The BMW Racer
When Oscar Liebmann wanted to do something special in the racing world he turned to BMW. The resulting RS500 OL Special built in the 1960s opened eyes and dyno charts in the pursuit for power that delivered a road racing champion to the top step of the podium.
Last September, after more than 10 years of trying, BMW finally scored a victory at the UK’S annual Goodwood Revival. The top step of the podium came courtesy of double World Superbike champion Troy Corser and his Austrian teammate Herbert Schwab on a supercharged 1929 R57 Kompressor. Now the German manufacturer’s BMW Group Classic historic vehicle division will be trying to make it two in a row in 2019, when the race reverts to its later format for bikes built before 1967. And they will be trying to do it on this – the RS500 OL Special.
To win at Goodwood, BMW are entering one of their most recent acquisitions, the legendary RS500 OL Special built in the US in the 1960s by German ex-pat Oscar Liebmann and raced to copious success in the US and Canada by his son Kurt. It was an honour to be the first person outside the Liebmann family to ride the Daytona-winning BMW. Kurt trucked the bike up to Pocono Speedway in the Pennsylvania mountains and it wasn’t until we started chatting about the OL Special that I realised just how much endeavour Oscar expended in creating the motorcycle bearing his initials. It might look like a BMW Rennsport externally but the engine owed more to OL’S talents as engineer and machinist than to the race shop of the Bavarian company. Oscar’s contacts in Germany remained close after he emigrated to the US in the 1950s, and they extended to the BMW factory’s Experimental Department. Among them were BMW technical director Dr Helmut Bönsch and famed engine tuner Ludwig Apfelbeck, who lent support and advice in the development of the OL Special in the 1960s. Kurt: “My father’s friends were eager to help improve the engine for solo racing, so in 1958 they sent him a batch of unmachined castings – enough to build five RS engines to my father’s own specification, though in the end he built just two.” One of these was closely based on the factory design internally, in adopting the ‘square’ 68 x 68mm configuration of German Rennsport engines that dominated Sidecar GP racing for two decades. It was known as the ‘longstroke bike’ as at the same time Oscar constructed his own short-stroke 70 x 64mm version to achieve more power through higher revs, but without sacrificing reliability. It’s this engine that powered Kurt to victory at Daytona in 1984, almost 30 years after his father began to work on the project, and it was this motorcycle I tested at Pocono.
Building it
Building the two engines occupied most of Oscar’s spare time over a 10-year period. Taking the raw factory crankcase castings, he machined them and fashioned his own three-bearing crankshaft from a solid billet of aircraft steel, with a centre main bearing to prevent the assembly flexing at the higher revs he intended the engine to run at. The 180° crank was polished and balanced, then fitted with Liebmann’s own polished conrods, each machined from a single billet of 8620 case hardened steel and fitted with three-ring Mahle pistons specially made to suit OL’S revised combustion chamber design. The compression ratio is 11.4:1 and the whole bottom end features extensive use of needle roller bearings, with all the bearing cages made by Liebmann himself. The original Rennsport cylinders were replaced by lightweight versions carved from billets of 7075 aluminium, but the original factory dohc cylinder head castings are used, although all the valve gear is Us-made. Oversize 42mm inlet and 38mm exhaust valves are fitted, using single coil valve springs, with the heads flowed to match and fed by a pair of 38mm Malossi concentric carbs. Liebmann made his own camshafts, offering greater lift and increased overlap compared to factory items, as well as all his own bevel gears and shafts in the single-shaft twin-cam design. He reshaped the combustion chambers to permit fitting the larger valves and also used dual ignition, with a second 10mm spark plug under each cylinder on the opposite side of the head from the bevel shaft. Points ignition was originally employed, using a 12v battery, but changed to the self-generating Kröber electronic magneto that allowed the battery to be junked, saving crucial weight, same as the original distinctive RS cambox covers recast by OL in light alloy. OL machined the square gearbox casting to fit his own five-speed gearbox, essentially a factory close-ratio item copied on the Amol Precision gear-cutter, but with improved shift action. The shaft final drive has straight-cut teeth on the drive unit, with a choice of four different ratios for the rear wheel, and Oscar made his own twin-plate dry clutch.
