Flexi-Flyers

Flexi-Flyers

TDC

Kevin Cameron

FLEXIBLE VEHICLE STRUCTURES DEform under operating loads. An example is found in the Space Shuttle’s controlled rate of main engine cut-off. At eight-and-a-half minutes of flight, the engines are operating at 65 percent thrust to limit Orbiter acceleration to 3 g. Velocity is close to 26,000 feet per second. Instead of simply shutting down the turbopumps supplying propellants to the main engines, the liquid oxygen flow is reduced at a controlled rate, to limit thrust reduction to a rate of change of 700,000 pounds per second. It takes roughly 1.3 seconds to reach zero thrust at this rate.

Why bother with such a complexity? The answer is that the thrust of the three main engines puts considerable strain into the Orbiter’s structure; that is, it deflects it like a spring, storing significant energy in the strained parts. If all that stored energy were released at once, the structure would “ring,” or vibrate, possibly damaging itself. The controlled rate of thrust reduction allows natural damping within the structure to absorb this energy harmlessly.

Consider a racing motorcycle. Former roadracer Dale Quarterley described his Kawasaki Superbike of the early 1990s as needing restraint during hard maneuvering.

“They make a big deal about chassis stiffness today,” he told me then, “but when you ride them hard, you can feel all the parts winding up and unwinding.”

He said the 750’s chassis was so flexible that you had to wait for it to finish “unwinding” from one move before you could start another-or there would be handling trouble.

Think of what this means. When the rider applies steering pressure on the bars to make the machine roll over for a turn, first the front tire’s tread starts to lay down footprint in the new direction. As it does so, it generates stress in the tire sidewalls, tending to pull the rim sideways to move in the new direction. This force on the rim passes through the slightly flexing wheel spokes and axle, and acts next on the fork legs, bending them in the new direction that the tire tread has taken. This force travels up to the steering head region of the chassis, twisting it slightly with respect to the rest of the motorcycle. Now all these parts-tire, wheel spokes, axle, fork legs and steering head-are deformed like springs, holding energy within themselves. As the tires drive out from under the bike, making it roll over, the energy in all these springs has to rebound. If the rider allows it to happen suddenly, it would be as if the Space Shuttle’s engines had cut off instantly, allowing the accumulated strain from 900,000 pounds of thrust to be released suddenly, vibrating through the structure, causing oscillations and high local accelerations. The motorcycle would wobble as all its parts snapped back to their unstressed positions and vibrated afterward.

So Quarterley and other top riders instead release those stored energies at a controlled rate. A good word for this is “grace.”

Cycle World's Off-Road Editor Jimmy Lewis has speculated about how one dirtbike, with quick steering geometry, may nevertheless seem sluggish in its response compared with another, similar machine with rake and trail figures that ought “by the numbers” steer more slowly. He believes this kind of surprise may be explained by differences in component stiffness. When you put steering pressure into the bars and the above sequence of events is set in motion, it takes time for tire flex to build up force on the rim, for wheel and axle flex to build up force on the fork tubes and so on. The sum of all these delays must be added to the time it takes the rider to turn the bars against gyro and trail/self-centering resistances, and to the time it takes for the machine to physically roll over once the applied forces have developed. In the case of a structurally “soft” machine with quick steering geometry, versus one with slow geometry but greater stiffness, it may be the slow-but-stiff bike that reacts faster.

Back on asphalt, this is complicated by the perceived need to make chassis less stiff, to enable them to act as supplementary suspensions when the machine is leaned far over in turns. In this condition, more flex helps to keep the tires in road contact, while greater stiffness leads to what Wayne Rainey once called “chatter, hop and skating.” The soft, slower-responding bike that results from reducing stiffness to avoid such problems must then be speeded-up in its responses by giving it quicker steering geometry.

All these flexible parts can be regarded as oscillators with very little damping. A more flexible fork or a twistier steering head may solve one problem but create another. The aforementioned tire chatter, largely a racetrack phenomena, is made much more likely when two or more of the motorcycle’s natural oscillators-the vertical bounce of the front tire and the back-and-forth bending of the front fork, say-come into step with each other. This allows the system to store so much energy in oscillation that the natural damping present-the friction between fork tubes and the crowns that clamp them, or the flex of rubber in the tire-cannot absorb it fast enough. Small driving forces in the system such as tire variation (outof-round or out-of-balance) put energy into the oscillators, perhaps sufficient to make the front tire begin to bounce up and down. If everything is oscillating together, the bounce builds until the rider sees double and the harder he twists the throttle, trying to accelerate out of the turn, the less time the tire spends on the pavement.

This is why racing engineers are no longer describing chassis stiffness in terms of deflection versus applied force, or chassis twist per foot-pound of torque, but in terms of oscillation frequency. Tuning fork, anyone? □