My first failure and what I've learned

I built this fork about two and a half years ago and I've ridden it pretty hard. It rode very nice, but Friday I went to stand up on a climb and my front end was lowered gracefully to the ground.
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The biggest thing I did wrong was drilling a vent hole in the underside of the cross tubes. That's where the cracks started.

I also saw a few months ago that the powdercoat was flaking off one of the cross tubes and didn't inspect it. I just chalked it up to a crap powdercoat job. (Which it is)

The cross tubes are 7/8" .049 4130. Since I built that fork, I've noticed that every other one I've seen uses larger tubing, like 1" or 1 1/8".


Opinions please: had I not drilled the vent holes, would the fork still have failed? I'm thinking of stepping up to 1" .058 or .065 for the replacement.
Also, I did one other fork like that but I haven't ridden it yet. It's a road fork, and the tubes are the same size (except the cross tubes are shorter due to the narrower tire clearance) but the vent holes are drilled into the leg and the steerer. Should I burn that one as well, or ride it?

First off Eric, props for sharing this, I think it will spark a good discussion.

IMSBHO (in my small but humble opinion) If the leg sleeves were .058 and the steerer was at least .083 then the short cross tubes should have been the same as the sleeves with the (necessary)vent holes into the sleeves.
I find it interesting that it folded the direction that it did, you would think that it would come back at you. I guess its good that it didn't go on some rippin descent. How far from your truck were you?

Eric,

First off, props to you for setting aside any prideful holdings and posting up your issue for the benefit of others. So many don't recognize the potential benefits of failure analysis, rather hiding broken equipment under the bench away from the eyes of others.

The strength of a segmented fork relies on a few design/fabrication parameters...the choice of the tubing for each part, the tightness of the miters, and the ability to join dissimilar wall thicknesses with proper heat proportion. Each of these elements act in a synergistic relationship to properly distribute stress and contribute to performance and durability.

As you surmised, there are some changes I would recommend to increase the longevity of your finished product.

- the selection of tubing for the crown pieces needs to lean towards a larger diameter and heavier wall. The pieces are so short that any weight gain is often insignificant, but the structural strength gained is measurable. I typically use 1.125 x .049 or greater wall dependent on the rider weight and anticipated riding conditions.

- The mitering process MUST provide for tight miters in the anticipated angle to create the desired offset. I've created dedicated fixturing to allow me to hold these short crown pieces during cutting.

- The creation of vent holes are placed inside the diameter of the crown sections in the steerer tube and leg, where the removal of material is reinforced by the larger surface area of the tube joined around it. Alternately, I've seen the leg vent hole replaced with the sequential welding of the top cap last.

- Joining methods are critical, as you must be able to distribute the appropriate amount of heat to the dissimilar wall thicknesses without weakening the thinner tube while still attaining the proper penetration of the larger member.

Forks are one of the most critical pieces to get right in both design and fabrication, an opinion often not recognized by many new builders as they seem so simplistic.

Your self analysis is right on target...the placement of the vent hole in the inferior margin of the crown pieces, where torsional stress is transmitted from the legs to the steerer was a critical mistake and led to it's ultimate failure. You are quite lucky that the yield was as gradual as experienced.

cheers,

rody

Posting this in the ever so overloaded opinionated thing known as the INTERNET, is brave. And now we can all learn.
Rody hit it right on. When I do a fork like this, if someone even says: can you use thinner tubes, to save weight ?" . I don't build it for them. 2 ounces vs new teeth, weight should never be a factor.
When I do make forks like this, I tend on the thicker side of things. I like to use at least one inch .058 / .069 for the struts. I use the bigeest I can.
Thanks for sharing, and we can all learn from things like this.
And thank the stars you are ok!!
erik

yes, thanks for sharing eric. screaming good luck with the way she unraveled.

and thanks for your insight rody.

as to the direction of the failure, that's the direction of the most regular stresses unless one has a frontal impact yo.

