Why Do Long Pimples Wobble?

Photo of Chen Weixing

I've heard of balls wobbling, but this is ridiculous! Chen Weixing gets tied in a knot at the 2003 World Championships!

Photo by: Mariann Domonkos, courtesy www.ittf.com

I'm now going to tackle the subject in table tennis of how the long pimpled rubbers can make the ball seem to 'wobble' in the air. Please keep in mind that this is my opinion on the matter, and I am more than willing to hear anybody else's point of view and add it to the article.

What is the 'Wobble'?

When we talk about the long pimples wobbling, what exactly do we mean? In most cases, players are referring to the ability of the long pimpled rubber to make the ball seem to dip, rise, and suddenly go to the side while in flight. When you are playing against a person using long pimples, and the ball coming towards you suddenly seems to drop six inches then go sideways for no apparent reason, that is the sort of behavior we mean. The best definition of 'wobble' I can come up with is this:

Wobbling occurs when the table tennis ball suddenly moves from its natural flight path without any obvious reason.

What Causes the Wobble?

It's actually easier to explain this in terms of why a table tennis ball doesn't wobble all the time. A table tennis ball is very light - the current 40mm ball must weigh only 2.7g! That means that any motion of the air around it will tend to affect the flight of the ball.

So Why Doesn't the Ball Wobble on Every Stroke?

Without trying to get too technical, it's the spin on the ball that keeps it on a true flight path. A topspin ball will tend to drop towards the table, while a backspin ball tends to rise. A sidespin ball curls in the direction of the spin. Combinations of topspin and sidespin will make the ball go down and to the side, and backspin and sidespin will cause the ball to want to go up and to the side before gravity takes over. (If you can work out how to put topspin and backspin on the ball in a significant way, please let me know!)

The spin makes the ball curve in a predictable manner, and also helps to prevent the ball being affected by the air currents that are moving around the playing area. If you want to know more try the following article written by Jonathan Roberts, explaining the basic physics and maths of table tennis - diagrams included!

But What if there is Very Little Spin on the Ball?

If there is little or no spin on the ball, then the air currents in the playing area have a chance to affect the flight of the ball in a significant way. This is where the ball may begin to wobble - you can't guarantee that it will. But if you can manage to kill the spin on the ball with your stroke, then if the ball hits an air current of enough size, the air current will be able to push the ball in the direction the air is going. Since gravity is at work at well, an air current going down will be working with gravity and make the ball seem to dip suddenly, while an air current going up will make the ball seem to lift as it works against gravity.

Note that it may not actually lift the ball up - but it will slow down the natural rate of falling and make the ball seem to lift up. Air currents moving sideways will make the ball go to the side, again changing the natural flight path. Two air currents moving in opposite directions will push the ball back and forth.

How Can I Make the Ball Wobble?

Well, technically you can't make the ball wobble at all - in order for you to able to make it wobble you would have to be able to change the spin on the ball after you hit it, which is impossible, or you would have to have control of the weather and the air in the playing area.

But you can give yourself the best chance of getting the ball to wobble by attempting to kill the spin on the ball and hoping that the natural air currents will do their thing and push the ball around. So the question really becomes 'How can you kill the spin on the ball?'

Killing the Spin

Naturally enough, some rubbers will be better at killing the spin than others - this is why you very rarely see a normal rubber player wobbling the ball - in order to completely kill the spin he would have to exactly match the spin on the ball from his opponent - too little and the ball will still have some of his opponent's spin, too much and he will put his own spin on the ball. Only if he gets it exactly right will the ball have the chance to wobble.

Long pimples are naturally better at killing the spin than normal rubber or antispin - this is because when used correctly, the long pimples will bend around the ball and tend to grip it from all directions, so when the ball comes off the pimples they will all be trying to spin the ball, and thus the overall effect will be a float ball. I've got a detailed explanation with pictures of how the long pimples can kill the spin here.

Maximizing Your Chances of Wobbling the Ball

In a nutshell, your best method of creating a wobbling ball will be to use a long pimples with pimples that can bend enough to grip all sides of the ball, and then attempt to hit the ball in a relatively flat stroke, or with a stroke going against the spin of the ball a little. This also explains why the stiff pimples with glassy, frictionless tops but grippy sides are better at creating wobbling balls from hard loops then from softer shots - they need a certain amount of force to be able to bend the pimples around all sides of the ball - without enough force the pimples won't bend and the ball will tend to keep spinning.

