Joined: Aug 2010
Posts: 12,727
Likes: 2,105
From: Madison, WI
Bikes: 1961 Ideor, 1966 Perfekt 3 Speed AB Hub, 1994 Bridgestone MB-6, 2006 Airnimal Joey, 2009 Thorn Sherpa, 2013 Thorn Nomad MkII, 2015 VO Pass Hunter, 2017 Lynskey Backroad, 2017 Raleigh Gran Prix, 1980s Bianchi Mixte on a trainer. Others are now gone.
That rack is very very stiff, but you do not appear to have the mid-fork braze-on fittings for the upper mounts. Thus, your play is probably at that location with the adapter fittings that you used. I used to use that rack.
Racks and side to side motion are one of those things that some people are more willing to live with than other people, so I can' really say what is acceptable and what is not.