2008.09.23

Hannover, September 2008. The only part with which vehicles can really move is the tyre. The appropriate attention to the tyre inflation pressure ensures noticeable advantages in the operating and environmental balance.

The term tyre pressure indicates the fact that a relative figure is actually involved. Firstly, there is atmospheric air pressure at every point on earth. That is the “hydrostatic pressure of the air column at this location”, i.e. the pressure created by the weight of the air above it.

When we talk about “air pressure” in relation to all of kinds vehicle tyres, we are referring to the relative overpressure in the tyre compared with the normal outside air pressure that is constantly present. The experts talk about tire inflation pressure to avoid misunderstandings. This is also the pressure that tyre inflation equipment and tyre pressure gauges show because they compensate the atmospheric air pressure before display.

Here are some simple figures to demonstrate the importance of this relatively small volume of gases that fill the tyres under high pressure: The contact patches of the tyres connecting a 40-tonne articulated truck to the road are about the same size as 16 soup bowls. On a bus, which can weigh up to 21 tonnes, with its sensitive “human” load, six to eight of these soup bowl-sized areas on the pneumatic hi-tech tyres form the connection between the vehicle and the road surface.

They absorb all of the forces, weights and movements that occur between the vehicle and road on every journey. It is important that these round load carriers are inflated to the correct pressure. The wrong tyre pressure will cause a commercial vehicle to quickly enter a spiral of costs due to a lack of safety and wear.  

This can soon occur if you do not pay attention especially with commercial vehicles. Since they are normally heavier, today’s truck tyres are still made from large portions of highly durable natural rubber. Due to the minute pores in its structure, this natural material is also slightly permeable for a small amount of the compressed air used to inflate the tyres. In addition, the valve, the rim and the rim seat of the tire are not completely airtight. The small disadvantage of the necessary maintenance work is therefore inevitable. Completely airtight tyres would also be hardly elastic with the high standard of material technology today.

The natural “diffusion”, the passage of tiny quantities of gas through the micropores of the tyre material, is not a problem if the tyres are looked after on a regular basis — if you top up the air on a regular basis, you can use the tyres for their full rated life. Company figures and road safety will only be affected if you do not make sure that the tyres are inflated to the correct pressure.

The noticeable consequences of a seemingly negligible tire pressure setting or incorrect pressure result from the complex technical and physical interaction of material properties that shape modern truck tyres. The multifaceted designs using special rubber compounds, tire cords and task-related treads are designed to carry their specific loads in the set climatic conditions. Their mileage, the ride and the noise need to be just as highly developed as the technology of the vehicles that they move safely. In addition to the qualification of the designers, it is the seemingly simple physical phenomenon of the inflation pressure that makes the decisive contribution to safe operation of the tire and that of the vehicle it is carrying.

Shaping with gas pressure

The latest commercial vehicle tyres keep their shape and thus their performance capability simply with the correct inflation pressure. The gas pressure of up to 10 atmospheres behind the sidewalls and belt layers in the black tyres press the inner tyre wall against the rim and ensure that costly tube designs are also no longer required even on tough terrain.

The correct tire pressure thus also keeps the tread in the right position on the road and thus makes a highly effective contribution to transferring the propulsion to the road surface. Steering axle tyres also have to deal with extreme lateral forces, which flow away via the tire sidewalls that are kept stable by the air pressure. Tires for trailers and semitrailers also deal with high shearing moments and are still capable of coping with this and other load moments with minimum rolling resistance and extremely quiet running.

The tolerance range for the “ideal” tyre inflation pressure is not large. Just half a bar above or below the target pressure is the range within which a truck tyre can perform its task reliably and ideally. Deviations within this narrow band can even lead to negative consequences in some cases.

If you increase the tyre pressure on a commercial vehicle by 0.5 bar over the minimum pressure specified for the respective axle load, you only have small advantages in the short term. Slightly less rolling resistance and slightly lower fuel consumption are the direct noticeable consequences in addition to steering accuracy and driving stability. However, the technical loading for the wheel and axle mountings, suspension, dampers and steering increases when the tyre pressure is increased slightly. For this reason, the tyre inflation pressure should only be increased if the vehicle manual or documents from the tyre manufacturer expressly refers to this option. Depending on the tyre model, more overpressure quickly leads to deformation. The first impairment occurs on the tread which arches in the middle of the rolling axis. As the contact patch becomes smaller, it increasingly loses its capability to transfer the acting force moments reliably, safely and easily. The uneven tread also wears unevenly, increases the braking distance to a level beyond that tolerable for the tire or insurance and normally makes early tyre replacement inevitable.