So a softer, more flexible long pimples will tend to better at creating wobbling balls from all situations, while the stiffer pimples will tend to only work against shots with a certain amount of force. You could probably try to hit the ball harder yourself to cause the stiff pimples to bend against a soft shot.

As I said earlier, this is only my theory of how long pimples can create a wobbling ball, so I'd love to hear any other theories you might have - and I'd be more than happy to add them - so please feel free to add any ideas of your own below.

Next: Tips For Using Frictionless Long Pips


Tuesday 21st March 2006

Bob Sollish wrote:

In reference to your article on what makes a ball wobble I would have to say that your explanation is lacking - moving air (stray breezes) doesn't cause it. The answer lies in an understanding of fluid mechanics - in this case the fluid in question being air. (my Mechanical Engineering degree comes in handy here)

When a ball is spinning the flow of air around the ball is laminar. When the balls is moving through the air without spinning - or very little spinning - the flow is called turbulent. (picture the smooth flow over an airplane wing in a wind tunnel - that's laminar)

The ball transitions from one to the other unevenly - one side or part of the ball will lose its laminar flow and start to become turbulent - this is called "boundary layer separation." This separation on one side of the ball can create the effect of the "bottom dropping out" where the ball drops down suddenly towards the table - shorter than expected. In effect the spin on the ball has "stalled". A normally struck ball never spins down enough to do this within the length of the table.

The second effect in play here is that a ball with no spin at all will "buffet" in the air - oscillating from side to side as well as up and down, as it moves - you can see this effect clearly with knuckle ball pitchers in baseball. (Tim Wakefield being the best modern day example) In a slow motion replay the ball can be seen to "dance" left and right as it moves towards the plate. The turbulent motion with no boundary layer - will cause alternating low pressure areas on the sides of the ball (perpedicular to its motion) causing the ball to be drawn quickly to the side as it flies. The effect is identical to that of table tennis "wobble", and a baseball is clearly too heavy to be blown around by stray breezes.

Sorry about the explanation being pretty technical - but the real answers for things like this usually are.


0 #1 RE: Why Do Long Pimples Wobble?Mark 2012-05-31 05:47
What a strange idea, that table tennis ball have to wobble without spin. In such case we could reproduce the effect just throwing a ball by hand. I believe that you can trrow a ball all day long without any wobbling effect.
The turbulent theory is also inadequate. The thing is that the speed, when the flow become turbulent is approximately 4-5 times more than one maximal achievable for tt ball. Baseball theory is not applicable because of significant difference in ball sizes and quality of surface.
Moreover many people claim that they got wobble on a fast spinning ball.
I suggest that wobble effect is just an illusion. The cause is in the difference between suggested and real trajectory.
0 #2 WobbleAlbert Garcia 2013-11-11 14:31
Wobble effect I think is the transition of ball spin in flight. Normal rubber reverse the ball spin once it contact the rubber, but in long pimpled rubber if you put top spin on the incoming top spin ball, the ball spin transition will occur most of the time occur half of the flight path. That explain why some ball looks got a flat approach but suddenly it will drop dramatically to the table. I can wobble the ball most of the time esp when the opponent give me a side spin, you can alter the x and y rotation of the ball when you give top spin stroke to the incoming ball, and you can see your opponents face full of surprise, and say "What the F..."
0 #3 RE: Why Do Long Pimples Wobble?Joel Lidstrom 2013-11-23 00:55
It seems that Mark is onto something here, even though I am not so sure that he can throw a ball without any spin no matter how many times he tries! Undoubtedly a spin-less ping pong ball will move oddly due to the unpredictability turbulence. But the peculiar thing about "wobble" is that it occurs at flight speeds that are probably too low for turbulence to have caused the movement and concomitant distress that longs pips inflict on an opponent. The fact that long pips "reverse" spin--actually they don't reverse the spin as would a conventional smooth rubber--when shots are played right, they provide no small amount of movement that is exactly opposite of what the stroke "should" have produced. That is the craziness of long pips; the movement is not "un" predictable, but "anti" predictable!

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