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Thornycroft Type QC Lorry

The upward pressure on capacity caused Thornycroft to drop the relatively youthful 10 ton (10,160kg) JC rigid six wheeler in 1930, and to introduce a larger 12 ton (12,192kg) rigid six wheeler known as the QC. Three models were offered, these being standard, forward control and long-wheelbase vehicles. A key feature of off-road rigid six wheelers was the use of gimbals for attaching rear spring extremities to both rear axles to avoid extra spring loading when running off-road. By contrast, springs were rigidly attached to the back axles of on-road rigid six wheelers, such as the QC which was, nevertheless, cleared for limited off-road use at reduced load. The rear suspension arrangement allowed a 6ins (15.2mm) difference in rear axle levels. A feature of the Type JC chassis frame was retained in the QC, namely, the placing of the main cross-members above the longitudinals. Extensions of the cross-members either side if the longitudinals formed body bearers, which could be deleted if not required.

The most striking feature of the QC was its newly-introduced NC/6 straight-six engine. This enormous engine, in unit with clutch and gearbox, had a capacity of no less than 11,330cc and gave 104bhp at 1,700rpm. This was the largest lorry engine used by Thornycroft when it was introduced, and shared its cylinder dimensions with the engine of the Hathi heavy tractor of the mid-twenties. However the QC’s engine was of very different design, with its six-cylinders cast in one block as against the Hathi’s cylinders, which were cast in two blocks of three, a less sophisticated arrangement. Also, the QC’s engine had an L-head, while the HhH Hathi’s had more sophisticated ioe valves. Conventional wet-sump lubrication was used for the QC, whereas the Hathi used a dry-sump system, perhaps in deference to the angles to which the vehicle might be subjected in punishing off-road terrain. The QC’s NC/6 engine retained a front-mounted vibration damper, also fitted to its JC predecessor, but moved the timing drive to its normal position at the front of the engine from the rear location of the JC. In doing this, Thornycroft were acting in concert with some other six-cylinder engine manufacturers, who found that a vibration damper produced satisfactory results with front-driven timing gears.

The QC followed Thornycroft’s usual lorry design formula. The vehicle had a steel chassis frame supporting the front-mounted, fore-and-aft engine, and power was taken to both back axles from a dry single-plate clutch via a sliding pinion gearbox (i.e. no synchromesh) and a two piece open propeller shaft, delivering power to each differential through overhead worm drives. A short shaft running between both overhead worms supplied power to the rear axle.  Metal universal joints supported the external shafting. Disc type wheels were fitted, twins on the rear axles and singles at the front; pneumatic tyres were standard. Braking was on the rear axles only using a Westinghouse servo system. The maximum governed speed was 29mph (46.7kph) and, on test with a simulated load over a 28 mile (45.1km) course, The Commercial Motor assessed the QC’s average petrol consumption as 4.62mpg (61.2 litres per 100km).

In keeping with Thornycroft’s new policy of naming its vehicles, while keeping alphanumeric designations, the QC was given the name Dreadnought. The type was still in production in 1932, the final year covered by this narrative.

Specification of Thornycroft Type “QC” Rigid Six-Wheeled Chassis (Petrol) January 1931

To carry a net load of 12 tons (12,192kg), with a body allowance of 30cwt (1,524kg). (A reduced payload is carried when operating over bad surfaces

54.1hp (RAC), type “NC/6” six-cylinder petrol engine, bore 4.375ins (111.1mm) x stroke 6.5ins (165.1mm), will develop 103.6bhp at 1,700rpm. Compression ratio is 4.75 to 1 and firing order is 1 5 3 6 2 4. The six cylinders are cast in one block, but there are two detachable heads. The barrels have dry, cast iron liners pressed into them. Actually forming a part of the block is the chamber for the camshaft; this operates the side valves through roller-equipped rockers; the camshaft is accessed by removal of the two covers so that its whole attendant mechanism can be inspected and adjusted. The distribution drive is by a triple-roller chain; this serves all the auxillaries, including the fan which has a clutch in its hub.  

Seven white-metal-lined bearings carry the nickel-chrome-steel crankshaft; between the front main bearing and the first big-end there is a vibration damper. The upper portion of the crankcase is ribbed right down to the depth of the bearing caps, which are of aluminium and secured by steel keeper plates. Drive for the oil pump is by skew gear from the camshaft; from the upper end of the vertical shaft runs the flexible drive for the engine speed governor.

Lubrication of engine
The crankshaft is hollow and is drilled to permit force-feed lubrication of the big-end bearings. The submerged oil pump feeds the main bearings and the camshaft; the overflow from the bearings of the latter falls into troughs in which revolve the cams; oil is thus flung on to the rockers and rollers, etc. The cylinder walls are lubricated by splash, as are the small-end bearings. All oilways are cored or drilled in the engine and there is a very deep Auto-Klean filter, coupled to the clutch pedal, in addition to the diaphragm filter above the sump.

A high-tension magneto is in tandem with the dynamo. The magneto is controlled by automatic timing.

