Why does this work?

Tokyo-dave

Tokyo-dave

AzB Silver Member
Silver Member
This shot came up in a game of 9-ball the other night, and as I was thinking there would be no way to shorten it up enough for a one rail bank, I reminded myself of when I was a translator for Mike Sigel when he did a couple of clinics in Japan and he showed me this shot. I figured I'd try it. I actually called the pocket, although obviously I didn't need to simply because it can look like a slop shot. And, I kinda thought it would be an opportunity to get into the guys head by calling it and making it.
Problem is, when I think about the physics behind the shot, I don't know why it works!!
I hit the shot very firm with a healthy amount of right. Now here's why I wonder why it works. Hitting it with right will transfer a little left to the ob causing it to bank short after the first rail right. Well if the ob still has left on it, shouldn't it go long after the 2nd rail? But no, not only does it go short, it actually banks reverse angle? What's going on with the 2nd rail? Is the ob taking right after going into the first explaining why it banks reverse?
Somebody explain. I can do it, but I just can't explain why it works.
Thanks,
dave

CueTable Help

 
My guess is you were playing on equipment where rail-induced spin dominated over spin transfer from the CB.
 
Reverse banks

Only a small amount of spin is transferred to the object ball.
There is none left when it gets to the 2nd rail.
Opposing rails always reverse,unless the table stinks.
 
are you hitting it enough for the OB to become air-born? If so the rail induced spin may be the strongest factor.
 
Tokyo-dave said:
This shot came up in a game of 9-ball the other night, and as I was thinking there would be no way to shorten it up enough for a one rail bank, I reminded myself of when I was a translator for Mike Sigel when he did a couple of clinics in Japan and he showed me this shot. I figured I'd try it. I actually called the pocket, although obviously I didn't need to simply because it can look like a slop shot. And, I kinda thought it would be an opportunity to get into the guys head by calling it and making it.
Problem is, when I think about the physics behind the shot, I don't know why it works!!
I hit the shot very firm with a healthy amount of right. Now here's why I wonder why it works. Hitting it with right will transfer a little left to the ob causing it to bank short after the first rail right. Well if the ob still has left on it, shouldn't it go long after the 2nd rail? But no, not only does it go short, it actually banks reverse angle? What's going on with the 2nd rail? Is the ob taking right after going into the first explaining why it banks reverse?
Somebody explain. I can do it, but I just can't explain why it works.
Thanks,
dave

CueTable Help

Click to expand...
That's what happens. If you shoot just the cue ball 3 rails around the table with center ball each time it hits a rail it picks up a little more spin. So if this bank is hit hard enough it picks up right hand spin causing it to reverse off the second rail.
 
This video explains and shows in super slow motion what goes on with these types of shots:


Check it out,
Dave

Tokyo-dave said:
This shot came up in a game of 9-ball the other night, and as I was thinking there would be no way to shorten it up enough for a one rail bank, I reminded myself of when I was a translator for Mike Sigel when he did a couple of clinics in Japan and he showed me this shot. I figured I'd try it. I actually called the pocket, although obviously I didn't need to simply because it can look like a slop shot. And, I kinda thought it would be an opportunity to get into the guys head by calling it and making it.
Problem is, when I think about the physics behind the shot, I don't know why it works!!
I hit the shot very firm with a healthy amount of right. Now here's why I wonder why it works. Hitting it with right will transfer a little left to the ob causing it to bank short after the first rail right. Well if the ob still has left on it, shouldn't it go long after the 2nd rail? But no, not only does it go short, it actually banks reverse angle? What's going on with the 2nd rail? Is the ob taking right after going into the first explaining why it banks reverse?
Somebody explain. I can do it, but I just can't explain why it works.
Thanks,
dave

CueTable Help

Click to expand...
 
Neil said:
You are putting right on the cb, which puts left on the ob, shortening the angle it comes off the rail. If it hit an adjacent rail, it would still have left. Hitting an opposite rail, the english reverses and it now essentially has right on it. Which reverses it off the second rail back to the pocket. The english is still on the ob when it hits the 2nd rail because you hit hard, and it does not have time to come off yet. To prove that, just try making it hitting softer, you won't be able to.
Click to expand...

I take it back about no spin left for the 2nd rail.
I was just having a shower and i realized that i won an 8-ball game
where the 1 and 2 across were blocked.I checked the ob on the first
rail and it ran slightly on the 2nd to find the pocket.
It also works at 1-hole with long 2-bankers (only way they can be made)
the ball must check on the 2nd rail....duh...
Well,my game still stinks but at least i don't.

This is a great forum.Every day you can learn something....thanx
 
pt109 said:
Only a small amount of spin is transferred to the object ball.
There is none left when it gets to the 2nd rail.
Opposing rails always reverse,unless the table stinks.
Click to expand...

