Natural Rubber in Engineering High-speed Tyres

(Paul Bremmer)

Introduction

The construction of passenger car tyres depends on the demands that can be stated. The car dynamics, its top speed and the desires of the consumer have to be translated to the functional demands on the tyre. This article illustrates the development of Vredestein Ultrac high-speed tyres, high performance tyres used for top speeds until 300 km/h. The high-speed tyres are designed for cars like the BMW M3. The M3 is the fastest and sportiest type of the BMW 3-series, with suspension and tyres optimised for handling. The engine performance numbers are impressive: the maximum engine power is 252 kW (343 bhp); the maximum engine torque is 365 Nm. The top speed is limited to 250 km/h by the manufacturer, without the electronic limitation the top speed will exceed 250 km/h by far. The powerful brakes and the tyre grip ensure high braking performance. The tyre sizes are 225/45 ZR 18 at the front, and 255/40 ZR 18 at the rear wheels.

Demands on high-speed tyres

On high speed tyres the demands arefocused on handling, grip, quick vehicle responses and safety at high speeds. Characteristics as comfortable ride and low wear rate are of less importance. The tyre must be constructed to perform well on:

• Braking performance
• Handling
• Aquaplaning resistance
• High speed behaviour
• Tearing resistance

Braking performance

The braking performance is determined by the grip of the tread compound on the road surface. The grip results from the total friction force of the tyre on the road surface. Therefore the dynamic footprint (the pressure distribution and the shape) and the friction coefficient of the tread rubber are crucial. The friction coefficient varies with normal stress and is directly related to the hysteresis properties of the tread rubber. For good braking performance, high-energy absorption as a result of hysteresis is required. The hysteresis can be attained by using the right elastomers with relatively high glass transition temperatures (Tg), and by using the right
amounts and types of fillers. SBR (Styrene Butadiene Rubber) type elastomers with relatively high amounts of styrene are normally used.

Figure 1. Components of a high speed tire

Because of their low Tg’s NR (Natural Rubber) and BR (Butadiene Rubber) are hardly used when ultra high grip is desired. The braking performance both on wet and dry roads can be improved by lowering the air ratio. The air ratio is defined by the ratio of groove area and tread area in the contact patch. For dry roads a slick (a tyre without profile, thus with air ratio zero) would give the best braking performance, but for the resistance to aquaplaning – as discussed next-grooves in the thread are essential.

Handling

Handling of the car is the way a car feels and behaves during driving manoeuvres. A car with a good handling has predictable behaviour and a reasonably quick dynamic response to the steering input. Furthermore it is easily controllable during extreme manoeuvres, and it offers a safe feeling to the driver. Additionally the car must be able to achieve high cornering speeds.

Handling of the car is determined by a series of tyre and car characteristics. Three important tyre characteristics for handling are the grip during cornering, the feedback from the tyre forces and moments to the steering wheel, and the dynamic tyre response to steering. These characteristics are influenced by the tyre components as follows (see also figure 1)

Tread

A stiff tread compound is beneficial for the grip during cornering as the stiff profile blocks show little deflection. A very stiff undertread compound is applied for raising the total tread stiffness, which refines the quick tyre response. The total tread height is minimized for improved stiffness.

Belt layers

Wide and stiff steel belt layers reduce the deflection of the tyre construction during cornering, resulting in a quicker response and more grip. For minimizing belt edge separation, a possible defect of a tyre after many million cycles, natural rubber is the basis for the belt compound because of its high resistance to tearing.

Apex and rim cushion

These components are manufactured from a stiff compound, normally consisting of NR (Natural Rubber) and BR. The stiff construction results in a short reaction time of the lateral tyre force on the steering input.
The dynamic behaviour of the tyres with regard to handling is tested on the N.rburgring, a racetrack that is frequently used by car manufacturers to make the final adjustments on the car suspension in order to get the desired handling. On this 20-km longtrack with various curves several tyres with varying constructions and compounds are subjectively tested to achieve the best handling performance.

Resistance to aquaplaning

When a tyre rolls through a water film with increasing speed, at a certain moment the tyre will loose contact and starts floating. This phenomenon is called aquaplaning. To prevent aquaplaning, water has to be transported from the contact patch through the grooves in the tread. For this reason a slick will perform worse on aquaplaning. The resistance to aquaplaning of a tyre can be affected mainly by the combination of design and dimension of the grooves in the tyre on one hand, and the shape of the dynamic footprint on the other hand.

Effects of high rotational speed

Centrifugal force

The contour of the tyre is influenced by rotational speed. Centrifugal forces act on the tyre construction and cause growth of the contour. The rolling radius increases, but moreover the contour could get an undesired shape by non-uniform growth. To control the grow shape and to strengthen the tyre construction in order to cope with high rotational speeds, a cap ply is applied. The cap ply (usually embedded nylon) is applied with a controlled amount of pre-stress at the desired locations. The cap-ply compound partly consists of NR, because of its low heat build up properties.

Heat production

A tyre deforms by the longitudinal, axial and vertical force acting on it. Rotating the tyre at a high speed causes fast changes in global and local deformation of the tyre. As rubber mixtures have hysteresis, cyclic deformation of the rubber in tyres results in heat production. Therefore the heat production increases with rotational speed, causing large thermal loads on the rubber compounds. Too high thermal loads cause a degradation of rubber mixtures. To reduce the heat production and

to allow better cooling, the amount of rubber material must be minimized, and the compound must be tuned for less heat production, for example by altering the stiffness, increasing the cross link density and by using less heat producing elastomers. Side walls, for instance, are made from NR/BR blends. As for natural rubber, BR has low heat build up properties in comparison to the common SBR type elastomers. To improve the cooling of the material, the heat transport can be increased by improving the heat conductivity and capacity of the several compounds. The heat stability of rubber compounds can be influenced by the use of anti-degradants and the type of vulcanisation system.

Uniformity

The uniformity of the tyre gets more important when the rotational velocity increases. A too large non-uniformity of the tyre has negative effects (vibrations, pulling, local heat build up) that will increase at high speeds. As a consequence this results in joints with some overlap. For high performance tyres non-uniformity is avoided by applying the nylon overhead as continuously wound jointless strip (cap strip). Advantage of this method is the possibility of changing the pre-stress and strip distribution during cap strip winding, offering further control of the tyre contour both at low and at high speeds.

Resistance to tearing

The friction forces at road level are transported through the tyre construction to the rim. During extreme cornering or braking the shear stresses in the several rubber compounds are considerable. The adhesion between the several compounds and materials, and the cohesion of the separate materials must resist the shear stresses. To investigate the resistance to tearing, driving tests are performed under extreme conditions at the N.rburgring. The tyres are subjected to much higher loads than in typical road use, providing a valuable means of testing tearing resistance in extreme situations. In this way the resistance to tearing during road use is guaranteed. Besides the handling and tearing tests at the racetrack several indoor and outdoor tests are performed to investigate safety and durability of the tyre, like endurance tests, aquaplaning tests and high-speed tests. In this way maximum effort is put in assuring the performance of the high-speed tyre, a crucial part of high performance cars.