BMW Kompressor
BMW spearheaded the pre-war shift to supercharging, with their boxer twin GP racer – and help from a surprising source of technical know-how
In 1939 Georg ‘Schorsch’ Meier created history when he won the Senior TT riding a works BMW 500 Type 255 Kompressor, the first time a supercharged machine had won the blue riband race on the Island. The previous year, Meier had ridden the same 1938 Type 255 Kompressor to win the European 500 championship (the pre-war equivalent of today’s world championship), but the history of the forced-induction BMW went back much further than that.
It was the brainchild of youthful Swiss-born BMW designer Rudolf Schleicher, who, in 1925 at the age of age 28, had designed the R37, BMW’S first sporting motorcycle. The R37 won more than 100 races, with its creator demonstrating its versatility (as well as his own) by winning a gold medal aboard one in the 1926 Isdt/international Six Days Trial. Schleicher left BMW in 1927 to work for the Horch car company (a forerunner of today’s Audi) after a disagreement with his boss Max Friz about the potential of technology the company had previously used on its aircraft engines – supercharging. Friz was unconvinced, but when BMW rider Ernst Henne beat the world land speed record in 1929 with a Schleicher-designed 735cc supercharged twin, it dropped a heavy hint he might be wrong – especially as works BMW rider Hans Soenius had won three successive German 750cc road racing titles in 1927-29 with a similar machine. In 1931, his 137.66mph record having been smashed by Britain’s Joe Wright on a 1000cc supercharged Zenith-jap at 150.73mph, Henne appealed to BMW president Franz
Josef Popp to persuade Schleicher to return, which he duly did, extracting more power from the bike with an uprated Zoller supercharger. Henne regained the world record in November 1932 at 151.77mph, raising it eventually to a remarkable 159.10mph without any additional streamlining!
In 1928, as the precursor to its return to the aviation sector, BMW had purchased a licence to manufacture American Pratt & Whitney aircraft engines, which happened to be supercharged. A closer study of these had overcome Friz’s objections, so after Schleicher’s return to head up BMW’S experimental department, he was given the task of designing a supercharged 500cc road racer, allowing BMW to compete successfully at European level – a task given added impetus with Adolf Hitler’s accession to power in 1933.
The pursuit of German success in European motorsports received government backing, with Auto Union and Mercedes-benz being the standardbearers on four wheels, and BMW alongside DKW on two. Schleicher’s new Type 255 Kompressor design was developed during 1934, and made its competition debut in May 1935 at Berlin’s Avusring races held on a highspeed, autobahn-based, banked track, to demonstrate the validity of BMW’S efforts to the country’s high command.
Things didn’t go completely to plan, though, with BMW’S Karl Gall finishing second and team-mate Ludwig Kraus fifth on the 500 Kompressor’s debut, behind victorious Swedish rider Ragnar Sunqvist on an unblown works V-twin Husqvarna. BMW focused on development for the remainder of that year, with no further outings until October, when Ernst Henne opened the BMW Kompressor’s victory roster by improbably winning a gold medal in that year’s ISDT held in Oberstdorf, Kraus also winning gold in the sidecar class.
Schleicher’s purpose-built new design was extremely avantgarde, clean-looking and light, with all major engine components including the crankcase and cylinder heads cast in light but expensive Elektron magnesium alloy, delivering an amazingly low dry weight of just 137kg for the whole bike – compared to 153kg for the much less powerful un-supercharged Norton single, its main rival at the time. Measuring 66 x 72mm for a capacity of 493cc (there was also an overbored 72.2 x 72mm 590cc variant for the 750 class), the engine’s one-piece magnesium crankcase weighed just 5.6kg, with a separate finned three-litre oil sump bolted beneath it. The built-up all-roller bearing 180° crankshaft ran on two 35mm main bearings, each pressed into separate housings (cast iron at the front, steel at the rear) spigoted and bolted into the crankcase. The 140mm long one-piece elliptical conrods carried four-ring flat-topped Mahle pistons, each with three compression and one scraper ring. These ran in seven-stud cylinders turned from steel, each weighing 3.4kg and lightly finned, and seated on aluminium plates. Compression ratio was a lowly 7:1, with the single 27mm Fischer-amal carburettor offset to the right, feeding a multi-cell vane-type Zoller volumetric supercharger mounted in the nose of the crankcase. Delivering 1.0 bar of boost (almost 15psi), this was driven at engine speed directly off the front of the crank, with long induction pipes swept down beneath each cylinder to the rearfacing inlet ports. These provided ample capacity for the incoming charge, with any potential drop in density minimised by the direct air cooling of the system. The opposite end of the crankshaft provided chain drive to a jackshaft positioned above it, which in turn drove the Bosch magneto mounted on top of the crankcase via a secondary chain, plus via bevel gears the horizontal shaft on each cylinder driving BMW’S unique double-overheadcam layout. Each shaft had a bevel gear at its opposite end which drove one camshaft in each cylinder head directly
GEORG MEIER
MEIER REGISTERED BMW’S FIRST VICTORY AT MONZA, TO CLINCH THE EUROPEAN CHAMPIONSHIP IN HIS ROOKIE YEAR
– the exhaust on the right one, the inlet on the left (reflecting the offset cylinder location of the 180º flat twin), with the second camshaft geared directly to the first.
