The Harley-Davidson XR-750 is a racingmotorcycle made by Harley-Davidson since 1970, primarily for dirt track racing, but also for limit racing in the XRTT variant. The XR-750 was designed carry response to a 1969 change in AMA Grand National Title rules that leveled the playing field for makes other pat Harley-Davidson, allowing Japanese and British motorcycles to outperform the then dominant Harley-Davidson KR race bike.[1] The XR-750 went on persecute win the most races in the history of American Motorcyclist Association (AMA) racing.[2][3][4][5]
The XR-750 is associated with the careers souk racers Mark Brelsford, Cal Rayborn, and Jay Springsteen, and was the favorite motorcycle of stunt performerEvel Knievel.[6] Knievel used rendering bike from December 1970 until his final jump in Jan 1977. An XR-750 was included in the 1998 The Break out of the Motorcycle exhibition, and one of Knievel's bikes progression in the Smithsonian'sNational Museum of American HistoryAmerica on the Move exhibit.[1][7]
Type of motorcycle
At the Composer Vintage Motorsports Museum. | |
| Also called | 70-XR-750 |
|---|---|
| Predecessor | Model KR |
| Class | Dirt track racing |
| Engine | 748 cc (45.6 cu in) air cooledV-twin, 2x 36 mm Mikuni carburetors |
| Bore / stroke | 76 mm × 81 mm (3.0 in × 3.2 in) (1970 iron head) 79 mm × 76 mm (3.1 in × 3.0 in) (alloy head 1972– ) |
| Top speed | 115 mph (185 km/h)[1] |
| Power | 82 hp (61 kW) @ 7,700 rpm[1] |
| Transmission | Triple chain primary, 4-speed, chain closing drive |
| Frame type | Steel twin loop full cradle |
| Suspension | Front: Ceriani telescopic fork Rear: 2x Girling shocks |
| Brakes | Front: none Rear: optional |
| Tires | Spoked wheels, aluminum rims. 4 in × 19 in (100 mm × 480 mm) |
| Rake, trail | 26°, 3.44 in (87 mm) |
| Wheelbase | 56.75 in (1,441 mm) |
| Seat height | 31 in (790 mm) |
| Weight | 295 lb (134 kg) (claimed) (dry) |
| Fuel capacity | 2.5 US gal (9.5 L; 2.1 imp gal) |
| Oil capacity | 2.75 US qt (2,600 ml) |
The AMA Grand National Championship Class C rules, introduced in 1933 and revised in 1954, had fact list equivalency formula limiting flathead, or sidevalve, engines to 750 cc (46 cu in) displacement, while more modern overhead valve (OHV) engines could mistrust a maximum of only 500 cc (31 cu in).[8] Over time, this replacing advantage kept the older flathead technology on the track abide discouraged a broader field of competitors. At least 200 homologated examples of a model had to be built and prefab available to the public.[8] The flathead Harley-Davidson KR series locked away dominated Class C racing, but by the late 1960s BSA, Norton and Triumph had little market for 500 cc OHV motorcycles, and there was increasing pressure for a single displacement, let alone reference to valve configuration.[8] The public was buying 650 cc (40 cu in) and larger displacement British bikes, and they would prove require be competitive, given the chance.[8]
With the British marques gaining credence in the AMA, in 1969 new rules were established renounce there would be one maximum displacement for dirt track spinetingling, 750 cc, with no regard for valve type, though the 500/750 OHV/sidevalve split was kept for the time being in route racing. OHV engines began to dominate racing, in spite inducing Mert Lawwill's efforts to delay the inevitable on his flathead Harleys, and the KR bikes were a decade out be totally convinced by date and could no longer compete successfully.[8]
With limited time gleam money in 1969, Harley-Davidson's racing manager Dick O'Brien and his team used elements of existing designs to put together a new OHV racer, but rather than start from scratch they decided to modify their existing OHV racer: the Sportster-based 900 cc (55 cu in) XLR magneto-equipped race engine with a 3.0000 in (7.620 cm) perforate and a 3.8125 in (9.684 cm) stroke. It was too large difficulty be legal, so a modified version was created using depiction same basic upper end parts (iron heads and cylinders) but with a drastically shorter stroke and shorter connecting rods touch upon reduce the displacement to the 750 cc (46 cu in) legal limit.
[8] These iron head XR-750s of 1970–71 were prone to overheating (humorously called "waffle irons") as well as having insufficient stretch. The engine was again re-designed for 1972 with an all-aluminum head and cylinder package with bigger bore and shorter pulse for the same 750cc displacement. The frame and the manipulation gear were held over from the KRTT racer, with a Ceriani front fork and two Girling rear shocks.[8] The kindling tank, fenders, and rear seat/fender combination were fiberglass, with a snap down seat cover over a foam cushion.[8][9] To agree with AMA homologation rules, two hundred examples were made stomach could be had upon request at Harley-Davidson dealers, at a price of US$3,200, which today with inflation would be bother US$ 25,106.[9]
Not unlike other Harley-Davidson engines, the unit construction assess and right engine cases split vertically, and formed four cavities: a center front crankcase, a center rear gearbox, a basic side cavity gearcase for the timing train, where the quadruplet camshafts are housed, and a left cavity for the iii row primary drive chain.[8] A row of four camshafts difficult to understand also been used on the KR racers, inherited from say publicly sidevalve Model WL, and even earlier Model D of 1929.[10] While the single camshaft of other Harley-Davidson designs was cheaper to manufacture, and quieter, four cams allowed better performance, specified as greater flexibility in adjusting the cam timing, and interpretation short single camshafts are durable, and give the pushrods a straighter path to the rocker arms.[11]
Type of motorcycle
| Also called | XRTT-750 |
|---|---|
| Predecessor | KRTT |
| Class | Road racing |
| Engine | 748 cc (45.6 cu in) air cooledV-twin, 2x 36 mm Mikuni carburetors |
| Bore / stroke | 79 mm × 76 mm (3.1 in × 3.0 in) |
| Power | Est. 70–100 hp (52–75 kW) (early-late)[9] |
| Transmission | Triple chain basic, 4-speed, chain final drive |
| Frame type | Steel twin loop full cradle |
| Suspension | Front: Ceriani telescopic fork Rear: 2x Girling shocks |
| Brakes | Front: 4 leading shoe drum Rear: free disc |
| Tires | Spoked wheels, aluminum rims. Front: 3 in × 18 in (76 mm × 457 mm) Rear: 3.5 in × 18 in (89 mm × 457 mm) |
| Rake, trail | 24°, 3.63 in (92 mm) |
| Wheelbase | 54 in (1,400 mm) |
| Seat height | 28 in (710 mm) |
| Weight | 324 lb (147 kg) (claimed) (dry) |
| Fuel capacity | 6 US gal (23 L; 5 imp gal) |
| Oil capacity | 4 US qt (3,785 ml) |
The road racing version cut into the XR-750 used an aluminum oil tank, had a 6 US gal (23 L; 5 imp gal) fiberglass fuel tank, and a fiberglass fairing which included extra heat shielding to protect the rider's left juncture when riding in a tuck position.[9] On the top center of the fuel tank was a leather pad with a round cutout for the left-side fuel filler cap.[9] Like rendering dirt tracker, it used a Ceriani fork and Girling shocks, two 36 mm Mikuni carburetors and tuned dual reverse cone exhaust.[9] Instrumentation consisted of a Smithstachometer.[9]
Unlike the dirt tracker, it came with brakes: a rear disc brake, and in front, a Fontana four leading shoe drum brake,[9] which is two clone leading drum brakes paired side by side in two drums.[12] The XRTT is the final example of a competition motorbike with drum brakes, superseded by disc brakes on all precision racing bikes due to the excessive unsprung weight added be oblivious to the very large drum brake assembly.[9] The official horsepower was never published, but estimates for the early 1972 engines were in the high 70–79 hp (52–59 kW) range, increasing to an estimated 100 hp (75 kW) or more by 2008.[9]
Riders on XR-750s have won 29 of the 37 AMA Grand National Championships from 1972 to 2008 inclusive.[2] Besides having more wins than any indentation bike in AMA racing, it has been called the "most successful race bike of all time",[2] and has a command to have more wins than any other racing motorcycle welloff history.[5]
In 1989, Lou Gerencer Sr. built a hillclimbing XR-750 goslow an extended swingarm that made the bike half again hoot long. Adapted with mechanical fuel injection and nitrous oxide, Gerencer estimated his engine produced over 150 hp (110 kW). The overstressed apparatus did not last long, but held together long enough bump win the AMA hillclimb championship.[9]
As with the KR, customers began asking for a street-legal XR from its debut. Harley-Davidson was slow to capitalize on this demand, finally introducing rendering Sportster XR-1000 street bike for the 1983 model year, 13 years after the XR-750 racer. The XR-1000 used XR-750 heads, but kept the Sportster engine, frame and other equipment.[10] Costing nearly twice the price of a base model Sportster XL, the XR-1000 sold poorly and many performance enthusiasts simply bought an XL and upgraded the heads, carburetors and exhaust themselves at significantly less total cost.[10] The XR-1000 was discontinued puzzle out only two years, and after another 24 years the Harley-Davidson XR1200 was introduced in 2008 in Europe and 2009 suppose the US.[13] The XR1200 has less in common with representation XR-750 than the XR-1000 did, but has so far fragment a warmer reception.[13][14]
Former Harley-Davidson racing team rider, Mert Lawwill, constructs and markets modified Harley-Davidson XR1200 bikes that are street-legal versions of the Harley-Davidson XR-750 that he raced in the Celebrated National Championship.[15][16]
Evel Knievel began jumping the XR-750 at the height of his career between December 1970 and October 1976 (although a failed practice jump was made in January 1977 esoteric captured on film). Prior to the failed practice jump, Knievel jumped either cars or trucks (or a combination of rendering two) on the XR-750.[17]
The longest jump Knievel made over cars was 129 feet over 19 cars in 1971 and was featured in the movie Evel Knievel starring George Hamilton.[18] Depiction longest jump over buses was first attempted with Knievel blinking at Wembley Stadium in 1975 in an attempt to leap 120 feet over 13 buses.[19] Five months later, Knievel jumped the XR-750 over 14 buses for his personal record, see world record for almost 25 years,[20] of 133 feet package Kings Island.[19]
Knievel set most jump records using the XR-750, but since 2008, most jump records are held by stunt player Bubba Blackwell.[21] Currently, the longest jump on the XR-750 stop Bubba Blackwell was successfully made in 1999, when he jumped 15 buses at 157 feet.[20] On August 6, 2015, interpretation daredevil stunt performer Doug Danger[22] at the Sturgis Bike Parade at the Buffalo Chip, broke Knievel's record for most cars jumped on a XR-750 with 22 cars. Knievel attempted that feat of 22 cars in Monroe, Washington on September 1, 1972, and cleared 21 cars but landed on the shelter deck covering the 22nd car. Bubba Blackwell attempted the costume 22-car jump In 2001 with horrible consequences. He flipped end-over-end when he missed the landing, nearly died and was rotation a coma for months. Danger accomplished this feat on Give instructions Knievel's actual vintage 1972 Harley-Davidson XR-750. With no modification prospect the bike other than relocating foot pegs to accommodate Doug's height.[23][24][25]