What affect does english (sidespin) have on the tangent line and the 90° and 30° rules?
Sidespin is used mostly to control the rebound angles of the CB off cushions. For more information and demonstrations, see the sidespin tutorial. But sidespin also has small effects on the directions the CB and OB head after impact:
“90° and 30° Rule Follow-up – Part IV: sidespin effects” (BD, May, 2005) shows the effects of sidespin on both the 90° and 30° rules. Here’s Diagram 1 from the article:
For a given contact point on the object ball (OB), the cue ball (CB) tangent line direction does not change with sidespin. Whether the CB has sidespin or not, a stunned CB heads straight down the tangent line (AKA “stun line”). However, the direction and amount of spin on the CB can have a significant effect on OB direction due to throw effects, as shown in the diagram. Here’s a good demonstration of these effects:
In addition to OB throw, spin can also get transferred to the OB. There are many shots where both throw and spin transfer have significant effects, allowing shots that would not be possible otherwise. Numerous examples are illustrated and demonstrated here:
The main purpose for sidespin (when not being used for throw or spin transfer) is to change the CB’s direction after hitting a cushion. For more info, see english terminology and uses.
Here’s Diagram 2 from the sidespin effects article:
Concerning the 30° rule, when inside sidespin is used, the deflected cue ball angle can be narrowed slightly, as demonstrated in the following video:
With outside english shots, the CB will swerve on the way to the CB, increasing the cut angle slightly, which prevents the CB from going forward as much as expected. And with inside english shots, the CB swerve decreases the cut angle slightly, which allows the CB to go more forward than expected. Sidespin also affects how the OB is thrown, but this is not being considered here. For more info, see the throw tutorial.
from 3andstop (in AZB post):
While side spin doesn’t (in and of itself) affect the tangent line from the point of contact on the OB, side spin can and does affect where you contact the OB in order to pocket it.
Therefore, in a tight spot (very common in straight pool) pocketing a ball that would send the CB down an undesirable tangent line, can be adjusted with side spin.
This, because now you are able to contact the OB in a slightly different spot, either thicker or thinner than the contact point with no side spin (depending on which english is used) to pocket it (turn it into the pocket) and change both the contact point and tangent line.
So, in a sort of indirect way, side spin one way or the other, can change the standard tangent line when speaking relative to the pocketing of an object ball.
It will not change the tangent line when speaking of a carom only, hitting the OB in the same spot regardless of english.
from BeiberLvr (in AZB post):
Where you strike the CB does not determine the tangent line. It’s where the CB makes contact with the OB that determines the tangent line.
So on shots with side spin (especially over certain distances), the contact area on the OB will be different when using left or right spin, then it would be with center. Now if you played a short distance shot where the spin might not have time to take effect, then the tangent lines would be the same, because the contact area on the CB wouldn’t change.
Same goes with hitting center on the CB, but playing the OB to different parts of the pocket. That will also result in a different tangent line, but it just goes back to the fact that you’re simply changing where the CB contacts the OB.