F 2 Camuter an integral
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vmidi from Marshall by...n.ayes
A NEW INTEGRAL midibus is being developed by Marshall of Cambridge for intensive city operation. But although planning is at an advanced stage — I was among journalists to drive a prototype last week — it cannot be completed in time for next month's Earls Court exhibition. It should, however, enter production next spring.
The bus, designed from scratch by Marshall, uses many components well proven in other applications. A market survey conducted for the company by Peter Vince, former chief engineer at Midland Red and a consultant on the project, shows a potential annual market of 250-500 buses, The Camuter, which as its name implies' embodies several of the construction techniques pioneered on Marshall's Cammair integral bus, is described by the company as the "busman's bus of the eighties". Certainly, Marshall has tried to over-specify on components and has used a guideline that no component should be working at more than 65 per cent of designed normal workload.
The main reason the company went integral was to get a low as possible floor height — and, anyway, there was no suitable chassis available. In the event it has achieved a creditable first step height of 320 mm (1ft 11/2in) and a floor height of 510mm (lft lOin). Allwelded construction is employed using square and rectangular steel tube sections Steel was chosen rather than alloy because of its greater rigidity.
There are four continuous jig-built frame sections located fore and aft of both axles. These are joined by longitudinal members located in the roof, the double floor structure and sides. Both the front and rear sections are supported by extensions of the floor members. The engine and gearbox are mounted on a separate cradle which is cantilevered over the rear axle and which, it is claimed, can be dismantled easily as a single unit for maintenance. Because of the rigid structure simple leaf-springing can be employed.
The basic framing is welded together and major sub-assemblies are treated in Marshall's own phosphate bath process developed for use in the company's military contracts. No more welding is performed after treating and the units are bolted together. Exterior panelling is in alloy with a one-piece grp roof and large windows.
The rear-mounted engine is a Perkins 4.236 fourcylinder diesel engine which is mated to an 'Allison AT540 automatic fourspeed gearbox. The combination was chosen because of its proven use (it is fitted, for example, in the soon to be discontinued Seddon midi). The drive is carried via a Borg Warner Morse 13y-Vo chain drive to a. spiral bevel transfer box and thence to the rear axle This is a Leyland Terrier truck unit turned through 90 deg to accept drive from the rear.
Terrier components
The Terrier is the source of several other vital components, including steering mechanism, front axle and full hydraulic braking system. There is no air supply on the bus; even the glider doors are operated electronically.
Cooling is by means of a radiator in a separate compartment in the rear and fed by thermostatically operated fans. It is intended, at a later stage, to encapsulate the engine to produce a quiet vehicle and this has been taken into consideration during design of the cooling system.
The vehicle as a whole has been designed with city centre operation in mind. GRSA recommendations have been followed and the cab layout follows the GRSA laid-down pattern. Co-operation with the DoE during design stages is expected to mean that there will be few difficulties in the vehicle being approved for bus grant purposes.
A variety of seating layouts will be.offered but a 27-seat, 10-standee arrangement will be standard. Seats will be slightly, narrower than standard to ensure an adequate gangway in this 2.28 m (7 ft 6 in) wide bus. An emergency exit is provided in front of the rear axle on the offside.
Another development which is planned is the use of a six-cylinder engine for which there is adequate space. The unit used is likely to be the Bedford 300 unit because it has already been mated to the Allison transmission. Manual transmission is not envisaged. The Allison unit is fitted with inhibitors to prevent engine overspeeding or inadvertent engagement of reverse gear.
On the road
At present, only one prototype Camuter has been built. This is essentially a frame and mechanical units test rig and is unpanelled. On the move, with, the vehicle loaded to simulate unladen weight, the ride was impres sive. The frame seems to be remarkably stiff with no detectable flexing. This no doubt plays an important part in the ride which I thought quite acceptable despite the simple springing. Obviously, full approval must await a chance to try the vehicle properly loaded at its true centre of gravity.
The Allision transmission was obviously in need of some attention to gear change speeds, which I am told is in hand. At present it refuses to change into top until more than 48 km/ h (30 mph) is reached. This transmission gives very smooth upwards changes but offers virtually no engine braking. The Clayton hydraulic brake system provides effective retardation but, certainly at present, a very sensitive brake pedal. There is a spring parking brake.
A brief spell at the wheel showed that the bus in unladen form is very easy to drive and performs well. I felt the none-power assisted steering to be on the limits of acceptability these days and with a full load may prove too heavy for drivers used to modern powered-systems. However, manoeuvrability is excellent with a swept turning circle of only 14.6m (48 ft).
Good potential
Taken overall,. the Camuter project is both enterprising and significant. It has few obvious competitors at present and should prove acceptable to serious busmen because of the obvious serious approach of its design. As far as mechanical units are concerned I fully accept that those chosen have given acceptable service in other applications. However, it will take time and operational service to show if the somewhat tortuous driveline can survive the rigors of intensive stop-start operation. The same conditions are also necessary to prove the efficiency of the rearwards-facing radiator. My only other reservation is about the ease of effectively quietening the raucous Perkins engine. However, it is returning 8.8 km/1 (25mpg) in its present unladen state around Cambridge and a figure of 5.3 km/1 (15 mpg) is eventually hoped for.
Operators already approached have shown substantial interest in the design and three of the country's leading city undertakings have taken an active part in its early stages. They obviously feel the need for a vehicle of this size and capacity built to big bus standards. The price too, at about 0,000 £10,000 is more like that asked for fullsize buses.