THE MI URBASPORT TRI-MAGNUM: HOME-BUILT EXCITEMENT (Feb, 1983)
THE MI URBASPORT TRI-MAGNUM: HOME-BUILT EXCITEMENT
By Robert Q. Riley and Dave L. Carey
WITH almost a decade of safety-and energy-conscious cars under our belts, we Americans have learned to accept the econobox theory of automotive engineering. What used to be one of our greatest pleasures—the car—has become a mundane, utilitarian device for economically carrying people and things from place to place.
Conserving fuel is fine. But having fun while doing it is even better.
It’s possible for a car to be fuel efficient and downright exciting. To prove that point, we’ve produced the latest in the MI series of Urba cars, the UrbaSport Tri-Magnum. Tri-Magnum is a direct descendant of the popular three-wheeler, UrbaSport Trimuter, which was featured in MI exactly three years ago. And just like the Trimuter, you build the Tri-Magnum yourself using Mi’s plans. The Tri-Magnum can be built for far less than any store-bought performance car— about $2,000—in your garage.
The name Trimuter came from the fact that the car was a three-wheeled commuter car. It was powered by a 16-hp industrial engine which gave it a top speed of 60 mph and mileage of about 50 mpg. Our new Tri-Magnum, on the other hand, is more like the high-powered magnum pistol, since it can get from 0 to 60 mph in 9.5 seconds and 0 to 100 mph in just over 20 seconds! What gets it there is the 81-hp, 76 Kawasaki KZ900 motorcycle plugged into the rear. And if that’s not hot enough for you, you can use the later-model l,100cc Kawasaki. In fact, any bike from 400cc on up will work. Just choose the one that best meets your personal performance and fuel-economy needs.
The marriage of a motorcycle to the chassis of a three-wheel car is a natural union. There are plenty of used bikes around at a reasonable price (we paid $800 for our ’76 KZ900); by removing the front-fork and wheel assembly of one of them, you end up with an integral, lightweight power train. It’s also a power train that, pound for pound, is as efficient and powerful as anything made anywhere. We used the plentiful and lightweight VW Beetle front-suspension assembly at the Tri-Magnum’s other end. Ours cost $150 at a local wrecking yard. The motorcycle and the VW front suspension are tied together with a simple steel framework to complete the chassis, which includes the steering, suspension, brakes, power train and all. This package is covered with a sleek, aerodynamic fiberglass body. The result is an all-weather, fully enclosed vehicle that combines the economy of a motorcycle with the safety and stability of an automobile. And the increased weight is nicely offset by the improved aerodynamics.
The styling of Tri-Magnum is both functional and in character with its aggressive performance. Aerodynamics, the cooling requirements of the air-cooled motorcycle engine, accessibility to the cockpit and engine compartment, ease of construction and safety considerations are all integrated into the design. The impact-absorbing foam-filled front bumper, which ties into the frame with a massive steel U-member, is designed to spill air onto the body. Body lines flow smoothly from front to rear where they are sharply broken around the taillight nacelles to create a clean separation point. The rear-facing duct on top of the engine cover and the two shark-gill side louvers are designed to draw hot air out from the engine compartment while cool air is ducted into it from underneath. A small fan mounted just ahead of the engine keeps it cool while idling.
The lift-up canopy, though exotic, is simple, functional and strong. It leaves the main body area integral for maximum strength and, when open, it presents an entirely open cockpit so you don’t have to duck under a low roofline when getting in and out.
The canopy has an internal, laminated steel framework that runs around its perimeter and down the windshield posts. Gaps between the fiberglass canopy and the steel framework are filled with fiberglass to form a solid fiberglass/steel laminate. This fiberglass/steel composite is stronger than either of the materials individually.
Nitrogen cylinders from a Toyota hatchback counterbalance the weight of the canopy so it opens easily; it stays open by itself. The steering wheel moves forward and out of the way, so you simply step in and slither down into the comfortable, form-fitting, space-capsule- style seat. There’s room for two people, side by side. When you’re in, the steering wheel returns and locks in place; then a gentle tug on the nearest nitrogen cylinder lowers the canopy, which latches if you let go of it just before it closes. Inside, the view is panoramic. The windshield begins at the top of your head and extends forward to within a few inches of your feet. The side windows cover even more distance, wrapping around the sides from the base of the windshield to the rear cockpit wall. To improve aerodynamics, all windows are mounted flush with the exterior skin.
