The Case of the Feckless Fork

I'm a ME who worked as a bicycle mechanic for ten years. My impression is that there are very few engineers in the bicycle industry. I would be surprised if the fork company did any stress testing to failure. The "sideways braking" idea sounds like an excuse for a poor design. As a mechanic I witnessed many broken components and experienced the fatigue failure of a crank arm myself after less than 300 hours of riding. The bicycle industry needs higher standards and actual engineers to work for it.

My 50 Lb 6-year old was just lucky. He was the third owner of a hand-me-down bike, and had been actively jumping up curb cuts, off sidewalks, etc. in the week he'd had this bike, and had stopped to talk with a neighbor kid. As he sat there straddling his bike, the front fork fell off. Looking at the deformed metal where the steer tube entered the fork and the broken weld, it was obvious that the damage had been in progress for quite a while, but was not obvious from an external inspection of the bike. He was lucky that his only injury was to his pride that his 'new' bike was broken.

Editor’s note: Thanks to all the mountain bikers who responded to this blog post. This column is authored by Ken Russell, an MIT professor and forensic metallurgist and is intended to inform design engineers about classic materials-related issues and design bugaboos. Since specific technical details and this level of detail about failures you read about in the current headlines news is rarely available, Ken draws upon his case files. Though some cases date back years, the lessons for engineers remain relevant as basic materials properties and good design principles do not change. We are grateful for the expert input from the mountain biking community.

Thank you for your responses to my recent mountain biking article. The bicycle was by GT Bicycles, Inc., Model Team RTS. The fork was an Answer Manitou 4. I am not a mountain biker and have no idea whether the bike and fork are high end, low end, or so-so. I was retained by the insurer of Answer.

A childhood friend of the plaintiff’s witnessed the accident. In his deposition he described the plaintiff as a “…trickster who liked to use his mountain bike as a pogo stick, hopping around on rocks and other obstacles.” Such behavior is expected of a 16-year old such as the plaintiff and his 200 lb weight exacerbates the results of such behavior.

Dremer03 raised the possibility of improper spring in the forks. Each leg of the fork contains a skewer of elastomer cylinders that can be adjusted to control the shock absorbing behavior. The plaintiff had been monkeying with the elastomers and could have put them back in wrong. There is still the matter of the fatigue striations that originated at the outsides of the forks.

Randy (by E-Mail) suggests the striations were not real, but were due to metal coating the fracture prior to SEM study. That argument will not fly, as we did not coat the fracture surfaces, though other investigators might.

SnarlyMarley suggested the pinch bolts caused the fracture. I don’t see how the circumferential stresses from the pinch bolts could cause the longitudinal stresses that led to the fatigue cracks. You overwhelmingly reject braking by tuning the front wheel crosswise. It sounds like a dumb idea, and if done at all is the province of knuckleheads. However a number of you mention turning the whole bike sidewise during maneuvering and braking.

Christenson and Garry mention stopping a bike by laying it down on its side. p_s mentions sharp turns during mountain biking. All these maneuvers,if done roughly, would give the lateral forces needed for the observed fatigue. I clearly bought into the “crosswise baking” argument much too easily and thank the mountain biker readers for correcting and educating me.

09/17/09

K.C. Russell Calamities Writer

kenruss@mit.edu

I don't know if front forks on mountain bikes are routinely anodized but anodizing greatly reduces aluminum's fatigue strength. That is why you don't see black or other anodizing colors on airplanes. Anodizing puts the surface of the aluminum in a state of tension with microscopic cracks that eventually propogate into the base material causing failure due to fatigue.

I've been riding mountain bikes for 20+ years and have never turned my wheel sideways to brake. When that has happened inadvertently I get a quick trip over the handlebars (I suppose if you think about it I do stop VERY quickly that way!).

feckless fork
yah, no suck thing as sideways braking. That sounds like a horrible idea. Although I often brake while turning on the trail or skid steer around corners. Mountain biking involves a lot of turning over hard terrain that stresses a fork from all angles. There is no reason why a fork would be designed only to withstand force from front to rear. Who ever heard of a mountain bike trail that just goes straight? If he did land a jump straight into a "sideways brake" he's going down regardless. The only thing the manufacturer is liable for is the bad press they will get for making a weak fork. Of course, you didn't mention the manufacturer's name.

Sideways braking? No.
Looking at the figure I bet you get the cracks on the side of the fork as crack initiation points due to the pinch bolts locally adding a stress riser.

I guess there are a lot of engineers who MTB! I race MTB bikes and weigh 220, maybe I should quit! LOL Poorly done article, but its an anomaly.

I've biked most of my adult life and have never heard of "sidewise braking". If I was to turn my front wheel while engaging the front brakes, I'd probably flip over the handlebar. When having to turn and slow/stop at the same time, I always use my REAR brakes. The article need couple extra figures.

Anyone remember "laying down" the bike as an emergency braking maneuver? You turn the whole bike sideways in front of you and try to end up on your feet. Both wheels are diagonal to the ground and sliding sideways. If you don't stop in time, you crush the bike between you and whatever was in the way (or at least move it some if it's a motorcycle).

All you'll do by turning the front tire while braking is throw yourself over the bars.
Sounds like a typical case of JRA. I'll bet he was doing freeride stuff with a xc bike. But it failed when he was just jumping a log.