Unusually for a twin-cam design, short I-section forged steel rockers with cast iron followers were used to actuate the two valves per cylinder, because the cams were too close together to operate the valves directly. Double hairpin springs controlled the valves, which were set at an included angle of 82º to each other within hemispherical combustion chambers, with side-mounted 14mm sparkplugs.
The magnesium housing containing the four-speed gearbox was bolted to the rear of the crankcase, with a single-plate dry car-type diaphragm clutch keyed via a taper to the rear of the crank, thereby transmitting the crankshaft drive to the gearbox mainshaft through the splined centre of the driven plate, with the clutch ventilated by air ducts in the housing. Shaft final drive was standard practice, but the positive-stop left-side foot gearchange was a first on a BMW.
This ground-breaking engine design initially delivered 55bhp at 7000rpm, later increased to 68bhp by the end of the decade, running on a 50/50 petrol/benzole mix with two per cent castor oil added to lubricate the blower. It was fitted in an equally avantgarde chassis design which featured hydraulically-damped telescopic forks for the first time on any motorcycle, personally designed by Schleicher. The arc-welded twin-loop tubular steel cradle frame employing a mixture of round and oval tubing was originally devoid of rear suspension, but for 1937 Schleicher introduced a sprung rear end featuring vertically-mounted plunger dampers. The latter was developed by the man who would later succeed him, Alex von Falkenhausen, who overcame the reluctance of the riders to race with the new design by doing so himself in the 1936 ISDT, at Füssen, 80 miles from the factory, winning a gold medal. Development was hastily stepped up after Hitler visited the BMW stand at the 1936 Berlin Show, and threw out the question: “When are we going to see some rear suspension on your bikes?”
In the 1936 Swiss GP at Berne, works rider Otto Ley finished second to Jimmy Guthrie on the un-supercharged Norton single. Two weeks later, Ley scored the Kompressor’s maiden road race victory at the Solitudering near Stuttgart in front of 200,000 spectators, but was beaten again by Guthrie’s Norton in the Dutch TT. Finally, at Saxtorp,
Sweden on August 30, Ley gave the new BMW its first GP victory, beating team-mate Karl Gall to defeat Husqvarna on its home ground. But in the Italian GP at Monza on September 27, Gall and Ley finished third and fourth behind winner Omobono Tenni and team-mate Giordano Aldrighetti on the V-twin Moto Guzzis. There was still work to do... It all came good for the BMW Kompressor in 1937, its rigid rear end’s lurid handling much improved with the adoption of von Falkenhausen’s ‘spring heel’. After a 1-2 victory for Ley and Gall at the Avusring on May 30, in front of a watching Hitler, the BMW Kompressor came sixth in its Senior TT debut in the hands of Jock West, the sales manager for BMW’S UK importer HJ Aldington.
Gall won the Dutch TT at Assen, with Ley retiring, but both BMWS retired with mechanical woes from the Swiss GP eight days later, leading them to withdraw from the next week’s Belgian GP. Gall won BMW’S home race, the German GP at the Sachsenring, though in tragic circumstances; reigning European champion Guthrie was holding a comfortable lead on his Norton entering the final lap, only to be forced off the road by another rider, crashing fatally. Ley had already retired from that race, but won the Swedish GP ahead of Gall, while Jock West took victory for BMW in the Ulster GP, where the Kompressor’s 140mph top speed on the ultra-fast Clady circuit with its Seven-mile Straight was a decisive factor.
BMW had turned the tables on Norton – but Gall could only finish fourth and Ley sixth in the Italian GP at Monza, behind winner Giordano Aldrighetti on the much-improved four-cylinder, watercooled and supercharged Gilera Rondine, which Gilera had purchased in 1936 from the Rome-based CNA design partnership.
For 1938, Ley was replaced in the BMW team by a strapping Bavarian policeman-turned-army sergeant named Georg Meier, who finished fourth on his debut outing on the Kompressor in the final round of the previous year’s German championship at Hockenheim.
In the first European championship race of the season, the Senior TT, Gall crashed in practice and sustained a fractured skull, while Meier stripped a thread changing from soft plugs to hard after warming up the bike for the June 17 race and non-started. It was left to the trusty Jock West to get some sort of a result for BMW, finishing fifth on a Kompressor behind Norton’s replacement for Guthrie, the victorious Harold Daniell.
Meier took victory in the Belgian GP at Spa, with West replacing Gall to finish third, with the Bavarian ‘iron man’ taking victory again at Assen, ahead of guest Dutch team-mate Bertus van Hamersveld in second. Next, Meier won his and BMW’S home GP at Sachsenring, with team-mate Ludwig Kraus fourth, followed by Jock West’s repeat victory in the Ulster GP. Finally, Meier registered BMW’S first victory at Monza with Kraus second, to clinch the European Championship for BMW and himself in his rookie year in GPS. He also won the German 500cc title.
