Development of the Honda CB750:
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.