Measuring and testing – Tire – tread or roadway
Reexamination Certificate
2001-04-23
2003-03-18
Williams, Hezron (Department: 2855)
Measuring and testing
Tire, tread or roadway
Reexamination Certificate
active
06532812
ABSTRACT:
The present invention is directed to a method for significantly improving tread wear of the tires used on all types of vehicles.
BACKGROUND OF THE INVENTION
Many improvements have been made over the years in durability of pneumatic tires used on cars, sport utility vehicles, trucks, trailers and other types of vehicles. While 20,000 miles was once considered excellent tire life for a passenger car, improved tire manufacturing and components have now extended this by a factor of two to three under normal conditions. “Normal conditions” include maintenance of recommended inflation pressures, regular rotation of tires, and driving principally on a relatively smooth road surface at moderate speeds. In the real world these conditions are rarely met and tires must be replaced far earlier than might otherwise be required. Of the conditions listed above, only the road surface is not under possible control of the driver. However, human nature being what it is, most drivers are lax about routine tire maintenance. Rotation is an occasional event, if done at all, and tire pressure checks are often left undone until it is obvious from tire appearance and tread indications that something is wrong. To further act against obtaining longer tread life, the inflation recommendations given by auto manufacturers often seem to be pulled out of thin air rather than based on their own product data publications. These pressures are usually found on a plate on a vehicle door frame or published in the Owner's Manual. Equal pressures are often recommended for all tires on a given vehicle regardless of the fact that in most instances front and rear tires carry significantly different loads. This condition is further exaggerated in front wheel drive vehicles. Strangely, even when they are inflated to the same pressures, front and rear tires are expected to perform equally despite these load differences.
Empirical studies have found that front and rear tires inflated to similar pressures show very different wear patterns on their treads after only a relatively few thousands of miles. Front tires usually show much more wear along the tread edges. This is a result of excessive tire deflection or deformation that takes place as the tire contacts the road surface. “Deflection” is the term used to measure the differences in radii from the center point to the top of the tire and the center of the tire to the road contact surface. Deflection is caused by the necessity for the tire to flatten as it contacts the road surface in order to distribute its load carrying capacity. This condition of excessive edge wear is often called “swiping” when tires wear unevenly across the tread due to under inflation.
In contrast to the usual pattern of front tire wear, rear tires frequently show greater wear in the center portion of the tread. In most instances this is due to over inflation. This can cause internal stresses within the tire, stresses which the tire is least designed to resist.
Under inflation in particular is destructive to tire life since the tire experiences much greater flexing and heat buildup during use. It must be remembered that at 60 mph the average tire rotates 14 times per second.
The set of conditions described above creates an undesirable environment for optimum tire life and performance. It is not the purpose of the present invention to recommend vehicle and tire size match-ups. The invention is based on the discovery that each tire size has its own optimum deflection which should remain constant, regardless of load carried. Stated otherwise, since deflection correlates directly with “footprint”, each tire should maintain a constant optimum footprint regardless of load carried.
SUMMARY OF THE INVENTION
The present invention offers a means of calculating optimum tire pressures to obtain maximum tire life and performance. It is based in part on correlating tire loading, tire pressure, and tire deflection in order to obtain optimum “footprints” for every combination of tire load and tire position on any vehicle. “Footprint”, also called “contact patch”, is the area of tread in contact with the road surface at any given instant. The method is useful not only for passenger cars but also for light and heavy trucks, trailers, or any vehicle which may be equipped with either single or dual wheels. It can accommodate any combination of axles, tire load, tire position and tire size. Rear wheels may be single or dual and the vehicle may have a plurality of rear axles. The method is readily applicable when replacement tires are of a different size than the original tires used.
The method involves several steps. The first step is to determine vehicle curb mass/weight distributed to the front and rear axles or any axle combinations. Curb mass/weight is the weight of a vehicle without occupants. This information is found within the extensive specification sheets published by vehicle manufacturers for every make and model they produce. Similar information for all nameplates can also be found in Motor Vehicle Manufacturers Specifications, published by the American Automobile Manufacturers Association. These publications are readily available in many libraries. The vehicle owner can provide the passenger and cargo weights and their location in the vehicle. Total operating dead loads and live loads and their distribution can then be determined. This will include curb mass/weight, occupants, and cargo. The percentage of this added weight to each axle is then assigned, again using manufacturers data; e.g., percentage distribution of weight in front and/or rear seats to each axle. Total weight is then easily converted to weight carried by each axle and on each individual tire. This information is part of that which is essential to derive optimum conditions of tire inflation for maximum tread life and performance. Optionally, the vehicle may be weighed on a scale to determine axle loads.
Extensive data sheets are also available for every size and type of tire produced. This data is available in trade publications of the Tire and Rim Manufacturers Association, Inc. (TRA), Copley, Ohio. Among the numerous parameters available in TRA publications is “grown tire diameter”. Grown diameter is tire diameter under designated inflation conditions and after a prescribed minimum amount of use. Any given tire size has a fixed section width, although this does not indicate effective tread width. Actual effective tread width should be measured on the particular tires in use at the time. The tread length in contact with the road surface can vary dramatically with the ratio of tire pressure to applied load. This then becomes the effective tread length in contact with the road surface under one condition of use. Effective tread length multiplied by effective tread width determines the footprint (or contact patch) area.
Critical to the present invention are empirically determined “tread length determining factors” which are key to determining optimum tire tread wear. These factors have been determined by extensive experimentation. The grown diameter is multiplied by the tread length factor to determine optimum tread length in contact with the road surface. The factors for both front and rear tires are dimensionless numbers within the range of about 0.190 to 0.210, most preferably in the range of about 0.200 to 0.205. The factor for front tires is normally somewhat smaller, being about 96% to 98% of that of the rear tires. Ideally the front tire factor will be about 0.200 and the rear tire factor about 0.205. The factor is smaller for the front tires due to the more demanding requirements, such as steering and a proportionately heavier load.
Tread width contacting the road surface must usually be determined by actual measurement of the tires being used. Multiplying tread width by calculated optimum tread length gives the area of tire contact with the surface or the tire's “footprint”. Finally, dividing the individual load on a tire by footprint area will give optimum inflation pressure. This method of optimizing deflection and corre
Allen Andre
Ambient, Inc.
Gehr Keith D.
Williams Hezron
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