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Is slightly crimped aluminum downtube dangerous?

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Is slightly crimped aluminum downtube dangerous?

Old 02-03-18, 09:47 PM
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Is slightly crimped aluminum downtube dangerous?

I'm working on a bike that has slight crimping or denting on both sides of the downtube. Specialized Sirrus hybrid, welded aluminum frame. I'm curious for some knowledgeable opinion on whether the bike is safe to ride. Is there meaningfully increased risk of the downtube caving in while riding? The dimpling isn't deep, and the indentation on the left is about an inch away from the one on the right.

I'm looking for your best estimate of the probable risk here, not a careful lawyer statement. I'm not looking for legal advice.
The frame otherwise appears in good shape. My best guess is the denting happened while transporting the bike with other bikes, but that's only a guess.

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Old 02-03-18, 10:01 PM
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Originally Posted by TallRider View Post

I'm looking for your best estimate of the probable risk here, not a careful lawyer statement. I'm not looking for legal advice.
The frame otherwise appears in good shape. My best guess is the denting happened while transporting the bike with other bikes, but that's only a guess.
Nope, and I don't think it would matter what you think/say to an insurance company lawyer, since you cede those rights when you ask them to arbitrate for you.

That frame is toast. It would appear that, since it appears to be in a shop, that they would agree with that.
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Old 02-03-18, 10:11 PM
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Just keep a eye on it for any hint of crack formation. Until you see a crack you're reasonably OK. At the first hint of a crack, you're well past the beginning of the end.
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Old 02-03-18, 10:21 PM
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Other than the dent, is the downtube generally straight?

The Downtube is generally under tension, except in certain bump/crash situations. Thus, it shouldn't collapse more unless you rear-end a car, or hit a very deep pothole/road crack.

The Slingshot.


It is hard to tell exactly where your damage is, but I'm guessing maybe 1/3 along the distance of the downtube, closest to the bottom bracket. That should be a fairly low stress area, away from both the bottom bracket and the headtube.

If it was otherwise straight, I'd probably ride it. But, also inspect it regularly for progression. One problem is that it appears as if the paint is cracked, but probably not the underlying aluminum. I wouldn't fill it (which could mask a problem), but might consider stripping and repainting in the general vicinity of the damage, just so one could notice potential cracks forming in the metal easier.
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Old 02-03-18, 10:31 PM
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Originally Posted by wschruba View Post
Nope, and I don't think it would matter what you think/say to an insurance company lawyer, since you cede those rights when you ask them to arbitrate for you.
That frame is toast. It would appear that, since it appears to be in a shop, that they would agree with that.
The "shop" is at my house. Again, I'm asking for an estimate of probable risk, not of liability. You appear to be giving a statement about liability.

Originally Posted by FBinNY View Post
Just keep a eye on it for any hint of crack formation. Until you see a crack you're reasonably OK. At the first hint of a crack, you're well past the beginning of the end.
Thanks. That's what I've been planning to do.

Originally Posted by CliffordK View Post
Other than the dent, is the downtube generally straight?
The Downtube is generally under tension, except in certain bump/crash situations. Thus, it shouldn't collapse more unless you rear-end a car, or hit a very deep pothole/road crack.

It is hard to tell exactly where your damage is, but I'm guessing maybe 1/3 along the distance of the downtube, closest to the bottom bracket. That should be a fairly low stress area, away from both the bottom bracket and the headtube.

If it was otherwise straight, I'd probably ride it. But, also inspect it regularly for progression. One problem is that it appears as if the paint is cracked, but probably not the underlying aluminum. I wouldn't fill it (which could mask a problem), but might consider stripping and repainting in the general vicinity of the damage, just so one could notice potential cracks forming in the metal easier.
Good point about the downtube generally being under tension --> less risk of tube collapsing. The tube is indeed straight, and the dents are about halfway between head tube and BB.

It's actually a hybrid that I picked up for cheap for when friends of the right height want to tool around town. So won't be used much.
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Old 02-03-18, 10:32 PM
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I'd worry about lateral flex working the dents.
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Old 02-03-18, 10:45 PM
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Originally Posted by CliffordK View Post
Other than the dent, is the downtube generally straight?

