I went touring last week and my bike did something it never did before: front end shimmy. The bike without any load feels a tad twitchy (but not even enough to be considered an issue) and putting the slightest load at the front usually fixes the “issue” and makes it very stable and it traces really well. Because of this, I tend to load it up more at the front than the rear when going on tour. But last week, for the first time I was a bit heavier at the rear than the front, and the shimmy appeared. Nothing crazy, but enough to be annoying. To be clear, by “shimmy” I mean the steerer having a hard time keeping straight, and doing slight but fast left-and-right after, say, hitting a bump or riding one-handed. On the second day I shifted a little bit of the weight at the front and it helped, but didn’t fix the issue. Now I’m wondering what is causing this? Could it be because the frame has a relatively low trail for a loaded bikepacking bike? I would think that the center of gravity in relation to the wheelbase might have something to do with it too? To me, instinctively it makes sense that rear load affects steering, but I just never really thought much about it and can’t really visualize how that works. I consider it a minor inconvenience, I won’t do anything serious about it, but I’m just curious to understand the phenomenon. Any insight would be appreciated!

Here are the numbers I assume are useful in this situation:

HTA 72deg
Rake 50
Trail 63
Tire size 650b x 2.3”

STA 73
Chainstay 440
Front center 627
Wheelbase 1070