Harley-Davidson XR-750
Encyclopedia
The XR-750 is a Harley-Davidson
racing
motorcycle
made since 1970, primarily for dirt track racing
, but also for road racing
in the XRTT variant. The XR-750 was designed in response to a 1969 change in AMA Grand National Championship rules that leveled the playing field for makes other than Harley-Davidson, allowing Japanese and British motorcycles to outperform the previously dominant Harley-Davidson KR race bike. The XR-750 went on to become the winningest race bike in the history of American Motorcyclist Association
(AMA) racing.
The XR-750 is associated with the careers of racers Mark Brelsford, Cal Rayborn
, and Jay Springsteen
, and was the favorite motorcycle of stunt performer
Evel Knievel
. An XR-750 was included in the 1998 The Art of the Motorcycle
exhibition, and one of Knievel's bikes is in the Smithsonian's
National Museum of American History
America on the Move exhibit.
, or sidevalve, engines to 750 cc displacement
, while more modern overhead valve
(OHV) engines could be a maximum of only 500 cc. Over time, this displacement advantage kept the older flathead technology on the track and discouraged a broader field of competitors. At least 200 homologated
examples of a model had to be built and made available to the public. The flathead Harley-Davidson KR series had 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, without reference to valve configuration. The public was buying 650 cc and larger displacement British bikes, and they would prove to be competitive, given the chance.
With the British marques gaining influence in the AMA, in 1969 new rules were established that there would be one maximum displacement for dirt track racing, 750 cc, with no regard for valve type, though the 500/750 OHV/sidevalve split was kept for the time being in road racing. OHV engines began to dominate racing, in spite of Mert Lawwill
's efforts to delay the inevitable on his flathead Harleys, and the KR bikes were a decade out of date and could no longer compete.
, aluminum head
748 cc V-twin four-speed engine of 1970 was based on the mass production Sportster XL dating to 1952, but with modified heads and cylinders, a magneto instead of generator, and improved oiling. The frame and the running gear
were held over from the KRTT racer, with a Ceriani
front fork and two Girling rear shocks. The fuel tank, fenders, and rear seat/fender combination were fiberglass
, with a snap down seat cover over a foam cushion. To comply with AMA homologation rules, two hundred examples were made yearly, and could be had upon request at Harley-Davidson dealers, at a price of US$ 3,200, which today with inflation would be about US$ .
Not unlike other Harley-Davidson engines, the unit construction
left and right engine cases split vertically, and formed four cavities: a center front crankcase, a center rear gearbox, a right side cavity gearcase for the timing train, where the four camshaft
s are housed, and a left cavity for the three row primary drive chain. A row of four camshafts had also been used on the KR racers, inherited from the sidevalve Model WL, and even earlier Model DL of 1929. While the single camshaft of other Harley-Davidson designs was cheaper to manufacture, and quieter, four cams allowed better performance, such as greater flexibility in adjusting the cam timing, and the short single camshafts are durable, and give the pushrods a straigher path to the rocker arm
s.
carburetors and tuned dual reverse cone exhaust. Instrumentation consisted of a Smiths tachometer
.
Unlike the dirt tracker, it came with brakes: a rear disc brake
, and in front, a four leading shoe drum brake, which is two twin leading drum brake
s paired side by side in two drums. The XRTT is the final example of a competition motorcycle with drum brakes, superseded by disc brakes on all other racing bikes due to the excessive unsprung weight
added by the very large drum brake assembly. The official horsepower was never published, but estimates for the early 1972 engines were in the high 70 hp range, increasing to an estimated 100 hp or more by 2008.
In 1989, Lou Gerencer, Sr. built a hillclimbing
XR-750 with an extended swingarm
that made the bike half again as long. Adapted with mechanical fuel injection and nitrous oxide, Gerencer estimated his engine produced over 150 hp. The overstressed engine did not last long, but held together long enough to win the AMA hillclimb championship.
XR from its debut. Harley-Davidson was slow to capitalize on this demand, finally introducing the Sportster XR-1000 street bike 13 years after the XR-750 racer. The XR-1000 used XR-750 heads, but kept the Sportster engine, frame and other equipment. 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. The XR-1000 was discontinued after only two years, and after another 13 years the XR1200R was introduced in 2008 in Europe and 2009 in the US. The XR1200 has less in common with the XR-750 than the XR-1000 did, but has so far found a warmer reception.
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 and captured on film). Knievel jumped either cars or trucks (or a combination of the two) on the XR-750. The longest jump over cars was 129 feet in 1971 and was featured in the movie Evel Knievel
starring George Hamilton
. The longest jump over buses was first attempted with Knievel crashing at Wembley Stadium
in 1975 in an attempt to jump 120 feet over 13 buses. Five months later, Knievel jumped the XR-750 over 14 buses for his personal record, and world record for almost 25 years, of 133 feet at Kings Island
. The longest jump on the XR-750 was made by Bubba Blackwell
in 1999, when he jumped 15 buses at 157 feet.
