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Thornycroft Strenuous Type KD Lorry

Introduced in late 1931 for the 1932 model year, the Strenuous Type KD in normal wheelbase form met the need for a four-wheel lorry with a 5 ton (5,080kg) capacity. The short wheelbase KD, with the greater weight of a tipping body and associated tipping gear, could carry 4.75 tons (4,826kg). Both KD models were powered by the four-cylinder L-head HB4 of 5,420cc, uprated to 60bhp at 2,000rpm. The specification soon included braking on all four wheels, with a Westinghouse servo system. The KD’s appearance was a leap in advance of some of Thornycroft’s previous vehicles, which had a rather pre-historic look, and those who remember lorries of the ’forties and ’fifties would not have found the KD out of place amongst more recent lorries during those decades. The driver was now protected from the elements by side glazing, in addition to the windscreen. Electric lighting was fitted as standard, as was a mechanical tyre pump driven from power take off.

The Strenuous KD 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 the back axle 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 the differential through overhead worm drive. Disc type wheels were fitted, twins at the rear and singles at the front; pneumatic tyres were standard.

In 1931, The Commercial Motor road-tested an example of the normal wheelbase KD with four-wheel braking, carrying a 5 ton (5,080kg) simulated load. Long gradients coming out of London were taken at 14 to 16mph (22.5 to 25.7kph), the automatic ignition control helped prevent knocking under these conditions. Two turns lock-to-lock of the steering wheel drew muted criticism, being regarded as unsuitable for a 5 tonner; this presumably only applied to manoeuvring as the steering ratio was passed off as satisfactory for main and secondary roads. Much was made of the MB/4 engine’s flexibility, a feature which might reasonably be expected from over five litres giving a modest 60bhp, on a compression ratio of only 4.4 to 1. The governed speed in top gear was around 30mph (48.3kph) and second and third gear ratios were found to be well suited to the vehicle’s gross weight.

The servo-operated four-wheel braking system was considered thoroughly satisfactory, and it was found that at low speeds a slight servo lag could be offset by use of both foot and hand brakes. However at high speeds the quickest deceleration was obtained solely from the foot brake, as the addition of the handbrake tended to lock the rear wheels. No doubt modern motorists will raise their eyebrows at the built-in servo lag on the KD! When restarting on Cocks Hill, near Elstree (gradient 1 in 6), the loaded KD got away easily thanks to the low first gear. Average fuel consumption over the mixed 23 mile (37km) course was 7.66mpg (36.9 litres per 100km) and the average speed was 18.6mph (29.9kph), attained at cruising speeds of between 20 and 25mph (32.2 to 40.2kph). The test team were generally impressed with the loaded KD’s performance, finding that hills could be climbed in top gear which called for third on most other comparable vehicles, a fact that was claimed to reduce driver fatigue. This comment is indicative of the potential for poor quality gear changes on sliding pinion gear boxes.

The KD was still in production in 1932, the final year covered by this narrative.

 

Specification of Thornycroft Strenuous Type “KD” Normal Control Chassis (Petrol)  December 1931

To carry a net load of 5 tons (5,080kg), with a body allowance of 20cwt (1,016kg).

Engine
36.2hp (RAC) type "MB4", watercooled four-cylinder petrol engine, bore 4.75ins (120.7mm) x stroke 6ins (152.4mm), developing 60bhp at 1,800rpm. Compression ratio is 4.4 to 1. The four cylinders are of the monobloc type with two detachable heads. The valves have double springs, and the Thornycroft standard roller tappets, with accessible adjusting nuts, are employed. The pistons are of cast iron with three piston rings, but, if desired by the purchaser, aluminium pistons can be supplied. The connecting rods are of duralumin, the caps being fastened by two bolts. Big end bearings are gunmetal shells with white metal linings. The crankshaft is carried in three main bearings.

