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Thornycroft Type JC Lorry

Thornycroft decided that the best way to meet an emerging demand for higher performance bus and coach chassis, with safety, was to use six-cylinder engines and all-wheel braking. Thus, in 1927 the firm introduced its Type A6 four-wheel passenger chassis with four-wheel braking, designed to carry 20 passengers and powered by a six-cylinder YB6 engine. This new six had 3.75ins (95.3mm) bore and 5.25ins (133.4mm) stroke, it gave 60bhp at 1,500rpm and 70bhp at higher speeds. This was not Thornycroft’s first automotive six-cylinder engine, because such engines powered the magnificent 36hp and 45hp Thornycroft cars produced before WW1. The firm also powered its Hathi heavy tractor during the mid-twenties with a 100bhp six. However, discounting the Hathi as a commercial vehicle, the YB6 passenger-chassis engine was Thornycroft’s first commercial-vehicle six, and, as such, its introduction was a major milestone in the company’s commercial vehicle history.


In deference to the torsional vibration characteristics of crankshafts of in-line sixes, Thornycroft put the YB6’s timing drive gears at the flywheel end of the engine, where, because of the weight of the flywheel, torsional vibration amplitude was at its minimum. This reduced timing gear rattle and premature wear at rpm values which induced vibration peaks. However the firm went one better on its first six-cylinder lorry engine, the 7,763cc WB/6 (4.375ins/111.1mm x 5.25ins/133.4mm), with: 1) a torsional vibration damper fitted to the front of the crankshaft, and 2) rear-mounted timing drive. This engine first saw the light of day as the engine for the JC rigid-six-wheeler, introduced in 1929.

Designing single vehicles to take on more work reduced haulage costs per ton-mile, and was an example of a technological development enriching the economy. The JC 10 ton (10,160kg) rigid six-wheeler was built in response to the demand for ever larger lorries, and was offered in both normal and forward control form.  It could carry a greater load than any previous Thornycroft rigid-frame lorry.

The JC was not designed to go off-road as were Thornycroft’s previous rigid six wheelers, however it had a feature in common with off-roaders. Thus, both the JC’s rear longitudinal leaf springs, on either side of the vehicle, were separately fulcrumed at their centres on brackets hung from the main chassis frame, one spring above the other. However, the JC parted company with its off-road stablemates in its method of attaching the spring extremities to the rear axles, i.e. rigidly on the JC, and with non-rigid gimbal mountings on off-roaders.

The JC 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 via a two piece propeller shaft, delivering power to each differential through overhead worm drives. Braking was on the rear wheels only. The JC’s WB/6 engine had an L-head, although overhead valves were used by some other manufacturers for their vehicles, and the WB/6 was built in unit with its clutch and gearbox, in keeping with Thornycroft’s decision, announced in 1927, that all future Thornycroft engines would have this layout. A short shaft running between both overhead worms supplied power to the rear axle. Metal universal joints supported the external drive-shafting. As with contemporary vehicles, the JC was fitted with a crude sliding pinion gearbox, meaning that gear-changes needed careful co-ordination between accelerator, clutch and gear lever, to ensure that the peripheral speeds of newly meshing gearwheels was equal – to ensure silent meshing. To this end, a clutch stop was fitted to slow down the clutch driven member. This was before the advent of constant mesh/synchromesh gearboxes which now make skilled gear changers of us all. Disc type wheels were fitted, twins on the rear axles and singles at the front; pneumatic tyres were standard. Thornycroft was mindful of the thrashing that lorries could suffer, and fitted a speed governor to the engine to keep rpm in check. Conditions for the driver in the JC’s cab were primitive, although, unlike some of Thornycroft’s earlier models, a windscreen was fitted. There were no side windows in the standard cab, no heater and no power-steering, although Westinghouse servo-braking was fitted as standard.

Drivers were advised that, in cold weather, if the lorry was to be left for any considerable period in a position where there was a risk of cooling water freezing, then the water should be drained out through cocks in the radiator and cylinder block. Drivers were also reminded not to neglect to refill the radiator before starting the engine again. How much simpler life is nowadays thanks to anti-freeze! When the engine was started from cold, it had to be run slowly for about ten minutes so as to be thoroughly warmed up before driving. Early starts must have woken the neighbours! Also, with a cold engine difficult to turn due to cold oil or new bearings, the long-suffering driver was asked to give the crankshaft of the 7,763cc WB/6 half a dozen turns on the starting handle, with the ignition off before starting on the electric starter.

