Hard Banks Curve Short - Why?

[dr_dave]

For gearing english, I use 1 mm of BHE for every 5 degrees of cut as my rule of thumb. So if I am cutting the OB ball right 30 degrees, I'm going to use (or try to use) 6mm of left (outside) english.

FYI, Diagram 2 in my September '07 BD article is a graph of how the amount of gearing English varies with cut angle. Your linear approximation method seems fairly good for cuts up to about 45 degrees. For example, from the graph, a 30-degree cut requires 40% English for no throw or transferred spin. That corresponds to:

tip_offset = percent_English * (maximum_tip_offset) = 40% * (R/2) = .4 * (1.125"/2) = .23 in = 5.7 mm​

which is close to your 6mm rule-of-thumb value.

Regards,
Dave

 

[Jal]

Yes, for example a 50-25 bank would be from the nose at the 5th diamond away from the target pocket to nose location midway between the 2nd and 3rd diamonds on the banking rail.

I usually have the OB one diamond away from the cushion. As you get into the shallow banks, it has to be moved a bit farther back to avoid the double kiss or hitting the collimator.

The reason I do this is practical. I'm not gathering data for science. I want to know how to play the table on that day. Example:

During warm up I place the OB on the 20-10 line about a diamond off the cushion and bank it with a firm stroke. It banks short, hitting the cushion about 5" away from the effective pocket center. I place a coin on the top of the cushion where it hit and repeat. If I hit the same spot, I have confirmed the bank is 5" short. With a little trig, this is about 5.5 degrees, which I will round to 6 degrees.

Knowing that, I want to test my result. So I put the OB back on the 20-10 line, about 1 diamond out, and put the CB on the 20-10 line, maybe 18" away. To correct my aim, I take the 6 degrees of expected pushback and multiply by 0.8 (because the OB is one diamond off the rail) and see that I need to cut the ball about 5 degrees. So my aim point on the OB will be about 5mm off center. I will then shoot, with a tiny amount of outside english (to offset CIT), and the bank should go.

Another example: 20-10 bank, OB two diamonds off banking rail where there is a 20 degree cut needed to send the OB along the 20-10 line. In this case:
pushback correction = 6 degrees x 0.66 = 4 degrees

aim point on OB = 20mm (cut) + 4mm (pushback correction) = 24mm off OB center

I will shoot this shot aiming 24mm off OB center, with about 5mm of outside backhand english. This should send the OB along the modified bank line without sidespin, and hopefully, it goes in.
***************

I came up with some of this independently, but Jack Koehler had the same basic approach described in his book. Some of what he has is over-complicated (I look only at the total pushback, not how much is related to specific components), and his chart for angle correction multipliers is not very accurate. But, the basic approach is sound.

I'm going to have to think about it a bit before I understand, but it looks as if you've supplied all the info necessary for that. Appreciate it and thanks again.

Jim

 

[Luxury]

Btw. I really appreciate your video on banks. That will really help me. I only need to hit hard if the object ball is far enough away from the rail to stop sliding and begin rolling forward.

 

[Mr. Wiggles]

short banks

Not so much on one railers but on multi rail shots put a small amount of running english(the direction naturally shooting towards) and this will true up the shot imo

 

[dr_dave]

on multi rail shots put a small amount of running english(the direction naturally shooting towards) and this will true up the shot imo

Running English certainly helps make the Plus System and Corner-5 System consistent and reliable.

Regards,
Dave

 

[Robert Raiford]

Marlow takes a basic look at some of these effects in his book, but I think a lot more can be done to improve understanding; although, I honestly doubt the study would result in any truly practical revelations

Unfortunately, Marlow's analysis is much less useful than it could have been because he assumed in all of his calculations that the 'correct' rail height is 7/10 of a ball diameter instead of what every pool and billiard table on the planet actually uses. Hopefully, future analysts will stop to consider why table manufacturers have empirically settled on current specs after what I'm sure was much trial and error. I'd certainly hate to play 3-cushion on a table with 43mm high rails.

Robert

 

[Robert Raiford]

Note: I use the diamond position projections on the rails (not shooting thru diamonds).

Thanks for posting the sample 'pushback' tables. I'm still unclear about your aim lines, though. Are you aiming the ball on a line at the corresponding point on the cushion nose (which means you contact the rail slightly before it), or aiming to contact the cushion at that point (which means aiming slightly beyond it)? Is the origin of your aim line the point on the cushion nose at the origin rail, or the center of a ball that would be touching that point when frozen to the rail? Since you're measuring so precisely, I'm sure you can appreciate the effects various combinations of these small differences would have on the results, and I want to make sure I'm comparing apples to apples.

When I do something similar incorporating roll, I call them 'rollspreads' (vs the 'stunspreads' in your case).. I give them positive values when they lengthen beyond ideal reflections and negative values when they shorten. Tournament conditions for 3-cushion (new cloth, new clean balls, etc.) always have positive rollspreads, but typical pool table conditions have negative rollspreads for very direct/perpendicular incidence angles. As friction increases due to balls and table getting dirty, I find tables can change significantly over the course of play. Do you see your banks progressively shortening during your playing sessions?

Robert

 

[Dead Crab]

snipped:
Are you aiming the ball on a line at the corresponding point on the cushion nose (which means you contact the rail slightly before it), or aiming to contact the cushion at that point (which means aiming slightly beyond it)?

Do you see your banks progressively shortening during your playing sessions?

Robert

I've played around with the different methods you mentioned, and (at least for the moment) I use center-ball aiming along a 2:1 ratio of the cushion points. I find that siting center ball is just easier for the great majority of my banks. As you mentioned, the actual impact point of the ball is higher-up the cushion, and this causes some of the shortening of the bank. This isn't much of an issue at one diamond (20-10 bank), where the actual impact point is around 7mm higher than the aim point, but can become very significant for longer banks, for example a 100-50 bank (off the 5th diamond) will strike the cushion around 36mm (1.4") above the aim point.



My banks shorten with prolonged playing time, but I think it is mainly because my stroke decays.

 

[Robert Raiford]

My banks shorten with prolonged playing time, but I think it is mainly because my stroke decays.

I think another reason why billiard players may notice the table changing more than pool players is that we're primarily concerned with accurately banking cue balls instead of object balls. Since the CBs are accumulating chalk with each stroke and OBs and rails are only picking up chalk and such indirectly, it makes sense that CB-rail friction gets progressively higher faster than OB-rail friction during play.

Also, we tend to clean the table and balls more often - ideally, after every game - so I think that makes the differences more apparent as well. Once ball-rail friction reaches the point where sliding ceases for a given angle, further friction coefficient increases don't really matter so much.

Btw, I like your banking approach. I use a similar method with additional allowances for speed, throw and spin transfer and it's very accurate (I'm sure you have similar estimations). I play much more one pocket than bank pool, though, so slow-rolling banks while allowing for rollspreads and spin decay over distance is critical.

Robert