Front end shimmy, trail and weight distribution

[Vincentsavary]

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

[hampco]

What's the frame material and, if steel, any idea what the tubing is? Under-sized top tubes can cause or add to shimmy, imhe.

[Vincentsavary]

It is indeed steel, fully Columbus Cromor, which I would think is adequate but I'm also not a framebuilder

[Mark Kelly]

Have you tried pumping the front tyre up to a higher pressure?

[hampco]

What Mark said - and is the headset spinning freely, not brinnelled? This was interesting: https://en.wikipedia.org/wiki/False_brinelling

Cromor is pretty beefy but if it's a 1" top tube that could be the culprit.

[Vincentsavary]

I checked the headset, no loose and spins freely, and it's a White Industrie headset so I would assume they use good quality bearing that wouldnt cause false brinelling (but interesting article indeed). I also checked the front hub and brake, and everything was fine. Front tire pressure wasnt particularly high but still higher than I normally run. And top tube is 1-1/8".

I could be missing something but I'm pretty sure the shimmy doesnt come from a mechanical issue, mostly because it never did this before and doesnt do it when un loaded.