A U-type Zenith carburettor is fitted. To assist carburation, there is an exhaust-heated hot-spot in the induction manifold. The exhaust manifold consists of two triple branches merging into a single pipe above the cylinder block.

Cooling system. A departure from previous Thornycroft practice lies in the water pump; this is now driven by the distribution chain, the pump body being carried on the front of the distribution case; the spindle forms a forward extension of the magneto-dynamo drive line. The impeller-type water pump does not impede thermo-syphon cooling when out of action.

A cowled, gilled-tube radiator is mounted upon rubber buffers.

Petrol tank
The forward-control chassis has one 50 gallon (227.3 litre) fuel tank, whereas both normal control vehicles have dual tanks containing 79 gallons (359.1 litres). The tank is carried on the near side of the frame

A dry single-plate clutch is fitted.

Mounted on the rear end of the engine crankcase, ensuring perfect alignment, the gearbox provides four forward speeds and a reverse.

Overall gear ratios are

  • First 46.17 to 1
  • Second 24.75 to 1
  • Third 14.4 to 1
  • Fourth (direct)9 to 1
  • Reverse 69.21 to l

Speed range in top gear, 5mph (8kph) to 29mph (46.7kph) (governed speed).

An open two-piece propeller shaft transmits power from the gearbox to the rear axles. The steady bearing of the propeller shaft is carried in a flexible support anchored to a cross-member of the frame.

Driving axle units
Overhead worm and differential assemblies are fitted to both rear axles.

The brake pedal, operating through a Westinghouse vacuum servo-assistance system, applies internal expanding shoes in the drums of all four driving wheels. A hand-operated lever controls external contracting band around a drum on the primary propeller shaft. There are no brakes on the front wheels. Total area of friction material is 750sq ins (4,839sq cm).

Steering gear
Steering is by worm and wheel. The worm revolves upon taper-roller bearings and the wheel is a full one so that wear may be countered by bringing into action a fresh portion of the periphery. There are 3.5 turns lock to lock. This vehicle has a turning circle of 78ft (23.8m) right, and 72ft (21.9m) left.

Each main member is of channel section pressed out of 0.375ins (9.53mm) thick steel. The main cross-members are placed above the longitudinals.

Three rubber-bushed trunnions are employed as suspension points for the gearbox unit. The rear trunnions are bolted to vertical faces on the unit so that rapid removal is facilitated.

Rear axle
As the chassis is intended for main-road service, the springs of the bogie are coupled to the axles without trunnions; the springs thus take whatever torsional loads may arise. They also cater for the driving and torque stresses. Four inverted, semi-elliptic springs are used; each pair is pivoted centrally upon brackets attached to the frame and allows a 6ins (15.2cm) difference in axle level. Means are provided to take the side thrust of each axle while cornering.

Springs. Each rear spring has 11 leaves, 4ft 6ins x 4ins x 0.5ins (1.37m x 10.2cm x 12.7mm), whilst the front springs are 4ft x 3ins x 0.3475ins (1.22m x 7.62cm x 8.827mm). All springs are flat under load and are enclosed by gaiters. The front springs are secured by Thornycroft patent relieving plates, which enable the holding-down bolts to withstand stresses due to the flexing of the springs.  

Wheels and tyres
Detachable disc wheels. Wheels. bolted to cast steel hubs which run taper roller bearings for the front wheels, and ball and roller bearings for the driving wheels.

Pneumatic tyres are 40ins x 8ins (101.6cm x 20.3cm) Dunlops, singles on the front axle and dual on the driving axles.


Standard-wheelbase vehicle

  • Mean wheelbase. 14ft 6ins (4.42m)
  • Wheelbase of bogie. 4ft 6ins (1.37m)
  • Overall width. 7ft 6ins (2.29m)
  • Overall length. 27ft 6.25ins (8.44m)
  • Body space behind cab 17ft 0ins (5.18m)

Long-wheelbase vehicle

  • Mean wheelbase. 16ft 9ins (5.11m).
  • Wheelbase of bogie. 4ft 6ins (1.37m).
  • Overall width. 7ft 6ins (2.29m).
  • Overall length. 30ft 0ins (9.14m)
  • Body space behind cab 20ft 0.875ins (6.12m)

Forward-control vehicle

  • Mean wheelbase. 14ft 6ins (4.42m)
  • Wheelbase of bogie. 4ft 6ins (1.37m)
  • Overall width. 7ft 6ins (2.29m)
  • Overall length. 27ft 6.25ins (8.39m)
  • Body space behind cab 21ft 7.125ins (6.58m)

General measurements and weights
Chassis weight 6tons 3cwt 3qrs (6,287kg).

Electrical equipment
A 12volt lighting set is provided, and an electric horn. A starter is bolted to the off-side of the crankcase. A dynamo is fitted to maintain battery charge.

A speedometer is supplied.

Optional extras -among the extras are the following:

  • Mechanical tyre pump
  • Trailer brake gear
  • Auxillary gearbox
  • Power take-off (28hp or 6hp)
  • Drawbar gear