Not sure I understand all of that…..but the basic problem with the longer biplane fork for me was that it raised the bottom bracket height too much.  The alternative 40mm shorter Thorn fork I now use allows me to have the correct pedal - seat distance and still be able to get a foot safely down when stopping.  Essential on sloping ground and when carrying luggage.  I am sure both fork types are well strong enough for my riding speeds and style!

Forks are measured crown race to axle (C>A) that's the only length that effects geometry, it makes no difference how much of that is crown and how much blade.  I think there must have been some misunderstanding with your bike, I doubt Thorn has ever offered forks with a 40mm difference intended for the same frame. A twin plate crown fork may need to be slightly longer to maintain the same tyre clearance, a few mm, but as this link shows they don't take up much more room than any other design.
https://www.framebuilding.com/NEWPARTSPAGES/Cast%20Fork%20Crowns.htm

I think their present use is largely aesthetic, they are measurably stiffer, but not as stiff as a unicrown and it's questionably how much benefit is derived from any extra stiffness beyond what's required.  They were popular at one time because frame builders could make their own rather than buying in cast crowns, there was also the idea that they could be used with lighter blades to add compliance. I think it may also be possible to weld the blades in place rather than braze them, which might explain their popularity with Thorn's frame builders.

I've got a barely-used C17 but know people who are happy with them. Those people weigh somewhat more than my 64kg / 142lb and I think that's a factor. Few saddles have versions to cater for different rider wights.

Forgive this being probably my first and only post, but I bought this on a whim hoping my wife and I could spend more quality time with each other. When I presented her with the idea along with a jaunty rendition of  “Daisy, Daisy, give me your answer do..”  – her answer was a hard “NO!”
So, ultimately what we have is a very redundant early-generation Thorn Discovery touring tandem, which you’ll doubtless know has a UK-built Reynolds 531 frame, finished in the factory original cobalt/metallic blue. It is a well specified example of one of Thorne’s excellent long distance touring tandems.
I don’t know the correct way of sizing these frames, but the captain’s frame tube is 22 inches from the centre of the bottom bracket to the saddle clamp and the stoker’s is 20 inches. The seats are both in excellent condition, the stoker’s being a sprung post item. The rims are wide Gordo items running straight and true, and both tyres are in excellent condition.
Brakes and shifters are Shimano Deore XT kit, and the chainset/deraillieurs are Deore XT 3×9. Everything works as smoothly and seamlessly as you’d expect. The crankset is Thorne/Talon, with 170mm cranks and TCM04-48 48-tooth outer chainring.
Being a pretty serious touring tandem, it comes complete with front and rear carriers, and the corresponding Ortleib Roller Classic bag system. Everything you need to just pack up, jump on and go. The icing on the cake must be the Mickey Mouse bell though!
The tandem had been in storage for some time but has cleaned up well. The blue tape on the captain’s top tube was to protect the paint from his belly bag – it’s just not been taken off. I’ve given everything appropriate lubrication and adjustment but it must be borne in mind that it’s not new and has been previously used, Please let the pictures tell their own story.
I’m not sure how many photos I can post here, but feel free to message me if you need more, I’ve got plenty of hi-res images I can send by email. Collection only from Wimborne for this one, but I work from home so am always around.
Price wise – I’m looking for around £950 for the whole lot.

Not sure I understand all of that…..but the basic problem with the longer biplane fork for me was that it raised the bottom bracket height too much.  The alternative 40mm shorter Thorn fork I now use allows me to have the correct pedal - seat distance and still be able to get a foot safely down when stopping.  Essential on sloping ground and when carrying luggage.  I am sure both fork types are well strong enough for my riding speeds and style!

Quote from: Andre Jute on August 12, 2026, 10:29:28 PM

Joe's photographs remind me how much I loved that biplane fork...

Interesting - why?

Partly aesthetics: the biplane crown looks good and adds interest to the frame. It's also good engineering in that the wider and deeper crown, fixed three times along its length across the bike and another three times on the second plane, adds considerable stiffness.

I changed mine to a shorter Thorn steel fork as that design of fork was too long for me.

A shorter fork with a single-level crown would by the dimensions be stiffer than a longer fork of the same material and general construction. The twisting force on a fork, say cornered hard on a twisting downhill, would have a lever arm of the length from the centre of front axle diameter to where the head tube meets the crown, with differential loads on the two legs of the crown. In the turn the outside leg will be less heavily loaded and perhaps would offset some of the stress on the crown bearing. With a loaded bike with lowriders the panniers would add stress, but with a shorter lever-arm from the mounting sockets of the lowrider frames to the same crown race/head tube interface.