De­vel­op­ment of the Honda CB750:

Classic Motorcycle Mechanics - - MIRA FILES -

The launch of the Honda CB750 at the Tokyo Au­to­mo­bile Show in Oc­to­ber 1968 was just the first achieve­ment in a re­mark­able de­vel­op­ment pro­gramme that had started in the sum­mer of 1967. Honda’s big­gest road bike at the time was its CB450 DOHC twin, which although ca­pa­ble of match­ing the per­for­mance of the mostly Bri­tish 650cc twins sold in the UK was none­the­less re­garded as a bit of a toy. The CB450 wasn’t a ‘big bike’. Com­pany pres­i­dent Soichiro Honda put Yoshiro Harada in charge of the project to de­velop a new bike. Dur­ing a tour of the US in 1967, Harada was told by deal­ers that the bike should be “the big­ger the bet­ter”. Harada wasn’t quite sure how big but it was clinched when he found out that Tri­umph was de­vel­op­ing a 750cc triple, and that Kawasaki had a project for a big bike. So by Oc­to­ber 1967, its ca­pac­ity was de­cided at 750cc. And its power was de­ter­mined by Har­ley-david­son’s 1200cc V-twins. They turned out 66bhp, so the 750cc Honda would have more. The tar­get would be 67bhp. A 20-strong team of en­gi­neers was mus­tered and de­vel­op­ment started in Fe­bru­ary 1968. Tar­gets were:

• That the bike should of­fer sta­bil­ity when cruis­ing at 90 to 100mph yet be flex­i­ble enough for traf­fic ma­noeu­vring

• It should have strong and re­li­able brak­ing

• There should be min­i­mal vi­bra­tion and noise to re­duce rider fa­tigue, a com­fort­able rid­ing po­si­tion and easy-to-use con­trols

• Large and re­li­able light­ing and in­stru­men­ta­tion should be used

• It must have an ex­tended ser­vice life and easy main­te­nance • Em­ploy orig­i­nal de­signs us­ing new and bet­ter ma­te­ri­als, and pro­duc­tion tech­nolo­gies. Honda had only re­cently stopped rac­ing in the world cham­pi­onships, so had huge amounts of tech­ni­cal in­for­ma­tion. To speed up the de­vel­op­ment pro­gramme it set up com­puter-based sys­tems to ease the plan­ning of the pro­to­type phase, de­sign changes, hard­ware mod­i­fi­ca­tions and test­ing. The team was mov­ing into un­charted ter­ri­tory. The CB750 four was Honda’s first bike with an en­gine us­ing plain shell bear­ings. This en­abled the en­gine to be man­u­fac­tured more cost-ef­fec­tively than those us­ing pressed-up cranks with roller bear­ings, and Honda’s en­gi­neers had to de­velop new ma­chin­ing and assem­bly tech­niques. Harada said that the pos­si­bil­ity of us­ing twin over­head camshafts, as on the race bikes, was an op­tion that was planned for three or so years af­ter the CB750’S in­tro­duc­tion. Re­li­a­bil­ity, and the lower cost, of a sin­gle camshaft open­ing the in­let and ex­haust valves through rock­ers with screw clear­ance ad­justers prob­a­bly took prece­dence. Many fea­tures of the CB750 de­sign would be fa­mil­iar to mo­tor­cy­clists of the day but they were more com­pe­tently ex­e­cuted, mainly be­cause Honda’s en­gi­neers were start­ing from scratch with no pre­con­cep­tions or cul­ture to in­flu­ence them. The CB750’S cy­cle parts were a mix of the tra­di­tional but with a modern twist. Wheels were the nor­mal 19-inch front and 18-inch rear steel rims

sup­port­ing bias-wound tyres with in­ner tubes. Ge­om­e­try, with a 1460mm (57.5in) wheel­base, only erred from the fa­mil­iar by the use of a slightly more steep steer­ing head an­gle of 27 de­grees and mod­est trail of 3.7 inches, cho­sen no doubt to pro­vide lighter han­dling at the speeds that new rid­ers would be first test­ing the bike. The steel tubu­lar frame was tough, a welded struc­ture with twin spars lead­ing back from the bot­tom of the pressed steel steer­ing head assem­bly to the top of the rear shock mounts and braced by a sin­gle tri­an­gu­lat­ing tube. Two more spars looped un­der the en­gine back to the swingarm pivot. Sus­pen­sion was noth­ing new ei­ther – a tele­scopic front fork and De Car­bon-style rear shocks – and would bear the brunt of later crit­i­cism. The de­sign was al­most com­plete by the time a pro­to­type CB750 was first shown in Tokyo in Oc­to­ber 1968. There were many dif­fer­ences be­tween this bike and later pro­duc­tion ver­sions: the car­bu­ret­tors were rac­ing-style Kei­hins with en­closed open­ing mech­a­nisms, the left-side en­gine cases were more squared off around the starter mo­tor drive, the disc brake cen­tre has fewer spines and the side-panel badg­ing is sim­pli­fied to a small Honda logo. But the spec­i­fi­ca­tion was fixed. The CB750 had a bore and stroke of 61 x 63mm, giv­ing a swept vol­ume of 736cc. On a com­pres­sion ra­tio of 9 to 1 and breath­ing through four Kei­hin car­bu­ret­tors it de­vel­oped a claimed peak power of 67ps at 8000rpm – enough, ac­cord­ing to Honda, for a top speed of 125mph. The en­gine had plain steel­backed shells for the five-bear­ing crank­shaft with a six-pint lu­bri­ca­tion sys­tem fed from an oil tank be­low the seat. A ma­chine like this, fit­ted with a tour­ing fair­ing and sad­dle bags, was tested back-to-back against a Tri­umph Tri­dent in the deserts of Cal­i­for­nia in De­cem­ber 1968. Four ma­chines were built for show­ing to US deal­ers in Jan­uary 1969 and two months later in the UK at the Brighton Show. Called ‘late-pre-pro­duc­tion ma­chines’ with en­gine numbers CB750E-2110 to 2113, th­ese ma­chines were very sim­i­lar to the first pro­duc­tion bikes, apart from a num­ber of hand-fab­ri­cated com­po­nents. Th­ese four ma­chines are highly prized by afi­ciona­dos. One of the late-pre-pro­duc­tion ma­chines, 2113, was ac­quired and re­built by Cal­i­for­nia ‘sand-cast’ spe­cial­ist Vic World and auc­tioned on ebay in 2014 for the equiv­a­lent of al­most £90,000. More re­cently, 2110 came up for auc­tion in the UK and reached £160,000. Of the other two, one is thought to have been crushed by Honda af­ter be­ing used as a train­ing school ma­chine, while the other is said to have ‘dis­ap­peared’ in France. Pro­duc­tion of the CB750 started on March 15, 1969 at the Saitama and Ha­ma­matsu fac­to­ries. With the price of the bike pitched at $1495 in the US (about half that of big Har­leys) the fac­tory was del­uged with or­ders. Pro­duc­tion was ex­pected to be 1500 a year but this soon be­came the monthly fig­ure. Yet this was still in­ad­e­quate, and was soon dou­bled to 3000 a month. Even af­ter pro­duc­tion had started, parts were reg­u­larly be­ing up­dated. Th­ese bikes are iden­ti­fied by their en­gine and frame numbers start­ing at CB7501000001, but the main clues to iden­ti­fy­ing the