A tyre pressure set below the target, which various tire manufacturers claim is normally the case following research, has similar consequences. The tread is deformed in the centre of the rolling axis towards the inside and loses contact area. As a result, the wear on the tire shoulders increases. The rolling resistance, the fuel consumption and the breaking distance also increase while the steering accuracy and driving stability are reduced. The load on the chassis and steering also increases due to the increasingly imprecise reaction of the tyre and its pneumatic damping properties. An additional risk is the increased flexing of the tire after driving below the target pressure. The energy resulting from the flexing is transferred to the tire material as heat that can destroy the tire on long journeys. Almost all tyre damage occurring on freeways results from tires overheating because the pressure is well below that specified. 

Keeping an eye on specified pressure

Tyre and vehicle manufacturers therefore specify for each vehicle which axle position can be driven at a certain minimum pressure. It should always be set and checked when the tyres are cold, i.e. before a journey or after a break. The reason for this is simple: After just a short distance, the tyre, on whose special gripping tread the weight rests, warms up while the propulsion and lateral moments act on the shoulders and sidewalls. This flexing increases the tyre temperature and causes the air inside the tire to expand. If the tyre inflation pressure is set properly, this rise in temperature will be within the design norm range and does not pose a danger for operation. If you measure the pressure once the tires are warm and release the apparent overpressure, you will set an incorrect pressure that should be avoided.

The multiple influence of the moments that are acting on the tyre and the tyre pressure and the possible consequences of an incorrectly set inflation pressure are clearly shown in the investigations that were carried out in the “land of highway transport”, the USA, and in Europe. According to the figures, an increased diesel consumption of around four billion liters results from the average lower inflation pressure of 12 percent in the USA alone. That corresponds with around an additional nine million tonnes of carbon dioxide emissions. Comparable investigations that Continental performed with their ContiFleetCheck tool for Europe produced results on a similar scale. In terms of business figures, incorrect tyre pressures cause around 12% noticeable additional costs for a 40-t truck driving 14,000km each year due to increased rolling resistance, greater wear and tyre damage. Dr. Jörg Lehmann, project manager at Continental’s development department, estimates the additional costs per year and truck “at around €1000 euro if we take an experience value of around 12% lower inflation pressure.”

Depending on the size of the fleet, substantial additional costs and environmental burdens can be avoided if attention is paid to the high-tech products on the wheels. Unnecessary additional costs for tyres, fuel and repairs will then be tackled effectively as well as avoidable emissions and accidents.

One new technical development makes keeping an eye on tire pressure even easier. The tire pressure monitoring system developed by engineers at Continental lets you check inflation pressure by looking at a display.

The direct measuring system will be available as original equipment and for retrofitting from the end of 2009. It consists of a replaceable, encapsulated sensor for pressure and temperature in the tyre, a receiver on the vehicle frame and a display unit. In the receiver, the respective working pressure of the tyre is corrected by the increased pressure resulting from the temperature produced as the tyre rolls. In new vehicles, the results will be shown in a diagram of the vehicle wheel configuration on the dashboard screen. There is a small external monitor for retrofit solutions. The system shows the current working pressure for each individual wheel with a quickly read traffic light scale (green-amber-red). The exact decimal figures can be called up if required via the menu of the respective display computer. The temperature-compensating real-time display allows you for the first time to react quickly to tyre problems as they occur.

Similar systems have been mandatory in the USA for original equipment on light commercial vehicles and cars for a few years. The introduction on heavy trucks will also happen once suitable systems are also available for heavy categories. The EU commission is backing a similar binding introduction in Europe. Once again, Continental is taking the needs of its business partners into consideration with the development of the system that has been tested in the field by Continental customers since last fall.

With targeted annual sales of more than €26.4 billion for 2008, the Continental Corporation is one of the top automotive suppliers worldwide. As a supplier of brake systems, systems and components for the powertrain and chassis, instrumentation, infotainment solutions, vehicle electronics, tyres and technical elastomers, the corporation contributes towards enhanced driving safety and protection of the global climate. Continental is also a competent partner in networked automobile communication. Today, the corporation employs approximately 150,000 people at nearly 200 locations in 36 countries.

The Commercial Vehicle Tyres division, which oversees the development, production and global distribution of truck, bus and industrial tyres, posted a turnover of approx. €1.5bn in 2007 with a workforce of over 8,000 employees.

The tyre divisions are an official sponsor of the FIFA World Cup 2010^TM, which will be staged in South Africa in 2010. You will find information on this at www.ContiSoccerWorld.de, www.ContiFanWorld.com and www.continental-corporation.com