After a relatively short innings, the JC was withdrawn in 1930 and replaced by the 12 ton (12,192kg) QC rigid six wheeler which, like its JC predecessor, was for on-road use only, was powered by a six-cylinder engine and had vacuum servo brakes.


Specification of Thornycroft Type “JC” Rigid Six-Wheeled Chassis (Petrol) October 1929

To carry a net load of 10 tons (10,160kg), with a body allowance of 25cwt (1,270kg)

45.9hp (RAC), type “WB/6” six-cylinder petrol engine, bore 4.375ins (111.1mm) x stroke 5.25ins (133.4mm), develops 78bhp at 1,500rpm and up to 90bhp at increased revolutions. The six cylinder barrels are pressed into a single casting which forms the water jacket and carries the valves; the detachable cylinder heads are in two parts. On the left-hand side of the cylinder block are mounted the side-by-side valves, operated by a camshaft driven from the rear of the crankshaft. The auxillaries on the near side, however are driven from the front end of the camshaft, and consist of the fan-driving pulley, dynamo and magneto. Valves are situated behind detachable oil-tight covers. Seven white-metal-lined bearings carry the nickel-chromium-steel crankshaft in the upper half of the crankcase. The bolts for the main bearings pass right through the crankcase. The exhaust pipe branches are in three portions.

A simple vibration damper is built into the front of the crankshaft to damp out torsional crankshaft vibrations. This consists of a flywheel, the centre portion of which is attached to the crankshaft. The rim of this flywheel, however, is frictionally-connected to the centre portion by means of two spring-loaded, sector-shaped members. Any uneven motion of the crankshaft causes the rim of the flywheel to spin relative to the centre portion, but the spring-loaded sectors act as a brake, permitting only a slight amount of movement.

Lubrication of engine
The oil pump is contained in the base chamber and immersed in the oil; it is driven by skew-gearing from the camshaft. Unusual attention is given to the oil filtering. Below the crankshaft there is a gauze tray in the sump to catch the carbon particles. Another filter is attached to the oil pump in the base, and a detachable, rapidly cleanable, three-stage filter is mounted externally on the near side below the centre line of the crankcase. This can be cleaned without loss of oil from the sump. When starting up on a cold day the oil in this filter is automatically by-passed if the oil is too heavy to pass through at the time. Oil galleries in the crankcase are used, the only external pipe being that to the pressure gauge mounted on the dashboard.

Oil passes from the pump to the camshaft bearings and main bearings, the big-ends being lubricated under pressure through the drilled crankshaft. The cylinder walls and pistons are lubricated by spray from the crankshaft bearings, whilst overflow from the relief valve lubricates the camshaft timing wheels. On the side of the engine is the oil filler funnel; there is also a flexible dipstick on the same side of the power unit

Chassis lubrication
Various parts of the chassis are lubricated with grease or oil, depending on location.

A Simms high-tension magneto with a vernier coupling for timing adjustment. Automatic advance is used and the timing can easily be set by removing the starter motor to reveal marks on the front of the flywheel. There is an asbestos heat shield above the magneto to deflect any heat from the exhaust pipe branches.

A Solex carburettor is located on the near side and is fitted with an AC air cleaner. To assist carburation, a hot-spot is provided in the induction manifold. The throttle has a hand-operated slow-running adjustment below the steering wheel.

Cooling system
Circulation of the water is effected by an impeller-type pump, which does not impede thermo-syphon cooling when out of action. The pump is driven by an extension of the cooling fane spindle. A cowled fan, which is driven by a V-belt and has an adjustable pulley, is situated immediately behind the radiator.

A radiator of the familiar Thornycroft type, having a gilled-tube core, is carried on cup-shaped rubber buffers on the front end of the frame. The header and base tanks are castings bolted to the tube block.

Petrol tank
A 50 gallon (227.3 litre) steel tank is carried on the near side of the frame, petrol being drawn from it by a vacuum tank on the front of the dashboard.

A single dry-plate clutch is mounted in the flywheel. Six springs are used to bear upon the pressure plate which traps between itself and the flywheel a driven disc faced on both sides with friction material. A clutch stop is supplied.

Mounted on the rear end of the engine crankcase, ensuring perfect alignment, the gearbox provides four forward speeds, and a reverse, controlled by a lever by the driver’s left hand.

At 1,500rpm of the engine the vehicle speeds is16.75mph (26.95kph) in top gear.

