Here's one more heavy riff. There are actually only a few 16th note triplets in this one, but it's a good exercise nonetheless. In the notation, look at the first beat in the first measure. The first half of the beat has an 8th note, while the second half of the beat has a group of three 16th note triplets. This also occurs in the 3rd measure.
Practice this one slowly, and make sure you can identify where the 16th note triplets are. Once you get it down, be sure to create some variations!
Also, this lesson's riff is in Drop D tuning. If you aren't exactly sure what that is, click here.