1131C in Perspective DEVELOPING THE ROUTEIV1ASTER COACH LL ONDON TRANSPORT'S decision,
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reported in .our issue of June 29, to introduce a fleet of A.E.C.-Park Royal doubledeck coaches of the Routemaster type on eight heavily used Green Line routes is another significant step in their policy of vehicle standardization. The adaptation of the Routemaster bus design ha; obvious manufacturing and maintenance advantages and only bridge limitations prevent this RMC vehicle from being brought into use on other heavily trafficked Green Line routes.
Double-deck working on busy Green Line coach routes has, of course, long been a London Transport aim, but early postwar experience has made them understandably cautious about adopting " bus" equipment simply in order to increase capacity without sacrificing standardization; they have been at pains to ensure that the double-decker chosen would prove at least as popular with passengers as the RF single-decker has done. This is one reason why the Routemaster coach prototype (RNIC4) Was run for about five years, covering 200,000 service miles, before a production order was placed; an exceptional amount of development testing has been put into the air suspension system, 50 Routemaster buses having been fitted experimentally with rear-axle air suspension of various types and various setting3. There is, incidentally, no intention at present of introducing this suspension generally into buses; the experimental sets will almost certainly be replaced with the standard steel coil springs as vehicles become due for overhaul.
Following a series of pre-war experiments on similar lines, in 1947 London Transport adapted an RT double-deck bus for Green Line coach service, but the reaction of the public very quickly made it clear that suspension which was adequate for stage-carriage services could provide an unacceptable ride on longer coach routes over sometimes winding and heavily cambered roads. It would not, perhaps, be unreasonable to say that the ride quality at speed of the standard RT bus, which is not fitted with dampers, is one of the less satisfactory features of an otherwise comfortable vehicle. The RT coach was withdrawn and the RF single-decker has become the standard Green Line coach.
But the RMC is a very different kettle of fish from the RT, and one has only to look at the structure weight per passenger to see what it may offer over the rather heavy RF in first-cost and running-cost economies. The integrally constructed Routemaster coach, at 7 ton 12 cwt. 1 qr., weighs about the same unladen as the RF; fully laden, it turns the scales at about 111 tons.
The difference in structure weight of 144 lb. per passenger between the RF and the RMC is far from negligible economically—always assuming that the latter vehicle will operate at reasonably high load factors, and London Transport are confident of this, especially as some routes will be served by the double-deckers on a lower frequency than with the RFs..
But it is largely on the riding comfort of the RMC that London Transport arc banking to fill the seats onthe Green Line double-decker services. Although the coil-spring suspension of the standard Routemaster bus provides sufficiently improved riding over the leaf-sprung RT to suggest that it would have proved popular on the coach routes, air suspension at the rear axle has shown a margin of comfort that is considered worthwhile. Where both the RM and the RMC score is in the absence of the body roll that is inevitably associated with a tall vehicle having leaf springs set conventionally inboard of twin rear wheels. This is achieved by setting the rear suspension units at the outer extremities of the underframe, so
giving an exceptionally wide-based suspension layout. ibis, and the fact that the subframe carrying the wheels acts as art anti-roll member. gives a remarkable freedom from body roll, yet on the RMC the ride is even softer than with coil springs. The front suspension, as on the standard Routemaster, is independent, employing coil springs and wishbones.
The air suspension finally selected for the ,RMC is the Firestone rolling-diaphragm type. Air bellows operating at 40-50 lb./sq. in. are fitted in place of the rear coiLsprings, each being connected at its upper end to a 540-cu.-in.-capacity surge tank. These units operate at a constant frequency of 70 cycles per minute; this ability to achieve a constant suspension frequency regardless of load is, of course, one of the advantages of air springing and contrasts with the frequencies of 85-90 c.p.m. at which the coil-spring units of a standard Routemaster operate at full load.
Air pressure for the RMC 'suspension is supplied from the normal gearbox operating system as fitted to the bus and is controlled by levelling valves, one on each side. A delay of 9 sec. makes the valves insensitive to normal road shocks and body sway when cornering.
The shock-absorbers, which on the bus are mounted coaxially with the coil springs, have been sited inboard of the suspension units on the coach. Half the coaches will be equipped with arm-type shock absorbers and the remainder with telescopic units similar to those on the bus.
Other types of air suspension tested on the Routemasters were triple-convolution units (as used on the new A.E.C. Renown) and a modified triple-convolution form incorporating, in effect, a rolling diaphragm, and although these gave satisfactory results it was considered that the Firestone type was best suited to this particular application. With all the systems a variety of suspension frequencies was tried, ranging from as low as 62 c.p.m. to 75 e.o.m.; the lower frequencies proved especially unsuitable with this vehicle, giving a " floating " ride rather like a ship in a swell.
Since the routes on which the RMC will be used call for only about four gearchanges a mile compared with the average of 15 gearchangcs a mile on buses in Central London, the Routetnaster's fully automatic transmission is considered unnecessary and the solenoid-operated pneumatic valves for each gear are 'selected by manual operation of the steeringcolumn lever. Neutral engages automatically when the road speed drops below 4 mph., the selected gear being re-engaged on depressing the accelerator pedal. This prevents wasting fuel through idling in gear and has been found to give about 1 per cent, better fuel consumption. As on the Routemaster bus, an epicyclic gearbox and fluid flywheel are used, but the final drive ratio has been raised from 5.23 to 1 to 4.7 to 1 as now Used on the RF single-decker. These vehicles are thus unusually high-geared by double-decker standards.
As reported in our June 29 issue, these changes, together with more deeply upholstered and wider-spaced seats in an attractively re-styled interior, make the new Routemaster 57-seat coach a very promising addition to London Transport's fleet
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