by Troy Johnsontulda_1

HOME-BREWED CLUBMAN RACER

The Tul-Da Eccentric 500

Half the fun of spending time at the track is walking through the parking lot to ogle the custom work put into the street-going race-replicas. The parking lot is where you expect to find the unusual. On the track, the bikes usually come from a stock chorus of characters. They are all the same models you see every month in the national glossies. At the CRA races this year, however, there was a bike eccentric enough to draw questioning glances from the crowd.

The Tul-da Eccentric 500 is a home-brewed single cylinder racer that occupies a space several giant leaps ahead of the typical “750cc Engine Stuffed into a 600cc Frame” inde racer bike. The man responsible for the Tul-da is Dr. Rob Tuluie. That “Dr.” doesn’t mean he spends his afternoon making house calls. Mr. Tuluie is an engineer, and he knows a thing or two about tying a pair of wheels together to make a motorcycle. The Tul-da Eccentric 500 nameplate derives from Tuluie, the Honda CR500 engine that turns the sprockets and the eccentric swingarm mount.

That eccentric swingarm mount is indicative of the unusual and thoughtful engineering displayed throughout this motorcycle. Those of you who ride late-model Kawasaki and Triumph sport bikes will recognize the mount at the swingarm pivot of the Tul-da. It looks just like your eccentric chain adjuster. The eccentric swingarm mount allows Rob to adjust the anti-squat. To you and me, that means he can dial-in the amount of traction the bike gets coming out of a turn.

tulda_2The forces that be are constantly messing around with a motorcycle’s geometry as it circulates a race track. Braking, accelerating and cornering upset suspension parts, change wheelbase numbers and shift the center of gravity. Most racers learn to perform in spite of and along with these problems. Not Rob Tuluie. After eight years racing everything from an AJS 7R to a TZ750, he decided to conquor them. He made everything adjustable on the Tul-da. For example, it takes just ten minutes to change the rake. Altering the steering head angle lets him stake out a balance between quick turning and stability depending on the race day variables.

Tuluie built his bike in the winter of 1993. It weighs 197 lbs. dry. The overall dimensions of the bike lie between those of a 125cc and 250cc GP racer. The frame is thin-wall (about 1.5 mm) chrome-moly tube. Tuluie outsourced the TIG welding for the frame. Welding chrome-moly changes the physical properties of the metal near the welds. Heat treating the chrome-moly renormalizes it, but where do you find a kiln large enough for a motorcycle frame? Well, you call your friends at the local university. They have big kilns. The heat treated Tul-da frame cooled to a T6 on the hardness scale. A time intensive process like this will not find its way into mass production any time soon.

Tuluie gas welded the subframe himself, but he did not renormalize it. This allows him to make changes that effect weight transfer. The Tul-da is very sensative to changes and options like this help him tweak the chassis to perfection.

The tank and seat are hand-laid carbon fiber, a month-long project.

The Eccentric is sporting its sixth or seventh home-made exhaust pipe. The unusual feature here is its routing–through the swingarm. The swingarm is an altered Japanese part. The forks come from a TZ250 and are conventional cartridge forks. A conventional fork has less unsprung mass than an upside down fork. A low ratio of unsprung mass to sprung mass allows the wheel to follow road undulations without disturbing the bike.

The front brake is a single 320 mm Brembo disc, and the front wheel is a Marvic. The shock is a Fox unit. The rear wheel is a modified TZ250 wheel that Tuluie cut and rewelded to move the cush drive and sprocket into position according to his grand scheme.

The engine is a CR500 water-cooled two-stroke. A two-stroke gives optimum performance at a specific temperature. An exhaust temperature gauge and a water temperature gauge work with the radiator to ensure the Tul-da runs at its preferred temperature. The radiator on the Tul-da is a Nissan IMSA racing radiator that Rob cut in half. The thick core of this radiator is more efficient than a motorcycle radiator.

tulda_3A 44 mm carb mixes the fuel, and a carbon fiber manifold directs the charge into the cylinder. Rob welded up the ports in the stock cylinder, resleeved it and put the ports where they should have been in the first place. A lightened, aftermarket piston means less moving mass, and a nickel carbide bore means no more reboring. Tuluie had Carbon Tech design carbon fiber reeds specifically for his hopped-up CR motor. The engine makes between 70 and 75 horsepower.

The transmission is stock Honda. Rob makes the sprockets.

A single cylinder motorcycle is going to vibrate. The way Tuluie has delt with the vibration was to put it where he wouldn’t use it–at idle. The Eccentric vibrates a lot at idle and smoothes out at 5,000 rpm. Through the power band (from 6,500 to 9,000 rpm) the bike is “supersmooth” according to Rob. “TZ250s shake more.” A TZ250 is a twin. The Tul-da redlines at 9,500 rpm.

You may be wondering whether or not this thing actually works. The proof is in the pudding. Rob Tuluie raced the Tul-da in the 1994 and 1995 WERA Clubman series winning ten of eleven starts and two grand national championships. The Clubman class allows any single cylinder bike to race. In 1995, he also raced in the Unlimited Twins class where he won three of five starts and placed third in the grand national race. He brought the bike to Daytona in 1995 and won the AHRMA Sound of Singles (SOS) race.

Tuluie moved to Minnesota this past season and raced the Tul-da at a couple CRA meets at Brainerd International Raceway. BIR has an extremely long and fast front straight and first turn. Racers must hold their bikes wide open up the mile-long straight and through Turn One, and the Tul-da did not like it. It siezed three or four times. Tuluie is still considering ways to fix this problem.

There is a second Tul-da Eccentric 500 in operation. Rob built one for a racer in New York. But due to the time and energy it takes to build each Eccentric, a third bike seems unlikely.

M.M.M.

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