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Old 03-20-05, 05:57 PM   #1
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Carbon Experts! - small dent in Carbon Frame

Guys,

In your experiences, would a carbon frame be structurally at risk with the following dent in it:

Size: About 10mm long, and 6mm wide

Depression depth: max 2 mm

Location1: on the seat tube two inches forward of the top wattle bottle boss.

Location2: assuming the water bottle bosses are at 12 o'clock, the dent is about 10:30 or 11:00 o'clock on the circumference of the tube.

The carbon IS NOT CRACKED, and feels solid directly around the dent.

Is it cosmetic, or is it likely the integrity of the frame is compromised?

Thanks
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Old 03-20-05, 06:07 PM   #2
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Yoinks!

I think experience is going to be pretty thin in this realm. A CF frame is a pretty high dollar item meaning the manufacturer probably takes a pretty active interest in the customers of same. I would get in touch with them and see what they have to say.

That sounds like a pretty significant dimple to this ferrous jockey.
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Old 03-20-05, 06:11 PM   #3
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Quote:
Originally Posted by Bike_13
Guys,

In your experiences, would a carbon frame be structurally at risk with the following dent in it:

Size: About 10mm long, and 6mm wide

Depression depth: max 2 mm

Location1: on the seat tube two inches forward of the top wattle bottle boss.

Location2: assuming the water bottle bosses are at 12 o'clock, the dent is about 10:30 or 11:00 o'clock on the circumference of the tube.

The carbon IS NOT CRACKED, and feels solid directly around the dent.

Is it cosmetic, or is it likely the integrity of the frame is compromised?

Thanks
I have a carbon seat post that has a deppression and I rode with it last year without a problem; although the deppression area is below the edge of the clamp.

I will assume that the frame was not struck with something; rather it is a deppression from a clamp?

Perhaps you should contact the manufacturer of the frame for advice. Some pictures of the deppression area may also be helpful.
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Old 03-20-05, 06:14 PM   #4
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I'm not sure how you "dent" carbon because it doesn't give in the same manner as a metal.

Closest thing I can relate for you is this - my CF bike had a small crush spot from the FD clamp. The surface was a bit fractured, but the damage did not penetrate the tube. The advice from the builder was that while the damage did not immediately affect the safety of the frame, it was best to get it repaired, since it would only get worse with time. And that's what I did.

Something for you to think about - a compression on the surface might very well mean broken fibers on the inside. As I said above, CF doesn't dent like metal, it breaks. Your surface situation might not be telling you the whole story.
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Old 03-20-05, 06:18 PM   #5
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Quote:
Originally Posted by Trogon
I'm not sure how you "dent" carbon because it doesn't give in the same manner as a metal.
You compress/break the epoxy.

All production CF frames are built with a certain number of redundant plies. The question is whether you've exceeded that number in this case.
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Old 03-20-05, 06:20 PM   #6
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Any kind of deformation in a CF tube makes the structure very suspect. It is very likely that there is a crack in the material only it won't be on the outside... it will be on the inside. If the dent was caused by an external impact then the part of the material that is failing is in tension on the other side and not compression on the impact side.

Also, note that many CF frames use a metal insert in the seat-tube to help reinforce the area for the seatpost. How far this goes down is unknown but I'd imagine it usually stops just above where the water bottle bosses are. It is likely the frame went into some sort of bending motion that caused the seat-tube to deform right below the reinforced section. If that's the case, it's very dangerous to continue riding the bike. I'm going to bet that there is at least partial localised delamination occurring with cracks that aren't visible externally.
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Old 03-20-05, 06:21 PM   #7
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just out of curiosity, how did this happen?
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Old 03-20-05, 06:30 PM   #8
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Then it's not a dent in the traditional sense of the word, is it?

The fibers are spread/compressed, their directionality is changed, etc.
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Old 03-20-05, 06:36 PM   #9
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Quote:
Originally Posted by Trogon
Then it's not a dent in the traditional sense of the word, is it?

The fibers are spread/compressed, their directionality is changed, etc.
Ummm... that is what happens in an analogous manner in metals too. The material plastically deforms and the crystal structure moves out of position.
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Old 03-20-05, 07:02 PM   #10
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Metal stretches, it's essentially elastic. CF doesn't and isn't. It fractures.

But, if all the structural engineers on Bikeforums want to call a depression in a rigid, CF structure a "dent", that's fine, I'm done playing with the semantics of it.
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Old 03-20-05, 07:08 PM   #11
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Quote:
Originally Posted by Trogon
Metal stretches, it's essentially elastic. CF doesn't and isn't. It fractures.
CF can stretch too. I've done creep-compliance tests on fibre-reinforced superconducting coils. The plasticity of CF while short is there.
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Old 03-20-05, 07:09 PM   #12
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[QUOTE=wildjim]I will assume that the frame was not struck with something; rather it is a deppression from a clamp?
[QUOTE]

OK guys, thanks for all you assistance so far.

Some further clarification:

1. yep, workstand clamp is responsible for the 'fracture'
2. Correction: fracture is on the down tube (not seat tube - my caffeine induced error).
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Old 03-20-05, 07:22 PM   #13
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You may have gotten lucky and only deformed the outer superficial lamination layers. If I were you, I would certainly get the frame inspected.
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Old 03-20-05, 07:41 PM   #14
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Look, the term "dent" is a loaded term. In general, the way we use it it means the tube was hit with something and the material stretched inward.

