Dr. Dave's answers to frequently-asked questions (FAQs), mostly from the AZB discussion forum
for more information, see Section 4.03 in The Illustrated Principles of Pool and Billiards
and Vol. I of the Video Encyclopedia of Pool Shots
Is a follow shot more accurate than a center-ball hit or draw shot?
This is definitely true for shots like the straight-up-and-down-the-table drill. Here are some possible reasons follow shots can be more accurate (especially wth long shots):
FYI, some of this is described and illustrated in "Squirt - Part III: follow/draw squirt and swerve" (BD, October, 2007) and "Squirt - Part VIII: squerve effects" (BD, March, 2008).
Follow shots also provide the best speed and distance control (see optimal tip height for speed control). Also, the "natural angle" of follow shots is very easy to judge and predict (see 30° rule for more info). These are the reasons for the classic saying: "Draw for show ... follow for dough."
Now, there are some reasons follow shots might not be as accurate for some people:
Draw shots can be more accurate for some people because more speed is used to retain backspin, especially with longer shots where much of the spin is lost due to cloth drag action. Because of the faster speed required:
Why does the ball sometimes hop on a follow shot, and what can I do to reduce it?
Because the cue always has some elevation (because the butt must clear over the rails, and your grip hand requires some clearance if it is over the table), there is always a downward component of force on the CB that causes it to hop.
Also, an above center hits causes squirt into the table that adds to the downward component of force. Here's a good video describing and illustrating the effect:
To reduce the amount of hop, try keeping the cue as level as possible (i.e., less elevated) at impact with the CB.
With draw shots, the squirt effect cancels some of the cue elevation effect, because they act in opposite directions; however, the CB will hop with added cue elevation. Here's an example: HSV B.24 - Draw shot with elevated cue and hop over an obstacle ball.
Friction force between the balls at impact can also cause the CB to hop, but this is small effect (see TP B.5). Although, the effect can be exaggerated when the balls are clingy (e.g., old, dirty, and/or chalk-smudged). This effect is demonstrated in the following video: HSV B.46 - CB and OB hop and spin transfer during follow shots. See the cling/skid/kick resource page for more info.
If the CB is airborne when it hits the OB, the CB and OB can both hop after impact. There are other effects that can contribute to CB hop. If the CB is slightly larger than the OB (e.g., with an older "bar box"), the CB will hit slightly above the OB equator possibly causing both balls to hop (especially at higher speeds). For more info and demonstrations, see ball size and weight effects. A similar situation occurs if the OB is resting in a dimple or small tear in the cloth.
Another thing to be aware of whenever the CB is jumped or hopped is: If the CB hits the OB while airborne, the effective cut angle is changed slightly. In this case, you need to aim for a slightly fuller hit on the OB, especially with fast-speed force-follow shots with the CB close to the OB. For an exaggerated example of the hop overcut effect, see:
For more info, see jump shot overcut effect.
Another important effect related to ball hop is the delay of follow or draw away from the tangent line. Here are some examples:
drag follow effects
What is drag follow, and how and when is it used?
A drag shot is where you hit the CB below center, and the bottom spin wears off and slows the CB down as it approaches the OB. With a drag-follow shot, all of the bottom spin wears off and the CB develops topspin roll before getting to the OB, resulting in CB follow. A drag shot can help reduce the effects of table roll-off (CB drift due to a non-level or dirty table), since the CB will be moving faster during the early part of the shot and be less susceptible to roll off.
Cloth drag also has a large impact on draw shots. It can also be used to increase the effect of sidespin. For more information, see:
effects of tip offset, speed, distance, cut angle
On a follow shot, does it matter how high I hit the cue ball, if roll develops naturally?
Yes, depending on the shot speed and distance to the OB. At low speeds and large distances from the OB, it doesn't matter how high or how low you hit the CB, it will be rolling when it hits the OB (due to drag action). However, for fast shots close to the OB, the tip contact height makes a big difference in how much the CB curves after OB impact. As I pointed out before, the other important thing with follow shots is how much spin vs. speed the CB has after OB contact. A fast follow shot will create more follow action when the OB is hit more fully. Most of the speed is lost, but most of the topspin is retained, and there is more topspin with more initial speed and tip offset.
What is force follow, and how and when is it used?
A force follow shot is a follow shot hit with maximum topspin and fast speed. This term is used mostly when referring to a nearly straight-in (small cut angle) follow shot. For more information and examples, see:
natural roll, maximum offset, and overspin
What tip height is required for a rolling cue ball, and is it possible to get more spin than natural roll?
Good explanations and illustrations of this can be found in the following instructional article:
"How High or Low Should You Hit the Cue Ball?" (BD, September, 2011)
Here's an illustration from the article showing some key tip reference heights:
Concerning the amount of center ball offset required for immediate natural roll, the answer (and analysis) are in TP 4.2. The technical term for the contact point for natural roll is "the center of percussion." The required contact point offset from center ball is:
2/5 R = 0.4 R = 2/5 (2.125 in / 2) = .45 in
The maximum recommended offset, beyond which miscues are very likely, is usually cited in the range:
1/2 in to 9/16 in = .50 in to .56 in
Therefore, follow (topspin) in excess of natural roll is possible (i.e., .45 is less than .5), but not by much. This is called overspin. With overspin, the CB will accelerate (i.e., speed up) as the spin wears off until the amount of topspin degrades to the "natural roll" amount. For more information, see "Coriolis was brilliant ... but he didn't have a high-speed camera - Part IV: maximum cue tip offset" (BD, October, 2005). The following video provides visual proof of off-the-tip overspin, and provides more explanation:
Also, NV B.36 shows a simple experiment to show how difficult it is to achieve overspin without miscuing.
Now, after a CB hits an OB, it can most certainly have overspin. For example, in HSV 4.3 and NV 4.8 the CB has topspin. Before it hits the OB, the amount of topspin is the natural rolling amount (i.e., there is no "overspin"). After the CB hits the OB, the CB has more topspin than the natural rolling amount (i.e., there is "overspin"). This is what causes the CB to "follow" the OB to the rail. After rebound off the rail, some of the spin is retained (which looks like bottom spin now) causing the CB to draw back to the rail.
The reason for extreme follow effects is all about speed. For a fast natural roll follow shot, the topspin angular speed goes up as the ball linear speed goes up. And for small cut angle shots, the cue ball loses most of its linear speed and retains most of the spin, causing significant follow effects. HSV 4.1-4.3 illustrate the principles fairly well.
For more info and illustrations dealing with ball sliding and rolling, with and without sidespin, see:
spin, slide, and roll
cloth drag effects
Why is it that with some follow shots, the cue ball stays close to the rail?
NV 4.8 - Rail dribble follow shot, NV 4.9 - Normal follow shot into a rail, and HSV B.21 - follow-the-rail trick shot illustrate and explain the effects fairly well. The following video contains a good demonstration also:
With almost all follow shots (regardless of speed), the CB is "rolling" at object ball impact. This is certainly the case in the "rail dribble" videos. With more speed, there is correspondingly more topspin, but the CB is still rolling (provided you are hitting the CB high enough).
The amount of follow action depends on how little CB speed remains in relation to the amount of topspin retained. With a fast follow shot, the CB has lots of topspin (as the ball rolls at a higher speed). With a smaller cut angle, most of the CB's speed is lost, but almost all of the topspin is retained after OB impact. That's what produces the follow "action" ... lot's of spin, not much speed. That's why both speed and cut angle are important in this type of shot.
If the cue ball has lots of topspin and very little speed when it hits a rail, "rail dribble" occurs (see HSV 4.3 - Rail dribble follow shot).
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