Harley-Davidson
Harley-Davidson , often abbreviated H-D or Harley, is an American motorcycle manufacturer. Founded in Milwaukee, Wisconsin, during the first decade of the 20th century, it was one of two major American motorcycle manufacturers to survive the Great Depression...
racing
Motorcycle racing
Motorcycle sport is a broad field that encompasses all sporting aspects of motorcycling. The disciplines are not all "races" or timed-speed events, as several disciplines test a competitor's various riding skills.-Motorcycle racing:...
motorcycle
Motorcycle
A motorcycle is a single-track, two-wheeled motor vehicle. Motorcycles vary considerably depending on the task for which they are designed, such as long distance travel, navigating congested urban traffic, cruising, sport and racing, or off-road conditions.Motorcycles are one of the most...
made since 1970, primarily for dirt track racing
Track racing
Track racing is a form of motorcycle racing where teams or individuals race opponents around an oval track. There are differing variants, with each variant racing on a different surface type....
, but also for road racing
Road racing
Road racing is a general term for most forms of motor racing held on paved, purpose-built race tracks , as opposed to oval tracks and off-road racing...
in the XRTT variant. The XR-750 was designed in response to a 1969 change in AMA Grand National Championship rules that leveled the playing field for makes other than Harley-Davidson, allowing Japanese and British motorcycles to outperform the previously dominant Harley-Davidson KR race bike. The XR-750 went on to become the winningest race bike in the history of American Motorcyclist Association
American Motorcyclist Association
The American Motorcyclist Association is an American not-for-profit organization of more than 300,000 motorcyclists that organizes numerous motorcycling activities and campaigns for motorcyclists' legal rights...
(AMA) racing.
The XR-750 is associated with the careers of racers Mark Brelsford, Cal Rayborn
Cal Rayborn
Calvin Rayborn II was a top American motorcycle road racer in the 1960s and early 1970s.Born and raised in San Diego, California, Rayborn began riding motorcycles at an early age. He began his racing career in dirt track events in Southern California and in 1964, he began racing professionally in...
, and Jay Springsteen
Jay Springsteen
Jay Springsteen is a professional motorcycle dirt track racer.He began his professional racing career in 1973 by winning the AMA's Rookie of the Year award. As a member of the Harley Davidson factory race team, he went on to win three consecutive A.M.A. Grand National Championship in 1976, 1977,...
, and was the favorite motorcycle of stunt performer
Stunt performer
A stuntman, or daredevil is someone who performs dangerous stunts, often as a career.These stunts are sometimes rigged so that they look dangerous while still having safety mechanisms, but often they are as dangerous as they appear to be...
Evel Knievel
Evel Knievel
Evel Knievel , born Robert Craig Knievel, was an American daredevil and entertainer. In his career he attempted over 75 ramp-to-ramp motorcycle jumps between 1965 and 1980, and in 1974, a failed jump across Snake River Canyon in the Skycycle X-2, a steam-powered rocket...
. An XR-750 was included in the 1998 The Art of the Motorcycle
The Art of the Motorcycle
The Art of the Motorcycle was an exhibition that presented 114 motorcycles chosen for their historic importance or design excellence in a display designed by Frank Gehry in the curved rotunda of the Frank Lloyd Wright-designed Solomon R. Guggenheim Museum in New York City, running for three months...
exhibition, and one of Knievel's bikes is in the Smithsonian's
Smithsonian Institution
The Smithsonian Institution is an educational and research institute and associated museum complex, administered and funded by the government of the United States and by funds from its endowment, contributions, and profits from its retail operations, concessions, licensing activities, and magazines...
National Museum of American History
National Museum of American History
The National Museum of American History: Kenneth E. Behring Center collects, preserves and displays the heritage of the United States in the areas of social, political, cultural, scientific and military history. Among the items on display are the original Star-Spangled Banner and Archie Bunker's...
America on the Move exhibit.
Rule changes obsolete KR racers
The AMA Grand National Championship Class C rules, introduced in 1933 and revised in 1954, had an equivalency formula limiting flatheadFlathead engine
A flathead engine is an internal combustion engine with valves placed in the engine block beside the piston, instead of in the cylinder head, as in an overhead valve engine...
, or sidevalve, engines to 750 cc displacement
Engine displacement
Engine displacement is the volume swept by all the pistons inside the cylinders of an internal combustion engine in a single movement from top dead centre to bottom dead centre . It is commonly specified in cubic centimeters , litres , or cubic inches...
