Energy tires : How to improve tire design for rolling resistance

Energy tires has become very important becuase with the goverment requirements for better fuel consumtion. The rolling resistance is performance charactoristic and energy tires have be low rolling resistance. The general ideas change to improve tires to energy tires but will certainly affect other properties as well , for better or for worse.

Effect of Rubber Material

ranked various rubber material regarding their effect on energy tire (low rolling resistance) . The rolling resistance index was as the table

Rubber material	rolling resistance index
Natural Rubber	100
Hight cis-BR	94
Solution SBR	93
Emulsion SBR	90
IIR	73

* high rolling resistance index indicated that low rolling resistance coefficient.

Energy Tires Design

Energy tires can be radial construction that it have better rolling resistance than bias construction becouase the radial carcass is more flexible. Energy tires can reduces the gauge thickness of tread to reduce energy dissipation to lead to poorer rolling resistance.

Loading

Energy tires rolling resistance improves when tire’s load is decreased

Inflation Pressure high or low?

To design energy tires to operate with a higher air pressure.

Vihicle Speed.

Energy tires rolling resistance improves when vehicle speed is decreased.

Running Temperature.

Tire’s running temperature has effect on impart rolling resistance. Lower running

temperature may impart more rolling resistance.

Rubber Seals : Stress Relaxation Model

Rubber material is classified as viscoelastic material that mechanical behavior depends on the time and temperature. The decreasing of stress in the rubber material while under the constant deformation, is one of the most important characteristics. Because rubber material is used as a rubber seals to prevent leakage of fluids such as rubber gaskets O-Ring, etc. Stress decreasing through the critical value during operation resulting in the leakage of fluid. The decreasing of stress in rubber material, while under the constant deformation is called stress relaxation of the rubber material.

Materials model

Linear viscoelastic model was developed to describe the behavior of rubber material such as Maxwell model , Kelvin-Voigt model as Figure 1.

Fig.1 Linear viscoelastic model

Equations of motion of Kelvin-Voigt model and Maxwell model is shown in equation (1) and (2), respectively.

Solution of equation (1) and (2) can be expressed as Figure 2, which showed that Maxwell's model can simulate the behavior of stress relaxation, while the Kelvin-Voigt model for the simulation is not consistent with the behavior of stress relaxation. Thus, in the modeling of stress relaxation behavior of rubber material is used Maxwell's model .

Fig.2 Response of Maxwell’s Model and Kelvin-Voigt Model

Generally Maxwell's model can not easily predict the behavior of stress relaxation of rubber material which is very accurate. It has developed the Generalized Maxwell’s model, which consists of a maxwell’s model to a simple parallel, as shown in Figure 3.

Fig.3 Generalized Maxwell’s model

Solution of the Generalized Maxwell’s model is described in a pronie’s series is shown in Equation 3.

We can describe the solution of the Generalized Maxwell’s model in the form of a relaxation of shear modulus in the rubber material, as shown in equation (4).

When

G0 is the shear modulus at time t = 0.

gi is a coefficient of pronie’ series order i

ti is the relaxation time order i.

To develop a rubber compound to analyze the behavior of rubber material or rubber products. Stress relaxation testing of rubber material in order to determine coefficient of Pronie series, this is what the design engineer will be doing before design rubber seals etc.

Relate topic

Rubber seals : Seal Friction

Stress relaxation

Rubber seals : Analyze the effectiveness of the rubber seals using Finite element method.

How to improve rubber process : Reducing Backrinding in compression moulding

In Compression moulding process , when the expansion of the cure rubber is concentrated in a limited area and is greater than the ultimate cure elongation of the compound the mould is opened after cure , the phenomenon is called Backrinding The resulting in a rupture at the part line (o-ring ,rubber seals, solid tire,rubber mounts ,rubber gaskets etc.) . There are some ideas which may help prevent or eliminate backrinding but these general ideas may not work in all specific situation.

1. Mould Temperature : Curing a compound at a lower temperature for a longer period of time will allow more rubber to exit the mould from thermal expansion before scorch and cure are reached. The rubber compound will most likely have better tear resistance at lower temperature.
2. Mould Design : Backrinding can be severe in the moulding of lage, bulky articles with a lage amount of mass .In design the rubber mould, if the ration of surface area to volume can be increased then backrinding may be less of a problem. In design the mould place the part line in a noncritical area the does not affect product performance .
3. Cooling the mould to lower the pressure that will be released on demoulding is another option to consider to reduce or eliminate backrinding.
4. Preheat the preform before placing it in the mould
5. Extending scorch time through compound modifications should favor reduced backrinding becouse it provides more time for rubber to exit mould cavity before curing occur.
6. Improving tear resistance of rubber compound (hot tear resistance)

Warning : Change to reduce backrinding will certainly effect other properties.