One of the big Tampa Bay Rays storylines last year was Grant Balfour. The f-bombing, hard thrower from Sydney, Australia captivated the Rays fanbase with his hard throwing, gunslinger mentality. Balfour easily had the best relief season in Rays history and he did it with essentially one pitch. Therein lies the question. Everybody loved what Balfour did in 2008, but can he live off the fastball or does he need to mix his slider in more? The answer seems to be a bit of both.

Below you'll find a list of Major League relievers (min.40 IP) who threw a fastball over 80% of the time.

Okay, now let's analyze the data. Off the bat there are three names that don't fit with the others: Brad Zieger, Joe Beimel and David Riske. Surprisingly, they rank second, third and fourth on this list, however, none of the three throw over 90 miles a hour. As you can see they are also the only three who have K/9's below 9.72. This gives some credence to the theory that the most important aspect in strikeouts is velocity.

Since Balfour is a hard thrower it doesn't make much sense to leave these relatively soft throwers in the equation. That leaves us with this list

Again, there is a name that sticks out as just not belonging. Even though the Orioles' Dennis Sarfate throws hard he doesn't throw with much control. Of this list he is the only one with a K/9 under 10 and a K/BB under 3.

After cutting Sarfate from the list we are left with five pitchers. From this list we can easily find another name that doesn't mesh well with the others. While Octavio Dotel and Matt Thornton are the only teammates on the list, he is the only one with a HR/9 greater than 1.0 and is the only one missing a sub 3.00 FIP.

We now have our final four: Balfour, Kuo, Thornton and Papelbon . Balfour, Kuo and Papelbon finished 2008 ranked in the 10 in WAR by a reliever. In fact, Papelbon and his 3.0 WAR and ridiculous 9.63 K/BB rate rank only behind the great, Mariano Rivera in each category. Speaking of Rivera, while he doesn't make the list of fastball throwers; He threw his famous cutter 82% of the time last year and has thrown the cutter/fastball a whopping 99.7% of the time in his hall of fame career.

Now back to the list:

Matt Thornton is a surprise to many, but his numbers have steadily improved each year and his BABIP of .276 in 2008 compared to his career .300 BABIP only suggests a slight regression in 2009. Hong Chih-Kuo is not a household name, but his 1.99 FIP as a reliever was tops in the National League and only 0.02 behind Joey Devine for the Major League lead.

The biggest difference between Balfour and the other three pitchers is the others had at least one other pitched they used over 10% of the time. For Papelbon, it was just another variation of his fastball, a split finger, but even that had about six miles in separation with his regular one.

Like Balfour, Kuo and Thornton featured sliders. The difference between Balfour and the others is they threw it over 12% of the time. For his career, Balfour has thrown the slider 9.1% of the time, but only 6.6% in 2008. Because of his expected BABIP regression we want Balfour to get closer to Thornton and Kuo who saw less drastic drops. While both saw decreases from their career BABIP's, neither was as drastic as the 69 point drop that Balfour had in 2008.

If we use the same data from 2008, 58.1 innings and 1027 pitches, Balfour would need to throw about 55 more sliders or roughly one more per inning to get to the 12% plateau. It hardly seems like a lot, but one extra bendy pitch per inning could go along way in Balfour sustaining his success as a fastball first pitcher by potentially keeping his BABIP in check.