Naked One

NAKED ONE

POWER PACKED SECTION

Tsukigi Racing ZRX1100 exposed!

DON CANET

THROTTLE EASED OPEN ever so gently, the ultrafat Dunlop slips ‘n’ grips under the heavy strain of cornering and acceleration. The bike’s twin-shock suspension momentarily unloads as the rear tire spins up, then squats abruptly when the sticky rubber grabs hold of the tarmac. A shock wave rings through the chassis, prompting a buck-and-weave display that’s as subtle as Mike Tyson’s traffic etiquette. Committed, the rider continues feeding in power as the bike’s brute strength maintains a low, crossedup wheelie while the rear meat scribes an arc out of the comer.

That was so much fun, I just may have to try it a dozen more times!

High-drama comer exits are typical of the action seen in Japan’s NK-1 roadrace series. With Japanese passion for American nostalgia running as high as ever, it’s no surprise that NK-1-naked bikes closely resembling the monster AMA Superbikes of the early 1980s-has become a popular form of racing in recent years. These are big machines packing big power, with retro-race styling that jars loose memories of the classic battles between Eddie Lawson, Freddie Spencer and Wes Cooley nearly two decades ago. Naked-bike pioneer Heroyasu Tsukigi, owner of Tsukigi Racingan Osaka-based performanceparts company specializing in Kawasakis-is credited with having started the trend back in 1993 with a Zephyr 750 racebike. Tsukigi’s Zeph captured the public’s imagination that same year when he fielded the bike in the prestigious Suzuka 8 Hours with Americans Scott Zampach and Mike Hale doing the riding. And even though the Tsukigi Zephyr was outgunned by the multitude of works Superbike entries and failed to qualify, a seed had been planted. The NK-1 class was formed the following year, and has since spawned a sizable aftermarket dedicated to performance retro-standards.

Other notable Japanese racing outfits such as Yoshimura, Moriwaki, Over Racing and Beet Racing have all joined in the fray, building bikes based on Suzuki’s Katana 1200, Honda’s CB1300 and Yamaha’s XJR1200. As the level of competition quickly grew, Tsukigi’s 25 years of experience in racebike development was put to the test. He rose to the challenge and claimed the 1997 NK-1 Championship.

Having conquered foes on the home front, Tsukigi (www.tsukigi.co.jp; fax 011-81-724-47-0648) is now training his sights on America, and plans to open a stateside manufacturing and distribution operation within the next two years. Along with performance parts for Zephyrs and late-model GPzs, Tsukigi also manufactures exhaust systems, rearsets, triple-clamps, swingarms and bodywork for Kawasaki’s ZX-R series sportbikes. Shipping his championshipwinning ZRX to the U.S. for display at select shows-and a possible race outing here or there-is the first phase in Tsukigi’s business plan.

Of all the nice bits on the Tsukigi ZRX, it’s the simplistic frame braces that first catch the eye. Made of aluminum tubing, the bolt-on braces bridge the front and rear engine-mount points for added chassis rigidity. Move in for a closer view and many other details pop into focus. The braced swingarm is very nicely crafted, and extends the wheelbase 1 inch. Its main spars are made of box-section tubing with an internal reinforcement wall. The ZRX’s 4-into-l exhaust system is lightweight titanium. The 39mm Keihin FCR carburetors run open velocity stacks with a heat shield in place of the stock airbox. A ZX-7R curved radiator has been adapted with custom brackets, and mates nicely with the Plot oil cooler mounted below it.

Within the 1130cc engine run 78mm forged Cosworth pistons with a 12.5:1 compression ratio. The cylinder head’s ports have been polished, as have the stock valves, which are actuated by Yoshimura cams. Stroke remains standard, although the lightened and polished crank and rods are GPzl 100 items used to alter the primary gear ratio. A ZX-7R back-torque-limiting clutch-with factory race-kit plates-drives a Tsukigi GPzl 100 close-ratio six-speed gearbox. The ZRX hydraulic clutch actuation is retained, although the slave cylinder is now fixed to a Tsukigi-designed mounting plate that subs for the countershaft sprocket cover.

Power is fed through a #520 O-ring chain to the 6.25 x 17-inch rear Marchesini wheel, lifted from a ZX-7R Superbike kit. The 3.75-inch-wide front Marchesini, as well as the six-piston Nissin brake calipers and 320mm iron rotors, are also kit items. The ZRX’s stock Kayaba suspension has been upgraded to an Öhlins 46mm fork and piggyback shocks. Triple-clamps are billet parts featuring removable stem inserts for changing steering offset.

As with any muscle-bound machine, proper chassis setup is critical. After a couple of sighting laps around the Streets of Willow road course, I returned the ZRX to the garage for adjustments. The suspension settings were extremely rigid-the setup used for Suzuka’s smooth surface. Having Tsukigi and assistants in attendance made my job easy, although a bit of hand gesturing and sound effects were required to convey my impression of the bike’s ride. Back on track with greatly reduced compression and rebound damping at both ends, the ZRX was now able to cope with Willow’s bumps and my own unfamiliarity with the machine.

With the Superbike-bend Renthal bar providing plenty of leverage, tipping the ZRX into turns required very little effort. In fact, I found it very important to maintain a relaxed hold to avoid unwanted steering input. Not an easy thing to do on a bike with a flat, spacious saddle that wants to accelerate out from under you. Factor in the tight nature of the Streets, with its endless series of firstand second-gear comers, and you quickly become adept at riding the rear wheel between comers.

Churning out 157 horsepower and 87 foot-pounds of torque on the CW dyno, the potent engine produces ample steam to motivate this 418-pound machine out of top-gear corners, too. To its credit, the Tsukigi ZRX motor is a beautiful beast that produces a very broad and flat torque spread that would do any streetbike proud.

Perhaps the arrival of the ZRX 1100 NK-1 Champion to America will stir the winds of change, as did the Tsukigi Racing Zephyr back in ’93. If so, can a Legends Race starring Eddie, Freddie and Wes aboard contemporary replicas be far behind? Now, that would be a memory in the making. □