dnomel

I picked up a trashed aluminum cannondale 3.0 mtb frame just for the parts still on it. It has a large dent and crack in the right side of the seat tube just above the derailer clamp. I was going to toss it but after stripping it and feeling the weight I'm kind of tempted to see how it rides as is. The rest of the frame seems fine, straight, and really robust, especially the huge downtube. Anyone have any experience with this sort of thing? It seems like of all the places to have damage to a frame, the middle of the seat tube might be the least critical.

AEO

generally, any crack in a frame is no good.
but if you really want to ride it as is, drill a pair of holes at the end of the cracks to stop it from propagating.

berner

I used to work building custom aluminum sailboats. Some of the guys in our shop were really good welders and could weld a beer can to the side of a hull. It may depend on the welder and the location of the crack as to whether it can be repaired. Find a good welding shop to check it out.

dnomel

Thanks. I'll try drilling holes but the crack is in the dent and extends out of it pretty long already. I was also thinking about jamming a wood dowel or something down into the tube. But just looking at the frame I'm also wondering how important the seat tube is structurally. It's hard to imagine the frame collapsing, and with the size of the downtube I wonder if the bottom bracket might be supported from pedaling stress well enough by it alone. I don't know what the normal stresses are on the seat tube, or how a lack of one might affect things.
It's just so much lighter than my steel frame that I'm transfering it's parts to, so it's hard to throw away and a tempting experiment from the trash pile that I can't justify spending money on.

cyccommute 

I'll bet some ham handed mechanic clamped the frame in a Park repair stand and crushed the seat tube. Of the tubes on a bike, I'd say the seat tube is the least important. A dent or crack in the downtube, top tube or chain stay (in that order) is a death sentence or at least should make the bike suspect. The seattube is less drastic but the bike is still very sick. Since your frame is cracked and dented, care should be used if you plan on riding the bike much. To me, a cracked frame is toast no matter where the crack is. Your mileage may vary.

3alarmer 

dnomel:

Since you have thrown this open for general comment
and opinion, here is mine. All aluminum alloy frames will
eventually fail, if ridden enough to subject them to
repeated small repetitive impact stresses that lead to
stress fatigue. This is simply a fact related to the
material and its properties, and is the primary reason
why airplanes get retired at a certain mileage/age
(or sent to third world countries where we, here in
the USA, don't care as much if they kill people by crashing.)

While it is possible

it is also quite within the realm of the possible that this
crack is a warning of such a failure --certainly it is in the
right place (near the BB)--although you gotta wonder about
the dent.

In my experience, bike riders are usually more on the risk
taking side of the population. If you are like me with regard
to thrift, I understand your desire to try this frame out
since you already have it and it costs nothing. Certainly
the odds are that even if it fails, it will not be catastrophic.

I have a Fuji mountainbike that is alloy framed. Even though
I don't believe in the material as a valid approach to bicycle
frame design (components are another issue), I figured I
ought to try it out at least. It rides very nicely, climbs well,
is lightweight -- surprisingly functional overall.

I own it early in its life cycle, so I think I'm reasonably safe
on it. In your case, just make sure your health coverage
is current.

Regards,
Mike Larmer

prathmann

The same is true of every other frame material as well since bike frames in normal use will regularly be subjected to stresses beyond the fatigue failure limit. Fortunately that 'eventually' for aluminum as well as other frame materials is usually long past the point where the frame has been discarded for other reasons. So far I have personally only experienced fatigue failures on a couple of my steel frames and still have an aluminum frame that has already far outlived those in mileage.

At least the seat tube crack is in a very visible location so the OP could keep a close eye on it and quickly spot any elongation.

BCRider

Actually if you study the forces at work the seat tube is one of the harder working parts of a frame. The viability of riding this frame will depend a lot on how big of a dent and crack it has. You haven't said anything about the size of the damage in terms of how big the dent itself is, how deep the dent is or how long the crack is other than "it's pretty long already". Depending on how you see such things this could be anywhere from a half inch to 3 inches. So the opinions on the damage, other than the gut reactions given so far don't really mean much without knowing some measurements of the damage involved or, better yet, seeing a picture.

Also being a mountain bike means that the damage could have occured from a nasty crash onto a rock. Or it may have occured from a car impact. Either way you would want to check the frame out carefully for signs of deformation in the alignment of the tubes. And that isn't the sort of thing you can do by eye. It requires at the very least the use of a couple of long and very good straight edges along with careful measuring.

LarDasse74

While I see the point of your question - it may be possible the bike can be ridden safely - I cannot help but wonder 'why' you would choose to ride a damaged ~20 year old frame when the new ones coming out of Taiwan and China are as good or better quality and can be purchased new for $100 or less.

3alarmer 

prathman:

While I dislike the oversimplifications encountered on
the evening news and in internet chat groups, in the
interests of diversity I'll toss one out here.

Young's Modulus: Aluminum = 69 GPa, Steel = 200 GPa

source:https://en.wikipedia.org/wiki/Young's_modulus

This, of course, makes no allowance for design modifications
with regard to the particular use of the material (e.g. larger
tube diameters, thicker tube walls, reinforcements, gussets,
etc.) nor does it allow for increased elasticity due to alloying.
So any extended debate is really beyond the scope of this
forum.

Nevertheless, as a member of the dinosaurs (or at least endangered
species) still around in Bikeworld, I make no apologies with regard
to my love of steel framed bicycles and my dismay at the continued
assault of the $3,000 carbon fiber Buck Rogers model ZX 3000.

"I grow old … I grow old …
I shall wear the bottoms of my trousers rolled."

