{"id":106,"date":"2020-12-24T19:56:44","date_gmt":"2020-12-24T19:56:44","guid":{"rendered":"http:\/\/www.wiredcola.com\/wifi\/?p=106"},"modified":"2021-04-21T17:34:17","modified_gmt":"2021-04-21T17:34:17","slug":"fork-failure-fun","status":"publish","type":"post","link":"http:\/\/www.wiredcola.com\/wifi\/2020\/12\/24\/fork-failure-fun\/","title":{"rendered":"Fork Failure Fun"},"content":{"rendered":"\n

A friend had an aluminum fork fail not too long ago, and let me take a look at the bits to analyze, so I thought I\u2019d share it with you. This fork failure was likely very typical, but it had some fun surprises.<\/p>\n\n\n\n

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The fork in question <\/figcaption><\/figure>\n\n\n\n

The fork in question is a name-brand all-aluminum cyclocross fork. Sized for 700C wheels, cantilever brakes. This fork was well-used for many years, and failed in classic \u201cJRA\u201d fashion, just as the rider was starting off from a stop while on an ordinary road ride. Both legs sheared off just below the fork crown, as you can see above, and the rider was not seriously injured.

Lucky thing, as fork failure is horrible and can wreck your whole life if you\u2019re unlucky. I do not recommend it.<\/p>\n\n\n\n\n\n\n\n

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A close up of the crown and the sheared areas. You can see the construction of the crown<\/figcaption><\/figure>\n\n\n\n

There\u2019s various ways to make a fork like this, but this one was built in a simply and effectively: the crown is a fairly regular cylindrical shape, and the pre-formed one-piece aluminum legs are welded to the crown. The welds are not perfect, but look strong and did not fail. <\/p>\n\n\n\n

There are other ways of building aluminum or composite forks. You can even glue the parts together, not as crazy as it sounds. This fork is built simply, but the design is sound.<\/p>\n\n\n\n

Both legs failing at the same time while under no serious load sounds surprising, and it should be. For one thing, IT DIDN\u2019T HAPPEN. This failure was baked in some time before the last ride, and this kind of failure is common, and you can see it in \u201csudden\u201d failures of metal parts in all kinds of situations. <\/p>\n\n\n\n

What follows is going to look like an engineering analysis but I can promise you: I am not an engineer, a technician, or in any way qualified to discuss this professionally. I can\u2019t even tell you the relevant math. With that disclaimer, let\u2019s do some non-engineering not-really-analysis!<\/p>\n\n\n\n

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Crown area left leg, close up view of the interior. I\u2019ve done a bad job of photographing it, but trust me that the bright white metal on the left side of the crack in this image is fairly contrasted with the dirtier metal seen on the right side of the crack (again, from this upside-down photo perspective)<\/figcaption><\/figure>\n\n\n\n
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Left fork leg top, the bit that stayed with the crown, looking at the exterior. Note the extremely meaningful paint bubbles. <\/figcaption><\/figure>\n\n\n\n
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Another view of the inside of the top of the left fork leg. Still hard to see the contrast in the metal coloration, darn it. And boy that sure looks like the crack extended past the failure zone, huh?<\/figcaption><\/figure>\n\n\n\n
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Left fork leg again, this time the broken-off leg, seen from above. Note the interior metal is fairly grey and clean, except for black discoloration in the top left of this view of the fracture area.<\/figcaption><\/figure>\n\n\n\n

When a specialist sees metal parts fail like this, the first thing they\u2019ll look for is signs of the initial failure, and this is a textbook example. If you look at those photos above (and the captions) you\u2019ll see I keep talking about dark discolored areas. That\u2019s where the part initially failed. You get a small crack that forms in the part somehow, but the part is still strong enough to hold together. However, the crack is there, and starts to flex, wear, propagate, and get dirt into it. So you get a distinctive area of the fracture zone that just looks older, because it is older. In this fork, the rear part of the left leg shows this distinct signature for a few millimeters of the fracture zone: on the inside and on the fracture surface itself, dirty areas indicating this isn\u2019t metal that was recently exposed to the world. On the outside, those subtle paint bubbles, which sometimes are just paint bubbles, in this case indicate a failure of the metal underneath. I don\u2019t know how obvious all of this would have been when the fork was cracked but not yet failed. The paint bubbles were definitely there to be seen, but the crack may have been nearly invisible without aggressive inspection.<\/p>\n\n\n\n

There\u2019s not much \u201cplastic deformation\u201d in this failure. The metal hasn\u2019t been stretched out of shape much from the original form, except at the very front of both fork legs, which I take as meaning the failures started at the back of the fork legs, and the fronts only broke loose when the crack was so far open the shape of the fork was deforming, which would be the fractions of a second before total failure.<\/p>\n\n\n\n

