Single-minded Daf
Page 21
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• Daf has produced a righthand-drive prototype of its SB220 single decker rear-engined bus underframe — launched at the 1984 RAI bus show in Amsterdam — to gauge reaction from UK Operators.
Last week representatives of major British bus operators, bodybuilders and Daf UK dealers were shown the ultralow-floor prototype at Daf Truck and Bus's Colchester vehicle preparation centre.
Their comments and observations have been noted for specification changes prior to a full UK launch — with an implied commitment to production from Eindhoven towards the end of the year. When the SB220 is introduced in Britain it will be a direct competitor to Leyland's new Lynx.
The UK prototype differs from current SB200 models built for the Dutch market. Its 5.3m wheelbase is shorter, to suit the Ilm bodywork being demanded in Britain, and provides a passenger capacity of 51 seats plus 22 standing. Kerb weight of the imbodied SB220 right-hand-drive chassis is 6,030kg.
Because an integral design requires the underframe/chassis and bodyshell to complement each other structurally, Dal Bus is collaborating with a number of UK coachbuilders including Duple, Plutons and Northern Counties, which are already working on suitable designs. Dal will also apply its own finite element stress analysis techniques — already used on Hainje, Den Odsten and Abenraa-bodied SB220 buses on the Continent — to ensure engineering integrity.
Compared with Dafs older SB201 and 210 chassis the latest SB220 is lower, quieter, smoother, more manoeuvrable and handles better. As before front and rear full-air suspension is standard but axle location geometry is quite different, with a new front axle beam which dips under the main longitudinals, and rises on the vehicle centreline, where it is partially located by a ball-jointed rubberbushed top A-frame.
By deflating the twin front air bellows — now located outboard of the frame — front entry-door floor height can be lowered by 95rnm for improved access.
With the air springs fully inflated, frame-top height above the front axle is just 510mm. This is maintained rearwards to within 870rnm of the rear axle, where the frame is stepped up 260mm over the four-bellows rear suspension.
The air spring configuration, compared with the older model's trailing-arm two-bellows arrangement, is said to afford better straight-line stability at speed and to reduce bounce induced by the long, 3,120mm, rear overhang, in which is concentrated the full weight of the engine and driveline,
Sloping floor
Though the frame is stepped, bodybuilders and operators can opt for a sloping floor or aisle.
Steering geometry has also been revised, with the ZF integral-power box now mounted in a triangular frame, with linkage joints positioned so that bumps are not fed through to the steering column. The front wheels turn through 540 to give a between-kerbs turning
circle of 16.5m.
There has been a diesel technology spin-off for the new SB220 from Dafs ATi truck engine development programme. Injectors, camshaft and turbocharger matching are changed in the latest 11.6 litre horizontal Daf bus power unit which carries a simplified designation, being known simply as the LC engine. It produces 162kw (218hp) at 2,000rpm, with a maximum torque of 970Nm (7151bft) at 1,300rpm.
Partial encapsulation of the power unit, combined with the internal engine changes, has reduced peak drive-by noise levels from 82 to 77dB(A).
Integral retarder
On the prototype, ZF's wellproven 4HP-500 four-speed automatic gearbox with integral retarder is fitted. In combination with a 4.1:1 single-reduction axle, it gives a modest maximum speed of 851an/h, but the 5HP-500 overdrive version can be fitted as an alternative, offering a 1061crn/h maximum for inter-urban working. Operators can also specify a Voith gearbox.
Dave Skinner, Daf Bus's technical manager, says the company's UK plans for the SB220 are clouded by the uncertainty of the single-deck city bus market, where only 50 units were sold in 1986.
Nonetheless he is confident that demand will grow in the next two to three years, as the limitations of the many miniand midi-buses ordered since deregulation become apparent. He cites cramped seating, absence of luggage room, marginally-acceptable noise levels and short vehicle and engine life.