Hard Banks Curve Short - Why?

The curving happens mostly when the spin from the OB is wearing off. You can see this on some of your multi-rail bank shots. On a 3 or 4 rail shot, the OB has picked up enough running spin that the ball will start to curve on its way towards the pocket at the end. Let me see if I've got this right without a table around.. Short bank to your right - hit the ball hard so as to shorten the angle a little (this will make it so that you don't have as much ball to go around) - use high-center/top to get inverse spin on the OB (stun/draw in this case) - top normally acts as a running english for rails, so the stun/draw will get enough of a change in spin that will cause a bend as it tries to come straight back.

Think of it like this.. when you're wide on a cross-side and the imparted spin brings it back in again to make it in the wrong side. Instead of having the rail there to grab the spin after the first rail, you're now adjusting it to have the spin grab the felt.

Can't verify what I said right at the moment, but maybe I got it.. either way, hopefully this helps out.
 
supergreenman said:
I think you're theory of the OB topspin going into the rail is sound.

As far as hard shots shortening off the rail because of compression, that is real.

I think the best way to describe it is to imagine the rail compressed pushing the ball back towards the direction it came. The harder the shot, the more the compression, the more it pushes back.
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Lookout! If Bob Jewett reads this you may have an argument on your hands. We've been arguing this point for 20 years.
 
Actually, now that I think about it - go to youtube and check out some of Semih Sayginer's videos to see the balls doing CRAZY things coming off the rails.

And I mean CRAZY in the sense of you can't believe what you are seeing.
 
Draw makes the eight ball go forward on that shot. I just set it up and confirmed what I already knew.
 
Luxury said:
Draw makes the eight ball go forward on that shot. I just set it up and confirmed what I already knew.
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Could it be that using draw doesn't "make" it go forward but instead just doesn't cause as much of a backwards reaction?

I remember playing with these shots a long time ago thinking that I could cause draw to happen on the object ball with enough topspin. Very difficult and inconsistent to get the object ball to back up off the tangent line with two frozen balls. Shooting it with four frozen balls seems far easier.

I would bet that if we put Mike Page on this then he could bust this myth in under ten minutes on YouTube. See his video on overspin for an example.
 
I was told that the extra balls give the spin transfer more time to happen because the balls are in contact more. And draw definitely puts follow on the object ball IMO. Could be wrong though.
 
Patrick Johnson said:
I'm thinking it's cross-table topspin caused by the cushion nose being higher than centerball (as if the OB was hit above center by a cue aimed across the table perpendicular to the rail). Topspin in this direction would be across the "natural" rebound path and would act like backspin put on a CB for a kick shot, causing the ball to masse short off its straight rebound path.

Anybody know for sure or have a reasoned opinion?
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Half assed attempt #1 :)


I think the cushion nose being slightly higher that the ball is a big part of it, but I think the difference between the top and bottom angle of the cushion adds to it as well. When you look at a cross section of a rail, it looks like the meat of the rail is pointed up.

I suspect as the harder the OB loads into the rail, more energy gets stored on the lower side of the rail and for a moment, there's no where for the nose of the rubber to go but straight up. As it releases, the nose actually snaps up vertically faster then horizontally?

I would also wonder if there might be an optimum speed that would affect the OB the most due to sort of a shock wave effect in the rubber?
 
My take on this is that when the ball compresses the rail hard it pushes under the nose of rail and thus transferring topspin to the OB as it leaves the rail. As the spin starts to take it curses forward thus causing the arc. The harder the compression the latter the spin takes etc.
 
Nick B said:
My take on this is that when the ball compresses the rail hard it pushes under the nose of rail and thus transferring topspin to the OB as it leaves the rail. As the spin starts to take it curses forward thus causing the arc. The harder the compression the latter the spin takes etc.
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Nick you are almost there with your answer to the question.you have one of the forces described.rethink the spin!


bill
 
Patrick Johnson said:
Why do hard-hit bank shots curve short after rebounding from the rail?
...

2. the rail cloth's friction shortening the rebound angle
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I mean the OB's rebound path starts out straight but then curves short as if it had draw on it (like a kick shot with draw),

I'm thinking it's cross-table topspin caused by the cushion nose being higher than centerball (as if the OB was hit above center by a cue aimed across the table perpendicular to the rail). Topspin in this direction would be across the "natural" rebound path and would act like backspin put on a CB for a kick shot, causing the ball to masse short off its straight rebound path.

