Automobile Repair/Tire pressure

Proper tire inflation is very important. All tires should be inflated to the pressure recommended in the vehicles owner manual. Cars sold recently that meet U.S standards will have a notice, often on the driver's door frame but sometimes elsewhere, like in the glove compartment, showing what the front and rear pressures should be, and they should be listed in the owner's manual. This should be between the minimum and maximum pressure ratings listed on the sidewall of the tire. Improper inflation may result in poor/unsafe handling, blowouts, poor gas mileage or excessive tire wear. Contrary to some expectations, blowouts are much more frequently a result of underinflation than overinflation; as tire pressure lowers, the sidewalls of the tire flex more during rotation causing more heat, which builds up and damages the tire. In contrast, tires are manufactured with a substantial margin of safety for overinflation. The manufacturer's suggestions are a good place to start, if it is intended to adjust pressures optimally. Historically American cars would usually have the specified tire pressure somewhat low, for a more "cushiony" ride, but this practice is reduced of late. If suggested pressures are not available, or if the vehicle is modified, Usenet quotes an algorithm from Oscar Pereda, an engineer for BFGoodrich, as a good starting point for pressure in pounds per square inch: (Vehicle Weight in lb/100) psi + 2 psi at heavier end + 2 psi all around if suspension and alignment are stock. If equipment is available to measure the temperature of the tire tread, they should be inflated so as to achieve even temperature distribution across the treads of all four tires; higher temperature in the center of the tread compared to the edges indicates overinflation, while the opposite indicates underinflation; higher temperatures at one end of the car similarly indicates that that end is overinflated with respect to the other end. Tire pressures can then be altered to tune handling. If, after the pressure is set up, the car seems to lack adhesion at one end, then air can be added to the tires on that end (one or two pounds at a time) until balance seems to be achieved to suit the driver. As an example, in the rear-engined Chevrolet Corvair, the normal oversteering tendency of a rear-engined vehicle was combated by keeping front tire pressures down to twelve psi, which was safe because of the low weight the front tires carried on that car; if the front tire pressure was raised to what would be normal for a front-engined vehicle, the car would become dangerously unstable. In the case of tires with very thick tread, such as aggressive mud or snow tires, the tendency described above to build up heat and deteriorate is that much greater, and therefore they should be inflated closer to the maximum pressure given on the sidewall, altered as necessary for safe handling, as described above. The almost universal adoption of radial tires has made adhesion on wet pavement less of a problem; however, where bias-ply tires are still used, the speed at which "hydroplaning" occurs, i.e. the tire loses contact with the pavement and begins to slide on the film of water, can be easily calculated as the square root of the tire pressure, times ten psi. If high speed driving on wet pavement with bias-ply tires is contemplated, this is something that must be taken into account. You will need a tire gauge. You can get a cheap pen style gauge for between $1 and $5 (USD) at a gas station or automotive shop. To check the tire pressure, locate the valve stem, take the cap off it, and firmly push the gauge onto it, and then remove the gauge and check the reading. The gauge should only hiss briefly while pushing it onto the valve stem. If it hisses more, then you aren't pushing on it hard enough, and are not getting an accurate reading. Spare tire Proper inflation of the spare tire is also very important. All tires will lose some pressure over time, but since the spare tire is usually hidden from view, many people overlook it. If you don't check it regularly, your spare tire could go flat, and cause further problems when you have a tire failure. The spare tire is often different than the regular tires. If the vehicle has a full-size spare, it's usually the same as the tires that were originally installed on the vehicle, which may differ from the tires currently installed. If it's a mini spare, the difference is obvious. So, make sure to check the pressure ratings on the sidewall of the tire to find the proper tire pressure. If you don't already know how to access your spare tire, you should check your car's manual. There are too many different configurations to explain it in this book. When to check Check pressures when the tyres are cold, before the car has been driven more than a kilometre or two. Driving warms the air in the tyre and raises the reading by 0.2 to 0.3 bar (3 to 4 psi), so a tyre checked hot and set to the placard figure will be under-inflated once it cools. Check at least monthly and before a long trip; a sound tyre loses roughly 0.1 bar (1 to 2 psi) a month through the rubber, and more as the weather turns cold, since pressure falls by about 0.07 bar (1 psi) for every 5 to 6 °C (10 °F) drop in temperature. Tyre pressure monitoring systems (TPMS) Cars sold in the United States since 2008 and in the European Union since 2014 must warn the driver of a significantly under-inflated tyre. Two designs are used: Direct systems have a pressure sensor inside each wheel, usually built into the valve. They report the actual pressure, often per wheel on the display, and their batteries last roughly seven to ten years, after which the sensor is replaced with the tyre. Indirect systems use the ABS wheel speed sensors to detect a wheel turning faster than the others because its rolling radius has shrunk. They need no sensors in the wheels but must be reset from the dashboard menu or a button after the tyres have been inflated or changed, otherwise the new pressures become the baseline. The warning threshold is typically about 25 per cent below the placard pressure, which is already low enough to raise fuel consumption and tyre temperature, so the light is a last warning and not a substitute for the monthly gauge check. A light that flashes for about a minute after start-up and then stays on means the system itself has a fault (commonly a dead sensor battery or a missing sensor after a tyre change) rather than a low tyre. Note 1: Direct sensors are fragile and are damaged by careless tyre fitting and by some tyre sealants. Tell the tyre shop the car has sensors. Note 2: After rotating the wheels on a direct system, some cars relearn the positions automatically while driving; others need the positions programmed with a tool, or the display will show the wrong wheel. Note 3: The spare wheel usually has no sensor, so the warning does not cover it. Check it with the gauge.