I just learned a bit about Specialized's "Plug and Play Fender" that apparently fits my bike by way of metal rod into the leftmost hole of the first photo, secured by the set screw in the second photo.

This made me insatiably curious as to what the other threaded hole is for. The marketing materials I read before buying the diverge said that only the aluminum model had eyelets for a rack, and in this carbon model there's no eyelet at the top of the seat stays for a rack - just that one lower eyelet. It does not appear to be 1/4-20 thread, for what it's worth.

What could it be for?

Outside eyelets Inside fender setscrew

  • 1
    It may be a threading to bolt on a rack. A feature that was added to the design after the information you have read was written. One of these 'subject to changes' things.
    – Carel
    Jun 8, 2018 at 17:57
  • Maybe you can mount a rack that clips to seatpost and those mysterious holes? Jun 9, 2018 at 10:05
  • I've never come across 1/4-20 on a bike. I'd expect rack mounts to be M5, though I've seen M4 and wouldn't be surprised by M6. Even American-spec bikes use metric fasteners for most things including all accessories (yes of course there are exceptions, but they're few and far between, as well as mainly vintage)
    – Chris H
    Jun 9, 2018 at 19:42

1 Answer 1


The smaller hole is for Specialized's proprietary "plug and play" fenders, while the larger is for a rear rack. The larger hole can also be used as an alternate mounting point for other types of fenders. For a rear rack on the carbon frame Diverge, it is intended you use the Specialized Rear Rack Seat Collar (second picture), which combines a seat collar and mounting eyelets for a rear rack.

Diverge mounting hole diagram

Specialized Rear Rack Seat Collar

  • 1
    I wish I'd known those collars exist a few years ago. That would have been a nice problem-solver
    – Chris H
    Jun 9, 2018 at 19:43

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