OBs CURVE short on hard-hit banks

Patrick Johnson

Patrick Johnson

Fargo 1000 on VP4
Silver Member
While playing banks recently I noticed something new (to me) about how the OB rebounds from the rail on hard-hit shots. On hard-hit bank shots the OB curves short, as if it has draw spin on it. This is clearly visible; I had noticed it before but just shrugged it off until now.

I'm sure it's not because of transferred draw spin from the CB; I can see it happening when the CB follows, draws or stops after the hit. So how does it happen? I think it must have something to do with the fact that the cushion contacts the OB above center, which tends to put some "follow" spin on it as it rebounds, with the spin pointed directly across the table (90 degrees from the cushion). The direction of this spin would be "draw" in relation to the rebound angle of the OB, and could cause the effect I see.

I wonder if my explanation is right, and why I don't notice this on slower banks. Is it always there but on slower banks the OB has some follow that cancels it out? Is it "rubbed out" on slower banks by other friction between the OB, rail and surface? Is this part of the reason for OBs banking shorter generally when hit harder (it's not just the lack of forward spin)?

Anybody? Bueller?

pj
chgo
 
Patrick Johnson said:
While playing banks recently I noticed something new (to me) about how the OB rebounds from the rail on hard-hit shots. On hard-hit bank shots the OB curves short, as if it has draw spin on it. This is clearly visible; I had noticed it before but just shrugged it off until now.

I'm sure it's not because of transferred draw spin from the CB; I can see it happening when the CB follows, draws or stops after the hit. So how does it happen? I think it must have something to do with the fact that the cushion contacts the OB above center, which tends to put some "follow" spin on it as it rebounds, with the spin pointed directly across the table (90 degrees from the cushion). The direction of this spin would be "draw" in relation to the rebound angle of the OB, and could cause the effect I see.

I wonder if my explanation is right, and why I don't notice this on slower banks. Is it always there but on slower banks the OB has some follow that cancels it out? Is it "rubbed out" on slower banks by other friction between the OB, rail and surface? Is this part of the reason for OBs banking shorter generally when hit harder (it's not just the lack of forward spin)?

Anybody? Bueller?

pj
chgo
Click to expand...
Can't give you an explanation for the effect, but I still recall a shot Eddie Kelly was showing some of us back in the 1960s. He set up a bank shot with an obstructing ball that appeared to be slightly in the way of the bank going in. After 5 or 6 of us looked over the shot closely and all agreed that the bank would not go, Kelly fired the ball into the corner pocket. It curved just enough to miss the obstructing ball.
 
Patrick Johnson said:
While playing banks recently I noticed something new (to me) about how the OB rebounds from the rail on hard-hit shots. On hard-hit bank shots the OB curves short, as if it has draw spin on it. This is clearly visible; I had noticed it before but just shrugged it off until now.

I'm sure it's not because of transferred draw spin from the CB; I can see it happening when the CB follows, draws or stops after the hit. So how does it happen? I think it must have something to do with the fact that the cushion contacts the OB above center, which tends to put some "follow" spin on it as it rebounds, with the spin pointed directly across the table (90 degrees from the cushion). The direction of this spin would be "draw" in relation to the rebound angle of the OB, and could cause the effect I see.
Click to expand...

The spin the rebounding ball picks up depends on the incoming vertical-plane spin (top/stun/bottom) and ball/cloth conditions. For more info, see:


This video shows straight-on shots only. As the approach angle increases, the rail has less effect on the vertical-plane spin (see HSV A.72-A.75). With a medium angle rolling bank or kick, the CB retains much of the top spin (which looks like pure masse spin on rebound), causing the rebounding ball to bend long (see HSV A.31-A.33).

Patrick Johnson said:
I wonder if my explanation is right, and why I don't notice this on slower banks. Is it always there but on slower banks the OB has some follow that cancels it out? Is it "rubbed out" on slower banks by other friction between the OB, rail and surface? Is this part of the reason for OBs banking shorter generally when hit harder (it's not just the lack of forward spin)?
Click to expand...
I think the main effect with slower banks is the roll action of the ball, as demonstrated and described here:


I think we are far from a complete understanding of the physics involved with kick and bank shots. There are so many variables (speed, spin, angle in, cushion nose height) and many of the parameters (friction at nose and rail groove, normal COR, tangential "throw-back" COR, cushion stiffness in vertical direction) change with all of the variables. For a long time, I have wanted to do a thorough physics analysis of all of this stuff; but, honestly, I doubt the model would be very accurate or useful without relying on extensive experimental data ... and even then, I doubt the results would justify the effort required (i.e., I don't think we would learn much more than what we already know about how kicked and banked balls respond).

