Yes Andy B says in the Mega Brochure for solos that he wants to keep the steering geometry of his bikes to himself. Nevertheless, that brochure must be by some margin the most detailed information ever supplied about a range of bikes.
The US brand Surly is sometimes compared with Thorn in so far as they build in steel and emphasise ruggedness. ...
In my opinion there is no comparison with Surly and Thorn.
My first and last Surly was a first year of production (2004) Long Haul Trucker. Surly refused to warranty the frame, said it was normal for a bike shop to have to clean out the bottom bracket threads. I explained that the shell was warped, Surly mailed me a pair of socks. The mechanic at the bike shop had to take a break half way through re-threading the bottom bracket threads because cutting that much metal takes a lot of muscle. He commented that he had never seen that much metal cut from a bottom bracket shell. I had to pay the bill since Surly said this was normal.
I tried it on a long bike tour, had a bad shimmy that I could not get rid of, when I got home I stripped the parts of the frame and put the frame in the metal recycling bin. Later when on a bike tour (I had my Nomad Mk II on that tour) I met a frame builder at a campground we were both staying at. I described my bottom bracket warped shell to her, she then explained in great detail how the welder had badly messed up their heat settings to get the shell that warped, and she said that it would have weakened that part of the frame and could have caused my shimmy problems.
Yeah, a lot of people swear by Surly, but I have not seen any desire by them to stand behind a product once it was paid for. I can only think of one exception to that, one color Long Haul Trucker had so much red paint (or powder coat) falling off their frames that they reluctantly warranted those frames.
My Thorn Sherpa, I bought the frame and fork used from someone in Canada (I am in USA), he said he really liked it but it was the wrong size. That was over a decade ago, I have done several tours on that bike, it handles rock solid.
My Thorn Nomad Mk II, I bought the frame and fork new in 2013. They listed one on the website as having a dent and was discounted. I bought that discounted frame. When I received it, it took me several minutes to find the dent it was so minor. And that frame is rock solid too, have done several tours on it.
To summarize, Surly sent me a pair of socks for my troubles on a frame that I eventually put in the trash, Thorn gave me a discount for a cosmetic blemish that I had trouble finding.
Thorn is not perfect, some of the clear coat on my Nomad Mk II has come off and the light mount on the fork is aimed off to one side instead of straight ahead. I did not pursue any warranty claim on the light mount or clear coat. From what I have seen they stand behind their product after it leaves the door.
I was disappointed in a pair of rims that SJS sold me, but that is a different story and that was not a Thorn product.