This guitar rhythm strumming pattern constantly goes between a dotted 8th note and 16th note.
I always teach students to do ghost strokes in between the chords you play for this rhythm in order to maintain the best rhythm possible. In other words, you strum down and hit the chord at the start of the beat, then move your hand back up and miss the strings, then move back down and miss the strings again, then, finally, hit the strings on an up stroke.
The count for this is 1__a2__a3__a4__a
Here is the Guitar Pro 6 tab for this lesson:
HEY!!! Does Your Rhythm SUCK!?