by Victor Wanchena
The saga continues with that crazy astrophysicist turned engineer, Dr. Rob Tuluie. The Tul-Aris is making the big transition from Rob’s vision into hard design and physical form. In part one Rob led us into the world of chassis and suspension design. In this installment Rob will dispense some wisdom on selecting the transmission and pushing, prodding and banging it into what he wants.
When I met to talk with Rob he was rather quiet, then he suddenly spoke as if inspired by some cosmic racing spirit. “Remember that a racing bike isn’t just jamming the biggest motor into the first frame that fits it. All the pieces must take shape together. Changing one affects the whole machine.” I wrote it on my arm, so I could remember the quote.
Rob was at a bit of a crossroads. Should he build his own transmission from scratch, or should he adapt one from another bike? If he builds his own transmission he could design it to fit all of his requirements, but that would mean countless hours spent starring glassy eyed at a computer. That precious time could be spent deciding a color scheme for the Tul-Aris that would coordinate with his race leathers, and he would be venturing on to the track to race at superhuman speeds with an untested design. Or, Rob could use a transmission that was already tested in battle, letting the major manufacturers spend the huge amounts on research and development, but that meant searching for one that fell within the parameters he had set.
Rob decided to travel the road more prudent and seek out a transmission that would fit all the specifications of the nasty job he had planned for it. He knew the transmission must be stuffed into certain physical dimensions, and this was a tough order to fill. It must be as compact as possible in length, so it can tuck up tight against the engine. But it still needs to be wide enough, so the front sprocket on the output shaft of the transmission and the chain will clear the fat and sticky tire needed to handle the 150+ horsepower motor. All those ponies also meant that Rob couldn’t scavenge the transmission off his Honda Trail 90. He would have to look for one that was proven to withstand racing’s grueling punishment. The last big concerns were six speeds (or at least five close ratios) and availability. Something vintage might be perfect, but would it be obtainable for a price less than the gross national product of Mexico?
Where to begin? There are countless different models that have been produced in just the past thirty years. With so many choices Rob had to narrow the field down to a few likely candidates. Well, big bikes = strong transmissions. This doesn’t mean cruiser or touring big, this means bikes like the KZ900, GSXR 750 and 1100, Ducati 916 and 750 F1, ZX10 and 11, GPZ1100, CBR900RR and 1000, etc.
With a few likely choices in mind Rob began the labor-intensive process of measuring transmissions. What exactly was he measuring you ask? There is obviously length and width, but also the distance of the input shaft from the output and the position of the shift drum. Some were easy to decide against – just a few outside measurements with a tape measure or just plain old eyeballing it. But others required a closer inspection. That meant tearing the transmission apart and making precise measurements with a caliper. This was time consuming, and Rob got odd stares from the folks behind the counter at the junkyards. A conversation with Rob always seemed to leave the guys at the yard curious as to why anyone would want to measure the transmissions on five different machines, but they’d point him in the right direction. Rob also got help with some transmission specs from Doug Lofgren and Reid Herman at Lofgren Motors, who let him measure up a couple of machines and they even loaned him a couple complete transmissions and cases. These became dining room table ornaments at Rob’s house for over a week. Rob even prodded a couple buddies on race teams for a few numbers.
Now a problem was starting to surface. The bulk of transmissions out there have their input and output shafts lying in a row along with the crankshaft. Tucked beneath these three lives the shift drum. This is a cylinder with grooves cut into it. Arms fit in these grooves and move the gears inside the transmission making gear changes a reality. This setup is almost an industry standard. The problem with this layout is that it makes for much too long a transmission, and that’s no good for our hero’s quest for the ultimate in compactness.
Our story now jumps to Italy a few years ago. The Ducati engineers, while designing their current crop of motors, decided that to make their 90-degree V-twin more compact they would have to shorten the length of the transmission. They made it shorter by staggering the three shafts into an inverted V shape and mounted the shift drum on the top of the transmission. This design tucked the transmission in behind the motor nicely.
Salvation appeared in spaghetti red bodywork. The Ducati 916’s transmission ended the hunt for Rob. It was compact in size and had proven itself on the bloody battle field of superbike racing. It also had the six speeds and smaller steps between the gear ratios that Rob wanted. This is important, since two-stroke motors make their peak power in a narrow rpm range. Six different gears would be able to better keep the motor spinning at its peak. To top it off, it is currently being manufactured, and there are various gears and ratios available at a reasonable price.
It wasn’t quite wide enough to accommodate the 190-size tire that Rob needed for the demon he called a motor, but otherwise it was a beautiful example of all the other virtues that Rob needed. This is where Rob received a call from Greg Ericksen, who had read about the Tul-Aris and was interested in helping the project. Greg is a master of the mystic art of computer design and, according to Rob, “able to make things not only function, but function elegantly.” Greg, in his work as design engineer extraodinaire, uses a program called Pro/Engineering 3D. The beauty of Pro/E is its ability to create three-dimensional drawings of very complicated bits, like a transmission, and then show how all the pieces interrelate. This saves Rob the daunting task of creating countless drawings, and is light years ahead of the 2D CAD program he initially used to design the Tul-Aris. When Rob and Greg finish designing, the Pro/E file can be loaded onto a disk and handed over to the two newest members of the Tul-Aris team, Mike “Dr. Metal” Weston and Tim “Tailpipe” Wirtz. As their nicknames indicate these two will be doing the CNC and manual machining of countless parts for the Tul-Aris and keeping Rob and Greg honest with their design feedback.
To clear the last hurdle, width, they decided to widen the transmission they would add extra bearings on the ends of the shafts in the transmission to cope with the higher loads on the transmission. According to Rob, “when dealing with 100+ foot pounds of torque, you better start adding bearings any place they fit.” The final product will be a modified version of the stock unit using aluminum and carbon fiber cases split vertically. It will be cassette style, which means Rob will have the ability to swap ratios without ripping the entire motor apart. Rob says, “this is the way carbon fiber is meant to be used, not as key fobs or triple clamp covers.” The actual gears and shafts will remain stock, but the shift drum, shift linkage, and drive sprocket are to be massaged extensively. Some 916 clutch parts will be tweaked to fit and the rest machined from scratch. The clutch will feature multiple dry plates and a cush drive, which are rubber pieces inside the clutch that absorb any nasty shocks that ripple through the driveline.
All this work may seem like overkill to get a compact transmission, but the smaller he keeps the motor and transmission the smaller he can keep the overall wheelbase of the Tul-Aris, and on the track, “short and light = fast and flickable.”
Rob is still looking for someone to help with the styling and mold making of the body work, anyone interested should give him a call at 612-529-5785.