Quick and dirty

Photo: Harley Davidson

 
The most successful racing machine of all time

Stuntbike for Evel Knievel and exhibit at the Guggenheim Museum

The successor's engine is based on the engine of the XG 750 Street

She is a child of the late sixties and her birth in 1970 owes the XR-750 to the fact that the American Motorcyclist Association (AMA) had changed the regulations in the flat track. The new guidelines left no chance for side-controlled engines, as Harley-Davidson has successfully used in its KR racers to date. A 750-ohV engine was needed – and Harley's racing manager Dick O'Brien built it with his team based on the V-Twins, which was used in the Sportster XLR racing machine. The new Flat Tracker found space in a modified KR racing frame, which was refined with Ceriani fork and girling struts and topped by a fiberglass tank and a brush in Harley's racing colour Jet Fire Orange. The lightweight 4x19-inch spoke wheels were pulled on to racing pneus and the 2.8-litre engine oil circulated through an optimised circuit. The machine weighed just 134 kilos and she was a truly fiery lady: a quarter-turn of the throttle was enough to completely open her throttle valve. 200 units were manufactured to comply with the regulations of the AMA. A racer had to put 3,200 US dollars on the counter for a copy – brakes not included, because in the flat track only one of them is needed and the teams could retrofit them according to their personal preference at the rear.

Two years later, the XR-750 had matured to perfection: in 1972, light metal cylinders and heads allowed higher compression and also successfully offered paroli to the thermal problems of the cast iron versions previously used. The engineers turned the rear cylinder around, so that two 36-carburetors on the right, equipped with massive air filters, and a raised exhaust on the left. Crankshaft, connecting rods, pistons and valves have been redesigned according to a modified bore-stroke ratio. More power and undisturbed stability were the reward of the effort and at the latest now other teams could pack, because the XR-750 was about to dominate the flat track. Between 1972 and 2008, she won 29 of the 37 AMA Grand National Championships, constantly optimizing, and her account was more race wins than any other motorcycle in the history of the AMA, which earned her the title of "most successful racing machine of all time". Stuntman Evel Knievel performed his most spectacular jumps on her and the Guggenheim Museum made her one of the jewels in The Art of the Motorcycle. Its road-approved derivatives include the XR 1000, introduced in 1983, and the XR 1200, which was launched in 2008.

Since the beginning of the 1980s, Harley-Davidson has only produced engines instead of complete XR-750 race bikes and secured the supply of parts for the racing teams, who now like to put their chassis together on their own. At the end of the eighties there were only parts instead of the engines. If you own one of the little more than 500 complete bikes from Milwaukee, you can count yourself lucky, because it is long worth many times its original price.

Even today, XR-750 racing machines thunder over American flat-track tracks, which have now grown to more than 100 hp. but now Harley-Davidson has a crown princess of the classic sports cannon at the start: The XG 750R, whose liquid-cooled 60-degree V2 is based on the Revolution X engine of the Street models, is preparing to follow in the footsteps of its great predecessor – a fascinating piece of technology, but unfortunately "for race use only"! If you want to get to know the road-ready and approved Harleys, check out the nearest dealer. You can find it under www.Harley-Davidson.com.