Historic Motorcycle Brands

Dunelt Motorcycles were produced by Sheffield engineering company, Dunford & Elliott (Dunelt is a combination of their names), from 1919 until 1935. 

Dunfield & Elliott had been kept busy during Word War One, supplying specialist steels for the War effort and found itself short of work when peace broke out. The company hit on the idea of producing motorcycles, but not just ordinary motorcycles. The Dunelt would be powered by a two-stroke engine, but with four-stroke-style low-speed torque.

Firstly, a little background of two-stroke engine design in the early 1900s:

Joseph Day’s patent No.6410 of 1891 covered two-stroke engine variants using a piston-controlled transfer port in the cylinder wall. The compressing effect of the descending piston forced the combustible petrol-air mixture from the crankcase into the cylinder, via that port.

This incoming mixture also pushed the exhaust gases out of the cylinder, via another cylinder wall port.

A problem with this layout was that some of the combustible mixture was lost out of the exhaust port, before the rising piston could seal it off, but that problem had been partially resolved by adopting a wedge-topped piston, known as ‘deflector piston’. This shape forced the incoming air-petrol mixture to flow to the top of the combustion chamber, away from the exhaust port.

The remaining issue with these early two-stroke engines was the limited charge pressure inside the crankcase that could be achieved by the descending piston. This problem effectively limited two-stroke, single-cylinder engines to a maximum 350cc displacement.

Dunford & Elliott had heard of a stepped-piston concept that could resolve this issue and the company set about implementing it. 

Dunelt two-stroke cylinder with stepped piston – Montagu Cars and Motor Cycles

The patented Dunelt 499cc engine had a cylinder barrel with different upper and lower bore diameters, in which slid a ‘stepped’ piston with two matching diameters. 

The pistons were double diameter and also bi-metallic, with an iron/steel section cast in for the ring grooves – not cheap to make. 

Where a standard-bore engine had 499cc-limited mixture induction and transfer ability, the Dunelt engine could suck into its crankcase and transfer into the cylinder about 770cc of charge petrol-air mixture.

A deflector piston ensured that most of that mixture remained in the combustion chamber.

This basic form of supercharging had the desired effect and, in conjunction with a heavy flywheel, earned the Dunelt a reputation for being a torquey, ‘slogging’ engine that lent itself to sidecar applications, including commercial service and delivery tasks.

Barnstormers NZ

Dunelt used that characteristic in its promotional material that  featured a cartoon character, ’Professor Supercharge’. Also, a Dunelt motorcycle was the first to cross the desert from Cairo to Siwa Oasis and back in 1924. 

A strong bottom end was key to its heavy duty roles, so the Dunelt engine had a one-piece, forged crankshaft and a one-piece conrod, with a big-end roller bearing. 

If that seems impossible, the inner diameter of the conrod’s big end was large enough for it to be fed over the crank web, into place on the journal. A groove in the web allowed the rollers to be inserted – one by one – between the rod’s inner race and the journal. The rollers were crowded, not caged and were retained by four semi-circular end plates that were fastened into holes pre-drilled into the rod big end.

After rod assembly, the crank balance weight was bolted into place.

Sheffield Star advertisment

By 1924, Dunelt was producing eight 499cc models, with various types of sidecars, including trays, boxes and vans that were joined the following year by milk carrying and fire engine bodywork.

1926 Model K – The Motor Cycle

A 250cc solo-bike, Model K version that was also aimed at women riders followed in 1925. It later set two Double Twelve-Hour World Records at Brooklands. In 1927 a young German woman, Suzanne Koerner, rode a Dunelt motorcycle from Berlin to Birmingham

The 499cc was dropped in 1927 and was replaced initially by Sturmey Archer side-valve engines, then by JAPs and Rudge Pythons.

1929 Dunelt 350 Majestic Twin Port Sports Sturmey Archer engine – Classic Style Motorcycles

The Great Depression had hit hard by 1930, when the Dunelt 250cc stepped-piston engine was phased out and replaced by Sturmey Archer, Villiers, JAP and Rudge Python engines.

In 1930-31, a 498cc, overhead-valve Dunelt won the Maudes Trophy on the Isle Man, by covering 13,119 miles at an average speed of just under 35mph, in 16 days.

1930 Dunelt 500 – Mecum Auctions

In 1931, manufacturing was transferred from Birmingham to Sheffield and the bikes were branded Sheffield-Dunelt for a couple of years.

By 1935, Dunelt was in trouble and produced its last motorcycle, reverting to the production of outboard motors and bicycles. Raynal purchased the rights to build Dunelt Cycles (bicycles) in 1937 and was bought out by Tube Investments in 1950. 

TI produced a series of prototype mopeds in the mid-1950s and a Dunelt moped was exhibited at the Earls Court show in 1956.

• BACK TO MOTORCYCLE BRANDS