World War 1 broke out in August 1914, and the War Office found itself seriously short of lorries which it could call up for service, due to the failure of the subsidy scheme. It obtained a large number of vehicles from manufacturers and private users all over the country. During the August Bank Holiday week, Thornycroft was busy delivering to a mechanical transport camp in Kensington Gardens, London, all vehicles which had been nearing completion at Basingstoke, and which could be completed quickly with temporary or permanent bodies.
Shortly afterwards, Thornycroft was instructed by the War Office to supply its entire output of J type lorries for military use, the first batch being delivered on 8 September.
Later on in the war, Thornycroft was able to supply small numbers of J types to private operators while most of its production was absorbed by the War Office. About 5,000 J Type lorries were built during World War 1 by Thornycroft.
After World War 1, Thornycroft continued production of its four proven pre 1919 models, including the 2 ton BT, 3 ton X, 4 ton 40hp J and 5 ton 40hp Q type models. Later on, these types were joined by the 6 ton 40hp W, Thornycroft’s first post war lorry design.
The Hathi heavy four wheel drive tractor was introduced in 1925, designed for heavy haulage of lumber, etc over difficult terrain. It could cross gullies with 3ft high vertical banks, exert a steady drawbar pull of 9,000lb, haul a 10 ton trailer up a gradient of 1 in 10 at 7mph, and travel through soft sand. Its built in winch had a pull of 5 tons. The British Army acquired a number of Hathis for artillery transport. The vehicle was powered by a giant 11.33 litre six cylinder engine developing 100bhp.
By now, Thornycroft had become a major firm whose extensive product range included motor vehicles for goods, passenger transport and municipal service, cargo and passenger ships up to a length of 450ft, shallow draft vessels, oil tankers, destroyers, ferries, tugs, yachts, marine and stationary engines, motor boats capable of up to 40 knots (74kph), water tube marine boilers, etc. In addition to its London premises and its Basingstoke and Southampton works, Thornycroft had several depots and branches both at home and overseas.
The Thornycroft company was well-established and respected by 1927, producing commercial vehicles, ships and other engineering products.
In many ways the companys progress in lorry design was far more impressive between 1927 and 1932 than it had been from 1919 to 1926. All Thornycrofts lorries were powered by four cylinder engines during the earlier period and had spark ignition.
However, the need for more load capacity without loss of performance resulted in their most powerful lorry engine having around double the output of the previous eras best. The requirement for more power meant larger engines, leading to the introduction of six cylinder units.
Mercedes Benz caused a stir when it exhibited a diesel lorry at the UKs 1927 Commercial Motor Show, held at Olympia. Other manufacturers producing and marketing IC engined lorries in the UK were then offering petrol vehicles.
It was only a matter of time before they, too, had to introduce fuel efficient diesel lorries in order to stay competitive. Thornycroft was not the first British manufacturer to offer diesels, and the company appeared reluctant to do so, waiting until 1931 to introduce a lorry diesel engine.
However, once on board the diesel bandwagon, Thornycroft demonstrated that it lagged behind no-one in applying the new (for lorries) diesel technology. Larger, six wheeled, vehicles were introduced to cope with greater weights, and Thornycroft introduced vacuum servo assisted brakes as well as all wheel braking, superseding the rear wheel brakes fitted to an earlier generation of lorries. In summary, the 1927 to 1932 period was an era of more rapid technological advance in lorry design than before, both for Thornycroft and its competitors.
Road haulage expanded from the 1930s, in some areas competing with railways for the carriage of freight. Thornycroft lorry sales news items were spread over a wide area of operators both at home and overseas.