Honda 750 Full Attack Interceptor (1983)

		Under the old Superbike rules, street bikes were hot-rodded into racers with special pieces top to bottom. Under the new 750 Superbike rules, if you want to build a race bike, you'd better build one for the street first. Honda did that. For starters.

►Debate has for years waxed hot over the simple assertion that "Racing improves the breed." With the coming of the Honda Interceptor, the definitive answer is in: it does. The road-racing technology Honda has had to put into its street Interceptor to permit legal use of certain features in Superbike racing has made the Interceptor almost embarrassingly good. People will notice this, and they'll want a lot more of it. They'll get used to confidence-inspiring responsiveness, quick steering, and stability over rough surfaces, and they won't accept less.

Changes in AMA Superbike racing rules demand that the Interceptor and the Interceptor Superbike be very close in design. Material may still be added to the race bike chassis, but now none may be taken away. This makes impossible special wide swing arms, relocated footrest brackets, reangled steering heads. If a company wants these features in a race bike for the new 750 Superbike class, it must build them into the street bike. That's just what Honda's done.

The Interceptor engine is a completely redesigned V-4 based on the 1982 Sabre, but with heavily reinforced cases and the engine rotated backward 15 degrees to permit a shorter wheelbase and steeper head angle. A chain replaces the Sabre's shaft drive. The chassis is a massive and heavy twin-loop design welded in rectangular steel tubing, fully as wide as the engine, and triangulated by it into great rigidity. The racer's swing arm, too, is stock Interceptor—a wide aluminum unit accepting the widest racing tires.

Away goes the production 39mm TRAC fork, replaced by a one and fiveeighths-inch Showa racing fork, the same used on other Honda racers from FWS to NS500. This fork carries two of the Nissin four-piston racing calipers, each bolted to racing TRAC anti-dive mounts—on both fork legs instead of the one-sided TRAC of the street bike. These fork legs are milled from solid stock on programmable metal-removal centers; metal ends up only where the designer wanted it—not where the manufacturing process happened to put it. The lower fork crown is the hollow casting with four pinch bolts that other Honda racers use—strong in torsion because it is a closed-box structure, yet light because it is cored out. At the request of Freddie Spencer, who likes solid bars, the upper crown, milled from an aluminum blank, carries mounts for normal steel handlebars instead of the street Interceptor's articulated clip-ons.

Riders like this Showa fork, and I'm told it contains only accurately made parts based on the familiar street-fork technology of check-valves and simple orifices. It has slippery Teflon-impregnated bushings and is very supple.

At the rear is the stock investment-cast Interceptor swing arm. Used without modification now, the arm was tested with a brace since discarded.

In place of the production shock and linkage, a combination of steel and aluminum milled and turned pieces makes up the Pro-Link assembly, all pivots turning on hardened steel pins and needle bearings. For use in highly leveraged suspensions, even the friction of a well-made bushing is too much. It produces stiction and unwanted torques that can cause rapid shock-rod wear. Needle bearings promise to eliminate such rubbing friction, but in early Honda racing suspensions, needles were used in forged links. Under the normal twisting of a race bike's chassis and swing arm, these stiff forged links transmitted torsion to the needle bearings, binding them up. On the Interceptor Superbike, Honda made the Pro-Link arms from paired, thin milled plates, separated by round spacers. These thin plate arms can twist with the chassis, maintaining strict center-distances while not binding up the needle-bearing pivots and thus eliminating even the modest friction of Teflon-fabric-lined ball bushings (Heim-type joints).

The racer's spring/damper unit mounts vertically and incorporates a thin extension which projects down through a hole in the swing arm to reach the apex of the Pro-Link assembly underneath. The shock body is machined from solid aluminum, as you would expect of a low-production special part. A nitrogen-pressurized damper, with its gas-pressure accumulator mounted remotely, prevents shock heat from raising the gas pressure. The pressurized gas prevents cavitation inside the damper oil, which would otherwise be pulled apart under the suction of the damper chamber's filling stroke. This Showa shock uses the bending washer technology of the De Carbon system, and its low-speed damping can be externally adjusted for both compression and rebound.

The Pro-Link maintains a low initial rate over most of the suspension's travel, then increases smoothly near full compression to absorb unusually large bumps without bottoming.

The wheels are the familiar Honda racing items fabricated from a cast hub, high-strength sheet-metal spokes and an extruded rim, all fastened together with titanium bolts. Such a wheel allows use of strong, pore-free material in the crucial spoke and rim areas, and the cast hub provides a complex shape without excessive machining. In Europe wheels of this kind have been run on the GP racer with light carbon-fiber parts substituted.

Each front brake caliper has two pairs of pistons—a leading pair of 27mm diameter and a trailing pair of 34mm—acting on a segment-shaped pad. As a disc moves through the grip of the pads, binder resin evaporates from the pad material, producing a thin gas layer which must escape from between the pad and disc. Combinations of holes and slots in the discs help somewhat, but this gas layer is so thin it can reduce the pads' grip considerably even so. To overcome the effect of this gas layer, designers enlarge the trailing piston, the result being even pad wear from leading to trailing edge. With only one pair of pistons, one on each side of the disc, pads wear into a wedge shape that can cock the pistons so much by the end of a long race that the brakes become ineffective.

There is another subtlety to the four-piston system. The velocity of the disc in inches per second, obviously much larger at the very outside of the disc than at the inner edge of the pad track, concentrates heat at the outer edge. This can distort the disc into a cone shape which pushes the pads and piston back, requiring the rider to pump them out again at every corner—a very unnerving business. To combat this, Honda made the pad very long in the circumferential direction, yet quite narrow in a radial direction. Consequently, disc speed—and heat concentration—at the outer and inner edges of the pad is more nearly equal and distortion is reduced. The same concept has been applied to clutch plates for years.

The excellent Nissin calipers swing on TRAC arms pivoted on the fork legs, their free ends bearing on the stems of the fork's compression damping valves. The harder the rider brakes, the more the calipers' torque reactions push the compression damping valves closed and the harder the fork resists bottoming.

Again Spencer's preferences influenced the design of the rear disc and caliper. Freddie does all his braking straight up, having no real use for the rear brake because under these conditions the rear tire carries no weight. The rear disc and caliper can therefore be extremely small, with the caliper carried on a floating mount as in motocross.

To obtain the benefit of easy response to rider control forces, Honda made the front wheel a 16-incher as used in GP racing for three or four years now. The rear is an 18-incher.

Between the suspensions lies the chassis—the great experiment of infusing race technology into the streetbike world. If this is the only outcome of the new AMA Superbike class, it will all have been worthwhile, for here at last is a chassis designed to produce outstanding handling qualities rather than low production costs. In fact, this chassis does not even represent the state of Honda's art in racing design; it doesn't have the double triangulated yokes that support the steering head on the Honda NS500 or other GP machines. Be that as it may, it is a huge advance over the older narrow-frame designs on street machines. Furthermore, constructed of steel rather than the aluminum now universal in GP design, it is also deceptively strong. The top frame loops join the top of the steering head, and the two downtubes angle in to the bottom of the head, the only connection between upper and lower loops being a formed sheet-steel gusset on each side.

To see the frame by itself is to miss the full picture; because this V-4 has no primary imbalance, the engine can be rigidly bolted into the chassis safely, providing the most massive cross-bracing any designer could hope for.

On the Formula One FWS, the obvious precursor to this design, even the front cylinder head bolts into the chassis and braces the steering head. The FWS, however, suffered from a defect now corrected in the Interceptor: it was hard to service. The more places the engine is bolted in, the smaller the chance of making it to the grid if time is short and parts are lying all over the shop floor. A bit of lost chassis stiffness is a reasonable price to pay for rapid serviceability. Phil McDonald, who will service Mike Baldwin's machines this year, estimates that an engine change on the Superbike will take just over 30 minutes. The FWS? Two to four hours. Even reliable racing equipment needs serviceability: stones still fly into carb intakes; other riders still crash in front of you in the last practice before the main event. The old romantic notion of ultra-techno race equipment that can only be built by masked technicians working in white rooms full of filtered air is silly nonsense. You must be able to fix your equipment anywhere they have races, and you must be able to fix it now. Honda has learned a lot from European GP racing, and this is one area where it shows.

The Interceptor's shortened wheelbase is made possible by the redesign of the Sabre-V-4 engine— Honda engineers rotated the entire engine up and back 15 degrees. Once this was done, Honda could push the front wheel back and steepen the steering-head angle. While making this change in the engine, it was natural to reinforce the cases where the stresses are largest—around the main-bearing saddles. Engine rigidity keeps shafts and bearings properly aligned, avoids overloads and reduces friction. These cases are heavy, but for a purpose.

Like the Sabre, this engine turns on plain bearings supplied with plenty of oil from a unique wet/dry sump system. The sump tray beneath the engine is divided into two sections. Oil falling away from the engine's parts drops into the front sump where a large-capacity scavenge pump picks it up and sends it to the oil cooler, mounted in a "chin" position just in front of the engine and a few inches off the ground. From the cooler it flows into the rear sump compartment, where the engine pressure pump picks it up and delivers it to the oil galleries. This system holds subtle advantages. Unlike a roller engine, a plain bearing engine cannot tolerate momentary variations, in oil flow. Rollers are always there, holding the shaft away from the bearings' outer races; the only thing separating a plain bearing crankshaft from its bearing insert is oil. Stop that oil for an instant, or reduce its density with a mass of bubbles, and that crank and bearing will seize, spinning the insert out of the crankcase and wrecking the engine.

In the Interceptor Superbike engine, the two sumps are small and deep, each fitted with a conical screened oil pickup that almost completely covers its bottom. This prevents oil from sloshing away from the pickups, as it easily can in many other designs. The two-sump system also gives entrained air two chances to escape from the oil. On racing cars and in aircraft, both subject to much more internal oil motion than motorcycles, engineers use extremely complex means to de-aerate oil before it goes to the engine, and in some drag-racing applications, motorcycle engines use once-through systems, in which a tank filled at the start of each run supplies solid oil. Such extreme measures underscore how crucial oil supply is to a racing engine, and Honda has carefully considered the matter in the Interceptor Superbike.

Some oil flows to the crank mains, and from there through cross-drillings to lubricate the two crankpins in the forged one-piece crank. Other oil goes to the centers of the four cams and to the lever cam followers. The quantities of oil going to the head drain back over the valve springs, cooling them. Oil is also supplied to the centers of the two transmission shafts and to the shift forks. Radially drilled holes in the shafts lubricate the free-spinning gears, sending oil outward to lube the teeth as well.

As with the other V-4 Hondas, the Interceptor Superbike is water cooled—not magic but a convenient means of controlling an engine's operating temperature. Given enough fin area, air cooling can work as well, but to make an air-cooled engine run hotter or cooler, you must add or cut off fins or control the air reaching them. A water system needs only a small thermostat to hold engine temperature constant. Constant engine temperature means fewer changes to carburetion in response to weather changes, and it also means constant oil viscosity—not oil that turns to water as the day heats up.

Water has a higher specific heat than does any solid engine material. Pound for pound, it requires more heat energy to raise its temperature than anything except ammonia. This makes local overheating of a water-cooled engine very difficult. There is an added fall-back, too; although water requires one calorie per gram to heat it one degree Centigrade, it takes a whopping 540 calories to boil a gram of water. This is what gives water its huge advantage over any other cooling medium—it can remove monumental amounts of heat by local boiling, the steam being recondensed almost immediately by mixing into the coolant stream. General boiling of such a system is prevented by pressurizing it. Aluminum radiators, carefully fitted into the spaces left by the front tire, front cylinders, and the chassis, deal with the hot water.

The crank is almost identical to that of the Sabre, except that spur gears replace the silent-chain sprockets in its center. It is a true racing engine, uncompromised by the weakness of chain drive to its cams. A noisy but reliable gear train runs through the spaces provided between cylinders. The gear-drive modules bolt into place. Each cam-drive gear contains a bonded-rubber torsional damper, there to reduce the peak forces that might otherwise require much wider and heavier cam gears. A nice refinement—a little weight here saves a lot elsewhere.

Four titanium con-rods swing from the two-pin crank, assembled onto it with split caps retained by high-tensile steel bolts. The rod inserts run at 0.0016-inch clearance—you can see how fast these engines might seize without steady oil flow.►

◄Each rod is pinned to an ART-made forged piston carrying three conventional rings. These pistons have very thin skirts, and Honda has put much effort into keeping the dome thickness constant, the underside contour reflecting the crown's shape. Pistons from the 1025cc FWS are the prototypes for these, identical except for size. I was shown an FWS piston which had completed last season's racing with hardly a mark on it—its original machining marks were intact. Oil is great stuff when there's enough.

Valve clearance in the street engine is handled by threaded and lock-nutted adjusters at the valve ends of the forked lever cam followers. Under the pressure of racing accelerations such threaded adjusters are unreliable, so Honda gave the Superbike followers that are plain but strong forgings, with radiused solid ends. Clearances are set by sliding selectively fitted lash caps over each valve stem. It's a long, exacting job, but seldom required.

The valves themselves are the now-standard titanium items, the intakes 28mm, the exhausts 25. Steel cotters join each valve stem to its titanium spring collar, sitting atop low-pressure valve springs. Although the current trend is toward springs with few coils (which resist surge—bouncing of the coils), Honda prefers the low stress level of conventional many-coil springs to the problems produced by super-high internal stress in the newer three and one-half turn type. It doesn't take a lot of pressure to control these light valves, for even the bigger intake valve weighs only 18 grams. You can open the valves with a thumb push, and the full-lift spring pressure is some 150 pounds. This valve train remains stable well past the point of rod breakage, so there is little point in applying more technology than necessary.

The Interceptor has the limited-slip, one-way drive pioneered in the Honda NR500 and FWS—a device which prevents engine-braking from dragging and hopping the rear tire during hard braking. On the Interceptor street bike this device is integrated into the main clutch, but on the Superbike, it moves to the crankshaft end, becoming a separate eight-pound unit which can be reached by removing three screws and a cover plate. This accessibility allows tuners to match the grip of the limited-slip clutch to track condition and to rapidly replace parts. Because race bikes are usually push-started, Honda gives the Superbike an arrangement of spring-loaded drive pins in the clutch unit. During push-starting, the pins are engaged by their springs, permitting the engine to be started. Once spinning, the pins fly out of engagement and the slip unit is ready to race.

As in the Interceptor street bike, the race bike gearbox has only five speeds—the same parts used in the FWS, provided with extremely large engaging dogs and wide tooth faces. The dry main clutch permits drag-free running during clutch starting and guarantees good shifting once under way. As with Honda factory motocrossers, the Superbike uses steel inner and outer clutch hubs instead of the more familiar aluminum for two reasons. First, a steel basket retains its strength even when the clutch is very hot; aluminum weakens drastically at even water-boiling temperature. Second, the steel outer basket never opens up under hard use, which permits the plates to become wedged. The hammering of racing shifts and engine power impulses soon causes clutch disc tangs and splines to beat little grooves in aluminum hubs, making clutch release irregular. With the harder steel this doesn't happen. As a result of Honda's efforts to lighten these steel parts, each is a lovely filigree of milled slots and drilled holes.

In place of the right-angle drive of last year's Sabre shaft-driven system, a steel bearing-carrier bolts to the left side of the gearbox, through which projects a long countershaft carrying the splines for the output sprocket. From this, the chain transmits the power to the rear wheel.

The engine's numbers reflect Honda's understanding that acceleration and not top speed wins modern road races. Those. who have so far ridden the Superbike say its top speed is almost disappointing (although it has since been improved), but they also say that acceleration is almost identical with that of the much larger FWS 1025 Fl bike.

Oddities reflect how Honda emphasized acceleration. The exhaust pipes have headers much longer than pure top speed would dictate, the length improving lower-speed torque. To prevent such long pipes from choking the engine at high speeds, Honda gave them a large, one and three-eighthsinch inner diameter, a size often used on 1000cc racing fours. The compression, valve timing, and ignition timing are also said to indicate Honda's intention to produce acceleration rather than top speed.

In testing, maximum speeds were reportedly not much higher than those already reached by the best 1000cc twins of recent years—perhaps in the low 150s. This would indicate something on the order of 115 horsepower from this engine. What does this imply?

Were it developed to an rpm and BMEP level that represents the current state of the art, this engine would peak at 14,000 rpm and deliver a BMEP of 190 psi, which would translate to 150 horsepower. The present engine is therefore stressed far below its maximum capability, and has room for much future development.

There are limitations, however. The Interceptor cylinder heads have a curious offset design—the carburetor's axis is aimed right down the centerline of one of the two intake valve ports, while the passage to the other valve branches off rather suddenly, leaving a large gallery in which airflow can slow down and lose direction. Which, then, is responsible for the present moderate power level? The desire of the engineers to develop the engine gradually? Or inherent limitations in the design of the cylinder head? Time will tell.

Certainly no one can complain about the combustion chamber—with intake and exhaust valves inclined to each other at a small 38 degrees, a central spark plug, and squish areas at front and back.

Carburetion at present is by the curious racing constant-vacuum instruments, developed because Japanese endurance-racing rules require similar carbs on racer and street machine. Their 34mm throat size is large for a 750, so perhaps the CV feature has been developed from a handicap into an asset. Certainly riders have highly praised some of Honda's CV racing carbs recently.

The magneto rotor shows mature design: instead of making it from a separate machined hub riveted to a stamped rim, Honda machined the piece from solid bar, with the variable-reluctance trigger poles furnace-brazed to the outside of the rim. In service on street machines, the riveted assemblies are fine; at racing rpm, crankshaft torsional vibration loosens the rivets and shears them off. It can't happen here. Inside the rim are the permanent magnets which generate the required power pulses.

Currently the machines weigh more than the AMA minimum of 390 pounds, but a weight-reduction kit is expected from the factory, containing such little goodies as thinner-walled exhaust pipes and front - fairing frames fabricated from aluminum rather than steel.

Like the FWS of a year ago, and like the many other Honda racers built currently, the Interceptor Superbike design is well integrated: every part is shaped so its neighbors will fit. If the design is right, it yields the advantage of compactness. If the design is flawed, it could require a time-consuming redesign of many components to permit even a small change in one area. The Superbike is so well thought out that to change it in any important way will require a complete rethink.

Maybe that's the way Honda likes it. They have always insisted on the latest in engineering and research equipment, and designing such dense packages as the Interceptor and FWS may make good exercises for modern techniques. With Computer Aided Design, for instance, engineers can check the fit and integration of complex parts in three-dimensional simulation on a computer screen before any metal has been cut. Potentially, CAD can greatly shorten the time needed to design or re-design any complicated device. Indeed, we may soon measure the difference between a simple design and a complex one in minutes of computer time, not in weeks and months of human labor.

Be sure that Honda engineers are working with such design aids now, and when you look at their complex new Interceptor Superbike you know you are peering into the future. ■

The Motorbike Archives

Under the old Superbike rules, street bikes were hot-rodded into racers with special pieces top to bottom. Under the new 750 Superbike rules, if you want to build a race bike, you'd better build one for the street first. Honda did that. For starters.

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