Lubrication of engine

Oil is carried in the sump for lubrication and the submerged oil pump supplies the oil to a channel which runs end to end of the engine at a pressure of 15 lb/in2 (1.034 bar). Lubricant passes from this channel to the main bearings and through crankshaft drillings to the big-ends; cylinder walls and small-end bushes are splash lubricated. Camshaft bearings are pressure-fed and the surplus oil passes to the timing gear. No copper pipes are incorporated in the lubrication system, except the external pipe to the dashboard pressure gauge. An oil filter is fitted.

Ignition
A high-tension magneto with automatic timing control is employed. The magneto is mounted on the left side of the engine in tandem with the dynamo; should it be necessary to remove the dynamo for overhauling or adjustment, the magneto can be moved to the forward position and coupled direct to the magneto shaft.

Carburettor
Zenith type. An engine rpm governor is fitted.

Cooling system
Water is circulated by a propeller pump mounted on the intake lead with an outside adjustment for the packing gland. The pump shaft is driven by a Whittle belt off the magneto shaft and the fan is mounted on a prolongation thereof.

Radiator
Front-mounted.

Petrol tank
Mounted externally, to the left hand side of the frame.

Clutch
Power is transmitted to the gearbox through a single-plate clutch of 12.8ins effective diameter (32.5cm). A plate-type stop is fitted.

Gear-box
Mounted on the rear end of the engine crankcase ensuring perfect alignment. Has four forward speeds and a reverse, the top gear being direct drive.

The gear ratios are

  • First 5.16 to 1.
  • Second 2.76 to 1.
  • Third 1.6 to 1.
  • Fourth 1 to 1.
  • Reverse 7.6 to l.

The speed range in top gear is from 5mph (8kph) to 30mph (48.3kph).

Transmission
The drive is taken from the gearbox through an intermediate propeller shaft, and thence to the rear axle through a secondary shaft.

Back axle
Power from the propeller shaft passes to an overhead worm gear in the differential which drives 2ins (5.08cm) diameter fully-floating drive shafts. The standard differential reduction is 7.25 to 1, and 6.25 to 1 is optional.

Brakes
Four-wheel brakes operated through a Westinghouse vacuum-operated servo-assistance system. If the servo is out of action, braking is still available but requires higher pedal pressure. Brakes comprise two 2.5ins x 15.1ins (6.35cm x 38.4cm) expanding shoes in the 18ins (45.7cm) drum of each front wheel, and two 4ins x 18.7ins (10.2cm x 47.5cm) shoes in the 18ins (45.7cm) drum of each rear wheel. Total facing area is 451sq ins (2,909sq cm). A hand-brake lever expands two 1.875ins x 18.7ins (4.76cm x 47.5cm) shoes in each rear-wheel drum, giving a facing area of 140sq ins (903sq cm).

Steering gear
Worm and wheel steering set is fitted. Allows 2 turns lock-to-lock and a turning circle of 67ft 6ins (20.6m) right and 66ft 7ins (20.3m) left.

Frame: Steel longitudinal and cross members.

Springs

  • Front and rear axles are supported by longitudinal leaf springs.
  • Front springs 36ins long x 3ins wide (91.4cm x 7.62cm).
  • Rear springs 60 ins long x 4 ins wide (152.4cm x 10.2cm).

Wheels and tyres
Disc wheels with pneumatic tyres, 36ins x 8ins (914.4mm x 203.2mm), singles front and twins rear.

Wheelbase: 15ft 6ins (4.72m).

General measurements and weights:

  • Body space behind cab 15ft 10.25ins (4.83m).
  • Frame height (laden). 2ft 6.25ins (0.77m)
  • Chassis weight. 4.75 tons (4,826kg)
  • Loaded weight. 10.75 tons (10,922kg)
  • Front axle load (approx). 3 tons (3,048kg)
  • Rear axle load (approx). 7.75 tons (7,874kg)

Lighting set
A lighting set powered by a 12v battery is provided. A dynamo is fitted to maintain battery charge.