I now practice doubles, and RLRRLL repeating patterns. I also practice heel down AND heel up. I start off each day very slow with 16th note doubles at 150 bpm and some days I finish off playing singles (double triplets) and the RLRRLL pattern at 210 bpm. The RLRRLL pattern sounds much like insane triplets when played at 210bpm. You can find videos of all of these exercises in the FREE section of this website.
My speed has jumped up dramatically since trying this regimen.
4 FOOT PLAYING
ROCK DOUBLE TRIPLETS
180 BPM DOUBLE TRIPLETS
HOW TO BECOME A PAID SUBSCRIBER
2 minutes and 40 seconds of UNCUT insanity!