CONDITION

FIELD OF THE INVENTION

THIS INVENTION relates to a method of maintaining a pneumatic tyre in its optimum operating condition.

BACKGROUND TO THE INVENTION

A pneumatic tyre, when in use and inflated to its optimum operating pressure, has the configuration shown in Figure 1 . The flat area is referred to as the tyre's footprint or patch and is the area which is in contact with the road surface. The radius of the tyre at the footprint is r which is less than the radius R of the part of the tyre which is not in the footprint. Each revolution of the tyre results in each part of the tyre's casing flexing twice, once as it enters the footprint and once as it leaves the footprint. Flexing occurs because, at the entrance to and exit from the footprint, the shape of the casing changes. The designed strength of the casing enables it to accommodate the flexing which occurs when the tyre is at its optimum operating pressure.

When a tyre is under-inflated for the subsisting operating conditions, it adopts the configuration shown in Figure 2. The footprint area increases. Also, the degree of flexing to which the casing is subjected increases as the difference between rx and R is greater when the tyre is under-inflated. The change in radius from R to rx and back to R imposes on the casing a degree of flexing which is greater than that for which the tyre is designed. This results in tyre fatigue which can eventually lead to failure of the casing's walls. Heating also occurs as a result of excessive flexing.

If the tyre is over-inflated, as shown in Figure 3, the footprint area decreases as does the flexing of the tyre at the entrance to and exit from the footprint. The difference between R and ry is smaller than the difference between R and r in Figure 1 .

There is a temptation on the part of tyre users to over-inflate their tyres to achieve better fuel consumption. This reduces any possibility of damage due to flexing of the tyre.

However, the tyre is running on a narrow central zone instead of on the entire profile. Wear on this central section exceeds that of the outer parts of the profile and the central part becomes "bald" before the outer parts of the profile are worn down. The small footprint also increases the possibility of skidding.

An additional problem is that with a small footprint the tyre is less able to distort in the event that the tyre hits a rock or other debris on the road surface. As it cannot readily distort to ride over the debris, the prospect of the force exerted on the tyre by the debris causing it to puncture increases.

Tyre manufacturers specify the pressures at which their tyres should be operated and these pressures are selected to achieve the optimum footprint size. It is then left to the users to adjust tyre pressures to the manufacturer's specifications. In the practical situation of the trucking industry, tyres are rarely operated at all times at optimum pressure.

Tyre pressure varies in use for a number of reasons. The first is the temperature variation that is inevitable from cold start up to hot running. As a cold tyre heats up there is an increase in tyre pressure. The ambient temperature also has an influence on tyre temperature as does the extended use of the brakes as the truck travels down an incline.

The load on the truck also has an effect on tyre pressure which increases as the truck is loaded. Usually the tyres are pumped to the specified operating pressure after the truck is loaded. However, if the truck is partly off-loaded at different destinations, and is then fully unloaded without deflating, the tyre pressure becomes greater than is needed for the load then being carried. An additional factor is that tyre pressure gauges at filling stations and depots are not necessary accurate. The pressure shown on the gauge as being optimum tyre pressure may be not in fact be that pressure but may be higher or lower than optimum.

To overcome the excessive tyre wear and damage that can result from improperly inflated tyres it has been proposed to install tyre inflating and deflating systems on trucks. These monitor tyre pressure and then pump air to, or allow air to escape from, the tyre to maintain it at the operating pressure specified by the manufacturer. The operating pressure selected by the manufacturer is itself a compromise intended to take into account variations in loading and in tyre temperature during use. It is not necessarily the pressure that is ideal for the subsisting conditions but is the best that can be achieved.

The present invention provides a method of maintaining a pneumatic tyre in its optimum operating condition.

BRIEF DESCRIPTION OF THE INVENTION

According to the present invention there is provided a method of maintaining a pneumatic tyre in its optimum operating condition which comprises storing the diameter of an accurately inflated tyre, measuring a distance travelled by the tyre, counting the number of revolutions made by the tyre in travelling that distance, calculating the diameter of the tyre from the distance travelled and the number of revolutions, comparing the diameter so calculated with the stored diameter of an accurately inflated tyre to determine any difference between the two diameters, and deflating or inflating the tyre to conform its diameter to that of an accurately inflated tyre.

The diameter stored and the diameter calculated are both the vertical diameters from the footprint through the axis of the tyre to the top dead centre point of the tyre.

The tyre diameter is preferably calculated at predetermined distance intervals which could be, for example every five kilometres. Tyre inflation or deflation can be undertaken only when the percentage difference between stored tyre diameter and calculated tyre diameter exceeds a predetermined percentage of stored diameter. It is also possible, but less desirable, to calculate the tyre's diameter at specific time intervals. Time is, however, less of an indication of vehicle usage than distance as the vehicle may have been stationary. Time will still elapse but not distance.

DETAILED DESCRIPTION OF THE INVENTION

A change in the pressure in a tyre results, as discussed above, in a change in its shape. This change in shape causes a small variation in the diameter of the tyre which in turn results in a small change in its circumference. A tyre as shown in Figure 3 has a larger circumference than a tyre as shown in Figure 1 . Conversely, the tyre shown in Figure 2 has a smaller circumference than the tyre when it is as shown in Figure 1 .

If the distance travelled by the tyre is measured and the number of revolutions made is counted then the diameter of the tyre can be calculated from the simple formula: Diameter = Distance travelled

Pi x number of revolutions

When the tyre is in the correctly inflated condition shown in Figure 1 it has a predetermined diameter which is r + R. If this diameter is maintained it follows that the footprint size and flexing of the tyre are in accordance with the manufacturer's specifications.

The distance that the truck travels can readily be calculated from the GPS system carried by the truck. Available GPS systems update the information on position between once a second and five times per second. To obtain an accurate measure of distance travelled it is possible to record multiple positions over a distance such as 10 kilometers. If the distance chosen is sufficiently great, the GPS error can be reduced to less than 0.5%.

The rate of rotation of the wheel can be counted using any one of a number of commercially available systems. Rotary encoders, accelerometers or gyros can be used for this purpose. The number of revolutions made by a tyre over a significant distance, again say 10 kilometres, reduces the inherent error of all these devices down to an acceptably small value such as 0.5%.

It will be understood that the distance travelled by an accurately inflated tyre as shown in Figure 1 whilst performing a specific number of revolutions can be calculated from the data on diameter at correct pressure which the manufacturer supplies.

Tyres of diameters which differ from the diameter of the tyre of Figure 1 will travel a greater distance or a less distance when rotated the same number of times as the tyre of Figure 1 . The distance travelled by the vehicle as determined by the GPS readings, and the number of revolutions of the wheel in travelling that distance, enables the actual diameter of the tyre to be calculated. If the diameter so calculated differs from the diameter of the accurately inflated tyre, this indicates over or under inflation. Inflation or deflation of the tyre is then undertaken to bring the calculated diameter of the tyre into conformity with the diameter of the property inflated tyre of Figure 1 . When the calculated diameter conforms to the diameter of the correctly inflated tyre, the tyre is correctly inflated for the subsisting conditions. It follows from this that the footprint of the tyre will be that which the manufacturer's specifications indicate provides the best operating condition.

Using cumulative figures of distance travelled and number of revolutions reduces the errors inherent in GPS positioning devices and in revolution counters. The tyre pressure can be adjusted as frequently as is deemed to be desirable. Frequency of adjustment can be predetermined and embodied into the controlling algorithm. Alternatively the algorithm can detect that the diameter is more than a predetermined amount different to the optimum operating diameter and institute inflation or deflation in response.