Reinforcing Gussets & Grand Bois Fork Crown

[CWinters]

My first question here, so please be gentle.

I am wondering whether anybody here has worked with Grand Bois fork crowns, and if so, do you add reinforcing gussets? I have gotten conflicting opinions on this, and am now thoroughly confused - so naturally it made sense to go on the forum and ask for yet more opinions.

Thanks in advance.

gbforkinside.jpg

[Andrew R Stewart]

Why would you think reinforcements are needed. Andy.

[CWinters]

Why would you think reinforcements are needed. Andy.

Well, left to my own devices I wouldn't. But then what do I know. It has been suggested to me by a couple of experienced builders that a fork like this ought to have reinforcements, particularly since the bike (low trail, 650Bx42mm tires) will mostly be ridden on unpaved terrain. The (gusseted) Pacenti Artisan crown is cited as a supporting example, as well as the Sachs crowns, which are sold with reinforcements. Still, others have assured me that the GB crown is fine without them. My preference would be to just use the crown as is, but I don't want to dismiss the warnings.

[Eric Estlund]

So long as your braze them up well, you'll be fine. It's a good quality blade and a deep crown.

And don't take my word for it, Grand Bois doesn't add one for their bikes, either.

[CWinters]

Thanks for the feedback Eric.

I am comfortable with the braze, at least to the extent that I am willing to ride the bike myself.

What you're saying is basically what I want to hear. But to play the devil's advocate: Of course Grand Bois doesn't add reinforcers, since they designed and produced the crowns to be used without. But some have expressed criticism of the design.

Have you used GB crowns in your own builds?

[Eric Estlund]

Those 7 forks are ones I just put together. This is my first time using them, but the sockets are quite deep.

Many inner tangs are throw backs and are not necessary with modern materials or crown designs. Many millions of forks have been successfully made without tangs, many with crowns with shallower sockets.

To be fair- some people will express criticism of every design. The buck stops with the person putting the parts together, so if you feel better adding an inner tang (which, by the way, would be an external laminate to the blade, and not a functional part of the crown) then that's up to you. I'd be curious to the specific nature of the criticism and what problems they are being purported to solve.

Edit- This isn't dogmatic- if I feel like someone has a convincing argument, I'm open to change.

[CWinters]

Those 7 forks are ones I just put together.

Ah, got it.

...the sockets are quite deep. Many inner tangs are throw backs and are not necessary with modern materials or crown designs. Many millions of forks have been successfully made without tangs, many with crowns with shallower sockets.

...I'd be curious to the specific nature of the criticism and what problems they are being purported to solve.

Yeah, basically the opposite of this argument. That the sockets are not deep enough and that crowns like this should either be made with tangs or ought to be brazed with reinforcers, or else the blades will be overly stressed right under the crown. It basically comes down to a difference of opinion.

I should add that I am actually riding a prototype city bike right now with the fork crown in question sans reinforcements, which I designed but did not build myself. I am just being extra cautious and trying to do due diligence. I am encouraged to know that you've just made 7 of these forks and are fine with the crowns as they are. Thanks again for the feedback.

[Andrew R Stewart]

I always questioned the need/purpose for add on tangs inside blades. It seems to me their value is found between the ears, as in more/bigger. When you look at the common fork failures with classic fork crowns you don't see many that fail right at the crown's edge. Lots of drop out/blade issues, blade bending at roughly the mid point from impacts, steerer that bend just above the butt, twisting bends from angular impacts. But cracks at the blade/crown edges? Very few. Now if you don't braze well all bets are off. Andy.

[Eric Estlund]

I just snapped a crown/ surface area photo with several fairly widely used crowns.

I'll post tonight when I get home that should help reduce any concern.

[Eric Estlund]

Some crowns, as promised.
Top L GB, R PCD
Bottom L old Columbus (designed with a slip in tang), R new MAX



The total surface area of the GB is way high compared to the bottom two, and the PCD is intentionally over sized to give lots of room for carving. If you look at some older Italian style crowns, they had even less surface area then the Columbus.

The GB has something like 13mm of insertion depth at almost a full wrap of the blade.

[CWinters]

Interesting, thanks for the pics.

I will try to get up the nerve to post pictures of the brazed fork. Hopefully crown will be okay as is.

[e-RICHIE]

If the reinforce-er plate isn't up inside the crown pocket and brazed simultaneously when the blade is joined, it's only a decoration atmo. The practice, while steeped in lore, is rooted in eras when fork building (and frames, too...) was more a blacksmith-y task rather than one carried out by, er - skilled and attentive cats like us. The change over happened during the Eisenhower era when none of us here were working or even alive. It's a charming feature to have (in the fork) because it essentially strengthens the joint without 1) using a heavier walled tube or, 2) using a crown with more pocket area or, 3) saving money on car insurance by switching to Geico. That last part was levity - disregard it.

If the crown doesn't have channels cast in allowing a smooth and seamless insertion of a reinforce-er plate, don't add one on the blade only.

[CWinters]

Thanks Richard, very helpful to read this coming from you.

[e-RICHIE]

Thanks Richard, very helpful to read this coming from you.

Thanks atmo I forgot to add that, when done correctly, the procedure adds strength to
the fork but there is not the weight penalty one would have with heavier or thicker parts.

[Jan Heine]

The Grand Bois crown is a replica of the crowns used by thousands of Herse, Singer and other bikes over the last 70+ years. Combined, these bikes have covered tens of millions of miles, if not more. I haven't heard of a single failure at the crown/blade interface, and these bikes were ridden very hard in their heyday. I have ridden at least 60,000 km on those crowns myself...

I was involved in the design of the crowns, and the issue of inside reinforcing tangs came up. It would have been easy to cast them into the crown. However, I don't like to mess with something that has proven itself for so long, without understanding exactly what you are doing. Otherwise, you might introduce new problems. So we decided to make a replica of the classic design that has proven itself so well.

The Grand Bois crowns also were tested to the EN standard for fatigue resistance, and they passed without problems. Based on all this, I have total confidence in the design, and as somebody who once did have a fork crown fail on me, I don't say that lightly.

Jan Heine
Compass Bicycles Ltd.