by Victor Wanchena
Dr. Rob Tuluie is at it again. The German astrophysicist turned engineer has a passion for little bikes with big horsepower. He is the creator of the Tul-Da Eccentric 500 (M.M.M., December 1996). For those who missed it, the Tul-Da is a wildly modified Honda CR 500 motor in a frame and bodywork designed and built by Tuluie. The bike incorporates some interesting features such as an adjustable steering head angle and eccentrically adjustable swingarm pivot, which lends the bike part of its name.
Now the Tul-Da isn’t just some one old engineering exercise. It was successfully campaigned by Rob in ‘94 and ‘95. During that time he captured two WERA National Clubman Championships, an ARHMA Sound of Singles race win at Daytona and, more recently, several CRA wins. Rob is still actively racing it today.
After all the time and energy he poured into the Tul-Da, Rob did not intend to build another race machine, but we are proud to announce the birth of the Tul-Aris.
This story began with Rob walking through the Polaris factory in Osceola, WI where he is a senior engineer working on the Victory V92C. He spotted an engine that caught his fancy. It was a 700cc two-stroke, narrow and light — just right to make a great power plant for a race bike. Rob has a serious love for big powerful two-strokes, and a little research revealed that a race prepped version of the motor was putting out close to 160 HP. Such power in such a small package usually makes people want to hide, but it is very tempting to Rob, “Who of us ever has the chance to ride a 500cc G.P. bike? Nobody does, but you can build something yourself, and race it for a reasonable amount of money.”
For the next few months we will tag along with Rob as he designs and builds the Tul-Aris.
Where does Rob start when designing a motorcycle from the ground up? Well he begins with the three most important words in racing: traction, traction, traction. Now, it’s more complicated than just fitting the fattest and stickiest tires you can under the bike. There is what’s called dynamic traction. That is the way the frame and the suspension interact with the surface of the road. It’s called dynamic, because the suspension is constantly moving in relation to the frame and the road.
A good example of dynamic traction is anti-squat suspension. Anti-squat works by using the pull of the chain to extend the rear suspension. To achieve this, the pivot point for the rear swing arm must be placed high, so when he rolls on the gas the chain pull causes the swing arm to want to move down along its axis. This has two effects. The swing arm moving downward actually pushes the tire on to the road surface giving him extra traction whenever he is on the gas. Also, as the rear suspension is extended, weight is transferred to the front wheel making the bike more stable.
Another consideration for Rob in the design process is how to use the entire frame of the bike as a shock absorber. As the bike enters a corner and leans over, the suspension is no longer parallel to the bumps and other irregularities of the road. In this position the suspension is unable to soak up all the force of the bumps, because the force generated rolling over them is directed straight up from the road surface. As a result, the frame and particularly the swing arm and forks flex and twist, absorbing a portion of what the suspension could not. Now this is not as important for average street riding and its lean angles, but on a race bike it can mean the difference between a stable and unstable machine. The difficulty is finding the right amount of flex, especially since the frame (unlike the suspension) has very little damping, which is the process of converting oscillations into heat. Pushing oil through small passages in a shock absorber accomplishes this. If the frame is too flexible it will keep oscillating after the corner. This makes the bike unstable and gives the handling an imprecise feel like the super bikes of the late 70s and early 80s, which had too much power and not enough frame. The opposite extreme is equally bad. This is evidenced in Yamaha’s older G.P. bikes, which were so stiff that they lost traction in hard corners.
Tires also help in soaking up bumps, but are much softer and also have very little damping. The research and development departments of big race teams work with tire companies to develop tires with the right characteristics for their bikes. For the average guy those big budgets mean a good selection of tires if you know what characteristics suit your needs.
For Rob everything begins with the motor. Spotting the motor in the factory got him thinking. First there is the height and position of the motor, transmission, and gearbox sprocket. Since they are the heaviest components of the bike their placement is critical to the position of the center of gravity or CG. Think of the CG as the balance point of all the bikes weight. The CG does not have to be in the center of the bike but imagine that it is. Move it up, and the bike is top heavy. Move it forward, and the weight bias is on the front wheel. The CG must be placed forward enough to keep the bike from wanting to wheelie all the time. Now I know what your thinking. A bike that wheelies is fun for all you hooligans out there, but not when racing. It affects the traction of the front wheel, and that hurts cornering speeds. Pushing the motor forward keeps weight and traction on the front wheel, but pushing it too far forward lessens rear traction and makes the bike feel heavy in the steering. Keeping the engine high maintains good cornering clearance and weight transfer when braking or accelerating, but if it is too high the CG will cause the bike again to be the wheelie king.
Once the motor is positioned Rob moves to the geometry of the front and rear suspension. These are fairly straight forward, as there are formulas for calculating the positions and angles as well as Rob’s own experience. In some ways the Tul-Da was a test bed. The adjustability of steering head angle and anti-squat has allowed him to find a setup he favors and works well on the track.
Rob is continuing to tweak this bad ass. As the Tul-Aris takes shape we will bring you insights into the design and construction of a roadracing monster. Anyone wanting to help in the Tul-Aris project can contact Rob through M.M.M. or by calling the Polaris factory in Osceola at 715-294-3370.