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Understanding Tyre Pressures 

A labelled diagram of a Nitto Trail Grappler Tyre.

Tyre Pressure

The tyres on your vehicle are the only contact your vehicle has with the road, so it only makes sense to service and manage tyres in the same manner as the rest of the vehicle. Luckily for vehicle owners, it isn't difficult to regularly check on the conditions of tyres and can be conducted when filling up at a petrol station, all of which contributes to minimising the risk of Tyre failure.

Contact patch diagram

It is important to understand why correct Tyre pressures are necessary and the repercussions of poor tyre maintenance. Australian Law also distances that every vehicle is fitted with correct tyre sizes and pressures for safety reasons. There are 2 key factors as to why maintaining tyre pressures are key: Safety and Economy. It is the drivers responsibility to ensure the tyres are safe and are conditioned correctly.

When to Check Tyre Pressures - "Check early and check often"

A phrase commonly used in the Automotive Industry and for good reason. A simple 5-minute inspection of the quality and condition of tyres at the petrol station could potentially be life-saving.

Image of someone checking the tyre pressure on tyres.
Most issues in a vehicle can be resolved or the repair can work out significantly cheaper if pre-empted and the same applies to Tyres. A car can lose pressure from everyday use, spirited driving or even if it is immobile for long periods of time. Varied road conditions also speed up the deflation process as hitting potholes, bumps and curbs compress the tire meaning some deflation occurs.

Low Tyre Pressure

Tire failure is usually caused by incorrect tyre pressures as it means there is either too much or too little contact with the road. Low tyre pressure means the tyre has excessive contact with the road resulting in overheating and accelerated degradation.
Overheating degrades the tyre surface drastically as the compound is being overheated then cooled when the car comes to a stop and the process repeats. Long distance driving is where the danger factor is accelerated as highways require drivers to maintain high speeds for long periods of time.
Low tyre pressures change the property of the tires in several ways. Tread patterns aren't reinforced to their full potential and blocks may either be stretch or compressed. Neither of which get the maximum performance, safety, and economy intended by the Tyre manufacturer.

Nitto Tyre Pressure chart to help drivers establish optimum tyre pressure.
Wet weather performance is significantly decreased as it can increase braking distance by up to 3 times jeopardising passengers, other road users, pedestrians and the driver of the vehicle. Emergency situations can't be prevented as easily as the vehicles stopping distance is unmeasurable and unpredictable to the driver. The vehicle is much more prone to Aquaplaning/Hydroplaning (where the vehicle loses traction in wet conditions and the driver essentially becomes a passenger and has no control of the vehicle) when vehicles have low tyre pressures and significantly increases the likeliness of over and understeering.
Response and performance are also impacted as the car will feel less precise and predictable and makes turning in low speeds much more difficult. Poorly maintained tyres increase the chances of tyre failure which could lead to the tyre exploding and increase the chances of punctures and other risks. Having tyres that are poorly inflated means the physical properties of the wheel are flattened. It essentially means the tyre is heavier than it needs to be which translates when turning the wheel. A correctly Inflated tyre will be much smoother when turning the wheel as there is less drag and resistance, opposed to a tyre with low tyre pressures which makes turning the wheel more difficult and can wear the steering components in the vehicle quicker.
Durability and tyre lifespan decrease heavily every time a wheel is used with low tyre pressures. Even the slightest drop in tyre pressures can reduce the lifespan of a tyre up to 20-30%. The shoulders of the tyre are pushed out more meaning they become prone to wear much quicker and can become potentially dangerous.

A chart showing braking distances based on tread depth.
It also increases the chances of irregular tyre pressure wear which means tyre wear is unpredictable and can the impact can leak down to the tyres internal structure. Tyres aren’t meant to be worn down to the edges, a strong possibility with low tyre pressures. Economical impacts of low tyre pressures include increased fuel consumption as the integrity of the tyre is compromised and heavier, meaning there is increased weight, drag and load by up to 15% on the tyres meaning the vehicle must exert more power to accelerate the vehicle.


High Tyre Pressures

The impacts of Low Tyre pressures are like those when tyres are overinflated. Accelerated tyre wear, loss of traction and harsher ride quality are all by-products of high tyre pressures.
Manufacturers state an optimum tyre pressure because they have developed the tyre with specific compounds, tread patterns, and blocks that are optimised at a particular pressure.
Overinflated tyres cause the tyre to round out the tread section which expands meaning the contact with the road is optimised, causing instability and accelerated tyre degradation.
The tread blocks and pattern are developed with calculated gaps in between each block to maximise grip, traction and to help disperse water. Overinflating a tyre means the gaps are no longer optimised, meaning the control of the vehicle decreases.

Pirelli Tyre Pressure Chart
Like most circumstances, poor weather conditions exacerbate the effects of overinflating as water isn't dispersed as well and traction is lost. Ride comfort is also compromised when high tyre pressures are used as the optimum tyre pressure recommended by the manufacturer allows some give and takes into consideration that tyres will expand and detract according to its temperature.
When manufacturers set the optimum tyre conditions, it is tested in cold conditions because when the environment is hot, the air in the tyre heats up, making it denser, adding weight up to 1kg per tyre.
The added weight and pressure within the tyre also mean it is more likely to burst or for a tyre failure to occur as there is no leniency within the tire to absorb impacts from potholes and bumps.

Ideal Tyre Pressure:

The unit which tyre pressure is measured in two units: PSI – Pounds per Square Inch and kPa - Kilopascal. Manufactures of vehicles and the tyre now state the ideal tyre size and tyre pressure for the stock wheels in the Handbook and on charts stuck inside the door panel when opening the door. Tyres have the ideal tyre pressure on the tyre itself. The tyre pressure stated are measured in cold conditions as the air in a tyre becomes denser when it is heated, giving possibly inaccurate readings.

Tyre Pressure chart on the Ford Ranger Door Sill.
To fill a tyre with air it is as simple as going to your local petrol station with an air hose (majority of petrol stations are now equipped), enter the desired kPa or PSI, and connect the hose to the valve of the tyre and it will do the rest. It will release air in controlled amounts and will stop and beep twice when it has reached the desired tyre pressure. Driver’s must not forget to check the spare tyre’s pressure as the air escapes over time in all tyres.