If there was none left at the second rail, it wouldnt reverse imho.

Edit-Sorry didnt see the above post.
 
Nostroke said:
If there was none left at the second rail, it wouldnt reverse imho.

Edit-Sorry didnt see the above post.
Click to expand...

Losing an argument creates a paradox.
The loser gains something...the winner only breaks even

...i could never convince my old lady of this...:eek:
 
I agree most with Dr. Dave and the others that suggest that the spin on the ob is reversed when it hits the cushion. It's even obviously proven in the videos. But then there are shots like this, where spin is not reversed. This is a shot on the Kamuii website where the guy hits it hard with tons of left, the cb comes off the first rail short because of the left,but then goes very long off the 2nd rail 'cause the left stays alive into the 2nd rail.
http://www.kamuitips.com/video/

dave

CueTable Help

 
Tokyo-dave said:
I agree most with Dr. Dave and the others that suggest that the spin on the ob is reversed when it hits the cushion. It's even obviously proven in the videos. But then there are shots like this, where spin is not reversed. This is a shot on the Kamuii website where the guy hits it hard with tons of left, the cb comes off the first rail short because of the left,but then goes very long off the 2nd rail 'cause the left stays alive into the 2nd rail.
http://www.kamuitips.com/video/

dave

CueTable Help

Click to expand...
I think the reason that this happens is that when the cue ball hits the first rail hard and loaded with left is that it jumps off the table ever so slightly and the spin increases to the point when it hits the second rail it spins off with a ton of angle.
 
The spin reverses only at steeper angles into the rail (i.e., at smaller "approach angles" relative to the cushion perpendicular), and with smaller amounts of spin, as is the case with the two-times and three-times across shots:

When the CB comes in at a shallow angle relative to the rail, the cushion has very little effect on English, especially with slick cloth (i.e., the spin is mostly retained, and certainly not reversed). For example, see:

Also, if a ball has a lot of spin, the spin won't fully reverses (although, it will diminish quite a bit), even at steep angles into the rail. For example, see:

Regards,
Dave

Tokyo-dave said:
I agree most with Dr. Dave and the others that suggest that the spin on the ob is reversed when it hits the cushion. It's even obviously proven in the videos. But then there are shots like this, where spin is not reversed. This is a shot on the Kamuii website where the guy hits it hard with tons of left, the cb comes off the first rail short because of the left,but then goes very long off the 2nd rail 'cause the left stays alive into the 2nd rail.
http://www.kamuitips.com/video/

dave

CueTable Help

Click to expand...
 
Tokyo-dave said:
I agree most with Dr. Dave and the others that suggest that the spin on the ob is reversed when it hits the cushion. It's even obviously proven in the videos. But then there are shots like this, where spin is not reversed. This is a shot on the Kamuii website where the guy hits it hard with tons of left, the cb comes off the first rail short because of the left,but then goes very long off the 2nd rail 'cause the left stays alive into the 2nd rail.
http://www.kamuitips.com/video/

dave

CueTable Help

Click to expand...
It depends on how much spin the ball has going into the cushion (actually spin/speed ratio) and the slickness of the ball/cloth. With enough sidespin/speed, it isn't reversed off the first cushion.

On the way to the second cushion, whatever sidespin/speed ratio it had after coming off the first cushion--whether reversed or not--is increased, since the ball is slowed down by friction with the cloth. Sidespin is not all that much affected compared to the CB's speed.

If it were merely simple friction and the normal (perpendicular) compression force of the rubber acting on the ball, all of this could be figured quantitatively without much trouble. But the cushion is more complicated than that, mainly due to tangential restoration forces acting as well.

Oops, I see Dr. Dave has already addressed your question (and happens to be right :) :) :) ...and fast!)

Jim
 
Saying the spin "reverses" when it hits the rail is misleading, since it suggests that if the ball didn't have spin going in it won't have any coming out because there's nothing to reverse. But a ball clearly has induced spin after hitting a cushion at any angle except 90 degrees even if you shoot with no spin going in.

I think the following is a more useful model for building better intuitions about ball-cushion interactions. It explains all of these and other cases well (although I may not explain it particularly well without diagrams :)):
  • Every incidence angle into the cushion has a certain natural english that causes the ball to roll against it without slipping. A ball with the natural amount of spin for a given angle does not increase or decrease its spin after striking the cushion at that angle.
  • When you shoot directly at the cushion (perpendicular) at 90 degrees, the natural amount is zero spin. The natural amount progressively increases as the incidence angle decreases. A glancing angle near 0 degrees (parallel) requires nearly 2/5 radius off-center outside spin (which corresponds to the same amount you'd hit above center for natural roll on the table).
  • Whenever you use less than the natural spin for an angle, that acts as reverse english. The friction with the rail increases spin on the ball towards the natural amount and shortens the outgoing angle.
  • Whenever you use more than the natural spin for an angle, that acts as running english. The friction with the rail decreases spin on the ball towards the natural amount and lengthens the outgoing angle.
  • The closer you are to natural english going in, the more likely you are to to leave with natural coming out, even if this means reversing the spin (small amount of left can become right, and vice versa).
  • The further you are from natural going in, the more likely you are to retain the original spin direction (left stays left and right stays right, just reduced).
In general, all spin changes are towards the natural english for a given angle: stickier tables induce bigger spin changes, and slicker tables induce smaller changes (allowing a ball to retain more of its original spin).

