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.

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

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

Do you know whar frame size it is, Joe? Usually stamped into the frame, under the BB.

This topic has been moved to Bikes For Sale.

https://thorncyclesforum.co.uk/index.php?topic=15755.0