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).
Article discussing the Pneugrippa process for improving tyre grip and mileage.
| Identifier | ExFiles\Box 136\4\ scan0131 | |
| Date | 9th February 1937 | |
| The Motor 56 February 9, 1937. Pneugrippa-ing New Tyres Better Road Grip and Increased Tyre Mileage Amongst Important Claims Made for this Process ALTHOUGH many people seem to think that the Pneugrippa process is for old or rather well-used tyres, this is actually only a secondary function. At the present time about 85 per cent. of the covers treated are new or slightly used; throughout their life such covers can be Pneugrippa'd three times before all the useful depth of rubber is worn away. Pneugrippa-ing is the treatment by means of which a series of transverse grooves is made in the cover, a special cutter being used for the purpose. Each cover receives about 330 cuts, 7 mm. apart and 3 mm. in depth. [Diagram showing tyre tread deformation] ROLLING (or at rest) ACCELERATING BRAKING DIRECTION OF ROTATION -> Movements of the Pneugrippa'd tread under various conditions of travel. Thus a series of sharp square-edged bars is formed on the tread and, although it may sound rather a drastic treatment, it has been found in practice to give a definite increase in the resistance of the tyre to skid, with a consequent improvement in braking. A marked improvement in tyre mileage is often obtained, which may seem strange in view of the proportion of tread rubber which is wasted in cutting the grooves. The answer is that tread wear is frequently increased by the abrasion which occurs during braking, if the wheels become locked, or when acceleration is fierce enough to cause wheelspin. If improved adhesion can obviate these sources of wear it may, on the balance, extend the life of a tyre. Much will, of course, depend upon the way in which a car is driven. Tests carried out by Tecalemit, Ltd. (sponsors of the Pneugrippa system) have shown a marked improvement in braking, whilst in view of the fact that the sharp square edges of the new-cut treads bite through a greasy road surface, braking power can be more freely employed. As giving evidence of the advantages of this treatment of the covers, suffice to say that many operators of big hire fleets of cars and heavy vehicles now treat all their tyres about three times during their period of service. Further evidence is provided in the fact that many racing car tyres are similarly treated, particularly on the Continent, in some of the most arduous road races where real surface grip is absolutely essential. This is in spite of the fact that these vehicles are designed to give the utmost stability and road holding. In England racing drivers have also made use of this treatment, and at Shelsley Walsh the majority of the competitors took advantage of the machine which was provided at the foot of the hill. The depth of 3 mm. has been found the best for all-round purposes; were it greater, heel and toe wear would occur. Sufficient flexibility is given to ensure a good grip on all surfaces; at the same time no grit or mud can become embodied in the slots to impair their efficiency as a pronounced flicking action which takes place as the covers rotate and contact with the road surface drives out all such foreign matter. It also assists in lowering the running temperature of the cover. This depth, combined with 7 mm. spacings, has been found to provide a tyre adhesion value which is greater than the actual load thereon, i.e., the force necessary to cause skidding is greater than the weight of the car. This should help to prevent wheelspin and assist in the transmission of every ounce of tractive effort in actual vehicle movement. Pneugrippa'd tyres, particularly at this time of the year, promote a high degree of driving confidence on all road surfaces and under all weather conditions. On ice, for example, whilst it is impossible to obviate all risk of skidding, the Pneugrippa'd tyre does give the driver an opportunity of correcting it instead of having to let the car get into an uncontrollable slide. [Image of a Pneugrippa'd tyre] The Pneugrippa process consists of a series of cuts in the tread, 3 mm. deep and 7 mm. apart. Transverse grooves, at right angles to the plane in which the wheel rotates, are obviously well adapted to reduce wheelspin, but their effectiveness against skidding may not be quite so clear. The point is that during a side-slip the wheel is also moving forwards so that the resultant line of motion is at an angle to the grooves. They also provide extra paths of escape for the muddy film which may otherwise be trapped between the road and the tread. There is no need to remove the tyres from the wheels for the purpose of Pneugrippa-ing them. Special machines are installed at many big garages and service stations throughout the country. All five covers can be treated within about half an hour and the cost is 3s. per tyre for cars up to 12 h.p., 3s. 6d. for 12 h.p.-15 h.p., and 4s. per tyre for those over 16 h.p. If no local service station has a machine installed, Tecalemit, Ltd., of Great West Road, Brentford, should be consulted. A Year's Service in a Few Days SOME of the most remarkable machines in modern motor factories are those employed for testing individual components and assemblies. These machines compress the equivalent of a year of service into a few days, and by applying destruction tests to representative samples of a consignment the engineer can forecast the behaviour of the whole under service conditions. A striking example are the presses used at Dagenham for testing the road springs fitted to Ford products. As each batch of springs is received from the assembly shops examples are selected at random and placed on one of the testing presses. The machine deflects the spring 90 times each minute, running continuously for 40 hours before the test is completed. The severity of the test is increased by the fact that, at each of the strokes, the spring is depressed to a far greater extent than it will be in service. In the case of the £100 Ford saloon, for instance, the spring is deflected 2 ins. at each stroke and a maximum load of 800 lb. is applied. Such a test is equivalent to many miles of running with shock absorbers out of adjustment or with the vehicle seriously overloaded, conditions which place entirely unfair strains on the suspension. This ingenious machine can also be used for rigorous tests of shock absorbers. | ||