Independent tandem for trailers
Page 36
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• The prototype of a semi-trailer equipped with a tandem-axle running gear with independent coil-spring suspension has been produced by Peak Trailers Ltd., Ashton Road, Bredbury, nr, Stockport, that basically comprises two of the company's singleaxle independent-suspension units.
Introduced at the 1968 Commercial Motor Show as a "last minute surprise", the single-axle suspension was fitted to a 28ft 24-ton-gross semi-trailer that was later tested for roll-over tendencies by Ron Cater (CM April 18 1969) in comparison with an identical semi-trailer with conventional beam-axle suspension. In his conclusions Ron Cater stated that the independent suspension was "definitely more stable" than the conventional type and provided a kinder ride to delicate cargoes and—or bodywork.
In prototype form the tandem-axle semi-trailer has a length of 33ft and weighs 4 tons 6cwt. The weight penalty of using independent suspension is about 5cwt (the penalty in the case of the single-axle type is about 11-cwt) but it is confidently anticipated that the penalty will be reduced to 3cwt by the time the production model is marketed. The kingpin-to-bogie-centre of the trailer is 22ft 8in.
Of the swing-axle type having fulcrum points close to the centre of the vehicle, the suspension is such that the camber or the wheels is positive (in that the wheels lean outwards) when the trailer is unladen and negative when it is carrying a full load. Because of the length of arms, the change in camber angle and resultant tyre scrub are, however, minimal and the height of the roll centre (about 2ft 2-1-in.) is increased compared with other types of suspension. The higher the roll centre of a vehicle the greater is its resistance to roll.
It is claimed by Mr. W. Bowden, director of engineering of the Peak company (who designed the suspension), that wheels haVing a negative camber provide greater resistance to roll than positively cambered wheels. And he also claims that the small amount of tyre scrub that is produced by the Peak system is more than offset with regard to its effect on tyre wear by the improved tyre adhesion afforded by the suspension, notably when the vehicle is unladen, and (when applied to a tandem axle) by the elimination of wheel hop on curves.
Damping is provided by telescopic shock absorbers located centrally in the coils. Braking torque is not transmitted to the springs, and the widely separated bearings of the arms spread the load over a wide area of the robust base frame. Mr. Bowden points out that both the tandem and sin
gle-axle units are produced as a package and are, therefore, suitable for fitting to a frameless structure such as a tanker or integral van. The brakes are operated by dual-diaphragm air actuators.
As shown in the accompanying drawing, the two systems of a tandem-axle suspension are linked on both sides by a compensator mechanism incorporating a tension rod. Increasing the spread of the axles is relatively simple in that the only modification needed is the fitting of tension rods of appropriate length. When the load on one of the wheels is increased the pivoted angle bracket, on which the coil spring is cone mounted, transmits the thrust to the tension rod, which is attached to pivots located at the lower ends of the brackets of both systems. Any increase in load on either spring increases the load on the other spring by a corresponding amount. The system is, therefore, independently non-reactive.
As a footnote, it may be mentioned that a number of works drivers have stated that the tandem suspension reduces drag on small-radius curves. No technical explanation of this claimed advantage is available.