NSU SPORTMAX 251RS TECHNICAL
After dominating the smaller GP classes in 1953/54 with its 125 Rennfox and 250 Rennmax factory racers, West Germany’s NSU – then the world’s largest motorcycle manufacturer – produced 34 examples of its 250cc Sportmax production racer for the 1955 season, after retiring from racing officially.
One of these was ridden to the 1955 world title by privateer Hermann-paul (aka ‘Happy’) Müller, and another to the British ACU Star Championship by John Surtees, and in doing so underscored the lasting qualities of the NSU Sportmax 251RS for its sterling combination of speed and reliability.
The NSU Max road bike debuted in 1952 as the first of a line of variations on a theme that later included not only the Sportmax road racer, but also the Geländemax enduro and the Spezial/ Supermax sports models. With its unit construction overhead-cam dry sump engine with caged double-row roller bearing big-end, dry clutch, geardriven primary, leading-link front suspension and monoshock rear end, the Max heralded the advent of the modern lightweight roadster.
Chief attention-grabber was NSU designer Albert Roder’s choice of valve-gear known as the Ultramax design, using a system of multiple eccentrics and con-rods, coupled with fullyenclosed hairpin springs, to drive the two-valve SOHC layout. No less than 81,500 examples of the various Max variants were manufactured up until the end of production in 1963.
NSU began development of the Sportmax in March 1953, initially using the Rennmax tubular-steel frame for early tests, while chassis expert Richard Emmerich adapted the stock Max pressed-steel frame for racing.this took place in NSU’S streetbike R&D division under its boss Karl Kleinbach, not in the race department, with Max engine specialist Gerhard Nied in charge of engine development. this resulted in the 69 x 66mm 247cc Max roadster’s 15bhp output being raised to 29.3bhp at 9000rpm by the time of the new model’s first race win in July 1954 in Oberreifenberg, where Walter Reichert scored its debut victory.
This improvement in performance over the road-bike was achieved with a bigger 1 3⁄16 th [30.16 mm] Fischer-amal GP carb with remote float, a larger diameter 36.5mm inlet valve (was 35mm), stiffer hairpin valve springs, a forged three-ring Mahle piston which together with a revised combustion chamber gave a higher 9.8:1 compression ratio (up from 9:1), a stronger con-rod made from case hardened steel, a larger triplex oil pump with direct delivery to the main bearings, and a lighter but more aggressive camshaft, with extra lift and dwell. Plus the bottom three gears of the four-speed transmission with gear primary drive – with the dry clutch now exposed to the elements for greater cooling – were all longer, with less of a gap between first and second, but still a 1:1 top gear, same as on the roadster. 80% of the Sportmax engine components were supposedly unchanged from the production Max, including the Bosch coil ignition with 41-degree advance.
In the Sportmax, the engine was mounted in a similar design of pressed steel spine frame to the Max, with an integral three-litre oil tank, but manufactured from lighter, stronger chromemoly material, not mild steel as on the road bikes.
The engine was not a stressed member, since while firmly attached to the frame at the rear, it was only held in place at the front by a bolted-up tension link to the cylinder head. Nevertheless, the frame structure had great inherent strength, as did the front fork, which also consisted of welded-up steel pressings neatly enclosing a leading link mechanism incorporating a sprung hydraulic damper unit in each leg, operated by aluminium links on the racer (steel on the Max).
18in wheels were fitted (19in on the Max roadster), with a distinctive 210mm single-sided SLS front brake – originally with a magnesium backplate, later aluminium – upsized to a 240mm unit on final versions – with a 200mm SLS rear. The stiff, ribbed, pressed-steel swingarm carried twin shocks of NSU’S own manufacture.
Wheelbase was a contained 1300mm, and dry weight just 112kg, with the 22-litre fuel tank empty.this was hand-made from 21 different aluminium parts including bosses, baffles and strengthening ribs by craftsman Georg Buss, who during the Second World War had worked for rocket scientist Werner von Braun at the V2 complex at Peenemünde. Buss also made the optional and very distinctive aluminium dustbin fairing which many owners purchased, having previously made the even more distinctive ‘bird beak’ bodywork for the 1954 factory Rennmax and Rennfox racers.
The full-enclosure Sportmax bodywork enabled the not exactly midget-sized Happy Müller to post a top speed of 212 km/h at Hockenheim on his 250 Sportmax, whose excellent power-to-weight figures yielded the performance of a 350 single, but the ease of handling of a lightweight.
“DESIGN BEGAN IN 1953. GOING FROM A TUBULAR TO PRESSED-STEEL FRAME. BODYWORK WAS ‘BIRD-BEAK’ OR ‘FULL-ENCLOSURE’.”