Dented Frame

March 12, 2006 7:25 PM

I put a dent in the downtube of my aluminum/carbon fiber bike, is the integrity of the frame compromised?

Cleaning my Cannondale Six13 the other day, I noticed that a rock or something had made a nice gouge in the aluminum part of my downtube, a few centimeters above the bottom bracket. There's a small nick in the metal, and a very shallow dent, about 2-3 cm long and 1 cm wide, probably only a milimeter or two deep.

I've always read that dented aluminum can fail catastrophically. But the bike feels as solid as ever, even when I'm accelerating hard out of the saddle, and the frame isn't cracked or anything. Should I be worried that it's going to fail at some critical moment, or should I just keep hammering away on it?

posted by dseaton to sports, hobbies, & recreation (9 comments total)

Your downtube is probably fine: it's under tension (the main forces acting on it try to pull it longitudinally). A ding like that would be of much more concern on the top tube, since it is normally under compression.

Think of it like this: Your down tube is like a soda straw being pulled from each end. The top tube is like a soda straw being pushed on each end- which will buckle first?
posted by pjern at 7:43 PM on March 12, 2006

Unless it's dented at a weak point (dead center) or stress point (near welds), or changed the angle of the tubing in question, I wouldn't worry to much about it.

Of course, it's a good excuse for a new frame ;)
posted by ilikebike at 7:43 PM on March 12, 2006

Okay.

1) That spot needs to be watched. It's probably not dangerous, but Al Alloys have this nasty habit of cracking and running quickly. They don't fail as quickly as carbon does, which is "Hmm, maybe that's a crack holy shit! wham wham scrape ow" but they're not like steel -- "hey, my forks cracked. I should replace that when I get home."

2) The real check. Get a straightedge, and check the rest of the tube. Is is straight? Do this as part of your maintenence checks. Is it staying straight?

If so, the tube is fine. If not, the tube is failing, and is no longer safe. Given that you're on a race frame, there won't be much extra material to work with, so there is a real danger that you've pushed part of the downtube to failure. But the first sign of this will be a bend in the tube. As long as it is straight, it's holding up.

As noted, that's a race frame. Race frames do not last -- they're light and stiff first, tough second. Look very carefully at the carbon parts, and the bondpoints between them and the alloy parts of the frame.

Carbon Fiber's biggest failing is that when it starts to fail, it fails completely and quickly. Any damage to the carbon means replacement -- and the right answer to noticing damage on the road is to walk the bike home.

Finally, don't bondo and paint that spot -- you can't watch it if you can't see it. Aluminum oxidation is really strong stuff, corrosion won't be an issue (unless something wierd, like Mercury, gets invovled. If you happen to ride the bike through a pool of mercury, first, call the EPA, second, throw the bike away - anything alloy is going to fail fast.)
posted by eriko at 8:03 PM on March 12, 2006

This is why I ride steel :D

Like other people have said, keep a close eye on it... Alu fatigues over time, and will eventually fail. If you hear creaking noises watch out.
posted by atom128 at 9:05 PM on March 12, 2006

I agree with the "watch it closely" advice. I started with a bigger, deeper dent (2" long, 1/2" deep) in the side of my downtube (caused by some idiot's unsuccessful attempt to use a car jack on my bike lock). Over the course of a couple of weeks, the tube slowly cracked at edges of the dent, and then the cracks spread toward each other. While I could hear the cracks creaking by the time I gave up on it, the frame never failed catastrophically.
posted by johnwilcox at 9:20 AM on March 13, 2006

I race BMX - which stresses hell out of parts - and have done so on bikes with dented aluminum tubes and in turn wouldn't be all that worried about this. Whatever did it didn't do so in such a way as to alert you at the time, which tells me it wasn't a grand mal strike of some sort, so I suspect you'll be fine and have many years of service left in the frame.

Keep an eye on the area, and if you detect bubbling of the finish or a crack emanating out of the affected area, then worry, but prior to that don't worry about it.

>>>>I've always read that dented aluminum can fail catastrophically.

Yeah, but, and I base this on many years of working on airplanes: While aluminum has less tolerance for stress damage, it does a great job of giving you an early warning. Which is to say it's not like glass or some such, it won't just shatter. Like any material, it may fail when stressed beyond its design limits, but that latter term is the key. The design of the machine and the method by which materials are applied will make far more difference than the materials unto themselves.

Keep an eye on the area, but otherwise don't worry too much about it. Send pictures and description to Cannondale if you're especially concerned, but otherwise go ride - and enjoy.
posted by Elvis at 9:50 AM on March 13, 2006

I don't like to answer you question without seeing the damage and what 'gouge' and 'nick' mean in visual terms. Having said that, I agree with eriko's answer, more or less. The problem with Al alloy frame components is that serious damage can sometimes be hard to spot. Hairline cracks strting at a damage site can be very fine. If you have access to a local bike shop who know what they're doing (by no means a given, I know), I'd strongly suggest asking thier opinion. If you can send a picture of the damaged part to Cannondale for an opinion, I'd maybe try that, too. Yes, they may use it as an opportunity to try and sell you a new frame, but they do generally have a reasonable customer service reputation in the bike industry, from what I've heard, and they know their own materials better than most.

Whatever you do, consider your frame compromised. It will fail. That's the nature of Al alloy parts under cyclic loads. The only question is whether it fails next week, or in 100 years time, given the operating forces. Keep checking for straightness and for visual indication of crack propogation around the damaged area.

As johnwilcox says, your ears can help as well. A faint clicking, creaking or ticking noise from the area may also be a givaway, although difficult to distinguish from other noises of your bike's gearing. But if you hear something new or unusual, certainly stop and check it out.
posted by normy at 9:53 AM on March 13, 2006

Thanks, everybody, for the great advice. I realize that the risk of owning a bike like this is that I've traded durability for stiffness and weight savings. But it seems like the general consensus is that, regardless of whether the frame is compromised, if it does fail, it probably won't do so in a dramatic and instantaneous way -- or at least, there might be some warning signs first that I can look out for. So I'll probably just keep riding, but here's a follow-up question:

I can get a replacement frame at a significant discount through a team sponsorship (I'm assuming that, since the damage was obviously caused by a blow by something and didn't just randomly appear, Cannondale isn't going to replace the bike under warranty). I'm a grad student, and even at a discount a new frame will be expensive, so if I bought it, I'd like to sell the one I have to cover part of the costs. So is it ethical, assuming full disclosure about the dent, to sell this frame? Would anybody actually pay for a bike they knew was slightly damaged?
posted by dseaton at 10:24 AM on March 13, 2006

I'm assuming that, since the damage was obviously caused by a blow by something and didn't just randomly appear, Cannondale isn't going to replace the bike under warranty

True, but if you asked nicely, as a loyal customer, I wouldn't be surprised if they might offer you a discount.

Would anybody actually pay for a bike they knew was slightly damaged?

I've bought superficially damaged frames before. It's a way to get a bargain.

If you're honest about the damage, and stress to the buyer that the sale is "as is" and once it leaves your ownership it's not your problem, I wouldn't consider it unethical. Just to cover yourself, I'd suggest only selling it to someone in person and not via ebay or somesuch, so they can make the judgement themselves. You'll also have to lower your price accordingly, of course.

Unfortunately and as you're probably already aware, unless it's something unusually collectable, bike frames and bikes in general lose most of their value from new. That's why most shops don't offer used bikes or take trade-ins. Even at the right price, finding a buyer for used bikes and parts can be challenging.
posted by normy at 11:12 AM on March 13, 2006