PITT TANDEM-AXLE SUSPENSION
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ANEW suspension for tandem-axle semi-trailers, announced by Charles Pitt (Barton Stacey) Ltd., has been designed specifically to overcome wheelhop during braking and also to eliminate the need for maintenance. Features of the design, which has a four-leaf-spring layout, include the use of rubber bushes at all pivot points and the connection of the rear ends of the springs oh each side through bell-crank levers and compression rods.
The forward ends of the springs are mounted in mild-steel fabricated hanger brackets with rubber bushes at the spring eyes. The rear eyes of the strings are mounted in the bell-crank levers which are also located in mild steel fabricated hangers. Large-diameter compression rods connect the upper ends of the bell cranks on each side. Rubber bushes are used also at the rear spring eyes, bellcrank pivots and compression rod connections. To ensure positive location of the axles transversely, necessary because of the use of rubber bushes at all mounting points, a Panhard rod is employed on each axle. This also is rubber bushed.
The new suspension is being offered by Pitt as an alternative to the Rubery, Owen leaf-spring suspension fitted on its tandem-axle models for some time. Rubery, Owen axles are still used, these being 20,000 series with 15.5 in. by 7 in. brake units. The axles are 5 in. diameter and the springs are underslung. The normal capacity of the bogie is 16 tons but versions for a bogie load of 18 tons are available for semi-trailer models to be used at the gross weights envisaged in the proposed regulations for vehicle weight limits. Distance between axle centres is 4 ft. 3 in. In the 16-ton bogie, each of the springs has 14 leaves thick, and deflection is 0.66 in. per ton.
Prototype versions of the suspension are said to have been rigorously tested at the F.V.R.D.E. proving ground at Chobham and semi-trailers with the layout haye also had extended operational testing. The initial problems that there must always be in develop ing a new suspension are said to have been solved completely. This was confirmed
011 demonstrations held by Pitt on a disused airfield near Barton Stacey. The surfaces of the runways there Were generally poor, and very rough in places. The suspension performed very well when the . semi-trailer used for the tests was driven at speeds of up to 40 m.p.h. Runs were made both laden and unladen and it was noteworthy that in the latter condition the suspension provided a relatively soft ride.
Braking tests from 30 m.p.h. were also carried out when laden and unladen. The first laden test with the outfit grossing 24 tons, was on a very rough section with uneven patches of tarmac and broken-up concrete. There was a certain amount of wheel hop, but this appeared to be mainly due to tyre bounce rather than brake reaction in the suspension. Later laden brake tests were carried out on a surface which, although by no means Smooth, was an improvement on the first one. On these there was no sign of hop in spite of a fair amount of wheel locking on the semi-trailer bogiemaximum deceleration on the tests as found using a Tapley
'LORRIES-SHOULD
ASKED for his opinions on unsafe loads, at'a Meeting of The Industrial Transport Association, Birmingham division, last week, Prof.. W. Gissane of the Birmingham Accident -Hospital said that lorries were designed to contain the load, not to retain it, and that . this was a shocking state of affairs. He added that he was 'making a special study of lorry accidents, particularly with regard to the problem of shifting loads.
Prof. Gissane had given a talk on " Some Aspects of Accidents" in which he described accidents as the man-made epidemic that was symptomatic of the machine age. Apart from major' rail and air crashes, the causes of. accidents were meter was around 65 per cent.
These laden teSts showed the suspension to be very well designed, but the unladen brake tests demonstrated more conclusively its effectiveness in preventing wheel hop. Efficiencies (by Tapley meter) of up to 90 per cent were obtained and although the semi-trailer wheels were locked for a considerable distance, none lost contact with the ground. The tyres seemed to cling to the surface and there was no sign of racking and juddering.
After the tests on the airfield, the semitrailer was driven over some very rough and pot-holed country roads and here again the suspension _stood up well to very harsh treatment.
The demonstrations showed most strongly the advantages of the new Pitt suspension. These will outweigh in very many cases the disadvantages in price and weight compared with the Rubery. Owen layout which is still offered. A.J.W.
RETAIN THE LOAD'
never investigated. Machines should be designed to reduce accidents and legislation should go hand-in-hand with education.
The danger to children resulting from the common lack of low forward vision from the cab of a heavy vehicle was particularly noted by Prof. Gissane and, referring to motorways, he contended that serious thought should be given to legislating against the use of fast lanes by commercial vehicles.
Elaborating on the value of safety belts during the discussion, Prof. Gissane claimed there was no known example of a belt increasing the extent of an injury. Seats should be rigidly attached.