腳踏車背後, 最不直覺的物理原理 @ FrankLu的發現之旅

你相信當你右轉時, 你的把手是先往左扭的嗎 ? (Countersteering)

你是否想過當車子靜止直立時, 把車體向右傾, 前輪就會向右轉動, 是為甚麼 ? 其實不是那麼直覺喔!

這部份的物理知識, 幾乎就是100多年腳踏車歷史的最根本.

其實小時後學習騎腳踏車, 摔了N次, 所學到的東西, 白話一點叫"平衡感". 而背後所支配的就是這原理.

要找到講的清楚的資料, 不上外國網站還真不行.
希望有一天能完全弄懂它.
http://socrates.berkeley.edu/~fajans/Teaching/Steering.htm
http://socrates.berkeley.edu/~fajans/Teaching/bicycles.html

發表於1970的論文, "The stability of bikes". 是第一篇分析此問題的專業論文.
http://www.phys.lsu.edu/faculty/gonzalez/Teaching/Phys7221/vol59no9p51_56.pdf

單車得已平衡行駛, 輪子轉動的陀螺儀效應並不是主因 (反直覺!) :
http://www.sheldonbrown.com/brandt/gyro.html
"A good example of a bicycle with no gyroscopic forces is the ski-bob, a "bicycle" with short ski runners in place of wheels. This bicycle, having no rotating parts, is ridden downslope easily by anyone who can ride a bicycle"

這Countersteering 與 Trial的觀念, 竟是造成單車得以穩定(平衡)前進的主要原因!
http://www.dclxvi.org/chunk/tech/trail/
"When the bicycle starts to lean, the handlebars are turned in the direction of the lean, and the bicycle is pulled in that direction as well. Since inertia causes the rider to continue moving straight, the rider is levered back over the bicycle's wheels, at which point the bicycle is upright again. This will continue to create another lean to the opposite side, as well as a slight change in direction, both of which must be corrected again."

Youtube上的Video介紹Countersteering, 讓你眼見為信!
http://www.youtube.com/watch?v=C848R9xWrjc

現在, 終於瞭解為何我那F-20R小折, 無法像700C公路車那般, 可輕易的放手騎.
注意看下圖的F-20R前叉. 最末端的那段向前延伸, 會等效減少"Trial"的值, 使轉向更零敏, 但不易stable.
"In bicycles, fork rake, often a curve in the fork blades forward of the steering axis, is used to diminish trail.^[7] "
"There are severalforks on the market that vary from straight to curved, thereby changingthe trail"