電單車轉彎為何不會反?

你無諗錯；但你是漏左考慮電單車是在運動狀態。如果電單車不在運動，原地傾斜，就一定跌低。
一號所答的frictional force (摩擦力)，其實本質是靜力(即外觀並不見運動，只是知道其理論上存在而已)。如果車軚向橫"打滑"(又稱為反向滑動)，該friction就會不存在了。
1. 而能夠阻止電單車反車之力(事實為跌車)，我們稱為離心力。當一部電單車以一個圓形運動時，就會產生離心力。離心力與重力剛好形成90度角。如果你將離心力 + 重力之力矩，就成為friction(劃圖顯示，就是貿與車軚同地面接觸點，形成一直角三角形)
2. 而電單車為何會產生向心力呢?
其實是因為車軚之圓形運動(motion)。當車軚垂直向前轆(與地面成90度夾角)，並不會產生向車轆兩邊之力矩(即力矩為0)。但如果車轆傾斜轉動，就會產生一個橫向力矩，令電單車轉灣；我們稱這為向灣心之力矩、或者稱為向心力。

1.為何法向反作用力和重力不會形成力矩，令電單車反

There is frictional force (摩擦力) between the tyres of the bicycle and the road. This frictional force is pointing inward towards the centre of the circle. It is such firctional force that provides a counter moment (反向力矩) about the centre of mass (質心) of the bicycler and its rider against the moment given by the normal reaction (法向反作用力). Hence, the bicycle would not overturn.

2. 為何沒有水平的(向心)力，單車可以轉彎

As said above, the centripetal force (向心力) is provided by the frictional force between the tyres of the bicycle and the road. A bicycle cannot turn a corner if the road is smooth (or slippy) as there will not be any frictional force to provide the necessary centripetal force.
.



2013-03-12 19:41:48 補充：
Your suppl. question:
The bicycle will overturn if it is NOT in motion because, as you said, there is a moment given by the weight of the bicycle about the point of contact between the tyres and road.

2013-03-12 19:48:20 補充：
But when the bicycle is under a centripetal acceleration (向心加速度). The moment given by the weight is balanced by the moment given by the centrifugal force (離心力).