Can Harley’s new Street 750-based Flat Tracker replace the XR750?

 

 

Kris Schoonover, Harley’s mightily bearded racing manager, threaded his words together carefully: “This is the first time… a new-design Harley has made the main… in forty-two years.” Factory rider Davis Fisher, by finishing third in his semi on the new XG prototype (he was briefly second), transferred to the 25-lap GNC1 main event at the Sacramento Mile.

 

 

The bike that last did this, 42 years ago, was the aluminum XR750 that has sustained the sport for over four decades. In racing, obsolete doesn’t mean a design can no longer win races. It means only that its development has fully exploited all possibilities. Then you need a fresh design with wider limits. The XG is Harley’s next step.

 

 

In the main, calm, inward Brad Baker would draw a roar from the crowd when he went into the lead on the traditional factory XR, but immediately his hand was in the air—his engine had crunched. The crew were saying, “Maybe something in the cam drive.” 

 

Fisher’s XG prototype was soon out as well—it began making weird noises and retired with no compression in the front cylinder. The hard ups and downs of racing.

 

Earlier, seeing Baker carefully eating a salad, sitting on the steps of Harley’s bus, or preparing his helmet and tear-offs, or just moving through, I thought of chess grandmaster Mikhail Botvinnik, who systematically moved everything irrelevant out of his mind before a match.

 

 

Davis Fisher explained to me that a flat-track twin is geared to accelerate off the corners, so when it reaches peak power, the next corner is still far in front of you. So your engine has to overrev its peak as you push into the corner. That’s hurting the still-fast XRs—Chris Carr said when he was riding them they needed a fresh big-end roller assembly every national. Former H-D team manager, the late Dick O’Brien was concerned in 1982 when some were running their XRs to 9200 (they began life at 7600). Today the number is more like 10,000! Plain-bearing engines today do this easily and reliably, as the six-time Sacramento-winning, Howerton-framed Kawasaki twin of Bryan Smith testifies. Matt Hines told me the teams talk rear-wheel horsepower (RWHP), and say it now takes 95 or 100 of them to run up front.

 

 

So here’s what’s coming. The XG750R, aka “the G-bike,” is a liquid-cooled 60-degree V-twin of 85 x 66mm bore and stroke, based on Harley’s ‘Street’ line, introduced a few years ago. The engine has four valves per cylinder operated by a single chain-driven overhead cam per head, driving a pair of forked roller rockers with screw-and-nut clearance adjusters. Intake valves are titanium and for the moment the exhausts are stainless. To clear the cam, the central spark plug threads in on the non-cam-drive side of each head—at an angle. Separate cylinders have iron liners. The race engine runs CP pistons and Carrillo rods on a stock crank, which is all plain-bearing. ‘Street’ was surely a source of division inside The Motor Company because it turns its back on the 110-year orthodoxy of 45 degrees, air-cooling, rolling bearings, and pushrods (all of which the XRs share). But there is no question that the changes are necessary to keep Harley in the all-American sport of flat-track racing.

 

 

The XG quit at Phoenix—a buried ECU instruction said, “If temperature exceeds X at Y rpm, kill the spark.” It was 107 degrees! Onlookers thought they saw a shortfall of acceleration but Davis Fisher said initial hook-up off corners was weak. At Sacramento it was better. Early days.

 

The XG quit at Phoenix—a buried ECU instruction said, “If temperature exceeds X at Y rpm, kill the spark.” It was 107 degrees! Onlookers thought they saw a shortfall of acceleration but Davis Fisher said initial hook-up off corners was weak. At Sacramento it was better. Early days.

 

An XR has a wheelbase just under 55 inches, with close to 50/50 weight distribution—so that’s where new projects begin. The XG prototype’s steel-tube cradle frame is simple and traditional. Suspension is Showa. A 20-inch swingarm of rectangular steel tubing acts through a linkless single damper.

 

 

This project has been handled by Vance & Hines, who have for years designed, produced, and operated Harley’s Pro Stock drag racer. Former rider Matt Hines said that when the ‘Street’ engines were delivered, “We looked at them and said, ‘there’s no way we can make a racer out of this—it’s so wide!’” But as they dug deeper they saw that by making their own closer-fitting right and left-hand case covers, the engine could become much more compact. There for all to see are the fine-pitch tool paths on those beautiful covers, attesting they were summoned from solid billet by CNC. Flywheels of various weights (essential on less grippy tracks) mount on the left.

 

 

The one item on the engine requiring a rules concession was the stock single-throat intake system, unable to deliver competitive airflow. In its place is a complex CNC two-throat throttle body with a single injector below each butterfly. Getting all the air it needs, what is the XG’s power potential? One thing 4-valve engines do well is overrev—a pair of small intake valves are easier to control at high revs than a single big one. At 10,000 rpm, with a developed level of net stroke-averaged combustion pressure, roughly 110 hp is possible, with plenty of valve control and bottom-end strength for over revving into the corners. At this level, piston acceleration is only 55 percent of that in a World Superbike ZX-10R, so there’s acres of room for power growth.

 

 

Stand at corner entry and you can hear top riders holding throttle as they run their bikes in, kicking the back ends out. Just as the rev limiters stutter, riders cut throttle, instantly switching that sideways attitude into a power consumer that slows the bike. Partner in the job is the rear brake, and because there’s only one (front wheels are brakeless), its reliability is paramount.

 

 

 

 

Information Source and photos: cycleworld.com / Harley Davidson