Regards,
Dave
 
Patrick Johnson said:
While playing banks recently I noticed something new (to me) about how the OB rebounds from the rail on hard-hit shots. On hard-hit bank shots the OB curves short, as if it has draw spin on it. This is clearly visible; I had noticed it before but just shrugged it off until now.

I'm sure it's not because of transferred draw spin from the CB; I can see it happening when the CB follows, draws or stops after the hit. So how does it happen? I think it must have something to do with the fact that the cushion contacts the OB above center, which tends to put some "follow" spin on it as it rebounds, with the spin pointed directly across the table (90 degrees from the cushion). The direction of this spin would be "draw" in relation to the rebound angle of the OB, and could cause the effect I see.

I wonder if my explanation is right, and why I don't notice this on slower banks. Is it always there but on slower banks the OB has some follow that cancels it out? Is it "rubbed out" on slower banks by other friction between the OB, rail and surface? Is this part of the reason for OBs banking shorter generally when hit harder (it's not just the lack of forward spin)?

Anybody? Bueller?

pj
chgo
Click to expand...

Interesting idea. I've noticed that something looks different about the path of a hard hit bank, but never been able to say what it was. Maybe this is it?

When do you notice the maximum effect? How hard are you hitting the shot?, Which angles? Have you noticed any relationship between the amount that the object ball curves and its original distance from the rail?
 
does this happen with hard banks that are sliding when contacting the rail? in that case, i like pj's explanation.
 
not exactly the same but maybe related, I always thought it was a little spooky how the side pocket 3 rail bank bites into the rail and comes back as if it has inside english. I'm in a rush so no diagram but CB and OB are in a line between the side pockets. Drill OB with firm centerball hit, about 2/3rds full, and it double banks into the side. Does the ball rebound off the first rail in a straight line heading a bit backwards towards the side? Or does it hook that way? If there's no hook, can someone explain what's going on? Is the compressed rail forming a sort of bowl around the CB and the CB is rebounding off the facing curve of that bowl?
 
CreeDo said:
not exactly the same but maybe related, I always thought it was a little spooky how the side pocket 3 rail bank bites into the rail and comes back as if it has inside english. I'm in a rush so no diagram but CB and OB are in a line between the side pockets. Drill OB with firm centerball hit, about 2/3rds full, and it double banks into the side. Does the ball rebound off the first rail in a straight line heading a bit backwards towards the side? Or does it hook that way? If there's no hook, can someone explain what's going on? Is the compressed rail forming a sort of bowl around the CB and the CB is rebounding off the facing curve of that bowl?
Click to expand...
i believe what you are describing is rail induced sidespin, which doesn't affect the angle off the rail (as described in this thread), but the angle off of the next rail.

like halfway through this video?
http://www.youtube.com/watch?v=Oii0UhnYjCc
 
CreeDo said:
not exactly the same but maybe related, I always thought it was a little spooky how the side pocket 3 rail bank bites into the rail and comes back as if it has inside english. I'm in a rush so no diagram but CB and OB are in a line between the side pockets. Drill OB with firm centerball hit, about 2/3rds full, and it double banks into the side. Does the ball rebound off the first rail in a straight line heading a bit backwards towards the side? Or does it hook that way? If there's no hook, can someone explain what's going on? Is the compressed rail forming a sort of bowl around the CB and the CB is rebounding off the facing curve of that bowl?
Click to expand...
FYI, I have demos (with slow motion footage) and basic explanations of two-times- and three-times-across bank shots here:


Enjoy,
Dave
 
CreeDo said:
not exactly the same but maybe related, I always thought it was a little spooky how the side pocket 3 rail bank bites into the rail and comes back as if it has inside english.
Click to expand...

It does have inside ("holdup") english.

... CB and OB are in a line between the side pockets. Drill OB with firm centerball hit, about 2/3rds full, and it double banks into the side. Does the ball rebound off the first rail in a straight line heading a bit backwards towards the side? Or does it hook that way?
Click to expand...

It really does that.

If there's no hook, can someone explain what's going on?
Click to expand...

It's a series of spin events:

1. The CB colliding with the OB puts some "holdup" spin on the OB, which shortens the angle off the first rail.

2. The OB colliding with the first rail at an angle reverses the spin that was put on by the CB (just like any ball picks up "running" spin when it hits the rail).

