This year we want to try to make reversible bumpers. We thought of doing that with zippers. However, we are not sure if we’re allowed to use zippers so if anyone knows if we’re allowed to please respond 
. Thanks.
I read R27 as disallowing this.
R27. Each ROBOT must be able to display Red or Blue BUMPERS to MATCH their ALLIANCE color,
as assigned in the MATCH schedule distributed at the event (as described in Section 10.1
MATCH Schedules). BUMPER Markings visible when installed on the ROBOT, other than the
following, are prohibited:
A. those required per R28,
B. hook-and-loop fastener or snap fasteners backed by the hard parts of the BUMPER,
and
C. solid white FIRST logos between 4¾ in. (~13 cm) and 5¼ in. wide (~13 cm) (i.e.
comparable to those distributed in the 2017 Kickoff Kit) and available in 2018 FIRST
Choice.
R27 clearly says “visible” so while zippers are not an exception, as long as they are not visible it’s Ok.
Looking at R27, I also agree that if the zipper is not showing it would be considered legal. However, I would very strongly suggest using hook and loop tape instead. It’s going to be much easier to assemble and work just as well.
Last year, we used reversible bumpers and had hook and loop holding the fabric to either red or blue, pulling it to the backside of the bumper. This worked well for us and we plan to use this method again for 2018.
Unless they’re in the inch nearest the robot, I would use a loop zipper - ziippers with teeth (especially metal teeth) might be considered “hard parts” by inspectors.
Here are some useful links that help you build or buy some. The bumper school link is by far the most helpful though which is this first link:
www.robopromo.com/category_s/1825.htm
These 2 have other stuff other than bumpers as well:
http://files.andymark.com/PDFs/AM14U3_UserGuide_2018withBumpers.pdf
http://files.andymark.com/PDFs/AM14U3_2018BumperAttach.pdf
My team usually use the bumper school design but we have some modifications to make it our own. Velcro works great but you need a lot other wise the bumpers might begin to switch color during a match. Also be careful with staples. My team goes overkill with them to make sure everything is attached because we drive very VERY aggressively and staples can break and pose as a safety risk but our bumpers always come out amazingly and we always get complements.
I don’t see any reason why you couldn’t come up with a legal way to use zippers, placement on the bumper could be an issue, you’d have to make sure it wasn’t a visible marking. However I wouldn’t recommend it. Think about how difficult it is going to be to zip/unzip the zipper after vigorous defense causes some of the teeth to pop open. I would just use hook and loop fasteners, they would be much easier and more robust in my opinion.