Front wheel wobble at speed

[gbb]

How to stop a shimmy on a recumbent will vary widely from model to model because there are so many different frame geometries and steering configurations out there. Resonance occurs because of a chance interaction between vibration and flexing in wheel, frame and forks. Changing components may help -a wider, stiffer wheel, different length or more rigid forks. Maybe even a different tyre. if there is a degree of adjustment available, even the riders seating position could influence things. Experimenting with these factors may produce a permanent solution.

While I dont ride.my roadbike anymore plus this isn't recumbent related directly, I came to suffer terribly from shimmy, it got to the stage I was braking on downhills to keep my speed down...ie, I lost confidence.

But it happened on two bikes, one a 531 with good rake on forks, the other a CF Ribble. The forks on the Ribble were considered 'a bit flexi' in reviews, it would have been interesting to fit something else.
Knee against the top tube never worked for me.

Ironically, my hybrids or ebike hybrids (heavier and much heavier) never suffer, not a hint of a problem on the same hills (such as they are round here)

 

[presta]

I ride an upright not a recumbent, but in over 40,000 miles I'd never had it shimmy, even racing down off Fleet Moss at 47mph, until one day I put a stem on with a shorter reach, and then the first time I went down even a modest little Essex hill (Market Hill in Maldon) it broke into a shimmy.

 

[a.twiddler]

The bike I did my most intense mileage on in the early 80s, A Viscount Aerospace Sport, was a terror for shimmying. I had to load it carefully and it didn't like a saddlebag at all, as with any weight in it the tail would wag the dog, especially off tarmac. So it was a couple of small panniers on the back, and a couple on the front. I did thousands of miles on that bike, and learned to manage it. It was what I had, so I rode wot I got. It gave a very nice ride when lightly loaded, and taught me to travel light. (Which I seem to have forgotten over the years)! Doesn't solve the OP's problem, but the discussion just jogged my memory. Perhaps if wider drops as per gravel bike had been available then it wouldn't have been an issue, but with GB Randonneur bars there wasn't much width choice back then, certainly where I lived.