, while more modern overhead valve
Overhead valve
An overhead valve engine, also informally called pushrod engine or I-head engine, is a type of piston engine that places the camshaft within the cylinder block , and uses pushrods or rods to actuate rocker arms above the cylinder...
(OHV) engines could be a maximum of only 500 cc. Over time, this displacement advantage kept the older flathead technology on the track and discouraged a broader field of competitors. At least 200 homologated
Homologation (motorsport)
In motorsports, homologation is the approval process a vehicle, race track or standardised part must go through to race in a given league or series. The regulations and rules that must be met are generally set by the series' sanctioning body...
examples of a model had to be built and made available to the public. The flathead Harley-Davidson KR series had 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, without reference to valve configuration. The public was buying 650 cc and larger displacement British bikes, and they would prove to be competitive, given the chance.
With the British marques gaining influence in the AMA, in 1969 new rules were established that there would be one maximum displacement for dirt track racing, 750 cc, with no regard for valve type, though the 500/750 OHV/sidevalve split was kept for the time being in road racing. OHV engines began to dominate racing, in spite of Mert Lawwill
Mert Lawwill
Mert Lawwill is an American dirt-track and road racer, born on September 25, 1940 in Boise, Idaho.He started his racing career as an amateur racer on the local TT track in Boise and, later, scramble races across the United States Northwest ....
's efforts to delay the inevitable on his flathead Harleys, and the KR bikes were a decade out of date and could no longer compete.
Development
With limited time and 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. The iron cylinderCylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
, aluminum head
Cylinder head
In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes in the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket...
748 cc V-twin four-speed engine of 1970 was based on the mass production Sportster XL dating to 1952, but with modified heads and cylinders, a magneto instead of generator, and improved oiling. The frame and the running gear
Running gear
The term running gear is used to describe the wheels, suspension, steering, powertrain & chassis/bodyshell of a motor-car or automobile, or the tracks and road wheels of a tank or similar tracked vehicle....
were held over from the KRTT racer, with a Ceriani
Ceriani
Ceriani was an Italian company that designed and fabricated motorcycle frames and suspensions. The company was founded by Arturo Ceriani in 1951....
front fork and two Girling rear shocks. The fuel tank, fenders, and rear seat/fender combination were fiberglass
Fiberglass
Glass fiber is a material consisting of numerous extremely fine fibers of glass.Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the invention of finer machine tooling...
, with a snap down seat cover over a foam cushion. To comply with AMA homologation rules, two hundred examples were made yearly, and could be had upon request at Harley-Davidson dealers, at a price of US$ 3,200, which today with inflation would be about US$ .
Not unlike other Harley-Davidson engines, the unit construction
Unit construction
Unit construction is a term used to describe the design of larger motorcycles where the engine and gearbox components share a single casing. The term is sometimes applied to the design of automobile engines and was often loosely applied to motorcycles with rather different internal layouts such as...
left and right engine cases split vertically, and formed four cavities: a center front crankcase, a center rear gearbox, a right side cavity gearcase for the timing train, where the four camshaft
Camshaft
A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...
s are housed, and a left cavity for the three row primary drive chain. A row of four camshafts had also been used on the KR racers, inherited from the sidevalve Model WL, and even earlier Model DL of 1929. While the single camshaft of other Harley-Davidson designs was cheaper to manufacture, and quieter, four cams allowed better performance, such as greater flexibility in adjusting the cam timing, and the short single camshafts are durable, and give the pushrods a straigher path to the rocker arm
Rocker arm
Generally referred to within the internal combustion engine of automotive, marine, motorcycle and reciprocating aviation engines, the rocker arm is a reciprocating lever that conveys radial movement from the cam lobe into linear movement at the poppet valve to open it...
s.
XRTT road racer
The road racing version of the XR-750 used an aluminum oil tank, had a 6 US gal (23 l) fiberglass fuel tank, and a fiberglass fairing which included extra heat shielding to protect the rider's left leg when riding in a tuck position. On the top center of the fuel tank was a leather pad with a round cutout for the left-side fuel filler cap. Like the dirt tracker, it used a Ceriani fork and Girling shocks, two 36 mm MikuniMikuni
is a leading Japanese carburettor manufacturing company that was founded in 1923 and incorporated in 1948. Their business activities are centred on carburettors, fuel injectors and other automobile related equipment...
carburetors and tuned dual reverse cone exhaust. Instrumentation consisted of a Smiths tachometer
Tachometer
A tachometer is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute on a calibrated analogue dial, but digital displays are increasingly common...
.
Unlike the dirt tracker, it came with brakes: a rear disc brake
Disc brake
The disc brake or disk brake is a device for slowing or stopping the rotation of a wheel while it is in motion.A brake disc is usually made of cast iron, but may in some cases be made of composites such as reinforced carbon–carbon or ceramic matrix composites. This is connected to the wheel and/or...