The Downtube is generally under tension, except in certain bump/crash situations. Thus, it shouldn't collapse more unless you rear-end a car, or hit a very deep pothole/road crack.......
With respect to Cliff, let me propose something of a thought experiment.

Stand an empty beer bottle on your kitchen counter, then carefully balance another upside down so they're neck to neck. These are to (very crudely) simulate a tube that's cracked all the way around. Now press then upper bottle straight down and note that despite being two parts, the "tube" can take a decent load.

Now carefully pick the pair up holding both and keeping them together upright. OK so far? Now let go of the bottom one.

The point is that if you're worried about a structural member cracking and breaking, worry about tension members which simply pull apart, not compression members that will still stay put because they have no place to go.
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Old 02-03-18, 10:50 PM
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Originally Posted by TallRider View Post
It's actually a hybrid that I picked up for cheap for when friends of the right height want to tool around town. So won't be used much.
Ahhh, yes, a hybrid.

The hybrids should be somewhat sturdier than road bikes which is a plus. But, there are also thousands of cheap hybrid frames and junk bikes out there, although perhaps not as pretty as that one.

I'm a little more risk adverse when loaning stuff out. At the same time, it is often better not to loan out one's best either.

Assuming you loan it out for no more than a week or two at a time, expecting maybe a couple hundred miles tops, you can make it a general rule to inspect before and after it is loaned out.
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Old 02-03-18, 10:59 PM
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Originally Posted by FBinNY View Post
With respect to Cliff, let me propose something of a thought experiment.

Stand an empty beer bottle on your kitchen counter, then carefully balance another upside down so they're neck to neck. These are to (very crudely) simulate a tube that's cracked all the way around. Now press then upper bottle straight down and note that despite being two parts, the "tube" can take a decent load.

Now carefully pick the pair up holding both and keeping them together upright. OK so far? Now let go of the bottom one.

The point is that if you're worried about a structural member cracking and breaking, worry about tension members which simply pull apart, not compression members that will still stay put because they have no place to go.
Hmmm... yes, if you have a crack or a break.

But another thought experiment.

Take an empty soda can.

Stack... hmmm, say a 20 pound steel weight on it. And it may well take the weight if generally balanced.

Now crumple the side of that can (without getting your fingers under the weight), and put your 20 pound weight back on it.

Can you figure out how to attach as string to your crumpled can and see if it can lift that 20 pound weight without disintegrating?

I suppose one thing. Pay attention if your bike is "talking to you". Which, of course, is easier if you're riding your own bike rather than loaning it out.
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Old 02-03-18, 11:06 PM
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Originally Posted by CliffordK View Post
Hmmm... yes, if you have a crack or a break.

But another thought experiment.

Take an empty soda can.

Stack... hmmm, say a 20 pound steel weight on it. And it may well take the weight if generally balanced.

Now crumple the side of that can (without getting your fingers under the weight), and put your 20 pound weight back on it.

Can you figure out how to attach as string to your crumpled can and see if it can lift that 20 pound weight without disintegrating?
Yes, there's that too.

But put both ideas into the context of the modalities of failure that actually happen on bikes. I doubt you'll find a single example of compression buckling of a tube, vs plenty of tension failures leading to crashes.

Frame tubes are greatly overbuilt for pure compression or tension loads because they have to take bending loads which are far more demanding. But when they fatigue and crack, they act like those two beer bottles and it's tension failures that lead to crashes.
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Old 02-03-18, 11:11 PM
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Originally Posted by wphamilton View Post
I'd worry about lateral flex working the dents.
Yes, the lateral flexing creates the stresses that ultimately will cause fatigue and cracking. My post wasn't about what would lead to fatigue and cracks as much as the consequences when it happens.

As one thinks about failure, think about what will happen, and whether the outcome is more likely to lead to inconvenience or injury.
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Old 02-03-18, 11:20 PM
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it would make a great permanent trainer or roller bike.
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Old 02-04-18, 08:01 AM
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Originally Posted by FBinNY View Post
Yes, the lateral flexing creates the stresses that ultimately will cause fatigue and cracking. My post wasn't about what would lead to fatigue and cracks as much as the consequences when it happens.
Yes, the consequences are why I'd worry about lateral flex. People seem to imagine other modes of failure, which then seem less likely.

