EQUAL TYRE PRESSURES.

New Zealand Herald, Volume LXIV, Issue 19762, 8 October 1927, Page 12 (Supplement)

EQUAL TYRE PRESSURES.

BALANCING THE AIR. Uneven braking, skidding, and undue stress on the differential results if one back tyre is substantially higher in pressure than the other. It is obvious that the friction of a soft and a hard driving wheel "ivill vary greatly. Apparent also is it that a soft tyre reduces the effective diameter j of the driving wheel by perhaps half an inch. This means that one back wheel is running faster than the other on a dead level straight. The differential thus gets no rest. It is wise to develop the habit of testing tyre pressures every Friday. This will ensure that the week-end mileage is run oil with the tyres in the best possible condition. Front tyre pressures should generally be a few pounds lighten than the rear, but they should be themselves equal. Steering wobble is usually attributed to incorrect or unequal front tyre pressures. A British, accessory maker is retailing a Ky re pressure equaliser. The device consists simply of a length of reinforced rubber tubing with connections at tho ends for coupling to the valves of two tyres; and a connection at the middle to which either the pump or a pressure gauge may be attached. When the connections have been made with the tyres two small screws on these connections the Schrader valves, the two tyres being are operated to depress the plungers of then in free communication with one nnother; the air in them cannot escape, however, as the equaliser is fitted with a tyre valve at the centre. When the connections have been made in this way both tyres automatically come to exactly the same inflation pressure. This prcs-

sure can then be measured Joy a gauge, and if it be too low a pump can be employed to inflate both tyres simultaneously and to exactly the same extent. MOTOR «■ TEADE ITEMS. The Nash plants are maintaining an output of from 700 to 80Q cars a day. Chevrolet production for the first months of 1927 was 697,318 units, as compared with 728/597 for the full year of 1926. A conference of Austin, distributors was held in Sydney on September 20. Mr. G. H. Scott, of Wellington, was the New Zealand representative. The Falcon Knight, a new American light six, is on the Australian market. It is the first low-priced sleeve valve car, the tourer being listed at £345 in Sydney. The Sunbeam Company announces that the 16 h.p., 20 h.p., 25 h.p., and three litre six-cylinder models and the 30 h.p. and 35 h.p. straight eights will be con- j tinued for 1928. No radical change will be made in the Rolls-Royce 40-50 h.p and 20 h.p. -cars to be exhibited at the British Olvmpia within a few weeks. The Rolls-Royce Company lias announced that its 1928 price for the 20 h.p. chassis will be £llßs, and that the W-50 h.p chassis will he £IBSO, the same as for the current model. The Corporation omnibuses, a suburban bus service, and several taxi companies are testing out a new battery electrolyte, introdaced into Auckland three weeks ago by Mr. E. B. McKay, who recently visited the Dominion in charge of the New South "Wales tennis team. The preparation has a sulphuric acid base, and is invention of an Australian motorist. The chief Claim for the mixture is that it prolongs the life of batteries by reducing snlphation. AITSWEES TO COSKESFONDE2TTS. X.Y.Z., Morrinsville. —The gear ratio is the number of revolutions made by the engine for every revolution of the road wheels when top gear is engaged. In the majority of cars now in production the ratios are such that the engine makes between four and five revolutions to every revolution of the wheels. There are two or three cases in which the engine runs slower than this, and quite a number of instances in which ratios exceed five to one. This tendency to run the engine faster with relation to the road wheels is the natural r&sult of the development of high speed power plants. Two cars may of course both have top gear ratios of to 1 and yet one engine will have made more revolutions after a given distance is . travelled. This would be duo to different tyre sizes. Manufacturers differ greatly in their choice of second and low gear ratios. Secand gears are usually between 6 and 1 and 10 to 1, a popular ratio being about 8* to 1. Low gears are found as low as 25 to 1. but most are. between 14 to 1 and 18 to 1. ' - * Following are examples of the top and low gear ratios of seme popular 1927 model cars:—

Austin twelve, 5.1, 19.8: Austin seven, 4.9, 16. .Clvno, 11 h.p., 4.7, 16.5: Arm-strong-Siddley,' 14 h.p., 4.7, 17; Morris, 4.7, 15.2: Rover, 9 h.p., 5.2, 17.7; Singer, 8 h.p., 5. 17.1; Standard, 14 h.p.,. 4.6, 18.2; Sunbeam. 16 h.p., 5.3, 20.9; Talbot. 14 h.p., 5.8. 23.2; Vauxhall, 14 h.p., 4.5, 15.9; Fiat, 9 h.p., 6.1, 20; Buick, 23 h.p., 4.9. 14.9: Chevrolet, 3.8. 12.6: Chrysler, "60," 4.9, —; Dodse, 4.1, 17.4; Essex, 5.6. —; Ford, 3.6, 10: Hudsqr, 4.5, 11; Oldsmobile, 4.7, 14.5; Oakland, 4.7, 14.2; Packard eight, 4.7, 15.7; Whippet. 4.5, 14.7; Locomobile Junior, 5.1, —; WillvsKnight, 20 h.p., 5.1, 16.1. If the gear ratios and tyre size are known it is possible to calculate the engine revolutions for any distance. The. following table shows the revolutions made by standard tyre sizes in travelling one mile together with the revolutions which the engine would make for the same distance with various gear ratios: — Tyre Size r.p.m, Engine r.p.m. Ratio of 4 to I. 30in. r.72 2.fiRR 32in. ? 631 2.5 _ 33in. Gil 2.44-t Ratio of 41 to 1. — 30in. 072 n.OvM 3'2in. cm "3in. Oil 2.749 Ratio of 5 to I.— no in. r~2 s r.r.i * . ."Kin. r.ri] 3,158 33in. Oil 3.055