As 1939 began, the clouds of war were getting darker. Norton withdrew from GP racing, although AJS replaced them with its supercharged V4 (see page 78), while the blown Velocette Roarer parallel twin, also being developed, would never race. This left Gilera as BMW’S main rival, but the Italian team opted not to travel to the Isle of Man.
As usual, BMW entered a three-man team for this race, but their participation was put in doubt when Gall crashed at Ballaugh in practice and again fractured his skull – this time tragically dying four days later.
After some soul-searching, the decision was taken that Meier and West should race, with the German rider leading his team-mate home in a 1-2 finish at a record average speed for the three-hour seven-lap, 264-mile marathon of 89.38mph. After the most important race victory of his career, Meier was drafted into the Auto Union racing car team to replace their star driver Bernd Rosemeyer, who had been killed during a land speed record attempt on a German autobahn. With BMW’S blessing, he raced the 478bhp supercharged rear-engined V12 Auto Union on alternate weekends to the BMW Kompressor. But in August Meier crashed the BMW in the Swedish GP at Saxtorp while trying to make up time after a pit stop, injuring his back and putting him out for what remained of the season.
This allowed Gilera’s race winner Dorino Serafini – second at Spa, after crashing at Assen – to overtake him in the points table by winning the German GP at Sachsenring which was held the following week, in spite of a four-rider blitzkrieg attempt by BMW to prevent him doing so.
Serafini then won the Ulster GP just six days later, but on September 3 war was declared, and while Serafini may be considered to have won the European Championship for Gilera, it was never officially recognised by the FIM.
With a vanquished Germany not readmitted to the FIM until 1950, the post-war international ban on supercharging was ignored in Germany and, with an updated 500 Kompressor now producing some 80bhp, Meier won four consecutive German 500cc National titles from 1947 to 1950.
By the time Germany was readmitted to the FIM, supercharging was banned and the Kompressor motor was
JOCK WEST
WEST WON THE ULSTER GP – THE KOMPRESSOR’S 140mph TOP SPEED WAS A DECISIVE FACTOR
redesigned into the normally-aspirated Rennsport engine which dominated sidecar racing for the next quarter-century. It powered to no less than 14 successive Sidecar World Championships from 1954 to 1967, and 19 in total, as well as permitting Walter Zeller to finish runner-up to John Surtees in the 1956 500cc World Championship on the Type 256 Rennsport, ahead of all the four-cylinder Gileras. Only between 12 to 15 true BMW 500 Kompressor Type 255 dohc models were ever made between 1935 and the outbreak of war in 1939. The supercharged bikes were only ever raced by the factory, and never sold to private owners. Those that survived the war, were often at the cost of their provenance, since some had been smuggled out of the country as war booty.
In the late 1970s, one dismantled Kompressor was tracked down in the USA by John Surtees, and painstakingly restored before he sold it back to BMW, where it forms part of the company’s Mobile Tradition historic vehicle collection. Though often presented as being Meier’s Tt-winner, the BMW factory has no records of race engine and frame numbers from the pre-war era, so unfortunately there is no way of authenticating this.
Two other Kompressor twins exist, built using genuine PRE-WWII Kompressor parts, with a pair of later un-supercharged 1950s plunger-frame RS500 Rennsport GP racers as the platform to create pretty authentic Type 255 replicas. One was sold for $480,000 at a Las Vegas Bonhams auction in January 2013, and was formerly the property of Walter Zeller, BMW’S 1950s factory GP rider.
Zeller had been eager to parade Meier’s late 1940s-era four-time post-wwii German championship-winning RS255 Kompressor, then on display at the BMW Museum in Munich, but when BMW politely refused his request in 1980, he combined a 1951 Rennsport plunger frame with a 1939-spec Type 255 engine. Gustl Lachermeier, a BMW race engineer for almost four decades, personally built the engine to 1949/50 specification, with a bigger Zoller supercharger than that fitted to Meier’s 1939 bike.
The 1951 chassis had a strengthened frame, while the leading-axle 28mm telescopic forks were very similar to the type used by BMW in 1939. Zeller modified the original 200mm single leading-shoe front brake for twin-leading
shoe action, and used clip-ons instead of the 1939 bike’s flat aluminium handlebars. The rear wheel was built with a 19in rim instead of 20in, as no new tyres of that size were available. But Zeller and Lachermaier otherwise used mostly original Kompressor racing components, which they accumulated from BMW factory and museum stock or private contacts, to create the bike Zeller rode in historic parades before he passed away in 1995. Lachermeier then assisted German BMW fanatic Wilhelm Schütz in creating a second identical such machine. “My dream was always to be able to ride a Kompressor,” said Wilhelm. “But except for the Zeller bike, the others are either lost or in the hands of people who don’t use them, but won’t sell them, either. Therefore I asked Gustl to adapt a Rennsport to Kompressor specification, for me to ride in the pre-war class at vintage events.”