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.

[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!

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.

My Raven Tour came with a biplane fork with L1 = 410mm but shorter unicrown forks with L1 = 370mm appear to have been an option - reference from Test of Raven Tour on SJS website.  ‘Our Tour came with twin plate 531 ST forks…….a shorter 531 fork is also an option’
The RST used the shorter unicrown fork (for sharper handling?) and Thorn were happy to sell me the shorter fork when I cycled into them and explained the problem of high BB.
Did the RST have the same frame geometry as the Tour?   I know it used lighter tubes in places.

[PH]

My Raven Tour came with a biplane fork with L1 = 410mm but shorter unicrown forks with L1 = 370mm appear to have been an option - reference from Test of Raven Tour on SJS website.  ‘Our Tour came with twin plate 531 ST forks…….a shorter 531 fork is also an option’

If it works for you, it works, I remain sceptical that the designer ever intended for forks of that difference to be fitted. It's the equivalent of a 2 degree head tube angle change, and the resulting difference in trail, I'd notice that.
However, that wasn't my point. Which was the forks were 410mm because that's the length the designer wanted them to be, not because of the crown chosen.  He probably chose that length as it offers the option to swap out for short travel sus forks.
The Raven Sport had considerably different geometry, the shorter fork had the option to be swapped out for an off the peg carbon fork.  When the Tour and Sport merged into the Raven, the 370 cast crown fork was kept and the 410 twin plate dropped.

[Andyb1]

Just to confuse a little more, another page on the SJS website says that the RT can have an 80 or 100mm suspension fork fitted!  I have no idea what the loaded / unloaded L1 range of that would be.
It seems like the RT was designed with a very docile geometry so will tolerate a range of fork lengths.

Perhaps frame design is more of an art than a science, with interactions between rear stay lengths, frame angles, fork lengths, tyre and wheel sizes  etc not always possible to predict.

I was surprised that the Nomad frameset I bought a few years ago could be used with 26”, 650 or 700c wheels.

[B cereus]

I’ve never understood why thorn have always been so coy about publishing geometry tables for their frames. Critical numbers that are conspicuously absent include both head and seat tube angles. Thorn claims it’s commercially sensitive information, and hint that they alone amongst bicycle manufacturers fully understand the relationship between head angle and fork offset that governs perfect handling. I really don’t buy that.

In fairness their customer care goes above and beyond, and they do offer extended test rides for Rohloff equipped bikes and a generous 100-day money back guarantee. I was a bit surprised to see that test rides for derailleur equipped bikes are not now available, and the money back guarantee only extends to 14 days. I’m sure this wasn’t previously the case.