3. The reversed spin throws the OB toward the side pocket off the second rail.

CueTable Help



pj
chgo
 
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Thanks for clearing up the double bank. I still don't get what it is about rails that allows balls to bite in and come off at sharper angles than the angle of approach, when hit with lots of force. The double bank is explained with various collision induced sidespins but what about when I pound the CB dead center into a rail and it comes off sharper than the angle of approach? Or is this something I imagine happens but doesn't in reality?

.,..
it's weird, I was watching the beard's "banks that don't go but do" dvd, while browsing the forums... and about 5 minutes after posting my question in this thread, he had a curving bank example on his DVD. The curve was used to beat a kiss. The camera angle shows the hook and it's very slight, not sure that the kiss wasn't beaten anyway, but it was interesting to see this in action.
 
what about when I pound the CB dead center into a rail and it comes off sharper than the angle of approach?
Click to expand...

When you don't pound it, the CB has some forward spin when it hits the rail, which makes it curve long after rebounding. Pounding it makes it slide all the way to the rail without gaining any forward rotation, and then rail friction holds it up (the ball has to move along the rail a little way while it's in contact).

This "draw curve" thing I noticed is also apparently a (little known) aspect of it.

Some people think the rail "pushes back" against the ball at an angle as you suggest, but I'm skeptical about that.

pj
chgo
 
so when a pounded ball bites into the rail and comes off short, that's entirely from the momentary friction between ball and cushion? I can't imagine it causing such a sharp bite. Is there a specific high speed video of a ball going into the rail hard on that colostate website?
 
CreeDo said:
so when a pounded ball bites into the rail and comes off short, that's entirely from the momentary friction between ball and cushion?
Click to expand...

No, rail friction has the same effect in both cases. The main difference between a softer bank and a harder one is that on the harder one the OB does not have as much forward roll as on a softer one.

There are at least three "friction factors" to account for in every bank/kick shot:

1. rail friction - shortens the rebound angle

2. forward/reverse spin - lengthens or shortens the rebound angle

3. sidespin - lengthens or shortens the rebound angle

The specific combination of factors present in a bank/kick shot determines how the ball will rebound. For instance, faster vs. slower bank shots rebound differently because of the presence or absence of forward roll:

A. Slower speed (rolling ball):
- rail friction shortens rebound
- forward roll lengthens rebound
- net effect: rebound longer than equal angle

B. Faster speed (sliding ball):
- rail friction shortens rebound
- no forward roll to lengthen rebound
- net effect: rebound shorter than equal angle

C. Faster speed (sliding ball) with a little transferred running english:
- rail friction shortens rebound
- no forward roll to lengthen rebound
- running english lengthens rebound
- net effect: rebound equals equal angle

The "curving short" effect I'm exploring in this thread seems to be a fourth factor, which may be always present but hidden by other factors unless the shot is hit pretty hard.

pj
chgo
 
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I'm still a little weirded out by pounded banks. The shortening of the angle seems stronger than can be explained by just a lack of natural roll (as compared to a 'regular' bank). I'd expect a sliding pounded ball to come off the rail close to the mirror angle, but it seems like it's coming off much tighter than the angle in. If that's not due to rail friction, what is it?
 
CreeDo said:
I'm still a little weirded out by pounded banks. The shortening of the angle seems stronger than can be explained by just a lack of natural roll (as compared to a 'regular' bank). I'd expect a sliding pounded ball to come off the rail close to the mirror angle, but it seems like it's coming off much tighter than the angle in. If that's not due to rail friction, what is it?
Click to expand...

I think I just answered this:

B. Faster speed (sliding ball):
- rail friction shortens rebound
- no forward roll to lengthen rebound
- net effect: rebound shorter than equal angle

The "curving short" effect I'm exploring in this thread seems to be a fourth factor, which may be always present but hidden by other factors unless the shot is hit pretty hard.
Click to expand...

I'm not sure what you mean by "seems like much tighter" - are you sure what you mean by it? Have you made a careful comparison of the outcome of different shots? Maybe "seems like" is the culprit here.

pj
chgo
 
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JoeyA said:
http://billiards.colostate.edu/normal_videos/new/NVB-23.htm

Dave,
In the video above, in the early part of the video, right at the 1:00 mark, the title is labeled , "slow top left" and I believe the cue ball actually has "slow top right" instead but I could be wrong.

JoeyA
Click to expand...
You are correct. I actually have the following text on the page for NV B.23:

Note: At the 0:55 point in the video, the text displays "slow, top-left English." It should be "slow, top-right English" instead, as the demonstration shows.​

but I know people don't always read words when there is a video to watch. I admit it ... I probably wouldn't either. :sorry:

I wish I could edit the video to fix the text; but, unfortunately, I no longer have the original files, so this is not possible.

Regards,
Dave
 
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