The interior has a definite jet-fighter feel to it. The elements are exotic in both look and feel, but they’re not designed for aesthetics alone. They are the natural result of the blending of motorcycle components and systems with the steering and seating of an automobile. For instance, the stick shifter, which looks as though it might have been removed from the nearest F-15 and bolted in place, is nothing more than a cutoff end of the motorcycle handlebar which has been fitted with the stock motorcycle handlebar-mounted switch assemblies and clutch lever. This control stick places all the controls within easy reach at a single location. It also saves money (because there are no extra parts to purchase) and simplifies construction.
Another cost-saving feature is the instrument cluster. The original motorcycle instrument cluster is mounted on a perch atop the steering column. The only new parts required are extra long cables for the tachometer and speedometer, plus a speedometer step-up gearbox and adapter so the speedometer can be driven from the standard VW connection at the left front wheel. Amidst all this there is one new gauge: a cylinder-head temperature gauge mounted on a perch just forward of the stick shifter. The perch also carries the fan switch, fan-on indicator light and the manual choke.
Driving the Tri-Magnum is similar to, yet different from, driving a car and a bike all at the same time. First of all, with the canopy closed there’s a totally encapsulated feeling. The thickly padded, form-fitting seat holds you securely in place. No sliding around or slouching with this design. The throttle and brake pedals are on the floor and the stick shifter/control column is comfortably at your side. The first step is turning on the key switch which lights up the oil-pressure indicator, near the bottom of a vertical light bar between the speedo and tach. The next step is to find Neutral so you can start the engine without having to hold in the clutch lever. The shift lever is spring-loaded to a neutral position. It pivots forward or backward about 1-1/2 inches off center to change gears. To switch to the next lower gear, move the shifter forward then release it to its neutral position. Each time you do this it downshifts one gear. To upshift, pull back on the shifter and release it. After a few pumps of the lever, the neutral-indicator light, a green light at the top of the light bar, comes on.
Pressing the shifter-mounted starter button brings the engine to life with a muffled, mellow purr from the rear. Throttle response is instant. Although the travel of the throttle pedal is a good 4 inches, just a slight movement of it sends the rpms soaring—at least that’s the way it sounds. A glance at the tach shows that these little taps on the throttle are revving the engine to a mere 3,500 rpm. The KZ900 develops its peak horsepower at 8,500 rpm and it’s redlined at 9,000. So what sounds like high rpm is just above idle for this engine. This is something that takes getting used to. If you shift gears according to how you expect the engine to sound, you’ll be lugging it.
Neutral is located between First and Second gears, so to start out, grab the shifter and the clutch lever, squeeze, then push the lever forward. Tri-Magnum lurches as it drops into First. The engine has very little low-rpm torque, so it needs more revs than feel normal as pressure is released on the clutch lever—at least until you get used to the sensitive clutch. Our first few tries either stalled the engine or laid a 20-foot patch of rubber as Tri- Magnum screamed out of the hole. (A lot more fun than stalling the engine.) It takes a few attempts to get the hang of coordinating the throttle and the clutch as you take off from a stop. But if you don’t want to fool around, just floor the throttle and let go of the clutch. The rear tire becomes its own clutch as it spins merrily along for the first 50 feet or so. Shifting gears is easy from there on out. During acceleration all shifts are up, so it’s second nature to squeeze the clutch lever as you grab and pull the shifter. You can do it almost more quickly than you can think it.
Acceleration is so tremendous that until Tri-Magnum hits 20 or 25 mph under full throttle, the rear wheel spins because the engine can produce more power than the single rear wheel can transfer to the ground. Once things get planted, however, the force nails you to the rear wall and keeps you there until you get out of the throttle. It takes only one jerk on the shifter, up to Second, to send Tri-Magnum flying past 50 mph. With each full-throttle shift the rear end floats a bit until it gets fully planted; not badly—just enough to tell you that the rear wheel is going faster than the car.