Riding it
When I straddled the BMW for the first time it felt tiny and very low, more like a 350 than a potent 500, despite the period 19in wheels with rather hard-feeling PZ2 Michelin tyres front and rear, which helped provide extra ground clearance. I didn’t manage to deck either of the horizontal cylinders, and I think the exhausts would touch down first, though Kurt claimed even he never did this. The chassis is a circa 1954 BMW works RS500 frame supplied by Dr Bönsch, which originally carried an Earles fork but was modified by the Liebmanns to carry a telescopic 35mm Ceriani fork up front sourced from a Harley
The racing issue
On completion the OL Special BMW had become an instant winner in Kurt Liebmann’s hands in the 1960s, carefully prepared by family friend Hugo Wolters. With its potent motor and a 135kg dry weight, the bike’s performance was highly competitive, so much so that the AMA refused to let it compete in National racing alongside the flathead KR750 Harleys and 500cc singles like Dick Mann’s G50 Matchless and Gary Nixon’striumph twin. They claimed the bike had been so highly modified that it no longer bore any relationship to a model of which at least 50 examples had been built and sold to the public. “We fought the AMA’S decision,” recalls Kurt: “But Harley had obviously decided they didn’t want to have to compete against us, and in those days they were the AMA. “We could have tried to persuade BMW to build a short run of homologation specials like Matchless did with the G50CRS desert racer, and since Dr Bönsch had only helped us so much with the project because he saw it as a valuable means of publicising BMW’S sporting image in the US, they might even have done so, but my dad decided he didn’t want to get involved with what he considered were dirty politics, so we left it at that. We were only a bunch of little guys who wanted to have fun.” Instead the Liebmann Beeem headed to Canada, where racing was run to FIM rules and capacity classes. Over the next few years Kurt became unbeatable on the bike north of the border, winning several Canadian championships in the late-sixties and early Seventies, as well as cleaning up in European-style club racing in the New England area. By the end of the decade he was ready to move on to other things (initially Formula 750 with one of the rare CR750 Hondas then, after that became uncompetitive, atz750yamaha).the Beeem was put to one side, but fortunately never disposed of, so that when the worldwide awakening of interest in Classic racing reached the US, the Liebmanns were ready and waiting with the right bike.
Kröber self-generating electronic ignition means no battery, so the bike tips the scale at 135kg with oil but no fuel
ERS, mated to twin Koni shocks at the rear. The BMW was very easy to flip from side to side thanks to the low-slung weight of the flat-twin engine and beautifully-balanced handling, though the rear suspension was a little stiff and skipped around a bit over the bumps. But I could see why Kurt was able to make up so much ground on Reiman’s less nimble Harley in the tortuous ‘box’ before the start/finish line on the Daytona short circuit. Less impressive was the front brake, a 210mm 4LS Fontana, which is usually adequate to cope with the 109kg weight of Harley Ers/aermacchi singles, but less sure of stopping the OL BMW scaling 135kg with oil and no fuel. Well used to such a brake on my own Aermacchis I raced back then, I found the action of the one fitted to the Liebmann BMW stiff and wooden, and it needed a really hard pull to stop it. Liebmann was permitted to retain the rear 310mm disc brake, just as BMW Group Classic will in the Goodwood Revival on the grounds that it’s a period fixture. The disc is tapered to create a venturi effect, with cooling slots let into the outside circumference. Gripped by an early Honda caliper, it works quite well, and helps to compensate for the inadequacies of the front brake. Blip the throttle at rest, and there’s hardly any of the wind-up effect you experience if you try the same trick with another flat (or vee) twin with a lengthways crank. Look in vain for it to rock from side to side as the engine idles, and try equally hard to unsettle the bike’s behaviour under braking by changing down at high revs going into a corner to extract the maximum from the engine braking – and you can’t. This made not only for notably quicker throttle response and improved acceleration, but more controllable handling both entering and exiting a turn, but be prepared for the front wheel to get light when you crack the throttle hard open. First gear is quite low, butut then the next three are close togeth her. Fifth/ top gear is quite a bit longe er than fourth, but with the turbine-like engine safe to 11,500rpm, you don’t need d to use more than 9500rpm m in the gears (the original rev-limit of the factory RS500 motor) to keep it on the boil. There’s pronounced megaphonitis lower down, which necessitates a firm touch on the clutch lever to get the revs up to 5000rpm, where the carburation cleans out and power starts to build up to the 6400rpm threshold. At this point the Liebmann camshafts start to work, and there’s strong acceleration thanks to the flat torque curve from then on up. Kurt proved the little BMW’S speed by entering it in several Battle of the Twins races as a ‘technical ’ 501cc machine, competing g against bikes of up to twice the engine capacity with considerable success. Opening it up on the Daytona banking revealed the engine could be run to 12,000rpm without damage. With the engine running at those speeds even after backing off the throttle, the 500cc flat-twin was still trapped in 1983 at 140mph,140mph a good goo turn of speed for a 500cc Classic, espe ecially a home-brewed one. “It’s a bike b that’s been such a part of m my life,” said Kurt. “It’s also brought t me satisfaction beyond anythi ing I could have hoped for. O Of all the bikes I’ve owned or r raced, this is my favourite. I’ve come to appreciate just h how much love and time my faather invested in this bike. It’s a work of art, and it’ll always rem ind me of the man I loved who o created it.” O Oscar would have been very proud to see his self-built bike in the BMW factory museum in Munich and to know that it’ll carry the German manufacturer’s colours in the Goodwood Revival. I suspect team m manager Sebastian Gutsch is going to have to ask Troy Corser, fas st lady Maria Costello and Kl aus Ottillinger to draw lots to de ecide who’s going to partner him in riding the OL Special again nst the Manx Nortons ridden by thhe likes of Dani Pedrosa.