Why does a Shimano centerlock rotor have free play? I have an electric road bike that has SM-RT64 Shimano centerlock
brake rotor in the front. I have noticed that when I press the front
brake and try to rock the bike back and forth, there's some free play
somewhere in the braking system.
I checked for free play in the headset to find nothing.
I checked hub (HB-R7070) bearings for looseness but they weren't loose.
I checked that the disc brakes have the pad retaining screw tight.
When I rock the bike back and forth, it doesn't seem to be the brake pads that
are loose. The looseness seems to be at the centerlock brake rotor. It is not
the fixed rivets connecting the spider to the disc that are free; instead, the
brake disc seems to be rocking back and forth at the hub, and the lockring
seems to be rocking along with it. Thus, the brake disc - hub connection is of
suspect.
Related information found online:


                
            
https://forum.cyclinguk.org/viewtopic.php?t=116894
https://forums.mtbr.com/wheels-tires/front-hub-play-980620.html
https://forums.mtbr.com/brake-time/xtr-disc-brake-play-386036.html
https://forums.mtbr.com/29er-components/disk-has-1-2-play-when-braking-914595.html
https://forums.mtbr.com/brake-time/linear-centerlock-play-792697.html
https://forums.mtbr.com/brake-time/centerlock-rotor-slop-1055472.html
https://forum.bikeradar.com/discussion/12768497/play-in-centrelock-rotors
https://forum.bikeradar.com/discussion/12719436/rotational-movement-on-centre-lock-disc-brakes-normal
https://www.bikeforums.net/bicycle-mechanics/360397-little-play-front-disc-normal.html
https://www.bikeforums.net/bicycle-mechanics/1121066-centerlock-rotor-slop.html

                
            
https://www.pinkbike.com/forum/listcomments/?threadid=177747
https://www.reddit.com/r/MTB/comments/3wajho/my_first_mountain_bike_has_a_loose_disc_brake_is/
https://forums.mtbr.com/brake-time/play-between-disc-hub-centerlocks-710730.html
https://forums.mtbr.com/brake-time/loose-rotor-centerlock-adapter-369055.html

What could be the cause?
Is the brake disc defective?
Are the aluminum centerlock splines in the hub damaged? (I hope not as it took
about 5 hours to build the front wheel, and swapping the hub to another would
take another 5 hours).
Or is the centerlock design just too weak for the massive braking forces I need
(my weight is over 100kg)?
The cause is a missing aluminum shim between the centerlock lockring and the
brake rotor.
I removed the brake rotor from the defective front wheel. Then I removed a
brake rotor from a known-good front wheel. When closely inspecting the
lockrings, I noticed something the known-good front wheel has that the
defective front wheel didn't: there was a very thin aluminum shim between the
centerlock lockring and the brake rotor.
When I built the front wheel, having had no prior experience with disc brakes,
so I thought the aluminum shim is something that should be removed and the
centerlock lockring should be tightened without it.
Now I have swapped the lockrings and the brake no longer has free play.
The missing shim probably has been thrown out along with garbage, so I have
to purchase a shim somewhere.