Anybody know for sure or have a reasoned opinion?
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I certainly agree with you and the other posters who attribute it to the nose of the cushion being above the "equator" of the OB. But just a slight note.

Ultimately, it is friction with the cushion that generates the spin. The ball and cushion interaction takes place at their surfaces. Normal forces (perpendicular) to the surface of the OB, generated by cushion compression, act through the OB's center. Thus, no torque is produced, and no spin can be generated by them.

The friction forces, however, act tangentially to the surface, and thereby create the torque - the moment arm is equal to the radius of the ball. In the case of a stun shot (no sidespin either) you can picture two masse type of spin components being produced by them, and three spin components in all:

1) A perfectly vertical spin component which does not contribute to subsequent masse action. Here the spin vector (arrow) is pointing straight up.

2) A spin vector perpendicular to the cushion and parallel to the table bed. This arises in tandem with (1) above, as the ball rubs against the cushion due to its parallel (to the cushion) translational velocity, and the cushion's nose overhang. In effect, this spin is equivalent to the ball being struck below center and driven parallel to the cushion.

3) A spin vector component parallel to the cushion, due to rubbing of the cushion as the ball "penetrates" the rubber. You can see this in Dr. Daves high-speed video on kick losses, HSV B15 (stun section, normal conditions). The examples are for perpendicular approach angles, but I would think it still applies at other ones too, to some extent. This spin is equivalent to being struck below center and driven perpendicular to the cushion.

I suspect that (3) would be dominant at approach angles not too far from perpendicular to the cushion, while (2) would dominate at more shallow approaches. As indicated, though, that is a speculation. Both (2) and (3) should generate these "backspin" components, but they happen to be at 90 degrees to one another. I guess it's optional as to which one might be considered acquired "draw spin."

Just some pedantic blathering, which you probably already knew.

Jim
 
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I'd go with Jal for the technical explanation. I'd guess that you probably want fairly sticky rails to get more spin onto the banking ball.

As for the cushion compression hypothesis on shortening banks, there's a related experiment described towards the end of: http://www.sfbilliards.com/articles/2003-07.pdf
 
I think the object ball slides on the rail abefore the rebound & the object balls picks up spin. Then the object curves on the rebound trajectory line like a slight masse'...
 
Jal said:
I certainly agree with you and the other posters who attribute it to the nose of the cushion being above the "equator" of the OB. But just a slight note.

Ultimately, it is friction with the cushion that generates the spin. The ball and cushion interaction takes place at their surfaces. Normal forces (perpendicular) to the surface of the OB, generated by cushion compression, act through the OB's center. Thus, no torque is produced, and no spin can be generated by them.

The friction forces, however, act tangentially to the surface, and thereby create the torque - the moment arm is equal to the radius of the ball. In the case of a stun shot (no sidespin either) you can picture two masse type of spin components being produced by them, and three spin components in all:

1) A perfectly vertical spin component which does not contribute to subsequent masse action. Here the spin vector (arrow) is pointing straight up.

2) A spin vector perpendicular to the cushion and parallel to the table bed. This arises in tandem with (1) above, as the ball rubs against the cushion due to its parallel (to the cushion) translational velocity, and the cushion's nose overhang. In effect, this spin is equivalent to the ball being struck below center and driven parallel to the cushion.

3) A spin vector component parallel to the cushion, due to rubbing of the cushion as the ball "penetrates" the rubber. You can see this in Dr. Daves high-speed video on kick losses, HSV B15 (stun section, normal conditions). The examples are for perpendicular approach angles, but I would think it still applies at other ones too, to some extent. This spin is equivalent to being struck below center and driven perpendicular to the cushion.

I suspect that (3) would be dominant at approach angles not too far from perpendicular to the cushion, while (2) would dominate at more shallow approaches. As indicated, though, that is a speculation. Both (2) and (3) should generate these "backspin" components, but they happen to be at 90 degrees to one another. I guess it's optional as to which one might be considered acquired "draw spin."

Just some pedantic blathering, which you probably already knew.
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No, I didn't know any of this. Thanks for the detailed explanation.

I take it "spin vector" is what I would call the axis of spin?

