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Old 10-13-08, 07:28 AM   #1
pgoat
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Can a CF fork 'bend'?

I've had steel forks get bent in that, with the front wheel off and the ends placed on a line on the floor perpendicular to the top tube, one dropout was further forward that the other.

My wife just did an endo and to my naked eye it looks like one tip is further forward slightly...she's gonna get the alignment and whatnot checked at the shop in any case, but I'm wondering if CF just fails or doesn't?
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Old 10-13-08, 07:34 AM   #2
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That fork is trashed. Get a new one. Don't mess around with carbon, it's failure mode isn't the same as steel. It'll disintegrate mid ride.
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Old 10-13-08, 07:46 AM   #3
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Many "carbon" forks have carbon legs bonded to an aluminum crown and that may be what bent in the crash. If it is damaged, the bonded joint can fail suddenly.

I agree with operator, if the fork is mis-aligned replace it.
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Old 10-13-08, 08:16 AM   #4
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thanks guys.

just to be clear, she hasn't had it checked yet at a shop; there are no visible signs of damage.

what happened was the front wheel turned as she went over and one brifter and the drop part of her bars got scraped a bit....the bike looked okay otherwise but we found the tire was rubbing against one side of the fork. I repositioned that (wheel- rim/hub/skewer - look fine, spins true) and then the brake was off center (assume it got knocked off center by the tire), so repositioned that too, and it was okay for her ride carefully back home.

The only marks we see are the slightest rub marks on the inner face of one side of the fork - just a smudge mark on the gloss outer coat. Does that sound like cause for concern iyo?

we're still getting it looked at for sure, and if there is is any sign of possible damage, she'll replace it.
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Old 10-13-08, 08:18 AM   #5
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Many "carbon" forks have carbon legs bonded to an aluminum crown and that may be what bent in the crash. If it is damaged, the bonded joint can fail suddenly.
I'd like to see if that's the case with hers (Cdale R1000, with OEM fork...)
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Old 10-13-08, 08:20 AM   #6
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On most carbon forks, the dropouts are Al and bonded to the lower end of the fork legs so one of them may have been pulled loose in the crash which would account for the wheel misalignment. Check them very carefully. The minor scuff mark inside the fork leg should be no problem.
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Old 10-13-08, 09:05 AM   #7
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cool, thank you...that is what I was worrying about - maybe steerer and crown and blades are fine but AL dropout tips took the brunt of the force....we shall see.
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Old 10-13-08, 09:15 AM   #8
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If in doubt, change it out. [in a Johnny Cohrane voice]

That is my usual advice on things that are paid for with your face, or in this case, your wife's. It just isn't worth taking any chances.
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Old 10-13-08, 11:57 AM   #9
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If the carbon or a joint let go enough to show this sort of deformation then the fork is toast. If the aluminium crown or dropouts bent without damage to the carbon legs or bonded joints then you won't be able to bend it back. With forged aluminium it's one bend per customer. So again the fork is toast. Bottom line is get a new one.

I guess the only other chance is that if the misalignment is small enough it may have been there all along as a manufacuring issue. But if you're sure the wheel misalignment post crash isn't due to the wheel itself being tweaked and that the fork is truly the item that was tweaked post crash then it's definetly new fork time. As mentioned carbon doesn't flex and yeild in the same way metal does. Carbon springs back completely. When it yields it's due to a destructive delamination in the layup. And once that happens it's like you just put a line of "tear here" serrations in a piece of paper.
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Old 10-14-08, 10:42 AM   #10
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we're bringing it in for a checkup - I'll let y'all know what the verdict is...

meantime - quick top recommendations for a good comfy CF fork? In other words, she wants something that will give maximum cushion for upper body jolts but not a piggish steel fork...maybe a good touring CF fork, assuming they exist? (sorry, I am not up on all this)
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Old 10-14-08, 07:49 PM   #11
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If a CF fork does not pass a dropout alignment test, then it's defunct.

