2008.09.23

Hannover, September 2008. The wrong tread has never done anyone any good — people make a bad impression and vehicles using tyres with the wrong tread sometimes do not just have bad economic consequences: Losing grip can also have fatal consequences when handling the high-tech rubber structures.

In both cases, the fault lies in poor coordination of the “active partners”. For the tyres, there are also sometimes planning, servicing and care faults.

To begin with, “road adhesion” and “grip” are the two key terms that sum up the requirements for tyre treads. The tyres are the only direct contact between the heavy vehicle and the road surface so they should adhere to the ground as best possible and ensure the contact is intensive as possible.

It is obvious that you then come up with soft round tires with an even, smooth surface and many minute surface structures. On dry roads, this kind of slick tyre takes hold of the asphalt so firmly that it transfers propulsion and steering movements ideally. The static friction is high, the vehicle does not slip and is easy to control and steer.

The first raindrop changes these properties dramatically, however. The slightest water film on the road reduces the static friction between the rubber and the road, however. Once the tire starts to “float”, i.e. cannot displace the film of water, despite the vehicle weight it is bearing, it will act as a slippery film. The steerability, grip and safety are thus gone — the vehicle sometimes too.

Furthermore the deformation of the soft tyre, which should also be elastic as a vibration damper, also produces resistance to rolling — the rolling resistance.

Something always gets in the way

The task for the tyre engineers at Continental is therefore to design the tires so that they also do not lose grip on wet road surfaces — or sand, snow, clay and other soft surfaces, have minimum rolling resistance and still transfer as much propulsion as possible.

The simplest way to grip a surface through a soft medium are fingers or studs — on tyres they do not move and are called “tread segments”. Their surfaces reach through water, sand or snow onto the ground and drive the vehicle forwards. Their invention also marked the beginning of a complex development: The smallest tread segments have a negative shape — pores or cylindrical microscopic holes, i.e. small recesses in the tread. These allow the slick to grip the asphalt, they absorb water and also sand or snow that is then no longer released that quickly. The tread segments therefore need to be larger and also active: protruding blocks, studs or sipes can be used to lead away interfering media like water, pebbles or sand.

There are also two disadvantages associated with this, however: Regular tyres have a smaller overall contact area than slicks when resting on all tread segments. It can thus transfer and control fewer forces.  This is also bad for the large tyres on commercial vehicles. The tread blocks should also be relatively large for this reason. However, the grooves between them should also not be too small because they will otherwise lose their cleaning effect.

To solve this problem, the Conti tire developers use inverse sipes: They are slightly larger and more mobile in the surrounding material than the mainly cylindrical microcapillaries of small pores. In the form of perfectly scaled and positioned grooves in the tread blocks, they can increase the grip and release any material picked up. In the tread grooves between the segments, standing sipes are used as small material tongues to briefly hold sand, snow or other media and then eject it from the tyre surface.

But that is not where it ends. Truck tyres, being elastic interfaces between the vehicle and ground, are flexible in themselves. During use, they deform themselves and thus also all tread segments that the engineers have come up with so far.

For this reason, the tire engineers at Conti have added the factor of deformation resistance to the interaction of rolling resistance-slip-traction. So that the smaller contact patches formed from the tread segments can still have an effect, their shape may change too much during use. This is intended sometimes, however. For example, a notched sipe opens wider when the tread runs out of the contact patch so that deposits are released more easily.

Simply complex tasks

For this reason, the Conti development engineers run through the different possibilities to optimize a tread for each new tyre generation with different kinds of tread segments. This is not always made easier by their colleagues from the “rubber kitchen” in Stöcken being able to trim the raw material as they wish for hardness, softness or elasticity.

It would otherwise hardly be possible to manufacture the high-quality vehicle carriers out of rubber in the special variety that is required by today’s logistics. To reach this aim, the test engineers coordinate their tyre series with different shapes and rubber compounds to surfaces, climate zones, optimum handling, economic aspects and also for several product life cycles. They want to make the movement of the tread segments as large as possible and as small as necessary.

