FWIW, I am very pleased with the LT Thudbuster on my Birdy. The supplied polyurethane bushes and preload work fine with my weight (88kg).

With the ex-Daewoo rear suspension bush replacement and the Thudbuster, I think the ride quality is now excellent.

Yesterday I rode a mixture of Australian country dirt and bitumen roads that I have ridden many times before and I'd rate the ride quality of the equipped Birdy as second only to my suspension recumbent trike, and perhaps better than the trike on coarse dirt (lots of little rocks and pebbles). I also find the Birdy suprisingly stable on gravel - much more so than a Brompton that I have also ridden a lot on these roads.

I had thought of swapping the rear rubber bush back to standard but I won't do that as the Thudbuster doesn't of course suspend any loads being carried on the rear carrier, so the advantage of the longer travel rear suspension still applies.

Yesterday while rolling quickly down long empty hills I also experimented with taking my hands off the steering. As others have reported, when this is done the front wheel of the Birdy instantly starts to shimmy very badly. The movement doesn't build up - it just immediately happens. It's weird because when being controlled by two hands, the bike's front end feels immensely stable. Even with one hand off the steering, at speed the stability feels marginal. Has anyone ever fitted a steering damper to a Birdy?