Regards,
Mike

cyccommute 

While the seattube works hard, it works in a mode that is inherently strong, i.e. compression. No other tube on the bike frame works in quite the same way. The forces on the tube are almost all vertical while all the other tubes have much smaller vertical components to their force vectors. The seat tube is very unlikely to fail in a manner that would cause injury to the rider. A failure of the seatstay or chainstay is likely to be more severe...and even that kind of failure is unlikely to cause any kind of injury. Many designs of bicycle are even missing the seat tube altogether.

I've crashed mountain bike spectacularly and never experienced any kind of damage to the frame tubes, especially to the seat tube. Any kind of impact with a car that would result in a bent or cracked frame would be evident in other bent tubes. I have seen pinched tubes on oversized Cannondale frames due to using a misadjusted Park workstand.

I would certainly agree with LarDasse that there is little point in riding an obviously damaged bicycle.

DiabloScott

It's kind of pointless to talk about fatigue failure when the crack is already there. It's incredibly UNlikely that this crack was caused by fatigue stresses.

What we're dealing with now is crack propogation speed and that has more to do with the wall thickness than the material. Since aluminum tubing is usually pretty thin, a moderate size crack can turn into a huge rip pretty quickly and that makes the bike a high-risk project.

3alarmer 

With this exception, I agree with everything you posted.

I fail to see the pointlessness of discussing bicycle design
and materials applications on a forum populated by a huge
number of bike dorks.

Regardless, perhaps by shear force of numbers we can
prevent dnomel from doing something of questionable
value for his continued well being.

Regards,
Mike

DiabloScott

OK, if you don't like "pointless"... how about "red herring"?

This is not an aluminum vs steel issue.
It's possible there's some aspect of materials science or frame design that hasn't been discussed to death in this forum; if so we should give it its own thread.

dnomel

The dent is almost the width of the tube and maybe almost a cm deep. The crack is horizontal in the center of the dent and extends out of it to over halfway around the tube, so I don't think there's really any strength in the tube there. The opposite side of the tube is fine and straight, and everything else seems straight and undamaged. I just assumed it hit something hard at that specific spot and that was why it was tossed, but maybe the crack showed up after the dent or slowly grew longer.
I've never had a big tubed aluminun frame, so I was impressed with this one. If I saw this kind of damage on a skinny steel tubed frame I don't think I'd think of it as possibly ridable, but this one just looked different to me.
I have good (but heavier) bikes, so this would just be an experiment. It's an interesting frame and very light but I do wonder how it compares to the new $99 ones available, not that I'd buy one.
If I do try riding it, I'll probably at least try jamming something down through the damaged area. I guess this would involve drilling out the water bottle mounts and hopefully a dowel or somethinig of the right size could push out the dent on the way down.

LarDasse74

Just to be clear, I was talking about budget aluminum frames like this one or this one. Not about the $99 X-Mart bikes that come with aluminum frames.

THe modern $99 aluminum frames are actually very good quality. Cannondale now get their frames made in Asia, probably in some of the same factories as the budget frames are made, as well as frames from Specialized, Kona, Trek, Giant, Jamis, Rocky Mountain, Iron Horse... (all major manufacturers, really) and there are fewer difference than you would think. Also, you would be amazed if you knew how much these good quality frames cost to manufacture in Asia

It's actually kind of odd that you are considering riding on a broken 20 year old 1st generation aluminum frame but would not consider buying a modern good quality aluminum frame (if you were talking about buying a $99 frame and not a $99 X-Mart bike)

cyccommute 

I agree with everything you've said but I do have a minor nit to pick. A Cannondale 3.0 isn't a first generation aluminum frame. It's a second generation frame and could be anywhere from 15 to 20 years old. Just sayin'.

LarDasse74

You got me there! But (if I may use a mathematical equation) broken 2nd generation < not broken modern

cyccommute 

To further reduce the equation:

broken<unbroken

BCRider

In a steady load situation with the rider just sitting or standing I agree. But when pedalling hard on a climb the side to side torqueing and twisting that occurs is contained by the main triangle. And a bad dent and crack in the seat tube being bent back and forth as well as loaded in compression is a combination that could work itself into expanding the crack and finally failing from the torque twisting the metal out of position and then the compressive loads allowing it to collapse. Will it? I don't know. None of us do. Is it stacking the cards in a way that encourages such a failure? Between the dent and the crack and if a picture shows them to be big enough I'd say so.

It's stil possible and even likely that a bad mechanic or an unknowing home shop newbie caused the problem. But it could still be due to a riding crash. I've crashed more than a few times as well and have not managed to do more than scratch the paint. But we've all seen pictures of crash dents here on the forum enough to know that it can happen. Again, until dnomel shows us a picture of the area the actual seriousness of the damage is all just hearsay and conjecture. It may be nothing to worry about at all or it may be the bicycle equivalent of a sucking chest wound.

DiabloScott's thinking that the torqueing loads from riding will make the crack propogate would be my chief worry if it were my frame and if the crack is more than a very minor one. Again, it won't be the compression from "just coasting along" but rather the twisting in the main triangle that will really work the crack in a way that makes it want to run like a crack on a frozen lake. If it's very tiny and contained within the dented area it may not be a big deal. But if it's beginning to run out of the dent and into the otherwise undamaged tube then I'd say trouble is a'brewing.

dnomel

I was just looking at that nashbar frame so I knew what you meant, and it looks like a good frame for cheap, I just meant that I don't need one. I just noticed this frame in the trash and almost didn't bother picking it up but it had good shifters and other usable parts on it. Now that it's sitting here stripped and lighter than any other frame I have I can't help thinking about it.

LarDasse74

Smash the top tube with a hammer or cut out the bottom bracket before you think too much more about it. Get it out of your mind and throw it in the garbage. The chance of immediate catostraphic failure is not great, but the consequences might be.