The Hewlett Packard 85 used a capstan in the disk drive to move the DC100 tape wheel forward and backward. The original rubber used on the HP-85 computer capstan becomes gooey and soft after 30+ years. (Below)
Below is a picture of the wheel on the DC100 tape that the capstan turns.
To make the capstan grip the wheel of the HP-85 capstan I used a 3/8 OD surgical tubing to create a new capstan wheel grip.
The inside of the tubing has more traction to it so I turned it inside out and cut a piece roughly 3/16ths wide to fit onto the capstan.
The wall of the surgical tubing is thicker than the original capstan rubber but it is far softer so it does not put too much stress on the wheel of the DC100 tape cassette.
There are other ways to fix the HP-85 capstan gooey wheel but this is fast, easy and works like a charm :)
Below is a video of the process.
I am surprised to discover that many of the original tapes like the Hewlett Packard Demonstration Tape and the Math Pac tapes work after 31 years (2011). Thanks really needs to go out to the people who created sites for the early Hewlett Packard computers. Listed below are the sites that have helped me out in restoring this computer.