When I first bought my Rocket in 2005 the first thing I noticed was that the rear disc was always hot to touch even with no brake activity.
I removed the pads and tied the brake lever up so I could not use it and went for a ride, after 20 miles stopped and felt the disc still the same, cut a long story short once I got rid of the cat box the rear disc did not heat up as nearly as much.
The cat box throws a tremendous amount of heat towards the rear of the bike.