Banking on Physics

I promised Tips on Banking to Andrea as a Christmas present. Andrea is a bit new to the game, and is starting the APA this January. (Good luck Andrea!)

Most people are taught to bank cross side with, "Aim the cue ball from the corner pocket at that 2nd diamond on the other side of the table, and the cue ball will bank off and come back into the side pocket". Well that's close. But there is more going on.

Geometrically speaking, the precise spot where the cue ball should hit would be the "nose of the rail" across from the diamond. And remember, the cue ball is coming in at an angle, so you need to aim over a wee bit so the side of the cue ball contacts at that point. Imagine a ball on the rail directly across from the diamond. That would be the spot where the cue ball would be the moment it rebounds. If you bank from the side to the corner, rather than from the corner to the side, you would aim at precisely that same spot. The fact that it is the same spot proves that the "angle in equals the angle out" regardless of which direction you bank.

But pool is not so much pure geometry, as it is physics. Balls spin. Rails squish. Balls rub against rails and cloth and other balls in such a way that energy gets transferred in ways you might not think.

Imagine the cue ball rolling from the corner pocket toward the nose of the 2nd diamond. It is going end over end, right? That means it is spinning with a bit of topspin. If there were no topspin, it would be sliding like your car tires, when you slam on the brakes on an icy road. The ball would not be rolling end over end if it has no spin.

What happens when a rolling ball hits the rail and starts back the other way? The rolling changes direction. Once it comes off the rail the cloth grips the ball until it goes end over end the other way. Get it? This is one of the reasons why a ball cannot bank a zillion times. The cloth keeps grabbing it to change its rolling direction, slowing it down every time the ball comes off a rail.

So, what does that imply? It implies that the ball loses energy (and speed) perpendicular to the rail. The component of speed parallel to the rail stays about the same. The result is that the ball "goes wide". So if you aim from the corner to the nose across from the 2nd diamond, the ball might miss the side pocket and hit the rail slightly past it.

This explains why it is a bit more prudent to aim more directly at the 2nd diamond rather than at the nose of the 2nd diamond.

But what if you hit hard? Hitting a ball hard into the rail does 2 things.

First, it gives the ball less time to grip the cloth to change its direction off the rail. If you hit it hard enough, the ball might just slide over to the rail and slide across toward the side pocket.

Secondly, rails compress. When a ball squishes into the rail and pops back off, the ball wants to come straight out a bit.

These two things cause a banked ball to come back narrow, rather than wide when you hit hard. If you bank hard from corner to rail to side pocket, you might come up short of the side pocket.

Okay, all this time we've been talking about banking just one ball. That's fine if you need to kick at an object ball (bank the cue ball first, so it hits an object ball). Usually though, you want to hit the cue ball into an object ball, and the object ball banks across. The difference is the object ball might not have that rolling topspin. Depending on how hard you hit, whether the cue ball hits with top or draw, affects whether the object ball has topspin or not. It could very well have backspin draw, if the cue ball hits with topspin and the object ball reaches the rail before the cloth has time to grip it. In that case, the object ball already has the spin it wants after it comes off the rail; it does not slow down. It fires across and might hit short of the pocket.

Okay, anything else? Yes. When the cue ball contacts the object ball at slow or medium speed, it tends to grab the object ball slightly. This is known as contact throw. If the cue ball has side spin on one side or the other -- or even if it hits the object ball at an angle, it can grip the object ball and drag it slightly with it and also cause the object ball to spin slightly.

If you are an APA skill level 5 or more, you probably know all this stuff. If not, let some of it sink in over time. Come back to this page as a refresher.

The best way to get a feel for banking is to practice over and over and over. See what happens when you hit hard and soft. See what happens when you hit the cue ball below center or above center into the object ball when trying a simple bank. If you have the opportunity in a match, try not to hit too hard or too soft. I suggest hitting a bit harder if you want the object ball to rebound narrow, or a bit softer if you want it to go wide (such as a cross corner shot).

In some cases, speed is what really matters most for the bank shot.

Consider the cue ball near the corner pocket. Your game winning 8 ball is almost on the far end rail, near the 1st diamond, but just an inch or 2 off the rail. The best choice is to bank the 8 back to the pocket where the cue ball is. Simply roll the cue ball at medium lag speed so it hits the 8 ball slightly off center. If you have the speed right, cut angle is not so critical. The contact throw and the way the 8 springs off the rail can make it go in. Experiment with this one. Banking "back at you" a table length can be easier than banking back up to the other pocket.

Mirror method

The simplest method for a beginner to line up a bank shot is to imagine a mirror placed along the rail, hung vertically. If you were to hit a ball at an angle into the rail, and watched the reflection as it rebounds, it would travel (pretty much) in a straight line to how it travelled (unreflected) into the rail.

So, let's try to use this. If you are banking into the side rail the distance it rebounds to reach the pocket would be the width of the table. On a 7 foot table, that is about 3 1/2 feet; on an 8 foot, about 4 feet; on a 9 foot, about 4 1/2 feet. If you are banking to the corner, imagine shooting at a point that distance away off the table, perpendicular from the opposite corner pocket. That is where you would see your goal pocket if a mirror was over the rail. (A picture would clarify this easily. If you study this and experiment, you will figure out what I mean.) Likewise, to bank into the side pocket, aim the object ball at a point, a table width past the table, perpendicular to the opposite side pocket. On league night, I help the beginner by standing a table width away from the side of the table and cup my hands precisely how they would see the pocket in a mirror. I then step away, and they bank the ball to where my hands were cupped, and have a pretty good chance at making it.

Dr. Cue

Dr. Cue (Tom Rossman) is considered one of the foremost authorities in banking 1, 2, 3, and more rails. His methods not only deal with geometry, but take into account physics. I could describe his methods, but it would undermine his thoughtful work. For more info, you can buy his small inexpensive Racking Up a Victory booklet or his 2 part video tapes or the equivalent 3 part The Monk Series 202 tapes. I think his little booklet is adequate enough. I have the 3 Monk Series 101 and 3 Monk Series 202 video tapes from Seyberts, but they are a bit costly. (I do not have the 2 part videos from Dr. Cue's site.) Some informative videos he has posted online are here.

Rail Conditions

Be aware that rails differ on various tables. Tables in a pool hall generally have firmer, more resilient rails, and cleaner cloth -- compared to the rails on bar tables. The balls tend to bank back a bit straighter (narrower angle, or short) on a clean, well kept table and a bit wider (long) on a dirty bar table with loose, dead rails. Sometimes a portion of one rail is totally dead while the rest is okay. (By the way, do not let friends sit on the edge of a pool table -- that can loosen the rails.) You can pretest for dead spots by bopping the cue ball several times on the rails as you walk around the table, before you start your match.

Note the weather outside. On warm, humid days, there is more cloth & rail friction causing a banked ball to go wider (longer) sooner. Cold and dry days will delay the time a ball skids after coming off the rail, so it will rebound narrower (shorter).

Rails should be installed so the lip is only slightly above the center of a ball. If the rails are too low, a rolling ball might tend to climb up onto it slightly, and hop off the rail as it rebounds. These are the tables that make it easier to "bank off the end rail, back at you". Rails that have a lip too high behave a bit strange. A ball with backspin (for example, hit with a cue ball having topspin) will fire back a bit straight (narrow, short) on a normal rail, but on a high rail, it can actually loose a lot of energy and come off dead (wide, long). If you want consistency on rails that are too high, roll into them with topspin.

Well, good luck Andrea! The more you practice, the better you can predict how balls will bank and move around a table.