As most of you who have found your way here probably have some idea about the Mini Motor I don’t propose to give a detailed history. The ‘Mini Motor’ section of the web’s ‘The Moped Archive’ will give you more information than you probably ever really wanted to know. ‘The Stink Wheel Saga – Episode 1’ by David Beare, Andrew Pattle and Philippa Wheeler (2004) describes the cyclemotor boom in book form and has a whole chapter on the Trojan Mini Motor.
Very briefly the Mini Motore was designed in Italy by Vincent Piatti as an auxiliary engine to drive lathes and similar devices. Its possibilities as a crude way to motorise a bicycle were quickly apparent and may have influenced the design. It is quite ‘large’ for a cyclemotor with a bore and stroke of 38 x 44mm giving 49.9ccs.
Its beauty is its simplicity. The crankshaft has a magneto on one end and the drive roller on the other. The two stroke engine is ‘piston ported’ so cut outs in the cylinder walls align with cut outs in the piston to allow pressurised mixture in the crankcase to be forced up into the cylinder. There are therefore just three moving parts: the crankshaft, connecting rod and piston. (This is not quite true as a set of points, activated by a camshaft cast into the magneto side journal of the crankshaft, time the spark.) The carburettor, which is needleless, is unusual in that turning the air filter adjusts the mixture to the engine and is used to fine tune the engine once warm, weaker on the flat and richer for the hills. With practice this can be done ‘in-flight’ by stretching a ‘blind’ left hand behind you while controlling the bike with the right.
Image from: http://www.Buy_Vintage.co.uk
The tank acts as the engine mounting frame and is hinged from the seat post. A cable to the handlebars operated by a large lever with a catch raises and lowers the engine. The engine was built from 1949 to 1957 and its ancillaries were developed and improved over the first four years. One of the most significant of these changes was a ‘toggle action’ mechanism which held the engine more positively on and off the tyre.
Mark 5 engine with ‘toggle action’ lowering mechanism. (Photo from the Trojan Motors Museum)
Probably because the position of the engine leaves the bicycle’s own transmission intact (and therefore able to assist the engine through an existing gear system) the Mini Motor was quite a successful as a long distance machine. Three Italian riders rode three machines to the UK in 1948 to find a British manufacturer willing to produce the engine under licence. Once Trojan started production George Murray Denton, an enthusiast for the engine having seen the Italian version in action on holiday in the country in 1948, became sales manager and with a team of other riders promoted the motor in a series of long rides, some against the clock.
A Mini Motor rider clocks in on a timed long distance event in the 50s (photograph believed to be from Denton family archive now owned by Tony Dent)
The engine I originally fitted in 2011 is a Mark 3 (prefix C) sold new in April 1952 in Leeds. ‘If it ain’t broke don’t fix it’ – so apart from adjusting the timing, cleaning the carburettor and changing the spark plug I have left it alone.
I bought it off the son of the original owner. As a boy the vendor lived in Bradford where his father was a school caretaker. At lunchtime the boy used to meet his father who worked in another school not far from the one he attended. When the boy was a young teenager his father was transferred to another school on the other side of the city and lunch together became impossible. Refusing to be beaten, the boy dragged his father’s old tandem from the shed which at that time had the Mini Motor attached. Ingeniously he covered the engine (which he was too young to ride) with some home-made cardboard panniers. Lunch with Dad was on the menu again. All appeared to be going well when an apologetic police officer overtook the innocent looking outfit and flagged the young cyclist down. “I am sorry sonny”, he said, “I had to stop you to let you know that there’s smoke coming from underneath your luggage. I think you tyre must be rubbing somewhere.” Further investigation led to the miscreant’s father being fined 17s 6d for allowing a motor vehicle to be used by an unlicensed driver.
Motor 2 (the spare)
I have built up a second engine to try and head off the bad luck that is almost certain to follow my having left Motor 1 to its own devices. It was an eBay special ‘with compression and a spark’. The seller neglected to mention that it also had a cracked cylinder head, damaged piston and broken Woodruff key which ensured that the spark wouldn’t actually have made the engine run. Luckily a misspent middle age has left me with a large plastic box of Mini Motor components and the engine runs now although not as well as the original.
One crucial factor in the LEJOG ride with be the roller’s relationship with the back tyre. Trojan produced three types of roller: the ribbed (as in the picture from Vintage Bikes), the carborundum and the herring-bone. The first grips moderately but wears very quickly, the second grips well but destroys the tyre and the third neither grips particularly well nor wears the tyre rapidly. Motor 1 has roller type one and Motor 2 has type three. This should ensure 850 miles of roller slippage but not too many new rear tyres (which are a pain to fit).
Motor 2 on its first proving run – that power! That sound! That vibration!