Terry Bahill
Dave Baldwin
Systems and Industrial Engineering
University of Arizona
Tucson, AZ 85721-0020
terry@sie.arizona.edu
http://www.sie.arizona.edu/sysengr/slides/eyeTellsBrain.ppt
© 2005, Bahill
For most pitches in baseball (excepting the knuckleball, etc.) the pitcher uses one of two grips: the two-seam grip or the four-seam grip. Pitchers often say that using a two- or four-seam grip causes the ball to move differently. When explaining baseball phenomena, we first use principles of Physics, then Physiology and finally Psychology. OK, Physics first. Most wind tunnel tests have shown little difference in forces between the two-seam and the four-seam orientation (Watts and Bahill, 2000, p. 75), although speed and spin rate produces variability (Alaways and Hubbard, 2001). Sawicki, Hubbard and Stronge (2003) summarize the literature with "the effect of seam orientation is only present at low spin" rates.
Physiological differences manifest in the grip. Pitchers can apply different finger pressures to the two-seam and four-seam fastballs, which could produce small differences in spin axes and spin rates.
However, the greatest difference is Psychological, specifically the batter's perception of the ball. The ball's appearance to the batter is different for the two-seam and the four-seam fastballs. We skewered baseballs on bolts in the two-seam and four-seam orientations. The bolts were chucked in electric drills and were rotated at 1200 rpm. The speed of rotation was measured with a stroboscope. The 2-seam and 4-seam simulated fastballs look drastically different. Critical flicker fusion frequency offers a partial explanation.
Because of the perceptual clues presented to batters, we advise pitchers to use the four-seam grip for fastballs and curveballs, but the two-seam grip for sliders.
Reference [85 and 89]. This lecture is it suitable for engineers and even the general public: it has no equations. It requires PowerPoint and a computer projector. This talk takes about a half-hour.