Tuesday, August 21, 2012

Fork vs Frame

Fork vs Frame
At the D2R2 last weekend I met a cyclist who was riding a hastily assembled spare bike, after breaking the fork on his custom randonneur just before the event. Riding off road, he'd hit a log at full speed and the fork cracked from the impact, right at the Grand Bois crown. However, the frame and front wheel appear undamaged. 

Fork vs Frame
His experience reminded me of a conversation about fork vs frame strength the guys at Circle A Cycles were having when I visited them a week earlier. They brought out an older racing frameset that had suffered a similar impact as the D2R2 rider's bike. However, in this case the fork was fine while the front triangle of the frame had buckled from the impact.

Fork vs Frame
Apparently, when a particular stye of Cinelli sloping fork crown came out in the 1980s with its corresponding straight, short fork blades, some framebuilders complained that the resulting forks were too strong - causing frames to suffer damage on impact. A less rigid fork would be able to absorb the impact and save the frame. And it is easier to make a new fork than a new frame.

Fork vs Frame
I just thought all of this was interesting, because the relative strength of the frame vs fork is something that even framebuilders don't always think about. But these dynamics are worth considering. A super-lightweigth steel frame paired with a super-rigid, strong fork may not necessarily be a great idea. The Circle A customer will be getting their frame rebuilt around the intact rear triangle. The D2R2 rider will most likely be getting a new fork made. I am curious and will follow up on both framesets.

31 comments:

  1. From a safety stand point, which is preferable? It seems like the rider would be safer if the frame bends and absorbs the force of impact, even though it will lead to a more costly repair. If the fork fails, it could cause the rider to pitch over the handlebars and get quite seriously injured. But maybe I'm wrong?

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    1. Good point, one is not necessarily "better" than the other - or, rather, it depends on your point of view. On the one hand, it is easier and less expensive to make a new fork than a new frame. On the other hand, a failed fork could be more dangerous. If you read the cracked fork report though, interestingly the rider did not crash.

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    2. I have a buddy who crashed & endo-ed, and after we looked at his bike we found that the fork was bent and the wheel would no longer pass under the downtube. It didn't get jammed, but it was forced to one side.

      It seems like that could happen if the frame bends, too.

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    3. Based on my knowledge of that incident, I am very certain the fork got bent from the force of his braking. It is known to happen with certain brake/fork combinations.

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    4. From a purely safety standpoint, I'd think you'd actually want frame and fork to both bend in order to absorb as much energy as possible. That's probably tricky to engineer, especially since different frame sizes will act differently.

      What I would want is a fork that is engineered to bend rather than snap off. I've seen both happen and a bend is definitely preferable.

      And I think I'd prefer a fork that didn't bend purely from braking. Yeesh.

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    5. He has modern koolstoppped sidepulls with grabby mountain levers. I would not be surprised if that gripped the dry rim pretty solid.

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    6. yikes, if you have the strength to bend a fork through braking, I'd imagine you're relative of the incredible hulk and that going over the bars at speed would be a small matter of leapfrogging..

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    7. It's easy to cobble together a brake system that's just too much. Anyone else remember the Scott Superbrake? Recalled after a large fraction of purchasers ripped crown from steerer.

      I'm riding mass-produced Weinmann centerpulls from 45 years ago. On a 50 year old bike. The limitation is available braking traction. Reinventing the wheel to generate braking force beyond available traction just makes no sense.

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  2. Why would they think about it -- you drive a car into a pole, are you worried about what to replace?

    Silly, who cares. The rider is on the floor either way.

    Checks out? You cut it, if possible, reattach new limb it rides far enough for the rider to smash into another log.

    This isn't rocket science. Bring the appropriate tool to the event, replace as necessary.

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    1. Actually, I know a lot of people who've bent forks and frames without going down. In fact, I suspect that the riders who know how to lower their center of gravity an brace themselves tend to increase the bend and decrease the 'going down' aspect. Going over the bars early on in the impact probably saves the bike from some force.

      That all said, I think your general point that it's probably not worth over thinking this. But what fun would that be ;)

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    2. Ground Round JimAugust 21, 2012 12:35 PM
      Why would they think about it -- you drive a car into a pole, are you worried about what to replace?

      That's silly. I've hit things head on and bent forks twice in my life and in neither case did I come close to falling or even losing my balance. In the later case I even continued my ride home some 10 miles after the impact: the frame and front wheel were fine, the fork was bent but not unrideable. In each case the repair was a new fork.

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  3. About 25 years ago I hit the rear bumper of a car. I on a light-for-the-era Gitane frame and wasn't going fast. The fork bent. I was glad it was only a $25 repair. The dynamics of absorbing force might get important at higher speeds.

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  4. I had a fork sacrifice itself for a frame once.

    http://blayleys.com/trips/2008/Accident/index.htm

    I have used this bike (with a new fork) on D2R2 and Green Mountain Double since...

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    1. Wow, what an ordeal!

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    2. Oh my God! Is this the same Redline you still have now?

