Our folding rehearsal chair has rubber stoppers which allow for the seat to be placed in many positions. The eight rubber bumper/stoppers are a wear item and can easily be replaced if it becomes necessary. So we have replacements available here if you want to add 'em to your gig bag!
In testing, I found that these replacements actually are more secure than the originals that come installed on the chair.
The rollers fit into the chair as shown - highlighted - to the right. Offered as a complete set of eight. You will re-use the bolts and washers from the chair, and you will need a small set of pliers and/or an allen wrench for installation; but replacing them is simple and self-evident, and should only take about 5-10 minutes.