If we simply take a standard combination of tractor unit and trailer for road use, we already have three different types of Continental tyre, some of which are in different sizes. On the steering axle, there are tyres that are made above all with longitudinally running tread rings that are only bladed in the center. The tyre developers thus create extremely good tracking with low rolling resistance and high stability against the torque occurring when the vehicle is steered. The outer areas of the closed seat rings are made movable by means of slight chamfering so that they cannot tear when turning. The drive tyres are different: pronounced tread blocks are mainly used here with partly strong siping that should transfer the traction to the road as the sum of individual access areas in a complex design.  They have larger grooves that also ensure stability by bridging the tread segments without hindering the ejection of sand, pebbles or snow. The tyres for semi-trailers and trailers clearly have a smoother pattern. Their function is mainly to support heavy loads and, depending on the axle, extreme shearing forces. For this reason, they mainly consist of tread rings even more than steering tyres — with less marked siping — and have more and stronger shoulder notches.

Suitably meeting tougher requirements

The climate and the ground are further modification parameters for tyre treads. All in all, winter tyres need to have more defined tread segments than simple summer tyres so that they can transfer the steering, propulsion and braking forces even on snow or ice. This creates new problems when designing the tyre — in addition to the question of rubber compounds that are compatible with the cold. The best way to get through snow is with powerful tread blocks between which wide grooves dispose of the loose frozen water. On the cold, solid frozen ground, you need lots of small pores with which the cold rubber can suck itself onto the ground. The useful combination of these properties comes together in the cold-resistant treads, whose architecture is very different in detail from the very smooth running summer tyre. On the winter tyres, the tread normally consists of lots of small tread blocks each with fewer sipes. The tread grooves and shoulder sections intended for material ejection and elasticity of the blocks among each other are kept more open and thus allow good clearance of any material picked up.

Even larger calibers are used for truck tyres that are used on loose surfaces off the road. The pure rolling properties are increasingly in the background while load bearing capacity and a gripping tread are becoming more and more important. If you have ever driven an empty dumper truck or an articulated loader in traffic on an asphalt road, you will be familiar with the vibrations that arise. Coarse-stud treads on construction site tyres with large blocks and grooves and normal coarse off-road tyres whose “blocks” look more like rungs on a ladder are designed to give the vehicle traction, steering capability and safe braking even on soft ground. For this reason, Continental makes these types of tyre almost without fine sipes and with wide open shoulder segments. Sipes would fill up while the grooves that run right into the shoulders keep the tyre tread clear and operable even in soft clay or deep sand.

Quiet performance....

Apart from all of this, today’s commercial vehicle tyres also need to be quiet. This simple and basic requirement for sensory environmental friendliness sets further complex tasks for the tyre developers in Hanover. The gripping treads above all on construction site tyres and traction tyres for drive axles are designed for smooth running and relatively low vibration for better handling and reduced running costs. However, it is above all the traction properties that set a high threshold for dealing with the additional acoustic requirement for the highly optimized tyres. If you alter the main source of noise emissions, the well-designed open edges of the tread blocks whose constant impact on the surface while rolling makes traction treads louder than steering or trailer tyres, you also have to change the complex tyre design in itself.

The tyre engineers then have to adapt the shape of the tread segments and thus also recalculate components like sipes, grooves, wall radii, rubber compounds, intermediate area topography etc. This is why the tyre engineers at Continental adjust their products to increased requirements in each development cycle and then present tires again that provide their high performance for longer than older types on a regular basis.

However, this not a matter of course — without any care, even the high-tech treads from Conti tyre engineering will wear out prematurely. It is therefore economically worthwhile to keep the expensive rubber grooves and blocks away from hard edges and sharp or pointed material whenever possible during use. After usage, the tires should always be checked and objects removed from the grooves, blocks and sipes. It is also very important to observe the minimum tread depth in addition to the tire pressure. The refined traction system of the tread can only work correctly if it has enough reserves. The specialist “workers” on the axles can only reliably do their job — produce turnover — if these few simple maintenance tasks are performed.

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