AJS Porcupine

The AJS 500 cc Porcupine was designed to be supercharged, as were a number of pre-war racing bikes, and was announced in 1945, but the FICM banned supercharging in 1946. [ [http://news.bbc.co.uk/1/shared/spl/hi/pop_ups/03/uk_vintage_motorbikes/html/2.stm] News-BBC UK "Vintage Motorbikes" (Retrieved 1 November 2006)] The Porcupine was preceded by the water cooled, supercharged 1935 - 1939 AJS V4.

The motor was then worked on to allow it to perform without a supercharger, and in this form Leslie Graham rode it to victory in the first GP World Championship of 1949. [ [http://www.teamobsolete.com/nmm/nmmrev.html] TeamObsolete "2000 and 2001 Review" (Retrieved 1 November 2006)] This was the first and only World Championship win for AJS.

Despite this victory, the bike is often cited as unreliable, and not living up to its promise. [ [http://www.pimpmymotorcycle.com/tribute/sammy-miller-museum.php] PimpMyMotorcycle "Sammy Miller Museum" (Retrieved 1 November)]

The Mechanicals

The Motor

The E90S Porcupine motor was a unit construction, aluminium alloy, 500 cc, DOHC twin, with horizontal cylinders and heads, to give the bike a low centre of gravity. A later version of this motor was named the E95, re-engineered to have its cylinders inclined at 45 degrees for better cooling and easier carburettor installation, and is claimed to have produced 55 bhp @ 7600 rpm [ [http://www.diseno-art.com/encyclopedia/vehicles/road/bikes/rare_classic_bikes/ajs_porcupine.html] DisenoArt "AJS Porcupine" (Retrieved 2 November 2006)]

The gear drive for the camshafts was on the right of the motor, while the gear primary drive was on the left. The geared primary drive meant that the motor ran “backwards”. The cam gear drive also drove a jackshaft at the rear of the cylinders, which drove an oil pump, a fuel pump, and (via a chain drive) a Lucas magneto. The four speed gearbox output was on the right.

As originally designed, the E90S was to be supercharged, with the blower mounted above the gearbox, and driven by the clutch. The loss of the supercharger meant that the design was compromised, lacking sufficient fly-wheel effect, which caused problems with magneto failure. The main initial change was to reduce the valve angle to 90 degrees for a more compact combustion chamber.

It had rubber mounted twin GP carburettors, inclined at 49 degrees, with an unusual float tank system used rather than float bowls. Plain bearings were used for the big end bearings and the centre main bearings. The outer main bearings were rollers.

One problem cited for the engine is the use of non parallel valves with conventional rockers. [ [http://www.freepatentsonline.com/6457444.html] FreePatentsOnline Background Of The Invention (Retrieved 2 November 2006)]

The 1947 model engine was mostly made of alloy, but, as an experiment, the head was cast in silver for increased thermal efficiency. Due to the softness of silver, it had to be alloyed to make it hard enough for racing use. By the time this was achieved the thermal efficiency gains were lost, and the experiment was abandoned. [ [http://www.mrs.org/s_mrs/bin.asp?CID=2984&DID=94886&DOC=FILE.PDF] MRS "The Art and Materials Science of 190 mph Superbikes" (Retrieved 2 November 2006)] There is dispute as to whether the silver cylinder head went beyond feasibility-study stage.

The Frame

The Porcupine used “Jam-pot” shocks and Teledraulic race forks. The E90 model had an open frame. The E95, introduced in 1953, had a loop type frame with the motor mounted lower. [ [http://www.mortonsarchive.com/viewer.asp?itemref=2&lasttype=Documentation] MortonsArchive "Last of the Porcupines" (Retrieved 2 November 2006)]

Only four E95 Porcupines were built. [ [http://www.teamobsolete.com/nmm/nmmrev.html] TeamObsolete "2000 and 2001 Review" (Retrieved 1 November 2006)]

The AJS Porcupine Today

Team Obsolete have restored a 1954 AJS Porcupine E95, using an ex-Tom Arter MKII motor, which features a unique Jack Williams built up roller bearing crankshaft. They use it for exhibition and historic racing. [ [http://www.teamobsolete.com/porc/porcupine.html] TeamObsolete "Porcupine" (Retrieved 2 November 2006)]

Tom Arter put a complete 1954 E95 AJS Porcupine up for auction, and it sold for $228,620. [ [http://www.collectorcafe.com/article.asp?article=455] Collector Café "Rare bikes and books" (Retrieved 2 November 2006)]

ources