This model suggests a better definition of running and reverse english. Instead of using the vertical center of the CB to define running vs reverse for a given angle, it shows how thinking about spin in terms of more or less than the natural sidespin for a given angle makes better predictions.

Of course, now you have to know what the natural amount is for each angle! :) It's a little more complicated than simply thinking in terms of left and right spin, but at least it's not friction dependent and is the same from table to table (to a very good approximation).

Robert
 
Excellent summary!

FYI, I have a good summary of all kick and bank effects (including the ones discussed in this thread), along with links to video demonstrations of each of them, here:

Check it out.

Regards,
Dave

Robert Raiford said:
Saying the spin "reverses" when it hits the rail is misleading, since it suggests that if the ball didn't have spin going in it won't have any coming out because there's nothing to reverse. But a ball clearly has induced spin after hitting a cushion at any angle except 90 degrees even if you shoot with no spin going in.

I think the following is a more useful model for building better intuitions about ball-cushion interactions. It explains all of these and other cases well (although I may not explain it particularly well without diagrams :)):
  • Every incidence angle into the cushion has a certain natural english that causes the ball to roll against it without slipping. A ball with the natural amount of spin for a given angle does not increase or decrease its spin after striking the cushion at that angle.
  • When you shoot directly at the cushion (perpendicular) at 90 degrees, the natural amount is zero spin. The natural amount progressively increases as the incidence angle decreases. A glancing angle near 0 degrees (parallel) requires nearly 2/5 radius off-center outside spin (which corresponds to the same amount you'd hit above center for natural roll on the table).
  • Whenever you use less than the natural spin for an angle, that acts as reverse english. The friction with the rail increases spin on the ball towards the natural amount and shortens the outgoing angle.
  • Whenever you use more than the natural spin for an angle, that acts as running english. The friction with the rail decreases spin on the ball towards the natural amount and lengthens the outgoing angle.
  • The closer you are to natural english going in, the more likely you are to to leave with natural coming out, even if this means reversing the spin (small amount of left can become right, and vice versa).
  • The further you are from natural going in, the more likely you are to retain the original spin direction (left stays left and right stays right, just reduced).
In general, all spin changes are towards the natural english for a given angle: stickier tables induce bigger spin changes, and slicker tables induce smaller changes (allowing a ball to retain more of its original spin).

This model suggests a better definition of running and reverse english. Instead of using the vertical center of the CB to define running vs reverse for a given angle, it shows how thinking about spin in terms of more or less than the natural sidespin for a given angle makes better predictions.

Of course, now you have to know what the natural amount is for each angle! :) It's a little more complicated than simply thinking in terms of left and right spin, but at least it's not friction dependent and is the same from table to table (to a very good approximation).

Robert
Click to expand...
 
Robert Raiford 's explanation opens my eyes to a lot of things that
I just accepted without a reason before.

It is hard to see on a cross side double bank because it happens so
quickly but on a long table double bank, a fairly common shot in
one-pocket, the angle coming off of the second rail is always less
than the angle off of the first rail. And , in my experience, that shot
is made with less force and less sidespin than the cross side double
bank.
 
12310bch said:
Robert Raiford's explanation opens my eyes to a lot of things that I just accepted without a reason before.
Click to expand...

That's great - I'm glad you found it helpful!

Accepting the complexities of reality and having a good memory for principles gained through experience is important to being a good player, but it's also nice when there's a theory that helps weave things together into a coherent whole. It doesn't necessarily affect our ability to execute in the known positions, but it can really help when creating new solutions in unfamiliar situations.

I distilled the 'natural english' principle from studying the comprehensive analysis of billiards by the French mathematician/physicist Coriolis (written in 1835, a few years after the invention of the leather tip). He was the first to create a useful model of ball-cushion interactions that accurately represents what you see on the table (running/reverse spin effects, curves due to follow/draw, etc.).

In addition to writing pages and pages of equations for his technical audience, he also invented many geometric constructions that beautifully illustrate the physical principles of billiards in an intuitive way for non-technical readers. I've created some dynamic interactive versions of his constructions that I'll be putting on the web soon for people to experiment with. They're a fun way away from the table to explore a rich model encompassing the subtle details of the game we all love. I'll make a post when they're available.

Robert
 
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