I Slackline Balance: How is it Possible?

[Komanchi]

I know bicycles and other single track vehicles stay upright with countersteering only, especially at lower speeds. But the guy rides his bike on a slackline that is obviously not wide enough to countersteer. Does his body movements work because there is slack in the rope so when he moves the point of contact go back under the system's center of gravity? Because doing that on a normal ground or a really tight rope wouldn't work I guess since professional riders turn their front wheels at a 45 degree angle and go back and forth to keep balance when at a standstill. I just can't fathom how he does it.

The video is this:

 

[jbriggs444]

Body movements work both on a slack line and on a tight rope. With a slack line, you are moving the point of contact right and left relative to your center of gravity. With a tight rope, you are pushing your center of gravity right or left relative to the point of contact.

 

[Komanchi]

Body movements work both on a slack line and on a tight rope. With a slack line, you are moving the point of contact right and left relative to your center of gravity. With a tight rope, you are pushing your center of gravity right or left relative to the point of contact.

My guess would be that the main objective in order to keep balance in either scenario is bringing the center of gravity above the contact patch since even with the slack line, the point of contact will move back to where it was anyway, you can't change it permanently.

But bringing the center of gravity above the point of contact would be easier I think on a rigid rope since it doesn't have any give, so it pushes back on the tires with a higher efficiency.

Do you reckon he could to the same on a tight rope without any slack?

 

[jbriggs444]

Do you reckon he could to the same on a tight rope without any slack?

Yes. Far easier on a tight rope.

 

[Komanchi]

Yes. Far easier on a tight rope.

Why can't people even pros can't stay upright when stationary if body movement works do you have any idea? They go back and forth when they do track stands

 

[jbriggs444]

Why can't people even pros can't stay upright when stationary if body movement works do you have any idea? They go back and forth when they do track stands

I'm sure they could. It's just that forward and back works even better.

 

[Komanchi]

Yes. Far easier on a tight rope.

Wouldn't the ability to move the contact point under the bike make it easier to retain balance on the slack line, even though it happens momentarily?

 

[jbriggs444]

If you've ever try balancing on a slack versus a tight rope, you'll notice the change in required reflexes.

 

[Komanchi]

Yes. Far easier on a tight rope.

Doesn't the ability to move the contact patch momentarily make it easier on a slackline though?

 

[jbriggs444]

Doesn't the ability to move the contact patch momentarily make it easier on a slackline though?

If you were in the middle of a very long (or very slack) line so that your center of gravity is well below the straight line between the support points then a slack line gives you more responsiveness. You can easily move your feet under your center of gravity. That responsiveness comes at a price -- it makes it easy to overcontrol. But that's not the tricky part. The tricky part is with a line that is only somewhat slack.

If you are in the middle of a not quite so slack line then the slackness has an effect which is difficult to deal with. It is difficult to explain. Let us dispense with the bicycle and consider an acrobat on a slack wire. Suppose that he is leaning right and wants to correct, centering himself again. He shifts his feet right. Now the support from the wire is further right and is pushing his center of gravity leftward. That's great, but there is a problem. It's also a clockwise torque tending to rotate him into the fall. He's corrected one problem and created another.

You have to be careful enough while taking care of your rotation so that you still have some freedom to keep your center of gravity where it belongs.

It's been a long time since I was an agile youth standing on cables or rolling on barrels. It's easier to feel what you have to do than to explain it.

 

[Komanchi]

If you were in the middle of a very long (or very slack) line so that your center of gravity is well below the straight line between the support points then a slack line gives you more responsiveness. You can easily move your feet under your center of gravity. That responsiveness comes at a price -- it makes it easy to overcontrol. But that's not the tricky part. The tricky part is with a line that is only somewhat slack.

If you are in the middle of a not quite so slack line then the slackness has an effect which is difficult to deal with. It is difficult to explain. Let us dispense with the bicycle and consider an acrobat on a slack wire. Suppose that he is leaning right and wants to correct, centering himself again. He shifts his feet right. Now the support from the wire is further right and is pushing his center of gravity leftward. That's great, but there is a problem. It's also a clockwise torque tending to rotate him into the fall. He's corrected one problem and created another.

You have to be careful enough while taking care of your rotation so that you still have some freedom to keep your center of gravity where it belongs.

It's been a long time since I was an agile youth standing on cables or rolling on barrels. It's easier to feel what you have to do than to explain it.

I guess the line in the video is in the category of "somewhat slack" which is very tricky

The main mechanism that is responsible for him keeping the balance is not the line moving under the center of gravity but moving the center of gravity back in line with the slack line correct? Which is easier to do on a tightrope since it applies the force back on the bicycle with greater efficiency and without delay.

So we are certain that if he can do this on the slack line, he sure can do it on a tight rope shaped more like a line so that his tires won't slip.

Thank you very much this really helped!