Energy tires : How to reduce rolling resistance (effect of speed)

The rolling resistance coefficient f generally increases with the speed V of the vehicle , at the beginning very slowly and then at an increased rate. The following expressions can be used

f = fo + KV

or

f = fo + KV^2

The value of fo and K must be measured on any particular tire . The speed at which the curve f(V) show a sharp bend upward is generally said to be the “critical speed of the tire” (Pic1) the critical speed is influenced by many parameter such as above that speed strong overheating take place; as most of increase of the rolling power is converted into heat. the increase of temperature can quickly cause the destruction of the tire itself ; the standing waves which propergate along the circumference of the tire from the contact zone are clearly visible. And it is of the factor which must be taken into account in the choice of the tire.

Pic1. Rolling resistance coefficient of tire

Energy Tires : Rolling Resistance

During the tire motion new rubber material enter continously into the contact zone and is deformed,to spring back to its initial shape when leaving it .To produce this deformation it is necessary to spend some energy which is not completely recovered at the end of contact zone due to the internal damping of the rubber material .

This energy dissipation is what causes rolling resistance. It is then clear that it increases with increasing deformation and mainly with decreasing elastic return.The distribution of contact pressure which at standstill was symmetrical with respect to the centre of the contact zone , become unsymmetrical when the tire is rolling and the resultant Fz move forword (Pic.1) producing a torque My = –Fz*dx whith respect to the rotation axis. Rolling resistance is due to this torque . For practical purposes , rolling resistance is usually expressed as

Fr = f*Fz

where the rolling resistance coefficient f must be determined experimentally. Coefficient f depends on many parameter as the tire travelling speed V , inflation pressure p , vertical force Fz ,the size of tire and contact zone ,the structure and rubber material of tire,working temperature and road conditions .(see to energy tire)

Pic. 1 Force in rolling tire

Rubber properties : Compressibility

Rubberlike material have very little compressibility compared to their shear flexibility. The numerical solution can be quite sensitive to the degree of compressibility for three-dimensional solid, plane strain, and axisymmetric analysis elements. In cases where the rubber material is highly confined (such as an rubber seals and rubber gaskets), the compressibility must be modeled correctly to obtain accurate results. In applications where the material is not highly confined, the degree of compressibility is typically not crucial; for example, it would be quite to assume that the rubber materials is fully incompressible : the volume of the material cannot change except for thermal expansion. Another class of rubberlike materials is elastomeric foams, which is elastic but very compressible.

We can assess the relative compressibility of a rubber material by the ratio of its initial bulk modulus, K, to its initial shear modulus, G. This ratio can also be expressed in terms of Poisson's ratio, , since

Table 1 represent some Poisson ratio

K/G	Poisson’s ratio
10	0.452
20	0.475
50	0.490
100	0.495
1,000	0.4995
10,000	0.49995

Since typical unfilled rubber material have K/G ratios in the range of 1,000 to 10,000 (Poisson’s ratio = 0.4995 to 0.49995) and filled rubber material have K/G ratios in the range of 50 to 200 (Poisson’s ratio = 0.490 to 0.497). If rubber material is modeled to imcompressible material it have K/G ratios in nearly infinity and Poisson’s ration = 0.5

Rubber mounts

Rubber mounts or anti vibration mounts can reduce the intensity , or more precisely the amplitude, of an externally imposed oscillation or vibration in two way

1. If the frequency of the oscillation is very different from the natural frequency of the system comprising the supported mass plus the rubber mounts support, the rubber material will not deform freely at this impose frequency ; we might indeed say that it is unwilling to vibration at an “unnatural” frequency
2. When the imposed frequency is very high , say 1000 Hz or more, the imposed oscillations are more or less damped out by the viscous element in the deformation of rubber material ; this effect is call “attenuation” and is particularly inportant with sound vibrations.

Pic rubber mounts sample

Relate Topic

The flat rubber mounts

carbon black on electrical resistivity of vulcanised rubber

Rubber material is of course commonly regarded and indeed widely used as an electrical insulating material and if insulation is what is required then rubber can be ideal material.It must be note however that by mixing the rubber material with certain types of carbon black ,this electrical resistivity can be enormously reduce ; Table A illustates the magnitude of this effect.

Table A Effect of various type of carbon black on electrical resistivity of vulcanised natural rubber : Data from Norman (1957)

Amount of Carbon black(phr)	log	volume	resistivity	,ohm-cm
	Channel black	Acetylene black	Oil-based furnance black	Gas-base furnace black	Thermal Black
0
10	15.5-15.8	10.3	14.5-16.0	15.0-15.5	-
20	9.5-13.5	6.5	3.5-15.0	14.5-15.2	15.0-15.5
30	7.2-8.8	3.4	3.0-14.0	10.5-15.0	13.0-15.0
50	5.5-7.2	0.4	2.3-5.0	4.0-8.0	10.0-15.0
85	5.2-6.2	0.2	0.5-3.0	2.6-4.3	8.0-14.5
140	-	-	-	1.8-2.8	5.5-7.0

When the resistivity is below about 10e7 – 10-e9 ohm-cm the rubber material becomes what is now called “anti static” , since it is able to conduct away static charges sufficiently rapidly to prevent any dangerous accumulation of charge.A further stage beyond this is “conductive” rubber with resistivity below 10e5 – 10e7 ohm-cm ,which will carry quite considerable currents.

It is important to note that the resistivity of an anti-static or conductive rubber is effected to a greater or less extent by deforming the rubber material and under unfavourable circumstances the resistivity may be so much increased that the anti-static properties are lost, an effect which is only slowly and pershap never completely recovable.

Another important point to bear in mind is that carbon blacks are commonly incorporated into rubber material in order to increase its strength and resistance to tearing and abrasion; indeed many of the rubber material suitable for engineering uses would be compounded in this way, with and inevitable reduction in their electrical resistivity. If therefore a really hight resistance is required the user should again make this point clear to the rubber manufacturer .

Of the nitrosamine in the condom. Risk or not?

Amount of nitrosamine with the highest set of teats is 10 micrograms per kilogram (or 10 parts in a billion parts), which means that the teat has a weight of 5 grams to allow the amount of nitrosamine is recognizes. maximum 0.05 mg, however, when heated chemicals that remain from the process vulcanized, animation can be changed to nitrosamine is as a means that sterile teat by means of a. Sterling slices in hot water could lead to increase the amount of nitrosamine compounds in the rubber. However, events like this are not occur with a condom because condoms as a product. Produced to be used only once. So the risk is caused by the substance nitrosamine is that there is behind. Process only Moreover, condoms are used with adults who are resistant. Nitrosamine is higher than the children up to about 100 times.However, for those who use condoms regularly, it might be the risk. Up as well. Therefore, I suggest using a condom with the amount of nitrosamine as low.

Why is the many application of natural rubber ?

Most natural rubber is used in tires (about 70% of the total production is natural rubber) because natural rubber has outstanding properties in terms of flexibility high mechanical properties and low heat buiding up during use. In addition to the properties of natural rubber and sticky stick (tack properties) makes a good tire assembly process to be easy. Therefore, natural rubber remains a key option in the production of tires. Especially truck tires, tire plane and tire forklifts etc.

In addition to tires . Many natural rubber to be used in the manufacture of rubber cushioning port (docking fender) and rubber cosapoan (bridge bearing) . Also has high mechanical properties and low creep under the force as well. In the same way natural rubber has been used in the manufacture of rubber to reduce vibration. (anti-vibration dampers) are due to these products to good mechanical properties and low heat buiding up accumulated during use.