What causes cue ball deflection (AKA “squirt”)?

Check out the following article: “Squirt – Part I: introduction” (BD, August, 2007). It explains and illustrates what causes squirt in a very easy-to-understand way. Here’s a simple explanation: With an off-center hit, while the tip is in contact with the CB, the CB starts to move forward and turn. The ball turn pushes the tip away sideways causing the end mass of the shaft to move. Mass doesn’t like to move, so it pushes back during contact (because for every action, there is an equal and opposite reaction). That’s why the CB deflects (squirts) off line.

In understanding the endmass effect, it helps to distinguish between the forward force (and impulse) required to deliver forward momentum to the cue and CB, and the sideways force (and impulse) resulting in endmass momentum and CB deflection. These are two different forces. The forward force and cue momentum is a result of what you develop and feel during your forward stroke into the ball. The sideways force (during the incredibly brief tip contact time) is a result of the interaction between the CB and endmass. This equal and opposite sideways force is what causes CB deflection and sideways momentum of the end mass, which in turn causes the cue flex and vibration (which you feel after the hit).

Here’s a diagram (still images from a 2000 frame/sec high-speed video) and an explanation from the article, providing a more-detailed explanation for what causes squirt:

super slow motion stills of cue tip compression

Still “a” is just before contact. Stills “b” through “e” represent a little less than 0.001 second (one thousandth of a second) during which the tip is in contact with the ball. In still “f” the tip hasn’t fully recovered from the compression yet as the CB is separating. Still “g” is after separation. The line and arc appearing in each still mark the initial cue stick and CB positions. Notice how much the cue tip deflects away (down in the diagram) from its original line of action. Also notice how much the cue tip deforms (e.g., see still “d”).

The black arrows in still “c” in the diagram illustrate the effect that causes squirt. While the tip is in contact with the ball, the ball starts rotating. This rotation (counterclockwise in the diagram) pushes the cup tip down a little during contact. Because the end of the shaft has mass (called endmass), it takes force to move the end of the shaft down as the ball rotates (because mass has inertia, and force is required to change its speed). And Isaac Newton said: “for every action, there is an equal and opposite reaction;” therefore, if the tip is being pushed down by the ball, the tip will push back with an equal and opposite force on the ball. This force is what causes squirt.

The amount of squirt (cue ball deflection) depends on the effective mass (“endmass”) being deflected in the shaft. The “effective mass” depends on how far the tip deflection is “felt” down the length of the shaft as a sideways “wave” travels down the shaft toward the butt. Because the tip is in contact with the CB for such a short time, the wave does not travel very far (only about 5-10 inches). The distance it travels varies with shaft stiffness some. It travels faster (and longer) in a stiffer shaft involving more “effective mass” in the sideways deflection, which causes more squirt. However, it is the shaft endmass and not shaft flex that results in squirt. The following article provides some evidence to back up this claim: “Return of the squirt robot” (BD, August, 2008).

The cue tip continues to move sideways and eventually springs back and vibrates back and forth, but the CB is long gone by then, so the stiffness and spring-back of the shaft has no significant or direct influence on squirt.

For more information and relevant demonstrations, see:

Does cue tip compression, tip hardness, and shaft flex affect CB deflection (squirt)?

Here are some example super-slow-motion videos showing how the tip deforms and how the shaft flexes during tip contact:

I know that when one looks at these videos, it is tempting to think that squirt (CB deflection) is caused completely by tip compression and shaft flex. However, it is best to ignore these effects when trying to understand the basics of squirt. Tip compression and shaft flex are really just side effects of the off-center-hit forces required to keep the tip from slipping on the CB.

Now, the more the tip compresses and flexes sideways, the longer the tip will tend to stay in contact with the CB. This would certainly result in more squirt (CB deflection) because effective “endmass” is larger with a longer contact time. Also, the more the tip flexes sideways, the more the endmass of the shaft moves sideways, which would also tend to create more squirt. A harder tip compresses and flexes less and results in a shorter tip contact time. Therefore, a harder would be expected to produce less squirt, assuming it is not heavier than the tip to which it is being compared (for more info, see cue tip hardness effects). However, the experiments documented in the Cue and Tip Testing for Cue Ball Deflection (Squirt) video seem to imply that tip type, hardness, and height have very little effect on squirt.

Shaft flex can also have an effect because it might cause some of the “endmass” to move faster than it would otherwise. This could contribute to more squirt, but I wouldn’t expect this effect to be very significant.

Again, the main effect that causes squirt is: During the very brief moment while the cue tip is in contact with the CB during an off-center hit, the CB starts to turn. This pushes the cue tip sideways aways from the CB giving the end of the shaft some sideways speed. It takes force to do this since the end of the shaft has mass. For every action (sideways force pushing on the tip), there is an equal and opposite reaction (sideways force pushing back on the CB), causing the CB to squirt sideways with “deflection” off its expected path (i.e., the CB doesn’t go straight).

The most effective way to reduce squirt is to reduce the effective “endmass” of the shaft (for more info, see low-squirt (low-cue-ball-deflection or LD) shafts). Keeping the tip contact time as short as possible (e.g., by using a harder tip) can also help.

Can the joint or butt of a cue affect CB deflection (squirt)?

No!

Physics and careful testing clearly show that squirt (CB deflection) depends only on the effective endmass of the shaft. Therefore, the butt can have no effect on squirt (CB deflection). For more info, see what causes squirt (CB deflection).

However, for a given stroke speed and cue elevation, changing the joint or butt can have an effect on CB swerve and therefore net or effective CB deflection (AKA squerve). For example, if the weight of the butt is different, the CB speed will be different (for a given stroke). CB speed does not affect squirt, but it does affect swerve. Also, the butt and joint can affect the efficiency of the cue’s hit, which can also change the CB speed and resulting swerve.

Swerve is a function of CB speed, cue elevation, and ball/cloth conditions. It has nothing to do with the properties of the shaft. That’s why when shafts are tested for squirt (CB deflection), the machines should keep the cue horizontal so stroke speed and the butt will have no effect on swerve or the measurements of the shaft characteristics. Results from some squirt-testing machines (like Meucci’s “Myth Creator” machine) can be misleading and seemingly in conflict with well understood and tested concepts. For more information, see the bullets on the squirt robot test results resource page.

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