AJS PORCUPINE
g nickname – and although development took place slowly rather than sharpish, it won the first 500c World Championship in 1949 before this distinctive racer’s career was punctured four years later
Development story of the spiky-finned, 500cc E90 twin Grand Prix bike
‘WHAT STOOD OUT WERE THE SPIKY FINS ON THE CYLINDER HEADS; THEY WERE 5½IN LONG’
The gentlemen of the press had to wait to get their first look at AJS’S new race bike, just a few weeks before the 1947 Isle of Man TT. They had been invited to Associated Motor Cycles’ factory in Plumstead to view it, but sales manager Jock West had to apologise for the delay – the 498cc E90 parallel twin was still on show in the canteen, where the workers were getting their first sight of the finished motorcycle.
Like the workers, the journalists were impressed with what they saw. While the factory Manx Norton was stuck with plunger rear suspension, the AJS featured a swingarm rear end and telescopic forks, with a full cradle frame constructed from a mixture of round and oval-section tube. The unitconstruction engine was laid nearly horizontal in the frame to give a low centre of gravity, with the cylinders canted up at 15°. Bore and stroke were almost square at 68 x 68.5mm and the pistons rose and fell together, firing at 360° intervals. Although the generously-finned cylinders were separate, there was a one-piece cylinder head. Sodium-cooled exhaust valves and standard inlet valves were closed by hairpin springs. Drive to the two widely-spaced overhead camshafts was by a train of eight gears running on needle roller bearings, hidden by a Y-shaped Elektron cover. The Lucas racing magneto was driven by the first gear in the timing train. Cam boxes were also cast in Elektron, with that same magnesium alloy used for the combined crankcase and gear case, along with the bolted-on sump.
The crankshaft, a one-piece forging, was supported on a single plain bearing in the middle and roller bearings at each end. The conrods were forged in RR56, a high-strength alloy developed by Rolls-royce for aircraft engines, and ran on Vandervell shell bearings. Primary drive was by two straight-cut gears, so the engine ran backwards. The four-speed close-ratio gear was a conventional Brit design, except that it featured a cross-over drive so the rear chain was on the right side. A massive clutch was mounted outside the drive case to keep it running cool – it would spin at 80% of engine speed. One journalist noted that the clutch was so liberally drilled for ventilation it looked like a crumpet. While most of the engine casings were cast from magnesium alloy, the heads and barrels were made in aluminium alloy. Because the heads and cam boxes shielded the cylinders from the air flow, they featured generous longitudinal fins. But what really stood out were the spiky fins on the cylinder heads. Those spikes were 5½in long and looked like porcupine quills – so it wasn’t long before the E90 became known as the Porcupine.
It was an open secret that the E90 Porcupine was originally designed to be supercharged. The laid-down flat-twin concept was the idea of Joe Craig who had left Norton in January 1939, fed up that it had shelved its racing activities, to go first to BSA and then AMC. Craig was a brilliant development engineer but no designer, so the E90 was laid out by Vic Webb and Phil Irving. Unfortunately for AMC, Craig returned to Norton in 1946 when they made him an offer he couldn’t refuse, and then the FIM banned superchargers.
The wide valve angles and hemispherical combustion chamber limited the compression ratio in the original design, which wouldn’t have been a problem if part of the compression was provided by the supercharger. But to compensate for the lack of a blower, the E90 had to use forged Hepolite pistons with high domes to increase compression, which meant that the space between piston and cylinder head was now shaped like the thin
rind of half an orange and the fuel would burn slowly and inefficiently. Long, curved inlet tracts, designed for their large volume to buffer the supercharged induction, were also a major headache and as a consequence the Porcupine struggled to carburate cleanly throughout the rev range.