CF does flex, as illustrated by the Cannondale Scalpel which uses the flexing of the CF chainstays as an integral part of the rear suspension.

CF forks are supposed to flex some small amount. This gives you a nice plush ride. But, they are supposed to flex back to a nicely aligned position.
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Old 10-14-08, 07:57 PM   #12
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contrary to previous posts carbon should not flex much at all. the modulus of carbon fiber is on the order of up to 20x that of steel. meaning that per unit force carbon will deflect 1/20th the amount of steel. When carbon products flex it is a directional property and there is actually no structural carbon in that direction it is just the polymer resin that is flexing.
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Old 10-14-08, 08:01 PM   #13
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contrary to previous posts carbon should not flex much at all. the modulus of carbon fiber is on the order of up to 20x that of steel. meaning that per unit force carbon will deflect 1/20th the amount of steel. When carbon products flex it is a directional property and there is actually no structural carbon in that direction it is just the polymer resin that is flexing.
I totally disagree. The amount of flex in any CF component is based on the lay up of the CF, which is based on the design of the component. As in the example I stated previously, the chainstays of the Cannondale Scalpel are intentionally designed to be a dynamic part of the rear suspension, and the CF is laid up, as such.

Yes, it's very strong and yes, it CAN be designed and laid up to be extremely stiff, but it can also be designed to be very flexy, as in the Scalpel.
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Old 10-14-08, 09:41 PM   #14
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Carbon fibre and metal alloys flex under load and spring back to their original position. When the yield load is reached metals deform and dont spring back, Carbon fibre composites shatter.
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Old 10-14-08, 09:56 PM   #15
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I totally disagree. The amount of flex in any CF component is based on the lay up of the CF, which is based on the design of the component. As in the example I stated previously, the chainstays of the Cannondale Scalpel are intentionally designed to be a dynamic part of the rear suspension, and the CF is laid up, as such.

Yes, it's very strong and yes, it CAN be designed and laid up to be extremely stiff, but it can also be designed to be very flexy, as in the Scalpel.
Correct.


Change that think out pgoat. As the rest of the posters have said, cf doesn't bend so it's the aluminum. (If it is bent) This will only cause worse things to happen if not changed out and you do not bend aluminum back into place.
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Old 10-15-08, 06:09 AM   #16
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What about Cro-Mo forks, I've got one that's bent and was wondering if it's ok to bend back, both sides are bent in the same spot, straight back-hit a tree, no cracking or dents.
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Old 10-15-08, 07:06 AM   #17
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contrary to previous posts carbon should not flex much at all. the modulus of carbon fiber is on the order of up to 20x that of steel. meaning that per unit force carbon will deflect 1/20th the amount of steel. When carbon products flex it is a directional property and there is actually no structural carbon in that direction it is just the polymer resin that is flexing.
I'm pretty sure that's not totally correct. I believe you're forgetting the denominator in the modulus calculation (ie, area) which allows thinner and lighter CF components to be used at a given amount of flex. I believe the overall strength (per weight) of CF allows designers to create parts that are so much lighter than other materials that the overall weight reduction outweighs the modulus difference. But that's a design choice - I could design a CF part that has a lot of flex, or very little flex.

As to the polymer flexing without the carbon moving...huh? Once cured, one isn't moving without the other - or I should say, if they do, your day is going to get bad real quick.
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Old 10-15-08, 07:26 AM   #18
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What about Cro-Mo forks, I've got one that's bent and was wondering if it's ok to bend back, both sides are bent in the same spot, straight back-hit a tree, no cracking or dents.
Cr-Mo (i.e.steel) forks can be realigned if the damage isn't too severe. The only areas of concern are the joints between the blades and the crown or the crown and the steerer tube. Be sure these aren't cracked or damaged.

One more thing, a frontal impact hard enough to bend both fork legs may also have "ovalized" the frame's head tube. Have theat checked before you put any money and effort into straightening the fork.
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