THE KING SLAYER
The Honda RS750 Dirt-tracker
Honda went dirt-tracking in the 1980s and did it well
Dirt-track racing—on mile and half-mile ovals plus Tt—is uniquely American. It comes to us from the earliest days of motoring, when the horse tracks of county fairgrounds were the natural place to race motorcycles from Harley-davidson, Indian, Reading-standard, and others. Dirt track survived competition from the super-fast and dangerous banked board tracks of 1910 to 1925, and it is having a rebirth today.
Why does a motorcycle manufacturer go racing? Harley keeps at it because, having been there forever, it would be seriously strange to stop, like a beautiful smile spoiled by a missing front tooth. Indian jumped in two years ago. Why? Inscrutable corporate reasons? To say they’re more American than Harley? Because it’s a way to get more people excited about motorcycles in general? Pick one.
Long before the present Indian revival, another company decided to go dirt-track racing, then rapidly refined what they were building until top riders could win the AMA’S Grand National Championship on it. That company was Honda, and the machine it built was the RS750.
The program began in earnest when Jerry Griffith built a framer around a Honda XR500 single for Jeff Haney to ride. It did well, coming to the attention of race boss Dennis Mckay, who wanted to see Honda get further into the dirt-track scene.
Harley had that scene pretty well sewed up with its aluminum XR-750 of 1972, a bike that every year became more and more intimately adapted to dirt as maybe a dozen ingenious tuners and the Harley factory team worked to improve it. No one aspect of bike design can win dirt-track races. The winning machine has to be adapted to dirt in all its parts. This meant that degreed engineers backed by computers were not a winning play.
What it takes is people who understand racing, backed by R&D. For Honda, those people included Gene Romero, the highly respected Jim Dour of Megacycle Cams, the late airflow maestro Kenny Augustine, and machinists and fabricators.
The first hardware was unlikely—honda’s CX500 V-twin, which mounts in its frame like a Guzzi, with its crank fore and aft. It was shorn of its shaft drive, then rotated 90 degrees to the left, and given a chain drive. Both pipes were on the left, both carbs on the right. Soichiro Irimajiri, the designer of Honda’s legendary RC165 18,000 rpm 250cc six, saw this “homebuilt” race and took an interest. Getting a full 750cc from the CX’S 500cc conflicted with good cooling. The necessary
oversize cylinder liners trapped steam bubbles, causing overheating that cut power after 10 to 12 laps. This was the 90 hp NS750 of 1981 and ’82—a tool for learning.
The tale I was told related a growing respect between the informal American team and the Japanese side, made up of Tokyo University graduate engineers. Parts and assemblies sent to the U.S. for evaluation were modified here in ways that earned respect once returned to Japan. A mutual understanding and cooperation developed.
The two NS years prepared Honda for the RS750. The game was to produce the same torque characteristics as the dominant XR-750 Harley, but with room for improvement. The advantages would be four valves per cylinder, which do what the cams tell them more obediently than the XR’S two heavier valves; plain bearings, which are immune to the fatigue failures that haunted the XR’S roller rods; and smoothness, coming from Honda’s offset crankpin balancing.
Honda people told me: “First you go for torque, and when you got it all, you spin it up. Then you get into a range where the combustion chamber [that’s best] for torque is in conflict with high rpm breathing.”
The engine was OK at 10,000 rpm with the same 79.5 x 75.5mm bore and stroke as the XR-750. Cylinders were chrome on aluminum—no low-conductivity iron liner to push up piston temperature. Team engines had titanium intake and exhaust valves, with exhausts being changed after four races.
The goals were simple. First, to make more power. The RS was up 5 hp on Harley-davidson peak power, and Honda was close on the bottom. Plus, because four valves make it possible, the RS had an extra 1,000 revs. Second, the RS750 had to be stronger mechanically than the Harley, which has crank and rod problems above 8500 (Harley race manager Dick O’brien didn’t like to see XRS on the far side of 9000). Finally, the Honda
needed to have a good dirt-track torque curve. The RS could be ridden away in fourth gear. On the same graph with an XR torque curve, the XR’S is higher but the RS turns more revs, so at the rear wheel it’s close. In both cases, the curves fall at about 6 pound-feet per 1,000 revs.
The RS was given a modern flat combustion chamber with forged, flat-top three-ring pistons fitted at the usual Japanese engine clearance of 0.0012 to 0.0016 inch. There are piston-cooling oil jets. Engine weight was 151 pounds, about 8 less than the H-D. Chain drives to the two single overhead cams are on opposite sides of the engine. Early engines had leakage-prone external oil lines, but they were later cast into the right-hand cover. Oil pressure was 80 psi—hot.
The C&J chassis looks like dirt-track chassis seen to this day, in that a single downtube drops from the steering head, then becomes two tubes that curve under the engine and back to the swingarm pivot. Steering-head angle is adjustable with offset cups between 24 and 27 degrees, with 2.3 to 2.6 inches trail. Suspension was Showa—a 41mm fork with U.s.-made crowns to allow offset adjustment.
The Japanese built what they learned into the RS750, which appeared in 1983, and Hank Scott won du Quoin that year.
For 1984, Honda did just what Indian did last year— they put front-row talent on their bikes: Ricky Graham and Hank Scott, who then finished 1-2 in the GN Championship. Then Bubba Shobert took the No. 1 plate in 1985, ’86, and ’87. The RS in these photos is his, and it resides in the Honda museum in Torrance, California.
How did folks see these newcomers? As adding welcome diversity to Harley’s 27 GNC titles in 38 years? Or as destroyers, unfairly buying success?
In 1986, the AMA engaged airflow pioneer Jerry Branch to study the practicality of “leveling the playing field” by the application of intake restrictors. Restrictors were imposed, and Honda left the series in 1988. Ricky Graham won one last AMA Grand National Championship on an RS750 in 1993.
This is the rider’s-eye view—brakeless front wheel, hot exhaust pipes, and Goodyear’s classic rain-pattern dirt-track tire. BELOW:
TOP: The RS’S engine is set high to transfer weight to the rear tire during acceleration. Inverted tuning-fork forward frame adds “softness” to keep the front hooked up.
TOP: The slender swingarm is part of the soft-chassis concept. Rear brake isn’t just for slowing down—riders use it to modify the engine’s torque delivery. ABOVE: A dzus quarter-turn fastener exemplifies what every race bike needs: quick serviceability.
OPPOSITE: The RS750 engine applied sportbike technology to the dirt track just as sportbikes were taking form.
45-degree air-cooled carbureted V-twin ENGINE: 4 valves per cylinder HEAD: 100 horspower at more than 8,500 rpm POWER: Dual crankpins at 90-degree offset CRANK: