Honda VT 750C Shadow

Road Test - Cycle Magazine

Okay, try this one for size: The VT750 Shadow is the most radical motorcycle Honda has ever built. Maybe it's not radical in a technological way, though there's plenty of innovation and bright-think to impress anyone who has more than a couple of engineering genes. The Shadow is the most radical Honda in the way it looks and in the way it interfaces with the rider.

As for looks - all the styling cues seem to have come from Milwaukee, and Willie G. Davidson must have been flabbergasted when he saw the Shadow. There is, after all, a Milwaukee look, in the same way that there is a Mercedes-Benz look, and it would be pretty startling to see a new Honda automobile that took all its styling cues from Stuttgart, nicht wahr? Frankly, we don't care how a motorcycle looks.

We do care about how motorcycles work with their riders, and here again the Shadow is the most radical Honda we've ever tried—because the VT750 is much less a rider's motorcycle than any Honda we've tried. The riding position is awkWard and uncomfortable. The way-out-front footpegs and controls draw the rider's legs forward; the rider's body positioning is certainly more passive than that required by the Magna, for example, and absolutely unlike the naturally active stance common to standard and sport bikes, which position riders aggressively.

It's hard to believe a company that builds Sabres and Magnas and Interceptors could produce a Shadow—until you think about it. Honda can take this extremist position because the company has its performance and touring and racing segments so well covered. As for the street-bike future, Honda obviously believes there's demand out there for great variety in machinery. The corporate honchos in Tokyo aren't betting on one particular look or style or type; Honda would no more gamble its street-bike future on Shadows than it would on Interceptors. In short, Honda can afford to bet many chips in many places.

Consider the Shadow a way-far-out Cruiser, a chip placed at the very edge of the marketplace checkerboard, far from the center squares occupied by the Magna and Sabre. Farther out, we think, than other cruiser-style motorcycles, including Yamaha's Virago. Now for the multi-million-dollar questions: Will the guy who already owns a cruiser-type motorcycle—be it American or Japanese, twin- or four-cylinder—trade it away to buy another, more radical cruiser? Or for that matter, how many guys who own any other kind or style of motorcycle would be interested in buying a way-rad Shadow? Only

Honda can afford to spend the millions to find out—by building the Shadow. Honda has played the edges of the board before. Honda put a chip down called the CBX, which was almost as far from center as the Shadow, but in the opposite direction. Though the six-cylinder Superbike was warmly received by the press, the CBX—long term—flopped.

But Honda is back to gamble once again, this time with a more time-proven engine configuration. What can be innovative about another V-twin? Plenty. This 750 twin is built around what is arguably the most technologically sophisticated Vee ever designed. Honda's choice of a 45-degree Vee configuration is initially surprising; 90-degree Vees have been traditionally chosen for smoothness because they provide perfect primary balance. All other Vee configurations inherently produce more primary vibration, often enough to turn a bike into a real shaker. Engine counterbalancing systems, sometimes employed to counteract primary imbalances, add weight, bulk, complexity and cost.

Unfortunately, a twin with cylinders offset 90 degrees takes up a lot of space, forcing other design compromises. In the past, engine designers have followed one of three paths. Build a Vee less than 90 degrees and live with the vibration. Or add counterbalancers and cope with the complexity, etc. Or build a 90-degree Vee and extol the virtues (and ignore the drawbacks) of a long wheelbase, typically unavoidable with that configuration.

Honda, with its technological expertise, searched for a different route. The factory amended the book on V-twin design by introducing to the motorcycling world a slick bit of established engineering: staggered crankpins. Typical V-twin motorcycles have a common crankpin for the two connecting rods; not the VT750. The Shadow uses two offset crankpins separated by an unsupported flywheel (like the old British vertical twins). Honda's engineers offset the crankpins to "fool" the engine into acting as if it has perfect primary balance even though the actual cylinder angle is narrower than 90 degrees. Honda's engineers developed the formula X=180°-28, where X is the amount of crankpin offset necessary with a cylinder angle of 0 degrees. So for a 45-degree Vee, X = 180°-(2 x 45°).  In this case, X works out to 90 degrees of crankpin offset. With this formula, Honda's engineers can develop a V-twin of any practical configuration with perfect primary balance. The new VT500 V-twin, as an example, has a Vee angle of 52 degrees and a crankpin offset of 76 degrees.

While this bit of engineering sleight of hand effectively eliminates primary imbalances, it aggravates another type of engine vibration: rocking couple. Stretching the offset between cylinder bore centerlines increases rocking couple, and thanks to the dual-crank pin and center-flywheel arrangement, the connecting rods in the VT's plain-bearing crank are centered about 38mm apart. The big Ducati 90-degree V-twin, in comparison, has a cylinder offset of only 22mm because it uses a common crankpin. So in this aspect Honda still struck a compromise.

The Shadow has bore and stroke of 79.5 by 75.5mm and 153mm connecting rods. A rod length two times the stroke distance is fairly long for a motorcycle, but not unusual; more important, it helps reduce secondary engine imbalances. A flat-domed piston further reduces engine vibration by trimming reciprocating weight—important when you're dealing with two big slugs.

The three-valve VT head uses a compact, heart-shaped combustion chamber featuring two 'spark plugs, one on each side of the head. A single ignition coil fires the two plugs simultaneously, enhancing rapid and complete fuel burning. Honda claims the dual-plug design improves fuel economy by up to 30 percent at low engine speeds. The two-plug setup may also reduce exhaust emissions; the VT ignition curve runs with a maximum advance of only 26 degrees before top dead center (35 degrees BTDC is the norm these days).

A quicker ignition burn also forestalls detonation; despite a relatively high 9.8:1 compression ratio, the VT ran on all grades of pump gas without pinging. The two intake valves each measure 31mm in diameter, while the exhaust valve is 40mm across; the included valve angle is a remarkably narrow 37 degrees.

Three-valve heads offer advantages over two-valve designs because they can flow better on the intake stroke, especially at low levels of valve lift. Still, the valve train is simpler and less crowded than a four-valve configuration, and forcing spent gases out on the exhaust stroke is seldom a problem. The single exhaust valve design also eliminates the difficult-to-control hot spot between the two exhaust ports. The Shadow's valve stems are long and slim, and the valve heads have a tulip shape for improved flow. Also, the ports are clean and shaped for optimum flow; each port rises straight out of the valve seat area to promote good breathing, then bends gradually.

The VT750 brings a long-overdue feature to motorcycling: maintenance-free, hydraulic valve adjustment. Although the Shadow uses a single overhead camshaft and rocker arms, it doesn't use a hydraulic tappet system like those common to automobiles. Instead of using camshaft-actuated tappets in the valve train, the VT has a variation of the eccentric shaft setup used to alter valve clearances in the old CB 350/450 twins.

Each of the VT's rocker arms mounts on an eccentric shaft that moves back and forth as the rocker arm pivots. As the eccentric rocker-arm shaft turns it raises or lowers the rocker to maintain a zero-clearance relationship between the valve, rocker and camshaft. Constant pressure on either the bottom of the rocker-arm shaft (from a hydraulic lifter) or the top of the shaft (from an assist spring and shaft) helps keep it moving in the proper direction.

In addition to being maintenance-free, the hydraulic valve-lash adjusters offer another bonus. The running tolerances necessary with a manually set valve train typically require camshaft lobes with long, gently sloped ramps to open the valves slowly; this can cause small variations in valve timing that hurt low-end power and make an engine idle poorly. With the valve train running constantly at zero clearance, valve timing holds constant and the cam profiles can be more radical, banging the valves open a little quicker.

Each head has its own silent-type chain to drive the cams; the front cylinder is driven off the left side of the crank, the rear cylinder off the right. Each camchain has its own automatic tensioner, ending adjustment worries.

The cylinders' bold finning is primarily cosmetic—the VT750 relies on liquid cooling. The long, narrow radiator mounts on the front of the frame where it looks almost like an overgrown oil cooler. An electric fan, mounted behind the radiator, supplements airflow.

Like its liquid-cooled relatives, the GL1100 Gold Wing and the V45 Sabre/ Magna, the Shadow has cylinders with cast-in liners to improve heat transfer. Unlike the other two bikes, the VT's cylinders are not part of the crankcases; the Shadow uses vertically split cases and separate barrels. To reduce production costs, the cylinders and heads each are milled from common castings; variations in machining differentiate rear from front to prevent accidental switching.

Large straight-cut gears pass power from the crank to the transmission. The big 66-tooth driven primary gear is actually two thinner gears "split" and offset with spring tension. This arrangement reduces backlash between the primary gears, cutting gear whine. Power passes through the clutch to a six-speed gearbox, the bottom five normally spaced, a bigger jump to the overdrive sixth.

The VT750's clutch is innovative in a number of ways. First, the clutch actuates via hydraulics rather than cable. In addition to providing light clutch pull, the hydraulic setup automatically compensates for wear, eliminating the need for adjustment. A diaphragm clutch spring replaces the more common coil springs, narrowing the entire unit. Although diaphragm springs hold advantages over coil springs (lever effort remains more consistent throughout the lever travel), the switch was largely dictated by the addition of a Sprague (one-way) clutch, increasing the diameter of the assembly.

The idea of adding a Sprague clutch actually came from the NR500 racing program. The one-way clutch is designed to eliminate rear-wheel lockup caused by high engine compression during rapid deceleration and quick downshifting. The Sprague clutch controls half the clutch plates, which work with the other clutch plates to deliver power in the normal fashion during acceleration, cruising and normal deceleration. But with heavy backloading, when engine braking would normally be sufficient to lock up the rear wheel, the Sprague clutch slips. This diverts the backloading force through just half the clutch, which keeps the rear wheel from locking yet still provides plenty of engine-compression braking. Although the clever clutch system makes much sense for race bikes and sport bikes, it seems out of place on the Shadow, obviously designed for cruising.

If you really want to ride, the Shadow's seating position is awkWard and irritating. The footpegs and controls are much too far forward to provide rider support and the sloped portion of the dished seat forces you into the "laid-back" riding posture whether you like it or not. The pull-back handlebar offers no help; it too forces you to sit back. Together, these pieces demand that you ride in one uncomfortable position; you can't lean forward, you can't move back on the seat, and there is practically no way to use the passenger pegs for relief. You're simply trapped. After an hour or more in the saddle it's time to find a rest stop. The Cruiser mode may work on the showroom floor and for quick trips around town, but the riding position defeats even medium-range touring and effectively discourages fast riding of any kind. Two-up hauling is also difficult; our passengers condemned the thin, short and narrow seat and complained about the short sissy bar, which offers little back support.

The VT's chassis and running gear limit the bike's versatility just as much as the seating position. The suspension is soft, conforming to the 750's boulevard cruiser application. Fork action is good on the freeway, but the rear end sacks so much under the rider's weight that the ride feels harsh; heavier testers could readily bottom the rear end over large bumps and holes. Overall, the freeway ride is acceptable—but not state of the art.

One fast trip down a twisty road reveals a number of handling shortcomings. The fork is underdamped for high-speed, backroad cruising. The shocks perform poorly. In fast sweepers, bumpy or smooth, the rear end hobby-horses. If this happens in a left-hander, things get exciting quickly; the sidestand grounds heavily and keeps bouncing off the pavement as the hobby horsing continues.

Considering its steering geometry, 32.0 degrees of rake, 5.5 inches of trail and a 60-inch wheelbase, the Shadow steers lightly. We expected truck-like handling, but the VT responds well to rider input. The front end, however, doesn't inspire cornering confidence; it gives little feedback through the handlebar, so you're unsure if the front is sticking or losing traction. The brakes are one of the Shadow's strong points; the front dual discs provide plenty of stopping power with good feel, and the rear drum works well.

The Shadow's engine is strong for a V-twin, though unremarkable for a 750. With a showing of 12.87 seconds at 101.92 miles per hour, the VT is quicker than Yamaha's Viragos, but it trails the current four-cylinder 750s. But big twins, of course, are known and appreciated for their low-end torque and overall power characteristics. In these respects, the Shadow does well. It has a wide power spread and revs willingly.

Despite Honda's innovative use of offset crankpins, vibration appears at certain engine speeds. At cruising speeds vibration is well controlled. Thanks to the overdrive sixth gear, when the VT turns a modest 3488 rpm at 60 mph, only an unobjectionable throb comes through the handlebar. After 4000 rpm the Shadow is downright buzzy. The mirrors blur and the vibration that works its way through the seat, pegs, bar and tank as the revs climb becomes impossible to ignore. We identify the fairly high-amplitude, medium-frequency vibration as a rocking-couple imbalance—the inevitable trade-off with this particular system.

Our Shadow's carburetion was spot on. Cold starting was hassle-free and there were no glitches or staggers through the rpm range. We averaged a frugal 51.7 miles per gallon in our testing; with a fuel capacity of 3.3 gallons, the VT should easily cover 150 miles before the fuel level warning light glows red. The petite tank doesn't hold all the fuel; a small interconnected auxiliary tank hides under the seat.

The Honda holds a number of additional labor-saving devices that make the Shadow almost maintenance-free. In addition to the automatic valve-lash adjusters, camchain tensioners and hydraulic clutch, the VT750 has a pointless ignition, a brushless generator, and shaft drive. Only the spark plugs, engine oil and filters require attention; no motorcycle has ever offered such hassle-free ownership.

All these features may well tip the scales in favor of the Honda when it comes time to buy a custom-style motorcycle. Anyone who buys a Shadow will, we think, do so on the basis of its radical looks and in spite of its radical ergonomics and compromised versatility. It is precisely this compromised versatility that we find troubling, because as a functional motorcycle the Shadow offers no better performance than what Honda believes the target audience needs. That's distressing because historically Honda has been such a strong engineering company—and it still is—and it continues to build a host of motorcycles devoted to function. We like motorcycles with almost decathlon broadness; but here Honda has given us a body-building champion. The Shadow guy probably won't be troubled by this show-time narrowness, this compromised versatility. He probably won't want to be bothered with his motorcycle's workings and is little concerned about its ultimate performance potential. For this guy, the Shadow is a suitable motorcycle—low on maintenance and high on Milwaukee style. We won't hazard a guess as to how many enthusiasts will go for Honda's radical new VT; we will say unequivocally that anyone who does choose the Shadow is sacrificing performance for aesthetics.
 

Source Cycle Magazine