Over this past off-season Brain Saeben surprisingly made a number of free agent acquisitions that were well received in the sabermetric community, including signing Jeremy Affeldt to two year eight million dollar contract. Affeldt has already provided 3.2 million dollars of value, so is well on his way to earning his contract. A big part of his value comes from the fact that he has allowed only one home run so far this year on the strength of his amazing 66% GB/BIP, as of last night good for third below only side-armers Brad Ziegler and Peter Moylan. That is way above his career average of 48%, although his GB rate has been increasing each year since 2006.
Usually such a high GB rate is achieved by throwing a ‘sinking’ fastball. Most fastballs ‘rise’ about 8 inches, that is they drop 8 inches less than you would expect due to gravity. A sinking fastball ‘rises’ a lot less than a normal fastball, so appears to sink to hitters. A sinker will generally have a vertical movement between 4 and -4 inches, so drops between 4 inches less to 4 inches more than you expect due to gravity. Here is the average vertical movement of the fastballs of the eight relievers with a GB rate above 60%. They are ordered along the right.
Affeldt’s fastball rises the most of the group, almost as much as an average fastball. Most of the group has heavy sinking action to their fastballs, as expected. In addition most of the group throws two-seam fastballs that tend to sink more, while Affeldt throws mostly a four-seamer that tends to rise. Affeldt’s high GB rate is very strange given this graph.
The next thing I thought was that even though Affeldt’s fastball rises he can locate it down in the zone. So I looked at the location of his pitches in 2008 and 2009.
The 2009 pitches are very slightly lower, but almost no different. Based on the type of pitches he throws, and where he throws them I do not think that Affeldt is a 60+% GB pitcher. I think that rate is very fluky and will settle back to his career average around 50%, but he is still a very good relief pitcher at that GB rate.