From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical analysis of factors contributing to automobile tire wear.
| Identifier | ExFiles\Box 45\2\ Scan075 | |
| Date | 1st December 1927 guessed | |
| 2. ROAD SURFACES.—The kind of roads over which tires are run has a large bearing on the rate of wear. The old dirt road which was fairly soft did not cause much wear, but improved roads with various gritty surfaces for traction purposes are sometimes quite hard on tires. Some improved roads provide long wear, but others that look just about as good will wear a tire 56 times as fast. In some places customers are satisfied with low mileage where they are used to difficult operating conditions, and in other cases the customer expects extremely high mileage without realising that the operating conditions have changed. The same types of roads show a difference in abrasiveness. For example, some tar-bound macadams are covered with sharp-edged gravel which is held in place by the tar. With this wide variation in roads, it is hard to classify which type of pavement is better than another. As a rule, paved roads are less abrasive than gravel roads. Gravel roads made from crushed rock are more abrasive than those from washed river gravel, because one type presents sharp edges and corners, and the other round. Highly-crowned roads have a tendency to “roll” tires to one side. The car shows a tendency to run toward the curb or ditch—more constant steering is required. Naturally, the rate of wear is increased—more so if the “crowned” road is rough. There is more abrasiveness, more pull on the tires and more wear. In some parts of the country abrasive roads are heavily travelled in the centre of the road; consequently, the tires on the right side wear out faster than those on the left-hand side. In addition to this, there are roads which have been improved, but are breaking up. Broken concrete, rough macadams and choppy gravel roads all cause excessive tire wear. 3. STARTING AND STOPPING.—With present traffic congestion in suburban as well as downtown sections, it has been necessary to instal boulevard stops and safety lights at street intersections. This has increased starting and stopping many times over, and naturally has a large bearing on the rate of tread wear. In recent tests a car was driven at 35 miles an hour, stopped every quarter of a mile without sliding the wheels, and then accelerated to 35 miles an hour again, wore one-half the tread design away in 108 miles. The same car, driven at the same rate of speed, but stopped every mile, wore one-half the tread away in 3,100 miles. This indicates conclusively the serious effect of starting and stopping. Added to this is the possibility of the present-day automobile to jump from a standing start to 30 more miles per hour in a few seconds, and of four wheel brakes bringing the car to a dead stop in half the distance that it formerly took. Even two wheel brakes are more powerful than ever before, and in this case to stop the car in a short distance like four wheel brakes it is often necessary to cause sliding as frequently, and distribute the wear more evenly; because of quick stops, the tread rubber is worn away rapidly in either case. 4. INFLATION AND LOADS.—Proper inflation and loading are so easily neglected that they are still very common tire abuses, in spite of the fact that much educational work has been done on this subject. Under-inflation allows the tire to squash out and gives wider wearing surface to the tire, thereby creating more road friction. This will cause fast wear. In addition to this, under-inflation may break the internal carcass by the excessive strain on the strands and cords. It is also liable to cause a rolling action of the tire. Two sets of tires, one run at a recommended inflation and one 6 lbs. under-inflated, show a difference in mileage of 25%. Overloading is much the same as under-inflation. It results in the air pressure being higher than necessary to carry the given load. It forces the tire out, increases the contact area and the pressure on the road, thus causing more friction and faster wear. Wheel Chatter, or a bouncing action of the wheels over a rough stretch of road, comes from over-inflation. This causes the tires to leave the ground, spin, then take hold again, thus grinding off the tread rubber. On over-inflated tires, the car has a tendency to weave on the road; that is, the series of small bounces caused by the tire being too hard to absorb the small inequalities of the road, thus the car dribbles from one direction to another, causing excessive abrasion, and naturally fast wear. Actual tests have proven that a tire over-inflated 10 lbs. will give only half the mileage it would have given had it been properly inflated. 5. TEMPERATURE.—One of the seasonal factors causing variation in tire wear is temperature. In cool, wet weather, tires show very little wear, but in hot, dry weather the rate of wear is many times greater. Of course, this temperature is always more or less existing in warmer climates, but in the Northern part of the country tires give an average of 60% more wear when operated on the same car than in Southern States like Florida or Texas. In order to appreciate the extreme changes in tire wear that are produced by changes in temperature alone, study the chart under subject No. 1, which ties up temperature with speed. This all has reference to atmospheric temperature, but the actual temperature of the road travelled is of major importance; as the temperature increases, the abrasive action also increases. 6. SMALLER DIAMETER WHEELS.—With the inception of smaller diameter wheels comes more rapid acceleration, quicker braking, and higher speed on curves. In addition to this, a smaller diameter balloon makes more revolutions in a mile than a larger one. In fact, a 28 x 4.25, for example, is 9½% smaller in circumference than a 31 x 5.25, but the actual difference in mileage under the same conditions is 22%. With smaller diameter wheels, there is more action per tread unit as it makes and breaks contact with the road, and there is naturally more scuffing and bound to be faster wear to offset the other advantages. 7. BRAKE ADJUSTMENTS.—This is a mighty important factor, as it has been found upon analysis of a given number of automobiles that 90% have the brakes very poorly adjusted and the balance is not perfect. The ordinary means of adjusting brakes by jacking up the car and running the wheels, and then stopping them, is not accurate, as the eye alone is the gauge, and may not catch the exactness of the stopping. Again, driving the car over a garage floor, and locking the wheels to see which one slides first, is not a good test, as the wheels may be locked instan- | ||