And why is it necessary?
In: 4
To simply turn the direction you want would cause you to fall over. Your wheels can move left while your body keeps going straight. You counter steer to use this to your advantage. You steer the wrong direction to cause you to fall in the opposite direction, the direction you want to turn in. Now your body is going left and you can turn your wheels left so you can keep your body over the wheels and stay balanced.
You know how when you go round a corner in a car, you are thrown to the outside of your seat? That is centrifugal force. If you were on a bicycle that was upright then that force would tip you over as you went round the corner, so you need to lean into the corner. By leaning inwards the force of gravity that is trying to tip you towards the centre balances the centrifugal force trying to tip you the other way.
But how do you achieve that initial inwards lean? You might think that you could just lean your body inwards, but that just causes the bike itself to lean outwards. So instead what you do is steer outwards slightly (without noticing). Effectively this moves the wheels outwards while your body carries on in a straight line – the overall result is that you are now leaning. Then you steer inwards to maintain the lean and turn the corner. At the very end of the turn, again without noticing, you steer into the turn extra to pop out of the lean and carry on your way.
You can do all this without your hands on the handlebars because the geometry of the bike (especially the way the steering axis is raked forward) allows you to steer the front wheel by leaning the bike frame.
The hard part about understanding this becomes clearer when you realize that the bike naturally wants to stand up as you are on the throttle.
[This Veritasium video](https://youtu.be/9cNmUNHSBac) should explain it very thoroughly.
The breakdown of it is this:
When you turn a bicycle in a certain direction, say left, it indeed turns in that direction. But only the lower portion of the bike does, the part where the tires contact the road. The top part (AKA you) keeps trying to go straight due to its momentum.
Since the bottom part of the bike started to go to the left, and the top part didn’t, we now have a bike that, if seen from behind, leans to the right. If you keep turning left like this, the lean to the right will only get worse and worse to the point you simply fall over. You’ve basically driven the bike out from under yourself.
To not fall over, you have to steer in the direction you are leaning. Steer the bike back under yourself. So you turn the handlebars left, and start leaning to the right. Quickly change direction and turn the handlebars to the right to maintain balance. You are now turning right.
On a pedal bike at low speed, the lean happens fast and you don’t need a lot to start turning. If you ride a bicycle, you already countersteer and don’t even realize it, it happens so quickly. The video I linked opens with experimentally verifying this–put people who know how to ride bikes on a bike that’s modified to only allow steering to one side, ask them to turn that way, and suddenly nobody can.
On a high speed bike, particularly a motorcycle on a highway, it’s harder to start a lean, so you have to very deliberately know to turn the opposite way first to start the lean and get enough of one going to be able to turn into it. It’s not that the bike just magically started to behave differently now that it’s going fast, the same rules as before apply. It’s just that the short and small automatic reflex you usually do to steer a bike isn’t enough.
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