, and in front, a four leading shoe drum brake, which is two twin leading drum brake
Twin leading drum brake
The twin-leading-shoe brake is a type of drum brake that has two leading shoes, rather than the single leading shoe and a single trailing shoe of a single-leading shoe drum brake. A leading shoe has a self-servo effect, so an advantage of a 2LS is that it provides the maximum retardation in its...
s paired side by side in two drums. The XRTT is the final example of a competition motorcycle with drum brakes, superseded by disc brakes on all other racing bikes due to the excessive unsprung weight
Unsprung weight
In a ground vehicle with a suspension, the unsprung weight is the mass of the suspension, wheels or tracks , and other components directly connected to them, rather than supported by the suspension...
added by the very large drum brake assembly. The official horsepower was never published, but estimates for the early 1972 engines were in the high 70 hp range, increasing to an estimated 100 hp or more by 2008.
Racing
Riders on XR-750s have won 29 of the 37 AMA Grand National Championships from 1972 to 2008 inclusive. Besides having more wins than any other bike in AMA racing, it has been called the "most successful race bike of all time", and has a claim to have more wins than any other racing motorcycle in history.In 1989, Lou Gerencer, Sr. built a hillclimbing
Hillclimbing
Hillclimbing is a branch of motorsport in which drivers compete against the clock to complete an uphill course....
XR-750 with an extended swingarm
Swingarm
A swingarm, or "swinging arm" is the main component of the rear suspension of most modern motorcycles and ATVs...
that made the bike half again as long. Adapted with mechanical fuel injection and nitrous oxide, Gerencer estimated his engine produced over 150 hp. The overstressed engine did not last long, but held together long enough to win the AMA hillclimb championship.
Street XR
As with the KR, customers began asking for a street-legalStreet-Legal
Street-Legal is singer-songwriter Bob Dylan's 18th studio album, released by Columbia Records in June 1978. The album was a serious musical departure for Dylan, who uses a large pop-rock band—complete with female backing vocalists—for the first time....
XR from its debut. Harley-Davidson was slow to capitalize on this demand, finally introducing the Sportster XR-1000 street bike 13 years after the XR-750 racer. The XR-1000 used XR-750 heads, but kept the Sportster engine, frame and other equipment. 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. The XR-1000 was discontinued after only two years, and after another 13 years the XR1200R was introduced in 2008 in Europe and 2009 in the US. The XR1200 has less in common with the XR-750 than the XR-1000 did, but has so far found a warmer reception.
Jumping
Evel KnievelEvel Knievel
Evel Knievel , born Robert Craig Knievel, was an American daredevil and entertainer. In his career he attempted over 75 ramp-to-ramp motorcycle jumps between 1965 and 1980, and in 1974, a failed jump across Snake River Canyon in the Skycycle X-2, a steam-powered rocket...
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 and captured on film). Knievel jumped either cars or trucks (or a combination of the two) on the XR-750. The longest jump over cars was 129 feet in 1971 and was featured in the movie Evel Knievel
Evel Knievel (film)
Evel Knievel was a 1971 motion picture starring George Hamilton as motorcycle daredevil Evel Knievel.- Story :The story is a biography of the famed motorcycle daredevil, much of which was filmed in his home town of Butte, Montana. The film depicts Knievel reflecting on major events in his life,...
starring George Hamilton
George Hamilton (actor)
George Stevens Hamilton is an American film and television actor.-Early life:Hamilton was the youngest son of bandleader George "Spike" Hamilton and his first wife, Ann Stevens . He was born in Memphis, Tennessee, and lived in Blytheville, Arkansas...
. The longest jump over buses was first attempted with Knievel crashing at Wembley Stadium
Wembley Stadium
The original Wembley Stadium, officially known as the Empire Stadium, was a football stadium in Wembley, a suburb of north-west London, standing on the site now occupied by the new Wembley Stadium that opened in 2007...
in 1975 in an attempt to jump 120 feet over 13 buses. Five months later, Knievel jumped the XR-750 over 14 buses for his personal record, and world record for almost 25 years, of 133 feet at Kings Island
Kings Island
Kings Island is a amusement park located northeast of Cincinnati in Mason, Ohio. Opened in 1972 by Taft Broadcasting Company and now owned by Cedar Fair Entertainment Company, Kings Island is the most visited seasonal amusement park in the U.S...
. The longest jump on the XR-750 was made by Bubba Blackwell
Bubba Blackwell
Bubba Blackwell is a stunt performer and motorcycle jumping world record holder who was sponsored by and promoted Buell Motorcycle Company...
in 1999, when he jumped 15 buses at 157 feet.