Polishing and Kicks
- Thread starter Scaramouche
- Start date
In the article, I see
Add to kicks the seemingly increased numbers of ‘bad bounces’ from cushions, with balls coming off them quicker than they are going on
Of course that doesn't happen. In a typical direct (straight in and straight out) cushion contact the ball loses 50% of its speed. If it only loses 70% of its speed due to the friction at the ball-cushion contact being high, the ball only appears to have gathered speed because of what you are used to seeing.
It is important to get all the polish off the balls after polishing and that is not easy. If they use polishing machines they either need to use several in series or hand rub with a clean, frequently changed towel afterwards.
On a related point, the carom people (61.5mm balls) have gone through several iterations of what to do about polish. I think the current policy in international events is to use only water and a clean cloth.
Ron Swanson
Banned
Um, did you see the recent UK champs? Bad bounces galore. They happen. No pixie dust required.
The first link (by Keith Stevens) has some good info but managed to get the numbers very wrong. The ball-ball contact time has been measured by Wayland Marlow (see his book) at around 200 microseconds. He also measured the variation in contact time with ball speed and found the expected decrease in contact time with high-speed collisions. For those interested in the details, look up Hertz' Law for contact between curved solid objects.
There are several other not-quite-right things in the first link. The contact patch between balls can be around 4mm for medium-fast speeds, for example.
Byrne suggested the chalked-ball experiment in the 1980s (or so) and Onoda did an analysis of the frequency of kicks (skids, cling, bad contacts) based on the observed chalk coverage in a typical cue ball (December 1991 Billiards Digest).
Recently Dr. Dave did a very complete set of experiments with different surface preparations of the balls, including with silicone. There were huge variations in the coefficient of friction between the balls depending on the preparation. A large consequence of this is that the cut angle for cut shots will vary with the preparation. (Stevens seems to be unaware of the usual "throw" in the American sense on all cut shots.)
There are several other not-quite-right things in the first link. The contact patch between balls can be around 4mm for medium-fast speeds, for example.
Byrne suggested the chalked-ball experiment in the 1980s (or so) and Onoda did an analysis of the frequency of kicks (skids, cling, bad contacts) based on the observed chalk coverage in a typical cue ball (December 1991 Billiards Digest).
Recently Dr. Dave did a very complete set of experiments with different surface preparations of the balls, including with silicone. There were huge variations in the coefficient of friction between the balls depending on the preparation. A large consequence of this is that the cut angle for cut shots will vary with the preparation. (Stevens seems to be unaware of the usual "throw" in the American sense on all cut shots.)