自行車筆直前進非“主銷後傾穩定效應(caster effect)”及“陀螺力騎過車的人都有過經歷,自行車總是趨向於筆直前進,只要速度足夠快,我們還可以放開車把。那麼,自行車如此穩定的原理又是什麼?研究人員通常推測是兩種機械原理促使自行車筆直前進,其一是“主銷後傾穩定效應(caster effect)”,它與轉向軸和前輪的位置有關;其二是“陀螺力”。然而,根據上周發表在《科學》雜誌上的報告,研究人員發現這些推測都是錯誤酒店工作的。康奈爾大學的工程師利用電腦設計了一種消除陀螺力和主銷後傾穩定效應的特殊自行車。他們的測試發現,自行車仍然能筆直前進。他們認為,真正的原因是前輕後重的品質分配。Source:solidot.orgHow to Keep a Riderless Bike From CrashingGive a riderless bike a push, and chances are it will cruise along for a bit without tipping over. Scientists have long thought they understood the physics of what keeps 結婚the bike upright. But a new study shows that much of what they believed is wrong. Researchers assumed that two mechanisms kept a riderless bike upright. First is the "caster effect," which relies on the position of the front wheel relative to the steering axis—the imaginary line that extends from the front forks downward. On most bikes, the front wheel meets the ground just behind this 租屋axis, so it tends to center itself like the casters on a shopping cart. The other mechanism is known as gyroscopic precession: Because the front wheel is spinning forward, any tilt to the left or right creates a force that will steer the bike in that direction, pulling it out of the fall.To show that neither of these mechanisms is in fact crucial, engineer Andy Ruina of Cornell University 買屋網and colleagues used a computer to design a bicycle that excludes both mechanisms. Unlike a normal bike, the front wheel of their vehicle lies in front of the steering axis, so the caster effect can't operate. On top of that, both front and back wheels are connected to duplicate wheels spinning in the opposite direction, so any gyroscopic precession is canceled out. The contraption 住商房屋doesn't resemble a normal bike. The wheels are small, and it sports heavy, metal weights close to the ground in front and atop a tall pole jutting up where the seat would normally be. "It looks like a funny scooter," Ruina says.Once the researchers had built the bike—described online today in Science—they gave it a push and discovered that it stayed upright almost until it came to a 禮服halt, correcting any leaning just as a normal bike does. Ruina's conclusion: "Gyro effects are important contributors to self-steering [and] so are caster effects. It's just that they are not essential."So what does keep the bicycle from falling over? The trick is its odd mass distribution: low in front and high in the rear. As a result, the front will try to fall faster than the 保濕面膜rear, just as a short pencil topples faster than a tall broomstick. Because front and rear are connected, this lopsided falling makes the front steer into the fall and in doing so pulls the bike out of it.Joel Fajans, a physicist at the University of California, Berkeley, who has studied bike stability, calls the work a "very thorough, well-done study." The most surprising result, he says, 代償is that the caster effect isn't necessary, as previous experiments had already suggested that gyroscopic precession isn't as important as many people think. "It's possible that these results might influence bicycle design," he says. Ruina and his colleagues agree that their findings could help professional bike engineers to explore other strange-looking designs that are just as 借錢stable. "With these changes of appearance, one might have bicycles that are easier for old people to ride, bicycles that are easier to maneuver but still feel stable when riding straight, bicycles that have a more comfortable sitting arrangement, or bicycles that have more convenient places for storage or carrying kids," Ruina says. .follow 節能燈具{
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