The tire pressure is decisive for the usability of a pneumatic tire. It is therefore set according to specifications and must be checked regularly and adjusted if necessary to maintain functionality. The tire pressure can also increase during operation due to warming. Incorrect tire pressure leads to premature wear or failure of the pneumatic tire. This is why tire pressure monitoring systems are increasingly being used in cars and have been mandatory for all new vehicles in the EU since November 1, 2014. In addition, the tire pressure can have a significant effect on the driving behavior of a vehicle. In case of doubt, you should always refer to the information provided by the respective vehicle and / or tire manufacturer.
The tire pressure is given in the unit bar . It is common not to specify the absolute pressure, but the overpressure compared to the ambient pressure ( air pressure ). The tire pressure specification 2.7 bar means that the tire pressure should be 2.7 bar above the ambient pressure of approx. One bar. The absolute tire pressure would then be 3.7 bar. The tire inflation gauges at petrol stations make it easy for the driver by only showing the relative tire pressure (i.e. the overpressure) in bar, which is also mentioned in the vehicle documents. In Europe, tire pressure on vehicles has been given in kilopascals since around 2012 . 100 kPa is 1 bar.
The maximum tire pressure is usually indicated on the tires in the unit of measurement commonly used in the USA, pound per square inch (psi) . To convert, 1 bar corresponds to 14.5 psi.
The tire pressure information provided by the vehicle or tire manufacturer always relates to the "cold" tire. This means the fact that the tire warms up with increasing distance and speed. In practice, a car tire can still be regarded as "cold" after driving 10 km at a moderate speed. If the tire pressure is checked after driving fast on the motorway, the tire inflation (air or nitrogen) has expanded due to the heating and the tire pressure rises by 10% for every 30 K, i.e. by approx. 0.3 bar at a tire temperature of 50 ° C.
Deviations from the tire pressure information provided by the vehicle or tire manufacturer may have functional reasons. Examples:
- Increased tire pressure reduces the rolling resistance of car tires and thus lowers fuel consumption . However, an internal pressure that is significantly too high reduces the power transmission potential and thus limits driving safety.
- In off-road vehicles , construction or agricultural machinery , the tire pressure for off-road driving is often reduced in order to increase the contact area of the tire. As a result, drive wheels can transmit more propulsion to the ground, while at the same time the ground pressure exerted vertically by the tire is reduced (reduction of harmful soil compaction ). In the case of vehicles that are operated on frequently changing types of surface, this can also be done while driving by means of a tire pressure control system.
Either air or nitrogen is used to fill the tire . The advantage of nitrogen filling is controversial for car tires. Allegedly, the tire holds the pressure better than with an air filling. For the normal operation of a car tire, however, the differences are negligibly small (see also tire gas ).
The nitrogen filling of the tire is usually indicated by a colored valve cap . Refilling the tire with air is possible without any problems; especially since air consists of 78 percent nitrogen. For the operational safety of a car tire, it is not the choice of the filling gas that is decisive, but rather the regular control and correction of the air or nitrogen pressure.
The tire pressure is temperature-dependent and should be measured when it is cold, i.e. at ambient temperature and before a journey. It is a few bar for conventional tires for cars and children's bicycles, for trucks and racing bikes to over 10 bar, and for tires for "fat bikes" on soft terrain, 0.5–1 bar.
- past, when air pressure was measured in atmospheres , tire pressure was given in units of atmospheric pressure (atü) .