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The supercharged AJS V4 – Britains most exotic racer, ever!

3 August 2023

In the late nineteen-thirties, continental European motorcycle manufacturers like BMW in Germany and Gilera in Italy were exploring the exciting possibilities presented by supercharging whereas their British rivals such as Norton and Velocette were still persevering with successful but unadventurous normally aspirated singles for their racing efforts. Whether this was conservatism or complacency is debatable but either way, it led to the surrender of their position as leaders in Grand Prix competition.

There was only one British manufacturer ready to tackle the continental teams head-on and this was AJS with its own 500cc supercharged V4. It was then Britain’s most exotic Grand Prix racer ever and remains so 85 years on.

The bike had a 50-degree V4 single overhead camshaft engine with water-cooling and a Zoller supercharger. It produced 55 bhp at 7200 rpm, which was an extremely high power output for the pre-World War II period.

The engine was mounted in the same frame used by the firm’s 500cc overhead-camshaft, single-cylinder TT racers, though it was slightly modified to house the bulkier and much more powerful motor.

It was no surprise, therefore, that erratic handling was the bike’s Achilles heel, even after the rigid frame had been modified to adopt pivoting link front forks and plunger sprung rear suspension, each fitted with adjustable friction dampers.

Two of these machines were entered by AJS for the 1939 Senior TT but the result was a disappointment as Walter Rusk and Foster were only able to finish 11th and 13th. The seven-lap race was dominated by the BMW ‘flat twins’ that represented small-engine supercharging at its best and which took Georg Meier and Jock West to a comfortable 1-2 finish.

Nevertheless at the following Ulster Grand Prix, held on the 20-miles long Clady road course that was one of the fastest on the GP scene, the AJS V4 at least proved that it was more than fast enough to take on the European opposition.

With the outbreak of WW2 less than a month away, the BMW team was unsurprisingly absent but from the drop of the starting flag the AJS riders Bob Foster and Walter Rusk easily pulled clear of a field that still included the works Norton team and Dorino Serafini on the Gilera with its supercharged four-cylinder engine.

But with the AJS already timed at 135 mph along the Clady circuit’s legendary seven-mile straight, Foster retired from the lead with plug trouble halfway around the opening lap

This left his brawny Irish teammate Walter Rusk to wrestle a bike which had already acquired a reputation as a wayward handler. Even so, Rusk had a 19-second lead at the end of the lap, which he had covered at of 100.01 mph. It was the first time that any road race circuit (as distinct from a Brooklands-style banked speed bowl) had ever been lapped at a three-figure speed.

Unfortunately, Rusk’s supremacy was short-lived as, halfway around the fourth lap, a link on the girder front forks of the fast but heavy AJS broke, quite probably due to the lateral strains of the rider wrestling with the bike!

In the end, it was Serafini on the Gilera who won this final race in the 1939 season, a victory which saw him crowned as European Champion (the equivalent of winning a World Championship in those days).

Only fifteen days after the race came the outbreak of World War II, a conflict in which RAF pilot Walter Rusk was sadly to lose his life. His historic Ulster GP aboard the AJS V4 was his last-ever ride on a racing motorcycle.

The resumption of Grand Prix racing after the war came in June 1946 at the Chimay circuit in Belgium and it was there, after emerging from its wartime mothballs, where the AJS V4 was to finally score its only victory.

The race was known as the Grand Prix des Frontieres as the circuit was located close to the border of Belgium and France and for that event the AJS V4 was in the hands of Jock West. He had naturally severed his connection with the BMW team at the outbreak of war and had joined AJS as its sales director. So his pre-war employment meant that Jock at least had a foretaste of what to expect when racing his new employer’s supercharged V4.

Seven years earlier, while riding the BMW in the Senior TT, West had watched nervously from close quarters as Bob Foster wrestled with the AJS. He had easily caught up with Bob from his later starting position but then had no option other than to stay behind the fast but wayward V4 for five miles until there was a safe opportunity to pass!

Luckily, the fast open roads of the Chimay circuit placed more of a premium on speed than on handling and West was able to use the power of the supercharged AJS to take the chequered flag in first place at an average speed of 79 mph.

Unfortunately, a second outing a month later on another very fast circuit at Albi in France resulted in retirement when the engine locked solid after seizing a big end and crankpin. This was on the penultimate lap and at the time West was well in the lead and headed for a repeat victory after setting a new lap record at 87 mph.

Sadly, the most exotic racing motorcycle ever built by a British factory was then rendered obsolete at the end of the season in which it had scored its only victory. The forecast of a ban on supercharging in motorcycle racing became a reality and that was the end of the line for what was the most ambitious project ever undertaken by a British motorcycle manufacturer.

Fortunately for today’s enthusiasts, the AJS V4 has been rebuilt and can be seen at the superb Sammy Miller Motorcycle Museum at New Milton in Hampshire.

Words by Bruce Cox.

Photographs courtesy of The Motorcycle Files library

About the author:

Bruce Cox has spent his business lifetime in the motorcycle media in both the UK and USA as a journalist, magazine publisher, international event promotor and TV documentary programme producer.

For the full story of the AJS V4 we recommend the e-book by Alan Cathcart in The Motorcycle Files series available from Amazon.

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