nearly 20 years ago I had a TerraTryke, "3.2" I think it was. The lack of high speed stability led me to do the same motorcycle steering damper thing you have done here. I felt like on that (non-motorized) trike, it helped but only marginally.
My current trike, still an older model, is direct steer, and was always very stable at speed. Interestingly, I did add a steering damper to it, not for the high speed stability, but because at lower speeds and over rough roads, the steering would develop a "shimmy" or oscillation - this because I also added a mid-drive motor on the boom and the weight shift forward was causing the oscillation. For this purpose, the damper entirely cured the problem. I had to mount it a bit strangely as the steering geometry is different than a TT, and the damper is not parallel with the cross bar or tie rod etc - works perfectly. I found an aluminum ring designed for motorcycle mounting of these things that was the same size I needed for my frame.
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