The Honda RS750 Dirt-tracker

Cycle World - - Contents - By KEVIN CAMERON

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:

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