The manufacturers of vehicles and tyres work in close collaboration to determine the best mix of comfort, road holding and the precise pressure required to achieve the best option.
The tyre pressure is in fact the pressure of the Air or Nitrogen (in some cases) that is acting in all directions on the inside walls of the tyre. This pressure forces the tyre into the required shape that allows the tyre to deliver the required service as expected.
The service expectation being lifespan in kilometers, and safe road holding during those same kilometers of travel.
If one explores the tyre structure it is made up of numerous layers of rubber, fibre materials, steel mesh and wire for the beading. The reason for the complexity of a tyre structure is to provide the strength needed to carry the weight of the vehicle, within speed limits (of the tyre itself, not only the legal speed limit).
The design needs to be flexible enough in the side walls to provide a softening of minor road shock, yet strong enough to resist rolling off the rim in a corner. The Tread path needs to be able to flex with the road, and to resist wear whilst at the same time providing the best possible grip on the road surface. (Note that not all roads have the same grip for the tyre to adhere to).
Therefore, the tyre pressure forms an integral part of the tyres ability to maintain the small contact area on the road for road holding.
Tyres can lose air pressure at a rate of up 0.14 Bar per month therefore they can become underinflated if not monitored. If the pressure is too low, the sidewalls can flex and allow excessive sideways movement of the vehicle on the tyre. In a corner, tyres will have uneven contact with the road causing the effect on the centre of gravity of the vehicle to be shifted to a position which can compromise the stability of the entire vehicle. Underinflation can be seen in the excessive wear on the inside and outside edges of the tread path, which wears out your tyres faster (replacement will be sooner than necessary), you will also have increased rolling resistance with the road. You may not feel this added resistance, but you will be using more fuel than necessary.
Under inflated tyres can also affect your steering. If the underinflated tyre is at the front of the vehicle, you are likely to experience a high degree of understeer. If the underinflated tyre is at the rear of the vehicle, they will increase oversteering.
An underinflated tyre raises the risk of a blowout dramatically, so it really is important to check regularly.
If the tyre pressure is too high then the surface area of the tyre that is in contact with the road will be reduced and lead to potential loss of grip on the road surface and skidding, or at least understeering. Both of which can be dangerous.
Additionally, the load on/in the vehicle is a determining factor in the correct pressure selection for the vehicle. In most vehicles, either in the owner’s manual or a decal located on the vehicle body (fuel filler flap or one of the door frame areas), there will be a guide as to the tyre pressure required for the specific configuration in which the vehicle is being used. The options vary mostly from one driver and passenger to four passengers with luggage in the case of motorcars. It is best to familiarise oneself with these pressures and/or the location of the information at least. Consultation of this data is important.
In the case of light delivery vehicles and/or our favorite 4X4 uses, these same principles apply in general. However the loads may vary considerably, thus affecting the required pressure accordingly.
4×4 Soft Sand
There are some exceptions to every rule though and this pertains more to the 4X4 community, where the practice of driving in soft sand requires a much lower than normal pressure. This low pressure is used to allow the tyre to provide the a bigger contact patch. Contrary to some beliefs it is not to make the tyre wider, the intention is to create a “tank track” type of contact surface.
However as with all motor vehicles it is imperative that this low pressure is corrected to normal road pressures as soon as reaching a solid surface again. Again, consultation of the owner’s manual will determine the road going pressures required.
In conclusion, do not trust just any pressure gauge, have your own one, make sure that it is calibrated and you are familiar with it. Use your own gauge to check the tyre pressure, that way you know it is CORRECT. Not all gauges are calibrated often enough and are often subjected to excessive use and sometimes even abuse in the workplace and on the forecourts. Be safe, use your own gauge and check the tyre pressures regularly. There is no normal or general pressure, use what is specified for your vehicle and get the best from your tyres. Do not underinflate tyres in an attempt to get a softer ride for you or your passengers, it can be very dangerous.