Automotive The "pendulum turn": angular momentum or rotational energy?

[vco]

There is a cornering maneuver in rallying called the "Scandinavian flick" or the "pendulum turn". It involves steering away from the corner before actually steering into the corner. This creates a pendulum effect which makes the car turn more sharply into the corner.

Sorry for the poor video quality:
Now, some sources state that is the increased angular momentum which explains how this maneuver works. However, I believe this is wrong. It is the increased rotational energy, not angular momentum, which explains the maneuver.

When a rotating body is stopped by applying a net moment, it is the rotational energy which determines the total angle the body travels before stopping. Therefore, the increased rotational energy is what makes the car turn more sharply when using the cornering maneuver.

Do you agree?

 

[A.T.]

It is the increased rotational energy, not angular momentum, which explains the maneuver.

What does "explain the maneuver" mean specifically? How would you tell the difference between the two options? Energy and momentum are physical quantities, so do you have a quantitative argument?

 

[vco]

What does "explain the maneuver" mean specifically? How would you tell the difference between the two options? Energy and momentum are physical quantities, so do you have a quantitative argument?

What I meant that the objective of the maneuver is to increase the rotational energy of the car. While the angular momentum also increases as a result of the maneuver, it would be incorrect to state that the increased angular momentum is the reason why applying the maneuver makes the car turn farther.

The increased rotational energy is what makes the car turn farther. This is because angular distance traveled is proportional to rotational energy, not angular momentum.

For example, it would be somewhat similarly incorrect to state that increasing the speed of a bullet makes it penetrate farther into the target due to the increased linear momentum. While the linear momentum does in fact increase when increasing the speed of the bullet, the increased kinetic energy is responsible for the increased penetrability due to distance traveled being proportional to kinetic energy.

 

[Vanadium 50]

makes the car turn farther

Farther than what exactly?

The car has a turning radius that's what it is because that's where the driver drove it. You need to explain exactly what you think the alternative is.

 

[vco]

Farther than what exactly?

The car has a turning radius that's what it is because that's where the driver drove it. You need to explain exactly what you think the alternative is.

By "turn farther" I mean smaller turning radius. Without the maneuver the minimum achievable turning radius at a given speed is greater.

Edit: Or maybe I should say that to "turn father" I mean that the cart oversteers more. The maneuver increases the amount of oversteer.

 

[A.T.]

What I meant that the objective of the maneuver is to increase the rotational energy of the car. While the angular momentum also increases as a result of the maneuver, it would be incorrect to state that the increased angular momentum is the reason why applying the maneuver makes the car turn farther.

But the point of the maneuver is not to turn farther.

By "turn farther" I mean smaller turning radius.

Then your bullet penetration depth analogy doesn't make sense.

Or maybe I should say that to "turn father" I mean that the cart oversteers more

That's again different from smaller turning radius and not really the point.

 

[Vanadium 50]

I think you are assuming facts not in evidence.

The driver turns outward, making the turn radius larger. Why do you think there is any more to it than that?

 

[vco]

I think you are assuming facts not in evidence.

The driver turns outward, making the turn radius larger. Why do you think there is any more to it than that?

The way I see it is that the driver utilizes the rotational energy built up in the pendulum motion to make the car oversteer more, which often allows the driver to clear the corner in less time.

 

[Dale]

I am with @Vanadium 50 on this. To me it looks like the purpose of the maneuver is simply to increase the turning radius. A larger turning radius allows the turn to be achieved with less centripetal acceleration.

 

[A.T.]

The way I see it is that the driver utilizes the rotational energy built up in the pendulum motion to make the car oversteer more, which often allows the driver to clear the corner in less time.

Less time is about reorienting the car faster. This also involves stopping the rotation of the car, not just rotating as far a possible. How does your bullet/energy argument apply here?

 

[Vanadium 50]

The way I see it is that the driver utilizes the rotational energy built up in the pendulum motion

It's not a pendulum. In any case "the way I see it" is not the way we do things in physics. If you think there's more to it than just v²/r, write down the expressions that you think are relevant.

 

[A.T.]

The driver turns outward, making the turn radius larger.

To me it looks like the purpose of the maneuver is simply to increase the turning radius.

For this, he could just approach the turn on the outside, instead of going outwards just before the turn.

I think the point is to initiate the slip of the back wheels, to reorient the car quickly towards the new direction of travel. This allows to apply thrust towards the new direction earlier, while the lateral resistance of the wheels removes the momentum in the original direction.

 

[anorlunda]

I moved this thread to ME Automotive because it may attract some different answers.

My interpretation of this is that it is like swinging on a swing by moving your legs. You start an oscillation, then by adding the force of the "kick" at just the right phase, you increase the magnitude of the following swing. We do that repeatedly on a swing to build and maintain the oscillation.

By analogy, the driver starts a so-called "fish tail" oscillation, then the "kick" is turning the wheel into the turn at the right moment. But this is a one-time kick and only a fraction of one oscillation cycle, not a repeated one. Whether or not that really works in the automotive context, I have no opinion.

 

[JBA]

By doing this the driver starts a wider radius turn that would not be attainable from the center of the road and in the process generates an initial controllable degree of lateral force on the vehicle's tires, starts reorienting the vehicle toward the intended new direction and makes a "barely" controllable final snap turn maneuver possible.This maneuver has been in use for decades in international road rallies, but only on unpaved surfaces, dirt, gravel, etc in dry, wet and snow conditions.
In actuality , it is just an extension of the induced oversteer used by both auto and motorcycle racers while making the turns on dirt surface race tracks.
A humorous exchange once took place between an international rally driver and a riding sports writer during the demonstration of this maneuver. Writer: "Have you ever crashed doing this?"; Driver: "Oh yes, many times".

 

[OCR]

Sorry for the poor video quality:

Here's a better one. . .

Dirt Rally Scandinavian Flick / Pendulum Turn - YouTube
And from the ultimate authority. . . .

Scandinavian flick - WikipediaSee also. . .

Moose test - WikipediaMost truck drives that pull livestock trailers (AKA - bullracks) make a some what similar

maneuver, right before entering a corner, called a false turn, . . . that causes every

thing to get "leaned over" before the actual turn, which improves stability.

.

 

[anorlunda]

Yeah, that flick test sounds very much like the OP's description. Good research @OCR .

 

[vco]

If you think there's more to it than just v²/r, write down the expressions that you think are relevant.

I don't think turning radius is the explanation. Even without the maneuver, when approaching the corner the car would still be positioned on the outer edge of the road (the "racing line").

The system is perhaps too complicated to be satisfactorily described with elementary physics, which may explain why this cornering maneuver is usually utilized on loose road surfaces only.

 

[A.T.]

which may explain why this cornering maneuver is usually utilized on loose road surfaces only.

That makes sense: If you know you going to slide anyway, it's better to slide in a semi-controlled way, that allows the quickest change of direction and keeps you on the road..

If you instead try to maximize radius by simply going outside all the time, any sudden sliding throws you off the road.