Thursday, December 24, 2009

The subtle art of counter-steering!

"If you want to turn left, push on the right side of the handlebar. If you want to turn right, push on the left side of the handlebar." The kind of intuitive steering advice that you got when you first started riding a bicycle.

All bikers learn pretty quickly that at at any speed faster than a snail, the only way to take a turn on a bike is to lean into it (the turn, that is). But what most bikers do not know is the subtle art of counter-steering.

The concept of counter-steering is a simple one.
If you want the bike to go left, what you have to do is push the right-side of the handlebar slightly forward: i.e. "Push right to go right"; or "Steer left to go right". Now this may sound counter-intuitive at first, (but then that is why it is called counter-steering) but it works. I'll keep the Physics simple.

First, let us look at why leaning into turns works, then I'll demonstrate how counter-steering achieves the same effect.

Rolling and turning: Angular velocity!
Anyone who has ever tried to roll a paper-cup will know that the cup rolls and turns at the same time. This is because the rolling paper-cup has an angular velocity and the larger radius at the open-end of the paper-cup implies that the open-end has a higher liner velocity than the closed end. A point on the open-end of the paper-cup hence covers a greater distance than a point at the closed-end. The only sensible way to achieve this is for the cup to roll as shown below.

sidetrack 1. Incidentally, cars actually use a differential to giving the outer wheels a greater angular velocity, and hence make the outer wheels go through a larger distance than the inner wheels, thereby accomplishing a turn. How do trains turn? Check out RPF's Fun-to-Imagine series, for a really elegant explanation to this problem!

Leaning a wheel:
Car tyres


Motorcycle tyres


Motorcycle turns are possible because motorcycle tyres are not flat (like car tyres), but rounded. When a motorcycle is moving in a straight line it is vertical and its contact patch is in the center of the tyre. When a motorcycle leans, however, it rides on a contact patch that is closer to the axis of the wheel itself. Since the parts of the tyre farthest from the axis of the wheel are moving faster than the parts of the tyre closer to the axis of the wheel (constant angular velocity but greater distance between the tyre-patch and the axis of rotation), the outside edge of the tire contact patch is moving faster than the inside edge. And as we have seen before, when one part of an object has a higher velocity than another part of the same object, the object executes a turn. If the shorter distance (between the contact point and the axis of rotation) is on the left, the wheel will turn to the left. In other words, leaning to the left, makes the wheel turn left! Hence leaning the bike to the left, makes it turn left.
Of course, you can take the force point of view as well! Leaning to the right but without sliding means that there is a frictional force to the right which is... yes! The CENTRIPETAL force required to keep the bike in a circular path. The force view-point begs for a more involved study of rake and trail and the fork design of the front wheel, all of which contribute towards making the bike turn into the curve, so that the centre-of-gravity is again under the bike.

Making the elephant lean!
Now, leaning into a turn can be done by physically shifting body weight, and it works... on light bikes. (My apologies to owners of pulsars and unicorns, but the facts speak for themselves.) Shifting the centre of gravity of bike-and-driver on a 130 kg pulsar, or a 11o kg RX100 is one thing. Trying the same technique on a 184 kg bullet is a whole different ball-game altogether. I'm a 90 kg (and not proud of it) mass and I find it extremely hard to get my RE Bullet 350 to change directions using the leaning technique alone. Imagine the difficulty a 70kg newbie would have! So how do we make the elephant lean?

Counter-steering: finally!
Iamgine you push the right-sdie of the handlebar forward. Initially, the front tyre of the bike turns left. The dentre of gravity of the bike and rider is no longer between the line joining the points of contact between the front tyre and the back. The cg is slightly towards the right of this line, because this line has shifted slightly to the left. Now consider what happens when you tilt something so that its centre of gravity does not lie between the points of contact. Put your cell phone on the table so that it rests  on its side. now tilt it to one side. You will see that it falls over because it is no longer in equilibrium.


The same thing happens to the bike. the bike starts to topple to the right, because the centre of gravity of the bike is to the left of this line joining its points of contact with the ground. So, pushing the right-side of the handlebar forward makes the bike lean to the right. Once the bike leans to the right, we know it turns to the right as explained above. Once it leans to the right, we know why it turns to the right.

Quad Erat Demonstrandum!

Using Counter-Steering

Most bikers use counter-steering unconsciously to take turns and are not even aware that they are doing it. This subconscious nature of the act of counter-steering makes this a hard skill to develop as an evasion tactic. Consider that you see a pothole a little to the right of you as you are driving. Seen you've seen the pothole late, you might try evasive action at the last moment. And the "intuitive" act of trying to steer away from the pothole by turning the handlebar away from it, actually causes your bike to go towards the pothole. But if you can learn to counter-steer consciously, you can avoid this kind a mishap. You will learn to make very sudden sharp changes in direction by actually turning your handlebar towards the pothole rather than away from it.

Statutory Warning!

  1. If you intend to learn how to use counter-steering, might I suggest that you do it on an empty road?
  2. Leaving the handlebar in the counter-steering position will end up in the bike crashing downwards upon you. And you don't want a 186 kg monster landing on your legs... So, use counter-steering to merely establish the change in direction.  Once the bike leans over, and starts turning, correct the direction of the handlebar to prevent the bike from falling.

2 Comments:

Blogger sethuram said...

very useful tip, i use a rx -100, my weight 45 kgs never ever tried the counter sterring method. i have noticed sudden baraking tends to crash the my to my right, it has happend a couple of times to me, any sugestions why it happens, changed rear tyres to a ceat. the only improvement i noticed is that when the air pressure is right the bike has less tendency to swing to the right, i also noticed that the bike behaves in a more diginified way with a heavy rider.
thankyou for the very useful tip
regards
sethu

7:00 pm  
Blogger Sriharsha Salagrama said...

Hi Sethu,
If the wheels are aligned properly, it is probably a question of weight balance and braking technique. Most riders learn how to do skids by locking the back wheel (slamming hard on the rear brake without touching the front brake) and shifting one's body weight to one side (by leaning). I'm assuming that the braking problem is primarily a problem of seating posture not being perfectly symmetrical or the wheel alignment being improper. Of course, the best way to prevent this kind of a skid from happening is to ensure that you are braking with both the front and rear brakes, and not just with the rear ones. Will probably be able to tell you what the problem is if i ride your bike, but it is going to be one of these: asymmetrical riding posture, wheel alignment or using only the rear brakes. It probably has nothing to do with the tyres themselves, if the treads are intact.

10:58 pm  

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