That's what I think too. I should probably do a scientific study comparing marks (or lack thereof) after cleaning/polishing the balls with different solvents/cleaners/waxes, but ain't nobody got time for that.
Ball Compression Contact Patch Marks Showing How Much Balls Compress and Flatten During Impact
- Thread starter dr_dave
- Start date
That's what I think too. I should probably do a scientific study comparing marks (or lack thereof) after cleaning/polishing the balls with different solvents/cleaners/waxes, but ain't nobody got time for that.
I was hitting the breaks mostly from the center with a square hit, and the racks were always tight (with my trained table), so there were large forces throughout the rack.
When I've used pieces of tissue paper between the balls to simulate gaps in different parts of the rack, the contact patch was clear in the part of the tissue that got smashed. It had a similar size and shape as the white marks on the balls.
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It's pretty easy to see that there has to be a contact patch of some size. Imagine two balls coming at each other at the same speed. There's a plane between them that neither ball will cross. Because the contact is not instantaneous, there must be a round flat spot (or nearly flat) of the contact between the balls.
If you know the size of the contact patch between the balls, you can fairly easily calculate the contact time if you know the speeds of the balls. Or you can measure the contact time directly, as Dr. Wayland Marlow showed in his book on billiard physics. That time is about 100 to 200 microseconds depending on the speed of the shot.
If you know the size of the contact patch between the balls, you can fairly easily calculate the contact time if you know the speeds of the balls. Or you can measure the contact time directly, as Dr. Wayland Marlow showed in his book on billiard physics. That time is about 100 to 200 microseconds depending on the speed of the shot.
For those interested, there is more data and info on this topic here:
. . . but ain't nobody got time for that.
Hah. I've always thought of you as the Project Farm (YouTube) of billiards. Or maybe he is the Dr. Dave of tools. ;-)
Would blue balls have more internal pressure, therefore would compress less?
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Das Cue-Boot
Active member
Hmm. If you look closely at the oval tennis ball marks you’ll see that they are all skids, usually accompanied by some yellow fuzz, amount depending on the surface. It is extremely difficult for us mere mortals to get enough topspin on a ball, especially on serves or overheads, to have the ball rotating at impact speeds.
With tennis balls you are also looking at the impact pattern from an obloid (that compresses perpendicularly to the direction of motion) and a plane ( might get a small amount of compression on a soft court, certainly some on grass) vs two spherical balls that would compress similarly. I have trouble visualizing how you can have long enough contact time on the balls to get an oval unless there is a lot of English on the impacting ball or the impacted ball is frozen to a rail.
There's a super-slow-motion video on the ball contact time resource page. Is that what you had in mind?
If you are instead referring to the video I am working on (for which I hit all the breaks), that will need to wait until I am done.
Yeah, like when you're walking.
Das Cue-Boot
Active member
If the contact time is linearly proportional to the speed, yes, but this whole discussion looks and acts non linear, hence the videos and the usual transition from theory to engineering. I was getting about 250 microseconds from the videos Dr Dave posted. No good way to relate that to speed with only one data point.
Didn't Max Eberle have a flat ball theory?
In case anybody wonders...
A ~1/8" contact patch (as shown), flattens less than 1/1000 inch (on each ball).
pj
chgo
i thought purple comes after red.......balls??Purple comes after blue……..balls