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 bulletin detailing the various factors that influence tyre wear.
| Identifier | ExFiles\Box 158\1\ scan0069 | |
| Date | 11th October 1935 | |
| The Washington State College Experiment Station studied the effect on tyre wear of speed and temperature in the summers of 1924 and 1925. The final results of their tests are shown in the chart below :— Atmospheric Temperature. | 20 M.P.H. | 30 M.P.H. | 40 M.P.H. ---|---|---|--- 40 degrees | 100% (normal) | 108% | 150% 60 degrees | 191% performance | 217% | 275% 80 degrees | ... | 317% | ... | 450% 100 degrees | ... | 491% | 558% | 717% Note that in every case independent of temperature change a very great increase in the rate of wear occurs with an increase of speed above 30 miles per hour. Keeping in mind that a 40-mile per hour average is no longer unusual on long trips, it is easy to account for the short life of rear tyres that have to force 3,000 to 4,000 lb. cars along at that or higher speeds. 2. TEMPERATURE AND WEATHER.—The chart under the preceding subject illustrates that tread wear is faster as the temperature goes up. In fact, the figures show that temperature has an even greater effect than speed on the rate of tread wear. Rubber softens as it gets hot and as it softens, the tread is more susceptible to abrasion. Moisture on the road or the tyre cools and stiffens the tread, so in wet weather tread wear is slower than in dry. Consequently, in some parts of the country having hot, dry climates, tyres are bound to average much lower mileages than where cool and wet weather prevail for long periods of the year. This faster wear due to temperature and weather alone may cut the normal mileage as much as 60%. When the effect of high speed and other tread wear factors are added, it is evident that some car owners are bound to have a high cost per tyre mile. 3. ROAD SURFACES ARE TOPOGRAPHY.—The kind of roads over which tyres are run has a large bearing on the rate of tread wear. A tyre which is capable of twenty thousand miles or more on certain kinds of roads might give only four or five thousand miles on other roads. In fact, in some instances this variation has been known to run much higher. The old dirt road, which was fairly soft, did not cause much wear. But improved or paved roads with gritty surfaces for traction purposes are frequently hard on tyres. As a rule, paved roads are less abrasive than gravel roads. Gravel roads made from crushed rock are more abrasive than those made from washed river gravel, because one type presents sharp edges and the other round. The same types of road even show a difference in abrasiveness. Broken concrete, rough macadam and choppy roads all cause excessive tyre wear. In some places, the inhabitants are satisfied with low mileage as they are used to the difficult operating conditions that exist, but customers who are accustomed to extremely high mileage in other parts of the country will be disappointed when operating their cars under different conditions because they do not realize that the road surfaces have changed so greatly. In addition to this, improved roads, instead of making longer tyre wear, have really had a tendency to shorten it because of the fact that they permit high speed for long periods of time. Highly crowned roads have a tendency to “roll” tyres 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, even more so if the “crowned” road is rough. In some parts of the country, abrasive roads are heavily travelled in the centre; consequently, the tyres on the right side of the car wear out faster than those on the left, due to the constant turning out for other vehicles. Hilly roads cause fast wear because of the extra traction needed to lift the car over a hill, and then hold it back on the descent. The effect of hilly roads on tyre wear might be compared to starting and stopping, as more horse power is transmitted through the tyres. In fact, experiments have shown that hilly roads wear tyres twice as fast as level ones. 4. 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 a recent test, a car driven at the rate of 35 miles per hour, was brought to a dead stop every quarter of a mile without sliding the wheels, but as quickly as possible, and then immediately accelerated to 35 miles an hour again. One-half of the tread design was worn away in 108 miles. The rush of traffic at busy hours, makes quick stops and quick starts very common, so that the possibilities of fast wear are very great. Added to this is the ability of the present day automobile to jump from a standing start to 30 or more miles per hour in a few seconds, and with four-wheel brakes, to come to a dead stop in half the distance that it formerly took. Even two-wheel brakes are more powerful and to stop the car in a short distance like four-wheel brakes, it is often necessary to slide the wheels. Four-wheel brakes do not slide as frequently, and distribute the wear more evenly, but because of quick, or many stops, the tread rubber is worn rapidly in either case. 5. INFLATION AND LOADS.—Proper inflation and loading is so easily neglected that it is still a very common tyre abuse in spite of the fact that much educational work has been done on this subject. Under-inflation allows the tyre to squash out and gives wider wearing surface to the tyre, naturally creating more road friction. This will bring on fast wear. An under-inflated tyre flexes excessively, and this increases the tyre heat. Heat, as previously explained, increases tyre wear, and so under-inflation brings on fast wear in various ways. Two sets of tyres, one run at recommended inflation and one six pounds under-inflated, have shown a difference in mileage of 25%. With over-inflation, the air pressure is too great for the load carried, so that the tyre bounces or chatters over a rough stretch of road, which causes the tyres to leave the ground, spin, and then take hold again, thus grinding off the tread rubber. | ||