What reasons might prompt a fielding side to request the ball be exchanged for one of comparable wear?

Question

Occasionally, the fielding side requests (sometimes repeatedly) of the umpires that the ball they are using be changed. This is not the request for a new ball regularly available under Laws 5.3 and 5.4, but a claim that the ball has become unfit for play under Law 5.5.

To my mind though, a mishapen ball would be more likely to assist the fielding side: I would have hypothesised that an uneven ball is likely to bounce and swing unpredictably.

Therefore, why are fielding sides often so keen to have the ball changed for another ball of comparable wear?

Answer 1

The movement created by swing bowling is dependent on the air flow across the smoother and rougher areas on the surface of a cricket ball (point 12 here: Physics of cricket). If the ball isn't symmetrical in shape then keeping a "rough" side pointing in a particular direction and therefore swinging the ball at all may be difficult to control/impossible to actually do.

Out of shape balls may also have lost some hardness and have a less pronounced seam, so the fielding side my be hoping to find a better alternative from the selection of used balls. Or they might be clutching at straws and desperately trying to do anything to get a wicket (would be interesting to know how often fielding sides try to get the ball changed when they are on top in an innings).