Although we didn’t try hard cornering at 90 mph, we did slow it down a bit for some sliding turns just to see what happened. Tri-Magnum basically understeers, which is what it’s designed to do. The center of gravity is located low and close to the front wheels in order to provide a large margin of safety against rollover. The trade-off in gaining rollover protection is inherent under-steer, which means Tri-Magnum acts pretty much like your basic Chevy when pushed to the max.
Directional stability is another benefit gained by placing the center of gravity up front. Tri-Magnum stubbornly resists swapping ends no matter how it’s treated. A locked-wheel, sliding stop from 50 mph produces a straight-ahead line of travel with the rear end floating first a little to one side, then a little to the other. Even full-power turns with the rear wheel spinning wildly across the pavement produce nothing more than a little sideways float at the rear.
Corners are taken flat with almost no body roll. This thanks to the stiff stabilizer bar on the VW front end. In our case it’s a necessary item because all the roll stiffness of a three-wheeler must come from the two side-by-side wheels.
Three-wheelers are considered motorcycles by the government, therefore they are required only to meet the legal requirements of a motorcycle. In effect, this means that just about anything with three wheels can be licensed for the streets. A three-wheeler does not have to meet any of the safety requirements of a car. It doesn’t even have to be equipped with bumpers or a windshield, and it can be licensed with only one headlight and one taillight. Our ideas are a little different where safety is concerned. Consequently, we’ve given proper attention to many design features not required by law.
First of all, we think that other drivers should be able to tell where the corners of the car lay, so we’ve designed Tri-Magnum with two headlights and two taillights. We’ve installed a foam-filled front bumper (a design proven to have extremely high impact-absorbing capabilities in safety studies) and tied it into the frame with a large steel supporting structure that is fiberglassed into the body for increased rigidity. The canopy is steel reinforced throughout. Side intrusion protection is increased by leaving the body unbroken by side-door openings. The bulkhead that forms the rear of the cockpit is built up with foam around the perimeter to a thickness of 3 inches, then fiberglassed over. This creates a built-in roll bar and adds even further to the capabilities of the body to withstand a side impact at the passenger compartment. A bump at the rear would be absorbed first by the foam-filled body and then by the wheel located at the rear of the car. Loads transferred to the motorcycle are stopped by the tripod structure that secures the motorcycle at the front. Potential whiplash is eliminated by the thickly padded rear wall of the cockpit which extends upward to the roof, completely protecting the occupants. Finally, there is no dash, so you can’t bump your knees.
Of course, the biggest safety feature is the ability to avoid a collision. Tri-Magnum has plenty of extra power to get you out of a tight situation, and it’s highly maneuver-able with virtually no unusual handling characteristics.
We estimate the cost of building Tri-Magnum at about $2,000, plus the cost of the motorcycle. The motorcycle is not altered except for removing the front-fork and wheel assembly and welding on two small brackets at the lower front of the frame. (Although we can’t imagine why anyone would want to do it, the motorcycle can later be removed from the Tri-Magnum, refitted with accessories and put back on its two wheels.) The cost of a used bike ranges from about $400 for a smaller and older machine, up to about $2,000 for a nearly new, late-model high-powered version. As mentioned earlier, we paid $800 for our 76 KZ900, which had only 17,000 miles on the odometer. It had a couple of broken lights, a dented fuel tank and a broken clutch cable, but was otherwise in excellent condition. The price was about right for what we got and we sold the fork and wheel assembly for $150 to recoup some of the cost.
You can assemble the chassis in about two weekends. It involves cutting and welding steel tubing. The body is basically a fiberglassing project. Figure on about 300 hours, start to finish. The plans, which take you through the whole project, step by step, with photos and drawings, are designed with the assumption that you have never fiber-glassed before, so illustrations and instructions are complete and detailed. The plans also describe the technique for establishing the shape and contour of body panels, so if you want to experiment with styling changes you’ll know how to get the results you’re after. And the result most of us are after is an attractive, personalized vehicle that delivers the mileage and performance we want at a price we can afford.
ROBERT Q. RILEY and DAVE L. CAREY together form Quincy-Lynn, the Arizona-based company that works with MI to develop and produce all the cars in the Urba series.