On the biplane crown: what does it add to the bike's stiffness?

[Andre Jute]

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.

[Andyb1]

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!

[JohnR]

The Mercury I bought from Thorn in 2020 had that type of fork but it also had 650B wheels which reduce the bottom bracket to ground distance relative to 700C with the same tyre size.

[Andre Jute]

Not sure I understand all of that

Sorry about that! Basically I'm just saying that theoretically a shorter fork, all other factors equal, is superior to a longer fork.

On a sturdy frame, like the Thorn touring frames, I sincerely doubt it matters much to the cyclist who stays within the generous load limits of the bike.

It starts to matter when in the search for the marginal gramme of weight-reduction a diamond-frame approaches noodle-like tube diameter and wall thickness.

[PH]

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.

[Andre Jute]

it's questionably how much benefit is derived from any extra stiffness [in the fork] beyond what's required.

The part-sentence quoted from your otherwise admirable post is debatable. Nobody in his right mind designs a touring bike down to some minimum, like a race-and-trash carbon bike. And if you follow that thought through, the strength* "required" for a touring bike should exceed the worst that can happen to it. The cyclist should be able to pick up his bike and continue the journey even after the earth parts and swallows him up.

* A bike or component like a fork that is stiff enough isn't necessarily strong enough: for instance the tube walls might be thin enough to dent, while a conservatively designed bike that is strong enough is usually also more than stiff enough.