This crack has formed at a pretty high load point: without getting over my head with load and force diagrams, there\u2019s a variety of forces that literally hinge on the top of the fork legs, above the cantilever brake posts and below the lower crown race, where the weight of the entire bike and rider meets the fork. The back of the fork leg, where I think the problem started, is under a lot of tensile loads even when a bike is just loaded up with a rider and not doing anything else, and I think that area gets a lot of dynamic loads when the bike is being used, so it\u2019s a natural weak spot for a failure. All that means in this case is there probably wasn\u2019t a factory defect in the metal: the most stressed part of the fork broke. You might think that\u2019s a failure of design decisions, but my guess is the fork is designed so that the place where it failed is naturally the strongest point: if you make that part of the fork leg strong, then the nature of economically building the part means you probably end up with a metal tube that is plenty strong all along the rest of the leg (even if you only need half the wall thickness halfway down the leg, it\u2019s a fussy operation to create a variable-wall-thickness formed aluminum tube like that, and by the time you cost out that part, your engineer tells you it would be cheaper, stronger, and lighter to make it out of carbon fiber). <\/p>\n\n\n\n

Anyway, this is how metal likes to fail, quite often: a small crack starts, it doesn\u2019t fail abruptly, but it does continue to fatigue, and cracks can propagate fairly effectively, so over some amount of active load cycles, the small failure becomes a total failure. In this case, the back of the left fork leg got a little crack, repeated use of the bike made that crack get a bit bigger and the left leg a bit weaker, until finally it was weak enough to really flex, at which point the load on both fork legs was too much and everything \u201csuddenly\u201d failed. QED, where\u2019s the tea?<\/p>\n\n\n\n

Now let\u2019s look at the boring right fork leg, which only failed because its twin cracked up.<\/p>\n\n\n\n

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the top right fork leg, the bit that stayed with the fork crown. You can see a bit of discoloration at the very back of the leg, but do you see the Easter egg yet?<\/figcaption><\/figure>\n\n\n\n

There is a small discolored area at the back of the top of the right fork leg, which leads me to believe this crack formed earlier than the last instants of failure, but the relative lack of discoloration in the fracture surface, the lack of external paint bubbling, and such makes me think this was the secondary failure, and probably didn\u2019t happen that long before the final failure. I am not an expert; an expert might be able to estimate whether this failed days, minutes, or seconds before the end.<\/p>\n\n\n\n

But we get a bonus in this right fork leg.<\/p>\n\n\n\n

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It\u2019s a crack!<\/figcaption><\/figure>\n\n\n\n
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Baby crack. Not as cute as Baby Yoda, also, it might try to kill you.<\/figcaption><\/figure>\n\n\n\n
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Baby crack from the outside. It was going to kill the fork, but got beat to the punch.<\/figcaption><\/figure>\n\n\n\n

But there was a second killer! It got beat to the punch. The right fork leg was grievously cracked in its own way, but hadn\u2019t failed before the left leg got around to shooting its shot. I find these photos especially interesting because we get to examine this crack in its glorious live state, very much like what you\u2019d see if you were carefully inspecting a metal part for cracks before the part had failed.<\/p>\n\n\n\n

As for this fork, well, I know most of the history. This fork had a long service life, I believe it was used in cyclocross races, and aside from the part where it dumped an unlucky rider on the road during its last ride, it gave reasonable service in semi-reasonable circumstances for a reasonable amount of time. <\/p>\n\n\n\n

It is possible all this damage was initiated by a single major shock, which then became a time bomb, but it\u2019s also possible this was the accumulated fatigue of hard service. An expert might be able to figure out the answer, but I don\u2019t know.<\/p>\n\n\n\n

This isn\u2019t a design failure, and I\u2019d say it\u2019s not an aluminum failure either: I\u2019ve seen steel forks fail in similar and different ways (indifferent quality control on brazed joints is a classic), and I\u2019ve replaced a well-used, non-failed composite fork as a preventative measure. Parts have a service life, and by their nature bike parts are built pretty light, because a limited (though not that limited) service life is more palatable than a heavy bike, for the buyers who buy this kind of bicycle.<\/p>\n\n\n\n

I hope these pretty photos of bike-part cracks and failures will teach you two things: how to spot pre-failure cracks in metal, and that it\u2019s worth inspecting your bike for possible failures on a fairly regular basis. The photo-essay you save may be your own.<\/p>\n","protected":false},"excerpt":{"rendered":"

A friend had an aluminum fork fail not too long ago, and let me take a look at the bits to analyze, so I thought I\u2019d share it with you. This fork failure was likely very typical, but it had some fun surprises. The fork in question is a name-brand all-aluminum cyclocross fork. Sized for […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-106","post","type-post","status-publish","format-standard","hentry","category-uncategorized","entry"],"_links":{"self":[{"href":"http:\/\/www.wiredcola.com\/wifi\/wp-json\/wp\/v2\/posts\/106","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.wiredcola.com\/wifi\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.wiredcola.com\/wifi\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.wiredcola.com\/wifi\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.wiredcola.com\/wifi\/wp-json\/wp\/v2\/comments?post=106"}],"version-history":[{"count":2,"href":"http:\/\/www.wiredcola.com\/wifi\/wp-json\/wp\/v2\/posts\/106\/revisions"}],"predecessor-version":[{"id":122,"href":"http:\/\/www.wiredcola.com\/wifi\/wp-json\/wp\/v2\/posts\/106\/revisions\/122"}],"wp:attachment":[{"href":"http:\/\/www.wiredcola.com\/wifi\/wp-json\/wp\/v2\/media?parent=106"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.wiredcola.com\/wifi\/wp-json\/wp\/v2\/categories?post=106"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.wiredcola.com\/wifi\/wp-json\/wp\/v2\/tags?post=106"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}