work your thumb out....

get one of those little squishy sand filled ***** you squeeze in your hand and work the heck out of your thumb...... build the muscles up enough and you can ride all day with no thumb fatigue at all............

 

Your thumb shoulg get used to it after a few hours. I am a new TAV rider and had that complaint for up to the 8th hour (accumulated hours) of riding. I've tried the thumb assist pro but didn't like it since I couldn't feel the throttle. And on the 9th hour of riding, my thumb finally got "brokien-in". It lost is soreness and most important of all the numbness. It's so much more fun now. Be patient with your thumb.

 

I modified my thumb piece, so it can be rotated to any position, and locked there. This way i can make the starting position as close as i want , to the grip. This has an added benifit, because now its easy to position my first finger, (from behind the grip), and under the thumb piece, so when my thumb gets sore, i can use my finger, and give my thumb a break...with out having to stop.
If I need to change the position, it only takes a few seconds, while stopped for a break along the trail.

 

X2