate individual tyres to prevent airflow from all tyres when one is being checked or inflated.
• They have a method for sensing the tyre pressures. This is addressed in most cases with central sensors that relay information to an electronic control unit and then to the driver.
• They have an air source, which is usually an existing onboard source such as braking or pneumatic systems. When using an existing system, however, they have to ensure that they don't jeopardize its original function. For this reason, there are safety checks to ensure that there is enough air pressure for the source's primary use before pulling air for tyre inflation.
• There has to be a way to get the air from the air source to the tyres, which is usually through the axle. Systems either use a sealed-hub axle with a hose from the hub to the tyre valve or else they run tubes through the axle with the axle acting as a conduit.
• There has to be a pressure relief vent to vent air from the tyre without risking damage to the hub or rear-axle seals.
CENTRAL TYRE INFLATION SYSTEM (CTIS)
The idea behind the CTIS is to provide control over the air pressure in each tyre as a way to improve performance on different surfaces. For example, lowering the air pressure in a tyre creates a larger area of contact between the tyre and the ground and makes driving on softer ground much easier. It also does less damage to the surface. This is important on work sites and in agricultural fields. By giving the driver direct control over the air pressure in each tyre, maneuverability is greatly improved.
Another function of the CTIS is to maintain pressure in the tyres if there is a slow leak or puncture. In this case, the system controls inflation automatically based on the selected pressure the driver has set.
There are two main manufacturers of the CTIS: U.S.-based Dana Corporation and France-based Syegon (a division of GIAT). Dana Corporation has two versions, the CTIS for military use (developed by PSI) and the Tyre Pressure Control System (TPCS) for commercial, heavy machinery use. In the next section, we'll take a look at the inner workings of a basic CTIS setup.