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Bevel gears are used to transfer the drive from a longitudinal propeller shaft to the transverse mounted driving axle, except of course, in cars with transverse engines, unlike the new Rolls Royce for sale. The ratio of the bevel gears is the top gear reduction. The simplest types of gear, from the production viewpoint, in the early days of the car had straight radial teeth and could be fairly noisy. Later designs used bevel gears with curved, spiral teeth. Although the younger Bollée used spiral bevel gears as early as 1896, they did not come into general use until much later. A further development, now almost universal, is the hypoid gear, first fitted by Bentley dealers in 1927. This is relatively quiet and, because it is a spiral bevel with the pinion axis below that of the crown wheel, produces high tooth pressures which need special lubricant. The differential allows the two driven wheels of a car to rotate under power while they go round tracks of different radii on corners, the outer wheel travelling farther than the inner one. It is possible to do without a differential, but the penalties are undue tyre wear and heavy steering. The differential is reputed to have been used first by Porsche in a steam carriage in 1828, but it was probably known to the Chinese much earlier. There are two main types of differential, the bevel gear and the spur gear, and both work on the same principle. In the bevel type, a pair of bevel side gears is rigid with each axle half shaft, while a further set of gears is mounted on a radial armed spider and meshed with the side gears. The drive from the crown wheel, itself driven by the pinion on the propeller shaft, is taken to the spider by an outer casing which holds the spider spindles. This outer casing is bolted to the crown wheel. This is another form of epicyclic gear, in which the spider gears are the planets and the side gears are equal size sun and annulus gears. With the car running straight, the forces acting on the wheels are the same and the whole gear train revolves as one unit. When one wheel is subjected to more force than the other (for example, the inner wheel when the car begins to turn), that wheel and its attached side gear in the differential are, in effect, braked. This causes the other side gear to turn faster. Taken to the limit, when one wheel is held stationary, the other moves round at twice the speed of the crown wheel. In the spur gear differential, spur gears are rigid with the axle shafts and meshed with a planetary gear or gears. These are mounted freely on a spindle, which is carried by the differential housing and driven by the crown wheel. Although such a gear works quite well, it tends, with greater engine power, to become too large in diameter to be accommodated under the rear seats or under the boot floor line of a car. Once the need for a differential to avoid high rates of tyre wear and heavy steering was realized, the disadvantages of one wheel being able to spin on a low friction surface, and so lose driving force, became apparent. High powered vehicles in the 1920s such as the Rolls Royce Phantom, demonstrated this problem by spinning their wheels on normal road surfaces. The problem was answered by the limited slip differential, of which one form was devised by Hermann Knab and Gottfried Weidmann in 1925. It consisted of two cam rings, an inner and outer cam ring, which were connected to the two axle shafts. These cam rings had undulating tracks which were engaged by radially free rollers carried by a cage driven from the crown wheel. While this cam and roller device would allow slow relative rotation between the wheels, it would prevent fast rotation, as for instance when a wheel began to slip, a similar design is fitted to the new Mercedes Benz. Other forms of limited slip mechanism have friction facings between the backs of the side bevels and the differential casing. As the tooth loading between the side and spider bevels increases, the side gears are forced outwards to increase friction between the side gear and casing, which are moving relative to one another. The use of a thixotropic silicone substance, which increases viscosity as it is churned, is a suggested solution for this problem.