The gear ratios are

  • First 5.12 to 1.
  • Second 2.75 to 1.
  • Third 1.56 to 1.
  • Fourth 1 to 1.
  • Reverse 7.69 to l.

A two-piece propeller shaft transmits power from the gearbox to the forward axle of the bogie. Metallic universal joints are used at all points. The coupling at the forward end of the propeller shaft permits sliding motion and a telescopic shaft is used between the two axles of the bogie. When normally loaded the transmission line is approximately straight.

Driving axle units
The driving axles consist of two cast steel casings with fully floating differential shafts driven by overhead worms, having a reduction ratio of 10.667 to 1. The worm and differential assemblies can be dismantled without removing the load from the axles.

The hand-operated set is of the external contracting type working on a wide drum fitted to the rear end of the intermediate propeller shaft; this set is intended as a parking brake, or for use in emergencies. Pedal pressure, assisted by a small Westinghouse vacuum-servo device supplied from a vacuum reservoir, goes through rods with separate adjustments to self-energising internal-expanding shoes in the four wheels of the bogie. The servo effect is proportional to the applied pressure upon the brake pedal. There are no brakes on the front wheels.

Steering gear
Steering is by worm and wheel. This vehicle has a turning circle of approximately 65ft (19.8m).


Pressed-steel channel frame members are used, the metal being 0.3125ins (7.94mm) thick, and the maximum depth of the channels 9ins (22.9cm), with flanges 4.5ins (11.4cm) wide. The cross-members, with the exception of the front one and that which supports the transmission brake, are hollow, of welded-up section, and are placed above the frame; they are made from 0.1875ins (4.76mm) steel.

The engine and gearbox unit is suspended at three points in the frame. At the front and rear trunnions are employed, one being carried upon a cross-member in front of the crankcase, whilst the two rear points consist of trunnions on brackets attached to the frame. Rubber buffers are used on all three. The rear trunnions are bolted to vertical faces on the crankcase so that rapid removal of the power unit is facilitated

Front axle
An H-section front axle is employed; the swivel pins move on taper-roller bearings. The track rod is behind the axle.

Rear axle
As the chassis is intended for main-road service, the axles are rigidly connected to the ends of the four springs of the bogie, no swiveling mechanism being interposed as would be necessary for rough country service. Four inverted, semi-elliptic springs are pivoted at their centres to brackets attached rigidly to the frame. The springs are also connected rigidly to the driving axles, and they take the driving and braking loads. The suspension is designed to permit 6ins (15.2cm) difference in driving axle levels, excessive movement being prevented by rubber buffers above and below the axles.

The springs and spring pins are completely enclosed by metallic and fabric covers. The brackets between springs and chassis frame are built-up to facilitate dismantling. 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.

Front springs Overall length 4ft (1.22m)

Rear springs Overall length 4ft 6ins (1.37m).

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

Tyres straight-sided pneumatic tyres, which are detachable, and interchangeable, front and rear. Tyres are 40ins x 8ins (101.6cm x 20.3cm) Dunlops, singles on the front axle and dual on the driving axles.

Front mudguards
Both front mudguards can be detached quickly to give easy access to the power unit.

Wheelbase. 14ft 6ins (4.42m) and 4ft 6ins (1.37m).

The tracks for pneumatic tyres are

Front 6ft 2.875ins (1.9m)

Rear 5ft 9.25ins (1.76m).

Ground clearance
10ins (25.4cm) under front axle.

General measurements and weights

Front axle (laden) 41cwt (2,083kg).

Rear bogie (laden) 80.5cwt (4,089kg).

Chassis weight 121.5cwt (6,172kg).

Body allowance 25cwt (1,270kg)

Overall length 26ft 8.375ins (8.14m).

Overall width 7ft 3.25ins (2.22m)

Body space (cab rear - frame end) 21ft 7.125ins (6.58m).

Frame height (laden)3ft (0.91m).

Electric lighting and starting
A 12 volt 5 amp lighting set is provided and the battery is of 105 amp-hours capacity. A starter is bolted to the crankcase and engages the flywheel teeth through a Bendix drive. A dynamo is fitted to maintain battery charge.

A gearbox-driven speedometer is supplied as standard.

Optional extras
The following are available as extras:

  • Mechanical tyre pump;
  • Spare wheel, tyre and carrier;
  • Auxillary gearbox;
  • Power take-off;
  • Non-skid chains to fit the outer of the twin-wheels only, or non-skid bands suitable for twin-tyres;
  • Radiator muff.