Test all limits in the Nissan Navara
Test all limits in the Nissan Navara

The importance of proper tyre pressure

During the past weeks, the issue of tyre pressure and specifically underinflation have been coming up on Facebook groups quite regularly and some tyre OEM’s even mention underinflation in their warranties. But what is underinflation and why all the fuss?

Did you know it’s not the tyre that supports the weight of your vehicle, but rather the air pressure inside the tyre? It’s no surprise then that tyre pressure influences many important characteristics of vehicle performance. This includes driving comfort, directional stability, cornering and braking grip, plus the general handling behaviour. Driving with incorrect tyre pressure will have a negative influence on one or more of these important characteristics. So, it’s important to monitor tyre pressure frequently to ensure optimal performance.

The tyre pressures specified for your vehicle are agreed upon between the vehicle manufacturer and the tyre producer. The pound per square inch (psi) is based on your vehicle’s total weight and size, towing weight capability, and recommended tyre size. These pressures are set to keep you safe and comfortable on the road so it’s important to follow them.

When it comes to education on tyre pressures, we need to keep in mind that the most important variables are:

• Tyre type (highway pattern, mild all-terrain, aggressive all-terrain, mud-terrain): each one will necessitate avariance in pressure and the ‘softer’ the construction,the higher the pressure required.

• Vehicle type and load (tare, GVM, GCM): the heavier the vehicle, the less the tyre can be deflated below the manufacturer’s recommended pressure.

• Terrain: compact and hard surfaces require slightly higher pressure to ensure that there is less tyre deflection in the upper sidewall area to avoid penetrations, but on sand, the pressure will need to be lowered to generate as large a tyre footprint possible.

• Speed: in a technical 4×4 context the speed is generally very low, but when driving on gravel at higher speed, you should not reduce pressures too much relative to your load as this can push tyre temperatures to an unsafe level.

The rise and fall of it all

Gas expands when heated and contracts when temperatures decline. This means that you should check your pressures in the morning before the tyre has been run, before the ambient temperature rises, and before exposure to direct sunlight. Hence, cold inflation pressure.

Temperatures rise and fall between day and night and as we move through the seasons. Tyre pressures can fluctuate by between 0.07 and 0.14 bar for every 10°C. So, if the outside temperature increases by 10 degrees, the tyre pressure could increase by as much as 0.14 bar. Conversely, if the ambient temperature drops 10 degrees, the tyre pressure will decrease by between 0.07 and 0.14 bar.

Because driving causes heat buildup, the pressure inside warm tyres will be higher. As such, you should never reduce the pressure inside warm tyres because when they have cooled down, the pressure could potentially fall to below the minimum required pressure.

It is especially important to check tyre pressures when temperatures drop in autumn and winter, as the resultant drop in tyre pressure will have an adverse effect traction, handling and durability. In fact, it is recommended that you check the pressures at the beginning of each month and before long road trips, where the additional load may require the pressures to be increased according to the specification. And don’t forget to check the spare too!

The dangers of incorrect pressure

Driving on overinflated tyres will have a negative impact on various factors. This includes driving comfort, directional stability, and handling (especially when cornering at speed). It will also cause irregular wear, which shortens the lifespan of the tyre.

By the same token, driving on underinflated tyres comes with its own set of pitfalls. In this case tyre durability, steering response and directional stability will suffer, not to mention the possibility that a tyre can become dislodged from the rim when cornering! Underinflated tyres flex more during driving, especially at high speeds, which causes the sidewalls to heat up. This can result in tyre failure or even a blowout. Remember that hidden tyre damage is not rectified by adjusting the pressure.

The ins and outs of TPMS

For a number of years now tyre pressure monitoring systems (TPMS) have been mandatory for all new vehicles in the USA and Europe. Although this is not the case here in South Africa, most modern vehicles will – by default – be fitted with such a system. Although many early models don’t have this feature, it is possible to add a system to old or classic vehicles.

A TPMS can be based on different concepts and sensors and there are basically two main types available. Direct systems have pressure sensors integrated with the valve or attached to the rim or inside of the tyre. On the other hand, indirect systems don’t have pressure sensors fitted to the tyre or rim. Instead, pressure differences are measured indirectly via for example tyre rolling circumference changes.

Vehicle manufacturers may opt for either, but these systems generally only issue a warning once the tyre pressure drops 25 percent below the vehicle manufacturer’s recommended figure. In many cases a slight drop in air pressure would not trigger a warning light and could cause a safety issue or adversely affect fuel economy. So even with a TPMS, tyre pressure needs to be checked on a regular basis.

The fitment and maintenance of TPMS systems should be left to the tyre specialists, though.

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