Yeah, this definitely goes into the "could have been much worse" file. I rode 8 miles to the trail head, but fortunately my girlfriend answered her phone and came to my rescue. I've got some 1 1/8" tubing on the way for another one, and I surely learned way more from breaking this one than I did building it.Hopefully someone else can learn from this too, without actually having to spend $80.
Yesterday I built a replacement with Verus blades and a Pacenti crown, but I didn't get good penetration cause I think the gap between the crown and steerer was too tight for brass and I struggled keeping everything evenly heated. I'm will most likely cut it up sometime this week, and give it another shot soon.
On the good side, the dropouts turned out really well so I think I can trim them just a tad for use as a bottle opener. Two very lucky friends are gonna get a very pimp gift:)

I was considering building some of these for myself, this is the last place I would expect them to fail, interesting. Of course, the vent hole was in a very stressed location, whereas inside the leg or the steerer wouldn't see as much stress.

I somehow got in the middle between a customer and a framebuilder in an incident where the fork deformed under braking. This customer was running around the interwebs getting revenge. I haven't seen that fork, but I'd really like to. Framebuilders should be very sure of their capabilities and design before they start selling forks. Of course, that's true of anything, but especially forks.

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Originally Posted by EricKeller

this is the last place I would expect them to fail, interesting. Of course, the vent hole was in a very stressed location, whereas inside the leg or the steerer wouldn't see as much stress.

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Yeah, I think I'd still be riding it had it not been for the vent holes. It'll turn out for the best, because the next fork will be an improvement. I'm going to run the vents through the steerer, through the leg and I'm going to drill vents in the bottom of the Breezer style dropouts, so they will all be wrapped by a tube joint. I have a decent camera now, so I'll probably document and post the build.

Btw, thanks for everyone's input, especially Rody. If you come to Austin, I'll buy you a couple of whatever you drink. (And bring a bike to ride)

Good Morning,

nearly all of my forks are segmented and I also repaired a few Fat Chance and european ones.

The Fat Chance ones have super thin segments: I found 1.125"x0.7mm here. They hold up fine until crashes. The forks were pretty short, though, which helps.
Another one was 0.8mm wall thickness and it hold up during a crash when the 1" steerer bent.

I use 1"x1.25mm for road and 1.125"x1.25mm for mountian usually with exceptions if I expect extremely heavy usage.

I would not place a vent hole in the steerer, because water will get in an will stay in the segments and blades. I have vent holes to the blades in the center of the segments. And I have vent holes at the bottom of the blade which I close on Yo Eddy style dropouts as the last step.

I think the Fat Chance ones are too thin for todays riding and todays often longer forks. Also the customers have changed, at least in Europe: Whenn I started Mountainbiking in 1987 it was rather cool to break stuff. If you bent your fork you upgraded to a Bontrager or something else. Today nearly everything is a warranty issue.

I think this threads fork would have hold up without the hole, but the 7/8" segment diameter is too thin too. There is no neutral zone on parts with torsional stress, so any defects of the surface a forbidden. (Of course the segments have not only torsional but complex stress). There is no "neutral zone" on them.

Greetings,
Georg

Interesting info on the Fat Chance forks, thanks.
On another note, one of these pics showed up on some Japanese guy's blog. The power of linkbacks, eh?
Anyone read Japanese better than Google Chrome?
chirosangaku@tumblr

Eric - Props for posting. Take heart in knowing that every builder on here has had failed frames, etc. It's all part of the process. We keep a wall of shame here to remind us of our mistakes.

I finished the replacement for the broken fork today. Lessons learned, all the vent holes are drilled internally, and the cross tubes are made from leftover steerer tube. 1 1/8" x .065 Plus, the fillets are smoother and the brake mount is one of those slick Paragon units that goes halfway up the fork leg.
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Planning a test ride tomorrow. I hope it's not too stiff.

hey, i'd ride that! sounds and looks well-re-engineered.

well. for a little bit..i beat my hands up enough off the bike that i rarely go off road w/o a squisher up front.