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本著實事求是的精神, 讓我們來做個簡單的實驗吧! 把後輪墊高, 模擬成一部頭管角更大(更陡)的單車, 如上圖所示, 此時的trail為0.
若把這樣的車傾斜, 前輪還會朝傾斜的方向轉嗎? 事實上, 當我們騎單車下陡坡時, 單車正處於這樣的狀態. 由經驗告訴我們, 此刻的車把會變得較為敏感,
於下坡過彎時, 我們得花更多的心力去操控車子, 以求平衡與穩定.
好吧! 回到正題. 當我們把沒有trail的車子傾斜, 前輪還會朝傾斜的方向偏嗎? 如下圖所示, 答案是不會. 很能想像吧.
人類從1880才年代才了解的操控原理, 著實困擾了我好久. 每隔一陣子就想拿出來想想這看似平常, 又不是很容易解釋的原理.
我並不認為它直覺, 不然人類不會到19世紀末才真正搞懂它.
在網路上爬過無數的文, 終於讓我找到解釋此原理的最佳解釋.
"...As the bike leans, the upward force of the ground on the wheel combined with this distance creates a torque about the steering axis that tends to turn the wheel in the direction of the lean."
http://wiki.answers.com/Q/How_does_a_bicycle_in_motion_maintain_its_balance#ixzz1FM8S5dlv
請看上圖左. 當我們把車身傾斜, 輪子與地面接觸點的反作用力(藍色)會產生黃色與綠色的分量.
其中垂直於輪面的綠色分量, 因trail不為靈而對轉軸產生力矩(綠色的力乘以紫色的半徑), 造成前輪朝車身傾斜方向偏.
當前輪偏到一定程度(如上圖右), 等效trail等於0, 此時力矩不覆存在(因已無紫色的半徑存在), 輪子自然就不再繼續偏轉下去.
(ps: 若在繼續偏下去, 我們不是等著摔車, 就是得耗多於心力把把手穩住, 無法人車一體)
這也同時解釋了
1. 上述trail等於0的的車子在傾斜後, 前輪不會朝傾斜方向偏擺.
2. Trail越大的車子, 在低速前進(或靜止)時, 身身傾斜後, 前輪偏擺幅度較大.
更完整與全面的解釋, 請猜參考拙著:
Trail/Head angle對單車穩定性與操控性的影響
http://www.wretch.cc/blog/franklu/11878227
Note: 除了Trail之外, gyroscopic precession也是造成車子傾斜, 前輪還會朝傾斜的方向偏的原因.
Another is gyroscopic precession. As gravity pulls down on the center of mass of a leaning bike it creates a torque about the line between the contact patches. The spinning wheels tend to precess in response to this torque, meaning they tend to rotate about an axis that is perpendicular both to their spin axis and the axis of the applied torque, an approximately vertical axis. The rear wheel is prevented from precessing by friction between the wheels and the ground. The front wheel, however, has some freedom to precess about the approximately vertical steering axis, and thereby tends to steer in the direction of the lean. These two effects, plus some other usually smaller ones, depend on the bike's geometry, mass distribution, and forward speed. For a traditional utility bike, the forward speed at which they effects are able to keep a riderless bike upright, even when bumped to the side, is between 4 and 6 m/s (9 - 13 mph).
Read more: http://wiki.answers.com/Q/How_does_a_bicycle_in_motion_maintain_its_balance#ixzz1FR7N7YtF
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