“With the price of the bike pitched at $1495 (about half that of a Har­ley) the fac­tory was del­uged with or­ders. The leg­end of the CB750 was born.”

very ear­li­est ma­chines are the sil­ver brake caliper with bolts on the back, painted head­lamp mounts and air-cleaner cases, tri­an­gu­lar side-panel badges, a spe­cific type of si­lencer (not with the HM300 stamp), an un­finned oil-fil­ter case. To speed up out­put, the crankcases from Septem­ber 1969 were made with pres­sure diecast­ing giv­ing a smoother fin­ish, ef­fec­tive from en­gine num­ber 1007415. At the same time the clutch cover was changed to use 10 rather than nine mount­ing screws. And so the be­gin­nings of the ex­clu­sive Sand Cast Only Club were es­tab­lished (see www.cb750sand­cas­tonly.com). Within this early pe­riod – which also in­cluded Dick Mann win­ning the 1970 Day­tona 200 race with one of four fac­tory-pre­pared rac­ers af­ter qual­i­fy­ing at 152mph – the pro­duc­tion bikes are var­i­ously de­scribed as CB750 or CB750K0, but some say that just 121 CB750K0 ma­chines were made. The early CB750 Fours were said to be the fastest. Re­port­ing for the weekly Mo­tor Cy­cle in April 1970, David Dixon tested one at MIRA’S tim­ing strip, achiev­ing stand­ing quar­ter-mile fig­ures of 12.6 sec­onds and a ter­mi­nal speed of 102mph. But the top speed fell short of Honda’s claim, with a two-way av­er­age of 115mph and a best one-way speed of 121mph with a strong tail wind. By Au­gust 1970 the pace of change hot­ted up and with CB750-1044650 the CB750K1 was in­tro­duced along with an­other raft of mod­i­fi­ca­tions such as the in­tro­duc­tion of the link­age for the car­bu­ret­tors mak­ing ad­just­ments eas­ier, a black brake caliper, slim­mer side pan­els with two em­blems, a 750Four above a Honda Wing di­a­mond. The novel chain lu­bri­ca­tor that drib­bled breather oil through the sprocket was made ad­justable. Fur­ther changes that would be car­ried over to mod­els made up to 1978 were in­tro­duced with the CB750K2 in 1972. Start­ing with en­gine num­ber 2000001, th­ese are iden­ti­fied by the chrome head­lamp mounts, black head­lamp shell and a re­vi­sion to the ‘750 Four’ side-panel badges. I road tested a CB750K2 in 1974 for Mo­tor Cy­cle and found its per­for­mance with higher gear­ing to have been soft­ened. While the av­er­age top speed at MIRA was slightly lower at 114.57mph, the stand­ing quar­ter-mile fig­ures had slipped to 13.65 sec­onds with a ter­mi­nal speed of 97.8mph. The CB750 was made in 17 dif­fer­ent ver­sions in­clud­ing po­lice, au­to­matic and sports un­til 1978, when the CB750F3 and CB750K8 were re­placed by a new range with 16-valve four-cylin­der en­gine with dou­ble over­head camshafts.

Just eight months af­ter Honda’s CB750 project was given the go-ahead, this pro­to­type was launched to world­wide ac­claim at the Tokyo Au­to­mo­bile show in Oc­to­ber 1968, half a cen­tury ago. Many dif­fer­ent ver­sions were made be­fore its pro­duc­tion line started up in March 1969.

1968 pro­to­type 1969 1975 K5 1975 P2B

1976 AU­TO­MATIC 1976 K6 1977 F2A 1977 A2

Chris Rush­ton has recorded more than 100,000 miles on this ‘sand-cast’ CB750 in the 28 years since it was im­ported.

All fig­ures compiled at Mo­tor In­dus­try Re­search As­so­ci­a­tion’s prov­ing ground, Nuneaton, War­wick­shire.

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