![]() One BAR is roughly equal to the atmospheric pressure at sea level, which is quite a bit of pressure! But when it comes to tires, we usually deal with fractions of a BAR. No, not the place where you might grab a drink after work, but something far more important for our tires! A bar, in our context, is a unit of pressure. What Does Bar Mean In Pressure?įirst up, let’s talk about BAR. ![]() These are the big guys in the tire pressure world, and they’re essential for keeping our wheels spinning smoothly. We’re going to focus on three main units: BAR, PSI, and KPA. They might sound complicated at first, but trust me, once you understand what they stand for, they’re as easy as pie. (Assuming standard sea-level atmospheric pressure of 14.696 pounds per square inch or 101.Hey there, tire enthusiast! Let’s delve into the nitty-gritty world of pressure units. Variation of tire pressure with temperature in Fahrenheit and Celsius However, a similar tire inflated at an ambient temperature of 250 K (−23 ☌ −10 ☏) only needs to be warmed up by 2.5 K (2.5 ☌ 4.5 ☏) to see the same 1% increase in pressure.ĭue to the temperature-dependent pressure changes, seasonal temperature fluctuations can result in appreciable changes in tire pressure. This 3 K (3.0 ☌ 5.4 ☏) increase results in a change of. The absolute pressure is the sum of the ambient atmospheric pressure (approximately 1.01 bars at sea level) and the gauge pressure, giving a total of 3.21 bar (321 kPa 46.6 psi). If the absolute temperature of the air in the tire increases by 1% to 303 K (30 ☌ 86 ☏), the absolute pressure also increases by 1%. If the volume of the tire remains constant, a 1% increase in absolute temperature results in a 1% increase in absolute pressure.Īs an example, a tire is inflated to 2.2 bar (220 kPa 32 psi) at an ambient temperature of 300 K (27 ☌ 80 ☏). Cold tire absolute pressure (gauge pressure plus atmospheric pressure) varies directly with the absolute temperature, measured in kelvin.įrom physics, the ideal gas law states that PV = nRT, where P is absolute pressure, T is absolute temperature, V is the volume, and nR is constant for a given number of molecules of gas. Tire pressure is commonly measured in psi in the imperial and US customary systems, bar, which is deprecated but accepted for use with SI or the kilopascal (kPa), which is an SI unit.Īmbient temperature affects the cold tire pressure. ![]() Excessive pressure, on the other hand, will lead to impact-breaks, decreased braking performance, and cause increased wear on the center part of the tread surface. ![]() Drivers are encouraged to make sure their tires are adequately inflated, as under inflated tires can greatly reduce fuel economy, increase emissions, cause increased wear on the edges of the tread surface, and can lead to overheating and premature failure of the tire. A 2001 NHTSA study found that 40% of passenger cars have at least one tire under-inflated by 0.4 bars (6 psi) or more. Most passenger cars are recommended to have a tire pressure of 2.1 to 2.4 bars (210 to 240 kPa 30 to 35 psi) when not warmed by driving. Recommended cold inflation pressure is displayed on the owner's manual and on the placard (or sticker) attached to the vehicle door edge, pillar, glovebox door or fuel filler flap. Inflation pressure of tires before a car is driven A tyre pressure gauge displaying bar (outside) and pounds per square inch (inside)Ĭold inflation pressure is the inflation pressure of tires before a car is driven and the tires (tyres) warmed up. ![]()
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