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    3. Yes, same Redline that I still ride today. As you can see from the photos, the fork sheared off at the crown. But the frame survived. At the time, I joked that it was an advantage of a carbon steerer - sacrificed itself to save the frame. Having experienced a catastrophic fork failure in 1998, I am well aware of what can happen if the fork isn't up to the job. But I don't think one can expect a fork to survive a head-on with a car, and feel like the way it broke was appropriate. As the comments on this post suggest, there are advantages and disadvantages. I ordered a new fork. It was over half the price I had just paid 3 weeks before for frame, fork and headset! We rebuilt the front wheel, and replaced the headset, bars and stem - possibly being overly cautious, possibly not. I have continued to use the bike for commuting, as well as D2R2, and Green Mountain Double this year. My dear old vitus did not fare so well when I slammed it into a big dog halfway thru a 300km, after logging almost 50,000 miles on it. That was in 1993. Fork suffered reverse rake, top and down tube were crimped. I also still have this frame - for display purposes only. I'm not terribly sentimental, but I did a lot on this bike, and can't bring myself to let it go. Someday I plan to make it a picture frame.

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  5. "I didn't find the cracked fork until after riding the bike home, including some big descents."

    Whoa!

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    1. you're lucky it did not fail catastrophically.

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    2. The back of that fork blade is rusty under the crown. It's not supposed to look like that. It looks like the braze didn't penetrate the joint.

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  6. Lightweight steel tubing is highly susceptible to the kind of frame damage your pictures show.

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  7. There are also other issues to be considered, stiff fork/ flexible frame will have different handling characteristics from flexible fork/stiff frame

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  8. Should the fork sacrifice itself to save the frame, it might be also at a cost to the rider. The fork is an anterior structure, and its loss should propel you forward and downward in a way that a buckle of the frame would not.

    In all cases, the first consideration would be rider safety. You may never ride that bike again, but you want to be able to ride the next bike.

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    1. If the fork snaps you are going to hit the deck but exactly the same can be said for the frame.

      If either bends instead of snapping, you'll stand a chance of staying up and I'd definitely prefer to bend a fork than a frame.

      That being said, if you are hitting something hard enough to destroy a frame or fork, the "falling on the ground" bit is probably the least of your worries.

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  9. Personally, I'd opt for whatever configuration of fork and frame would keep me OUT of accidents. Flexible forks can shimmy. Flexible frames can be somewhat flexy. Eliminate both and you have an unwieldy tank.
    I want balance where nothing goes until I've run completely out of luck, and then stay well back from that cliff.

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  10. I am looking at the green Circle A frame and thinking that the rider just hammered that bike.I am a steel guy, but any bike can only take so much abuse. Drive your Mini Cooper on logging roads and it won't last very long. DUH

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  11. in a reality where it is possible to pick up a tolerable carbon fork for £40 (www.planet-x-bikes.co.uk) I'd go with a less rigid fork (not to mention less vibration) rather than totalling a bike I've been fettling lovingly to near perfection for 18 months..

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  12. I am the rider that cracked my rando fork at the crown, the week before D2R2. The impact was 100% my fault. Treating a rando like a mtb is not the way to go.

    I will have a replacement fork soon.

    The frame is very thin wall 0.7-0.4-0.7mm non-heat-treated ishiwata tubing. I was very surprised that the downtube didn't buckle.

    Fixie Pixie - I am sorry to hear about your harrowing accident.

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  13. Some years ago, commuting home on my gofast Rivendell custom, stupidly having forgotten to bring lights, I misjudged a curb and hit the rise, bending the front fork. The frame and front wheel were fine -- and, in fact, I was able to ride the bike home, tho' handling was rather weird. This all thanks to Rivendell's policy of ensuring that the fork was designed to bend before the frame.

    The bike handled impeccably and the replacement fork, courtesy of Riv, didn't change that.

    Second of two: 39 years earlier, age 12, silly little git, riding my Hero and trying to annoy another cyclist by tailgating him -- gazing closely at his rear fender -- I kept going straight as he veered around a pedestrian ice cream cart. This was Delhi, 1967 or so, where ice cream carts were built from sturdy balks of timber, and the resulting impact of my front wheel against said cart left the fork bent but the frame unscathed.

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  14. To add some clarity to the issue it's not frame strength vs. fork strength, the issue is the fork MANUFACTURE.

    And pilot error.

    Holy Moly.

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  15. In days of yore if you wanted fast tires you rode tubulars. Period. You learned how to care for them. You learned how to ride them.

    The tubular tires mounted on tubular rims. Which were very light and mostly made of pretty soft alloys. Bang them around and you got flat spots. Thump thump thump down the road until you built new rims to your hubs. You learned to ride 'light'. Or you just didn't get to ride the fast stuff. Many failed the initiation. Too bad. The sport was about grace and style.

    The model these days seems to be that riders should be able to do any damn thing without any consequence. If a sufficient quantity of money has changed hands then the bike is held to be 'bulletproof' and the rider can use it for a sledgehammer or a crowbar or a punching bag and that's all OK.

    If you want to jump logs get a mountain bike.

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