As for whether (2) or (3) would be considered "draw spin", I think it would be both (the sum of them), since they would each, if acting alone, "masse" the ball off its rebound path in the direction of draw spin.

They each produce spin that increases as its masse effectiveness decreases (due to the changing bank angles), so I'm guessing the maximum combined masse effect is achieved at about the middle of the range of angles, or about 45 degrees. Does that sound right?

pj
chgo
 
JB Cases said:
I don't see any back spin. I think that the ball goes back towards you because the force of pushing against three other balls forces it back.

Hard to tell anything with that video though.
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I can get the same "backing up" action when the OB is frozen to only one other ball, but not to the same degree. With only one other OB behind the first OB there would be no rebounding, so that effect must be from transferred backspin. So I think the action in the video is a combination of the ball rebounding off the greater mass of multiple other balls plus some transferred spin.

Therefore I think some backspin can be transferred to the first OB this way, but very little. And when banking there's no sandwiching, so much less backspin can be transferred. And I doubt that any of it can survive sliding across the cloth to the rail, not even counting the friction of the rail itself.

In other words, I seriously doubt that transferred backspin can possibly have any effect on the path of a banked ball. My guess is that the force (speed) of a "force follow" shot causes the effect, not the follow.

pj
chgo
 
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Bob Jewett said:
I'd go with Jal for the technical explanation. I'd guess that you probably want fairly sticky rails to get more spin onto the banking ball.

As for the cushion compression hypothesis on shortening banks, there's a related experiment described towards the end of: http://www.sfbilliards.com/articles/2003-07.pdf
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The only way to evaluate the effects of compression is to change the compressibility of the cushion, while holding constant other factors, such as cloth friction, tension, speed and angle of impact, ect.

The stated experiment seems to fix everything except velocity, and is therefore designed to evaluate that parameter.
 
Dead Crab:
The only way to evaluate the effects of compression is to change the compressibility of the cushion, while holding constant other factors, such as cloth friction, tension, speed and angle of impact, ect.

The stated experiment seems to fix everything except velocity, and is therefore designed to evaluate that parameter.
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The experiment actually varies both speed and forward roll on the OB in various combinations (by varying the starting distance of the OB from the rail). The results are that changes in forward roll alone do change the rebound angle as predicted, but changes in speed alone do not. That indicates that cushion compression is not a factor.

pj
chgo
 
Patrick Johnson said:
The experiment actually varies both speed and forward roll on the OB in various combinations (by varying the starting distance of the OB from the rail). The results are that changes in forward roll alone do change the rebound angle as predicted, but changes in speed alone do not. That indicates that cushion compression is not a factor.
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If people are curious about this effect, here's a video demonstration:

Regards,
Dave
 
The one dynamic I think that may be missing is that I truly believe that a ball struck hard enough to shorten the angle is actually off the bed of the table.

The video evidence has the ball too close to the rail to actually elevate before it makes contact.
 
Patrick Johnson said:
The experiment actually varies both speed and forward roll on the OB in various combinations (by varying the starting distance of the OB from the rail). The results are that changes in forward roll alone do change the rebound angle as predicted, but changes in speed alone do not. That indicates that cushion compression is not a factor.

pj
chgo
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The correct experimental design for evaluating the effects of compression is to hold other variables constant. This is the way science is supposed to be done, and with good reason.

Suppose a misguided physiologist did a similar experiment with the following analogues:

output angle = blood pressure
velocity = heart rate
forward roll= volume of blood pumped with each heartbeat (stroke volume)
cushion compression = vascular resistance

Now suppose they experimentally increased heart rate in a subject and noted that blood pressure stayed about the same. Aha! that must be because increasing the heart rate decreases the stroke volume (it can), and therefore, we conclude that vascular resistance cannot be a factor in blood pressure!

That would certainly be an inappropriate conclusion because it is well known that vascular resistance is a very important factor in blood pressure. If they held heart rate and stroke volume constant, and varied vascular resistance, the effect on blood pressure would be dramatic.

This is why you have to isolate variables.

Back to pool:
If you were to compare similarly covered (e.g. Simonis) Artemis and Superspeed cushions as a paired study with identical variations of velocity/distance as input, you might reach a very different conclusion regarding the effects of cushion compressibility.
 
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