If a player can bowl above 85mph – or if the ball has aged – this
makes it possible for the bowler to “reverse swing” the ball. In simple
terms this is exactly what happens – a delivery that is expected to
swing away from the batter (an outswinger), instead swings in towards
them (an inswinger) – and vice versa. This happens when the seam is
angled for conventional swing but this time the rough side faces the
batsman. If the ball is bowled fast enough (above 85 mph), the laminar
layer goes into a turbulent state before it reaches the seam. The seam
actually has a negative effect here and affects the turbulent layer so
it separates earlier compared with the bottom surface.
Like in conventional swing, there is a lateral force created because
of the pressure difference but this time the lateral force acts in the
opposite (reversed) direction. As mentioned, there is a critical speed
to help generate reverse swing but in cases where this speed can’t be
reached, the roughness of the ball can help reduce this critical speed
needed to create this reverse swing.
As the ball gets older (which means the surface is rougher) it
becomes easier to deliver balls with reverse swing. The fact that more
roughness makes it easier to create reverse swing introduces the
temptation for players to increase this roughness illegally and thereby
make it easier to generate reverse swing at lower speeds.
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