When that dent goes, it will feel soft briefly and then break. I would not trust it.
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Old 02-04-18, 08:55 AM
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Originally Posted by CliffordK View Post
But another thought experiment.
Take an empty soda can.
Stack... hmmm, say a 20 pound steel weight on it. And it may well take the weight if generally balanced.
Now crumple the side of that can (without getting your fingers under the weight), and put your 20 pound weight back on it.
Heh... this reminds me of when I was younger and somewhat lighter (~150 pounds). I'd take an empty aluminum soda can and *very slowly and carefully* stand on it with one foot. Then I would lightly tap the side of the can with the other foot. The can would *instantly* collapse into a thin disk. A pretty dramatic demonstration of what happens when a highly-stressed unstable system is perturbed.

No relation to the OP's bike; I would have no reservations about riding that frame. A little dimple in a tube that's mostly under tension isn't likely to cause trouble.
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Old 02-04-18, 09:36 AM
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Originally Posted by FBinNY View Post
Just keep a eye on it for any hint of crack formation. Until you see a crack you're reasonably OK. At the first hint of a crack, you're well past the beginning of the end.
+1. Also I would guess that being a hybrid, Specialized probably used some thicker, sturdy tubing that will hold up to the dent better than some lightweight road frames.
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Old 02-04-18, 09:57 AM
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It is aluminum and will break eventually, but not catastrophically. Inspect it after each ride for a crack.
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Old 02-04-18, 12:45 PM
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I wouldn’t ride it. There are many frames available that are not damaged. Not worth the risk, however slight IMO
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Old 02-04-18, 01:02 PM
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wrap it in fiberglass..
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Old 02-04-18, 01:24 PM
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Originally Posted by fietsbob View Post
wrap it in fiberglass..
Wrapping could hide potential problems, although a good fiberglass (or carbon fiber) job should add strength to the area if applied well, and thus should be a positive.

Tape, decals, or body filler on the other hand could potentially hide the formation of cracks.

Could you fill the tube with long fiber reinforced resin?
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Old 02-04-18, 01:43 PM
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Myself, hybrids are a dime a dozen. I would pick up a replacement hybrid frame and do a parts swap. A bike with that kind of damage can typically be purchased at a low price anyway, so the economics after the swap should still be OK. I would then donate the fork to a co-op (and hopefully find a hybrid frame at a co-op too).

I wouldn't call that damage slight. At the same time, I don't see it suddenly ass-ploding either.
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Old 02-04-18, 02:15 PM
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Originally Posted by wrk101 View Post
Myself, hybrids are a dime a dozen. I would pick up a replacement hybrid frame and do a parts swap.
I think I mentioned that. Although I think the Sirrus is at least on the moderate quality scale. And, of course, there are many versions and models... not necessarily the ones one picks up for $20.

Still, one could clean up a $20 frame. Give it a clean paint job. Throw on the Sirrus fork, and build it up and it might come out pretty nice.
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Old 02-04-18, 07:36 PM
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If it were my bike, I'd still ride it and just watch it more closely for signs of impending failure.

Might be a little cautious about letting others ride it just because of the additional legal liability. Though I can't remember when someone else ever rode my bike. Probably not since junior high 50 some years ago.

I'd never buy one that's dented that badly.
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Old 02-04-18, 08:26 PM
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Dont strip the paint as any anodized areas removed takes away the treated aluminum coating. If anything, leave it be.
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Old 02-05-18, 10:51 AM
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TallRider, No doubt the frame's longevity is compromised. My experience with aluminum frames has been that those with a sharply creased dent are more likely to fail than a rounded dent, which may never fail in our lifetime.
Regardless of frame material damaged, it is the owner's judgement call. Anyone the bike is lent to should be aware of the damage.

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Old 02-05-18, 02:26 PM
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The experiment I recall from high school was to put a small (3"x3") piece of wood or a book on top of a beer can. Stand on said book, balancing yourself (and at the same time the forces) on the beer can. I weighed 220, and could easily do this, balancing on one leg. Then bend over while you are still on the beer can, and gently tap opposite sides of the beer can, drawing your fingers away as quickly as possible. The can collapses in a heartbeat.

In a normal bike, your peddling action will induce bending moment on that down-tube. Given that I still weigh over 200 (more than 220!) and I get up to over 40mph, I would not trust that frame. That's just me, though.
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