SIMPLIFIED CHASSIS DESIGN.
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Details of the Model 502F Fiat, with 10-15 h.p. Engine, Four Forward Speeds and Worm Drive.
MHE LATEST product from the won derful Pint works at Turin is a light chassis, designed especially for the carrying of 15-cwt. loads or 'gress loads, including the body, of 22 cwt. This chassis is partieularly suitable for serving as a basis of light, fast vans or commercial travellers' vehicles, providing ample accommodation for -samples, etc.
One of the most important points in the running.of a commercial vehicle is to prevent over-speeding when the vehicle is light, as this has a most deleterious effect on every part. In order that this shall be prevented in the new model, the Marelli magneto, with which it is fitted, embodies a neat centrifugal governing device, which, at an engine speed of 2,250 r.p.m. or a road speed of approximately 25 m.p.h. on top gear, earths the primary circuit and thus slows down the engine. A curious feature of the device, and one which is somewhat difficult to explain, is that • this cutting out of the ignition does not %use back-firing in the silencer.
In addition to this feature there is another which deserves special mention, and that is the controlling of the strength of the mixture in the carburetter. On the dashboard is a small lever, connected by Bowden cable to an enclosed rack, which rotates a screwed needle entering the orifice of the jet. When much power is required, or if the weather be cold, the needle is raised to its fullest extent ; when the engine is warm, and if the power required be little, as, for instance, in running along a level road, the needle may be lowered until the effective area of the jet orifice is reduced to the minimum. This control makes the carburetter so sensitive that it is quite an ordinary occurrence for the 15-ewt. vehicle to obtain 35 m.p.g.
The power unit is of notably neat construction. It is a four-cylinder monobloc with a detachable head, in Which is cast a neat easing for the high-tension leads. There is a cross-drive at the front to the centrifugal water pump at
the near side and the magneto at the off side, whilst the lour-bladed east aluminium fan, which is mounted on an eccentric spindle for adjustment, has link-belt drive. At the near side also are the dynamo and the starting motor, both of which are totally enclosed.
The bore and stroke are 65 ram. and 110 mm. respectively. The crankshaft iS supported on three white-metal-lined bearings, the big-ends also have bronze shells lined with white metal. Aluminium is employed for the pistons.
The lubricating system is of the drysump type, the submerged pinion pump being driven by skew gearing from the camshaft. The oil is sent first through an effective strainer, then through leads in the crankcase casting to the main bearings, from which it is conveyed through the drilled crankshaft to the big-ends, the pressure being regulated by a release valve. The engine and gearbox are formed together as a three-point-suapended unit, thus • aimiding straining through , frame flexion.
The clutch is enclosed in the casing and is of the .multiple steel-disc type with central spring. It can be inspected by removing the cover of the gearbox unit.
The arrangement of the gearbox is somewhat peculiar. It is what may be termed a box within fi box, and the inner box has a separate cover, which prevents oil being sprayed over the clutch. By removing this inner cover it is possible to take out the striker rods without • using any tools. An advantage of this system is that it gives a very rigid construction with short shafts.
Behind the inner box is the selector mechanism, which is of the simplest possible kind. The same may be said to apply to the connection between the change-speed control lever and the control lever on the gearbox. The layout of this can clearly be seen., in one of our illustrations. It is so arranged that free movement is allowed in the vertical direction, but when the change-speed lever is moved through the gate the rod carrying the striker arm is moved to the same 'degree. Pour forward speeds and reverse are provided; and the gears are of niche], steel, hardened and ground and running on chrome-nickel steel shafts supported on hall bearings.
From the back of the gearbox the drive is conveyed through a cardan shaft with two joints of the flexible-disc type to an overhead worm carried in a caststeel axle casing, the actual housing for the worm and differential being of cast aluminium. A filler cap at the rear gives the correct height for the lubricant.
Braking is effected through two sets
of shoes in each rear-wheel drum, each shoe taking up approximately a quarter of the total area. Opposing shoes are used for the hand brake and foot brake respectively. They are operated through -the medium of Cables, and, as showing the thoroughness of Fiat workmanship and design, we noted that each of the cable loop 8 has its brass -wearing strip, so that the cable is not cut through by
friction at the points where it is secured to the levers.
The adjustment of those brakes is simplicity itself. In the case of the band brake the two cables are brought round and secured to sleeves sliding on a-cross-shaft. These sleeves are screwed externally, one with a right-band thread and the other with a left-hand thread, and joined by a third sleeve, which is screwed internally in the same manner, so that turning the external sleeve has the effect of making the sliding sleeves move away from or towards each other, according to tbe sense of rotation, and
thus tautening or loosening the cable as required.
In the case of the foot brake the cable is passed right through the tube which carries the lever and thence over a small pulley carried on a spindle passing through the lever. Into the end of the tube is screwed a small hand-wheel, the cable passing through this also. It will clearly be seen that by unscrewing this hand-wheel the effective length of the cable is decreased ; consequently, the shoes are adjusted closer to the drums.
The wheels are of the detachable steel-disc type. carrying 820 mm. by 120 mm. MiChelin pneumatics.
The front axle is of the reversed Elliott type, and the steering is by worm and complete wheel.
Lubrication throughout is on the grease-gun principle.
Careful attention has been paid to springing; thus the half-elliptics at the rear are particularly long, giving easy riding.
The chief feature which is noticeable in the chassis is the lack of complication, the whole chassis being of clean and workmanlike design. Some of the smaller details are novel and effective. For instance, in the case of the adjustment of the chain drive to the dynamo, this unit is secured in its housing by means of a bolt having two different threads, one of which is for the nut locking the dynamo in position, whilst the other engages with a rack cut in the body of the dynamo. The shaft of the last-named is slightly eccentric relatively to the body and, consequently, rotation of the body, which is effected by turning the bolt, causes a displacement of the Sprocket and gives more or less tension to the driving chain, according to the direction of movement, which is controlled by the bolt.
The leading dimensions of thc chassis are :Wheelbase, 9 ft.; track, 4 ft.
74 ; space available for body, 9 ft. 10 ins.; weight, approximately, 16 cwt.; turning circle; 30 ft. diameter ; road clearance, 94 ins.
The price of this chassis, complete with electric lamps, horn, tool kit and spare wheel, Without tyres, is £270, and it can be obtained from F.I.A.T. (England), Ltd., 43-44, Albemarle Street, London, W.1.