I too have never heard of this "sideways braking" thing, and I learned mountain bike techniques from the Marin County gang that started it all -- Gary Fisher, Charlie Kelly, Charlie Cunningham, Jacquie Phelan, et al. None of them ever mentioned it.
I go with Brian's explanation: "the UNforeseeable forces stemming from the abuse doled out by a fat 16 year old is uncalculable!" When I was 16, I broke a lot of equipment. Now I'm in my 50s . . . and far, far heavier than when i was 16. I almost never get so much as a flat tire, let alone the smashed-up pedals, forks and wheel rims of my youth.
I served on an ASTM subcommittee that worked on these issues. There's a huge difference in use classification when a bike's wheels leave the ground.
And there's a huge difference in the stresses different riders will put on the bike. That's where the heavy 16 year old comes in. Few riders that young have developed the finesse to be kind to the machinery while jumping it in competition. The kid may have been very much abusing his bike without even realizing it.
To comment further, I'd want to know more specifics about the bike and about the kid's history of riding on it.
John Schubert
Limeport.org

All the engineer/mountain bikers in our group today were aghast at this article. We've been RACING mountain bikes for years and have never seen nor would consider "sideways braking". It is NOT routine; that would NEVER be believed by people who RIDE bikes; guess those folks were excused from jury duty that day.
"Billy the biker" said it so very elegantly.
The only case we know of where a front fork failed was a teenage cousin of a coworker who ran into a parked car while texting (!!!), then proceeded to "Bend it back"...the fork later failed in a race, but he took it (the blame) like a man.

Where did these guys get this? NOT A SINGLE mountain biker does that. The guy was heavy, fork broke. period. Todays brakes have plenty of power to not have to do anything to help braking.
Jezus.

Beaterdriver, you gotta be a complete idiot to think your car braking technique carries over to MTB braking.

The only fork I've ever seen "break" was a result of operator error - a kid went off a jump, gaining around 6 feet of elevation. he landed front first, with the wheel turned almost a full 90 degrees off of his original direction. Typical speed for that section of trail is somewhere between 15 and 18 mph.
The result was some bending and a couple of cracks - no shearing. This fork belonged to a rider that regularly crashed and generally abused his equipment (also lied to his father about it, though we in the shop usually got him to confess - grinning all the time).
If a fork was so worn as to undergo the catastrophic failure related here, I cannot imagine that there would not have been some damage readily visible well before the actual accident.
Beyond all of that, the idea that a racer would turn his wheel sideways to increase available braking force is absolutely laughable. The front wheel is turned IN ORDER TO STEER. Braking while turning the wheel guarantees a crash. NO ONE would "routinely" do such a thing.
Finally, a 200lb rider is WELL beyond what most designers would consider appropriate for their "race weight" forks. PEDALS (yes, those things you stand on!) are capped at 185 for the lightest versions available. Something else is or was going on here.

Brilliant in it's concept! Try it with your car or your motorcycle too!
That fork design is over 10 years old, and I declare this type of fear mongering unnecessary BS.
Jim

Honestly this whole article sounds uneducated. I am a mountain bikeer, I dont brake and turn my tire. From the description of the accident the rider landed on his front shock. Typically you want to land a bit more on your back tire than your front preventing a endo situation. That combined with the fact that he was racing the bike shows that the fork had been abused more than it was designed for. The article also doesnt explain who the manufacturer is or what type of shock. For all we know the kid was riding a comfort bike with 63mm of front travel. He probably landed so hard that is bottomed out, causing the whole fork to fail on impact. It is also completely plausible that he was not running the proper spring, or air pressure to support him doing such a landing causing it to again bottom out and fail.

wow, I can't believe you guys don't get it.
The kid ws simply using the turning resisteance to increase his braking power. I do it all the time with my beater cars, why?, because it saves on break wear with the trade off being incresed tire wear. obvious that you don't have to worry about tire wear on a MTB. How hard is it to understand that a tire going sideways through the muck is going to stop you faster than one going straight? Maybe this will help, think NASCAR, start of race, everyone weaving back and forth, why? to heat up tires, why do tires heat up? friction ! Friction means force ! Guess which direction some of the force is going? Backwards ! thats right, it's BRAKING the race car !! Come on you Rally drivers , help me out.

I think the "sideways braking" simply refers to kicking the back tire around perpendicular to the velocity, leaning the bike back and skidding to a stop.

Just talked to son who races MTB, does bike mechanics and is studying Mech Eng aeronautics, about how this might happen. He like Billy, says that it would not be normal to 'brake' with a sideways turned wheel, but that it is possible when doing stunts that the wheel is being returned to the forward position as first contact is made, so that there is an initial 'catch' on the ground. However that being said, it still suggest that the real side loads (MTB can be rough and tough on equipment) hadn't been fully understood. Other abuse cases are also possible!

Never heard of a technique where you turn your front wheel side ways. Sounds like a good way to get your front teeth knocked out and a broken back.

I've been racing mountain bikes for the better part of a decade and have seen riders from beginners through world-class cyclists. I've been in over 50 major events (with 500-1,000 racers per event) and have never seen/done/heard-of/read-about and cannot image this so-called "sideways braking". I've also never seen anyone break a fork, so it is a rarity.
The only foreseeable significant lateral force on the front wheel comes from glancing blows when riding through stuff like big rocks (and also from crashing).
On the other hand, the UNforeseeable forces stemming from the abuse doled out by a fat 16 year old is uncalculable!

Where's Figure 1? Are you afraid to reveal the manufacturer of the fork?
Mfgrs cannot safely assume 200lb riders are more than 3 SDEVs from the mean in their market - the 135lb pro riders get your stuff for free, us recreational, beer drinking slobs are the paying customers.
I like the run down on the testing. It would be good to have a regimen for testing forks before they break.

Sideways braking?
Benn mountian biking for almost 20years and have never heard such a thing. Not so clearly a foreseeable misuse. Bet you cant find mention of this technique in any of the many publications.