If you hit a CF tube with something, it's not going to dent, it's going to chip or fracture. The "dent" this guy is talking about is almost certainly due to the tube being bent in some direction or an over-tightened clamp. Those kinds of deformations are serious. A "dent" in an aluminum or steel tube due to impact is not nearly as serious because of the elasticity of the materials. You could hit CF bike tubes with rocks all day long and they would not "dent."

Of course the matrix has some elasticity, otherwise his tube would be broken. But I'd be willing to bet the inside of his tube is fractured by that deformation.
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Old 03-20-05, 07:46 PM   #15
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Quote:
Originally Posted by Trogon
Of course the matrix has some elasticity, otherwise his tube would be broken. But I'd be willing to bet the inside of his tube is fractured by that deformation.
I'm not saying it's not. In fact, I said that most likely it was. Denting a material and causing a fracture are not two diametrically opposing concepts.
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Old 03-20-05, 07:57 PM   #16
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Traditionally, I consider a dent to be some kind of deformation where the structure isn't macroscopically broken but rather just shifted. Fracturing is when you're actually breaking the structure.
The easiest way I can see this as is between a piece of metal and glass. You can bend a sheet of glass a certain amount just as you would a piece of metal. But put a blunt force on a piece of metal and it will deform and "dent" inwards while a piece of glass will completely shatter.
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Old 03-20-05, 07:59 PM   #17
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Quote:
Originally Posted by slvoid
Traditionally, I consider a dent to be some kind of deformation where the structure isn't macroscopically broken but rather just shifted. Fracturing is when you're actually breaking the structure.
In a heterogenous anisotropic material such as CF, the distinction is less clear. You can perform both a dent and a break simultaneously.
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Old 03-20-05, 08:00 PM   #18
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Quote:
Originally Posted by Bike_13
Some further clarification:

1. yep, workstand clamp is responsible for the 'fracture'
2. Correction: fracture is on the down tube (not seat tube - my caffeine induced error).
Further info if anyone thinks it'll help.
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Old 03-20-05, 08:50 PM   #19
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Quote:
Originally Posted by khuon
In a heterogenous anisotropic material such as CF, the distinction is less clear. You can perform both a dent and a break simultaneously.
That's true, I've seen big gaping dents in f1 car compartments but in a charpy test where the force is applied, I think carbon would definitely break where a soft and warm enough metal will deform, though I've never done charpy on carbon before.
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Old 03-20-05, 09:09 PM   #20
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this is why i can't see getting a carbon frame. i'd be afraid to ride it. i seem to dent anything with two wheels. or four for that matter.
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Old 03-20-05, 11:55 PM   #21
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The downtube is the most heavily stressed tube in a traditional diamond frame. I would definitely get a first-hand expert opinion.
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Old 03-21-05, 12:01 AM   #22
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Originally Posted by Surferbruce
this is why i can't see getting a carbon frame. i'd be afraid to ride it. i seem to dent anything with two wheels. or four for that matter.
Any force that would create a light dent in thin walled steel or aluminium should only cause a superficial surface scratch in carbon since a lot of carbon is overengineered.
So while you now have a light dent in metal, carbon will only have a surface scratch.
Anything forceful enough to put a huge compromising dent in metal will likely snap a carbon tube. Either way, both are ruined. See, nothing to worry about, go for that sweet carbon bike.
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Old 03-21-05, 08:13 AM   #23
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The better question is, is this guy asking because he wants to

a) buy said bike with damage
b) sell said bike
c) ride said bike
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Old 03-21-05, 04:56 PM   #24
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Quote:
Originally Posted by operator
The better question is, is this guy asking because he wants to

a) buy said bike with damage
b) sell said bike
c) ride said bike
This "guy" is Bike_13, and he want's to ASSESS IF THE BIKE IS STRUCTURALLY AT RISK. That's what was posted in the first post.

IMHO it makes absolutely no difference which of the following I would want to do:

a) if I wanted to buy said bike with damage, i'd want to know if it is safe or repairable, before I bought it. If not, I wouldn't buy it.

b) If I had a bike to sell with this damage, and I ascertained that it was damaged to the point of being dangerous or not repairable, I would throw it away.

c) if I wanted to ride said bike, I'd like to know if it is safe or fixable.

That's it. Don't hijack a forum with a veiled question trying to ascertain what I am doing with a "dented" frame - you know what I am trying to do .... get some opinions on whether it's okay or not!

Jeez, you ask for an opinion ....

Anyway, for those who may be following this post, I have has a composites engineer (manufactures and repairs carbon helicopter blades) look over it, to see how far his assessment varies from those here:

1. Any dent, impression, crack in carbon is a weak spot (and a hidden fissure), and will have to be repaired.
2. This dent is small, and not in a highest stress area - it may take time to weaken further, but it will require repair.
3. Riding the bike is not recommended (even with such a small issue)
4. A repair can be done easily, would cost about US75, but will be visible (unless the frame is painted). The outcome would be a good as new structurally, though likely to be a bit heavier (almost negligibly though.

Thanks for all your contributions.
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Old 03-21-05, 05:36 PM   #25
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Best of luck. I would be careful about #2. I will repeat that the downtube is the most heavily stressed tube in a diamond frame (that's why it's the first tube to get the ol' oversized treatment) and the most likely to fail. While he may have plenty of CF experience, I would be cautious unless he also has a corresponding amount of bicycle engineering experience.

Anyhow, good luck. If you go the repair route just keep an eye on it. While CF failure is much more sudden than Al and certainly steel, it's still not entirely instantaneous so you might get some warning if it ain't holding up.
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