The 1947 Senior TT didn’t run to plan for the AJS team – hardly surprising when you consider that the two bikes had never been ridden before they reached the Island. Jock West, who was works rider as well as working in sales, struggled to bump sta t his because of oil drag in the plain bearin effect, and then his clutch started to sl riding, but had to call into the pits for taken 1hr 24min. Meanwhile, Les Gra riding with a severely damaged left han place, but his chain jumped the sprocke to push in from Governor’s Bridge to cla finished 1-2, with Velocette in third plac fixed, West had made the second-fastest 40bhp Porcupine, only 3sec slower than A
Reliability problems would slowly be so Porcupine E90 would go down in history won the first 500cc World Championship got off to a bad start. With just two miles a two-minute lead over Harold Daniell’s N nothing could stop an AJS victory in the Se magneto’s long drive shaft sheared and Les push home to a heroic 10th place. He also f Belgian GP. But with wins at the Swiss and U a second place at the Dutch, Graham did eno the world title by one point ahead of Nello P Gilera Four. AJS also won the constructor’s c
The Porcupine lost its spikes in 1951 when casting for the cylinder heads was ditched in f two separate castings with horizontal, transve
better combustion chamber shape and power climbed from the original 40 to nearly 50bhp.
Instead of being mounted in the conventional position under the seat, the oil tank became a long boat-shaped magnesium alloy casting bolted under the engine. The wheelbase was shortened by an inch, and the forks lowered by the same amount, but these were minor modifications. Only four E90 Porcupines had been built before AJS produced an updated version for the 1952 season.
Designed by HJ ‘Ike’ Hatch, the E95 would still be a unitconstruction twin with the same 68mm bore, but stroke reduced by 0.25mm. The Y-shaped gear train to the overhead camshafts remained, but the barrels and heads (two separate castings) were inclined at 45° to the horizontal, which allowed the area around the exhaust ports to run at a much cooler temperature and the wheelbase to be shortened. The cylinders now had conventional axial fins while the heads had transverse fins across the top of the combustion chamber, bent into a V-shape to clear the spark plugs. Fitted to short, almost straight inlet tracts the Amal GP carburettors shared a single float chamber. The Lucas rotating-magnet magneto was now chain driven. A new frame used the redesigned engine as a stress member .
The 1952 championship season started well for AJS, with the spikeless Porcupines finishing first, second and fifth at the Swiss GP in Berne, but that success wasn’t repeated, even when AJS reverted to a full-cradle frame the following year. In a final attempt to regain its competitive edge the E95 was updated one last time for the 1954 season, with changes to reduce height and improve wind penetration. The headstock was lowered and the forks shortened, while the frame was also lowered about 1½in. AJS bosses insisted on using ‘jampot’ shock absorbers, although none of the works riders appreciated the importance of promoting the roadster’s rear suspension. But it was the 6½-gallon alloy fuel tank that was the star of the MKII E95. With sides that reached nearly to the swingarm spindle, it was really two pannier tanks with a bridge over the top. Thanks to recesses in the panniers, the tank provided effective streamlining for the rider’s legs.
Because the petrol taps were now below the carburettors, fuel flowed from the main tank to a float chamber mounted under the back of the gearbox. A pump, driven by the magneto drive, fed the fuel from the float chamber to a header tank inside the main tank. Gravity then fed petrol to a pair of weirs each side of the carbs which also set their fuel level. Surplus fuel fell over the weirs and drained back to the float chamber. To prime the system, the bike had to be stood nearly vertically on its rear wheel to get the fuel over the barrier between the header tank and the main tank!
But despite its impressive looks, the E95 was no match for a Norton Featherbed or a Gilera Four. Rod Coleman, Derek Farrant and Bob Mcintyre were the works riders for the 1954 Senior TT. On the first lap Farrant slid off his E95 at a wet Quarter Bridge, only a mile from the start. Coleman held fifth place for three laps but retired with a split fuel tank. And Mcintyre eventually finished in 14th place. Five days later, at the Ulster GP, the world’s fastest road race, Coleman finished second with an average speed only 0.65s slower than Ray Amm’s winning Norton. The Porcupine was giving 55bhp and 145mph, but it wasn’t enough. At the end of the year, AJS announced they wouldn’t be sending works teams to the 1955 TT or contesting international road races. The Porcupine had been well and truly spiked.
‘TO PRIME THE FUEL SYSTEM, IT HAD TO BE STOOD NEARLY VERTICALLY ON ITS REAR WHEEL’