Mattyb13

2017 Specialized Diverge Comp Carbon.
Shimano 105.

Weird issue.
When I am standing and lock my rear brakes I can still rock the bike back and forth a little bit. The caliper is holding the disc tight but there is play somewhere. My thought is the disc might be loose, but I dont think so. Could it be deeper in the hub or cassette? thanks.

Caliper

There is likely a bit of play in the pass inside the caliper.
Also, watch the chainstay and caliper mount, carbon will flex some when you do what you describe.

Psimet2001 

1. Figure it out and don't ride the bike until you do.
2. Headset. - With brakes on - rock the bike with your fingers over the transition between the head tube and the fork. If anything moves while rocking then it's your headset. Too many shops don't tighten them enough and they do settle in a bit through riding on a new frame.
3. if nothing in the headset then look at the rear of the bike while rocking it. See what moves and what doesn't. If the wheel moves and the rotor doesn't then you have a problem. If the caliper moves - you have a problem.
4. Check the axle for play. Make sure it's tight.
5. Hold the bike up by the saddle and grab the rim in the rear. Push it to side to side and see if there is any lateral play int he wheel.

isolate the issue and fix it before riding it again. Death is a bad thing while riding.

RedBullFiXX

I've seen this issue with hubs designed for Centerlock rotors, mounted with 6-bolt adapters
Some bikes come out of the box with this unfortunate setup