I'm looking at an aluminum mountain bike frame on ebay that has a crack in the headtube. The crack is on the top of the tube near the weld to the top tube and probably will not extend beyond the weld. It extends about 3/8 inch. This is a frame I am very interested in, but not if I'm simply buying something I'll have to throw in the recycle bin with old Coke cans. I'm concerned about the stress on the head and will that cause the crack to separate or is this nothing to be concerned about. Any experience with cracks in frames, especially aluminum or other comments are appreciated.

One word: Junk.

a pic would be nice, but i think i'm speaking for everybody when I say this:

run far, far away and don't look back.

yep, its junk. its worth its weight in scrap aluminum.

If it's a "Crack N Fail" (Cannondale) you might be able to get a discount on a replacement frame, but that takes time and the discount isn't any better than what you'd pay for a closeout or used frame anyway. I wouldn't buy it.

Never had that happen to any of my steel bikes. They can be repaired if it does though. I always wondered what the attraction to aluminum was. It's just as expensive as steel and can't be reapaired. Is the lighter weight worth it?

Tim

fatal

I used to argue with a guy on the internet. His alum mtn bike fell apart at the head tube while he was downhilling around 40 mph. He died a couple of days later.

One more vote for DON'T DO IT. A headtube failure is more than a minor inconvenience.

As to the attractivness of Al, yes it strictly a weight issue. Al frames can be much lighter than steel frames for the same stiffness. Durability is an issue but the ultra-thin wall steel tube sets that try to compete with Al have their durability issues too. There is no free lunch.

+1

Don't you go near that bike, but if I were the guy selling it I would try to see if there was a manufacturer warantee.

You could buy it, give it to your worst enemy, and watch him do a face plant, but that would be mean...

My problems with aluminum as a frame material are the very low elongation property of aluminum compared to steel or titanium, and the fact that most aluminum alloys don't have an endurance limit.

Elongation is the property of the metal that determines how far it can be bent before it breaks (brittleness), while lack of an endurance limit means that even a minuscule load, if applied enough times, will eventually result in a fatigue failure, often without any warning. Sudden, catastrophic frame failure can ruin your day.

Steel frames, OTOH, have higher elongation percentages and do have endurance limits, so failures are predictable instead of sudden or catastrophic. Because steel has endurance limits, repetitive small loads that aren't big enough to deform the material permanently (bend it) can be imposed on steel frames over periods of many years without failure, and if failure does occur it invariably does so with some advanced indication that it is about to fail (fatigue cracks, etc.).

For me, carrying a few extra ounces of weight in a steel frame is cheap insurance. New high strength stainless steel alloys like Reynolds 953 make possible thinner walled (lighter) steel frames that compare favorably with titanium and carbon fiber composite frames in terms of weight without compromising strength or durability, and without the low elongation (brittleness) of carbon fiber.

Just my opinion; I could be wrong.

One thing I like about aluminum for MTB is that the protection from rust since my bikes tend to get dirty and muddy on rides. Aside from that, they're economic and light. I still prefer steel, though, especially on road bikes. The ride is smoother and the frame lasts longer.

Stainless steel tube sets have been available for some time (Columbus offered one called Metax, IIRC, for a while) and the new Reynolds 953 is stronger than most. However the tubes are very expensive and difficult to machine and form. They also require similar inert-gas welding techniques to Al and Ti so the fabrication and joining processes are at least as costly as Al or Ti. They have never caught on.

Al does have it's mechanical limitations but they are more academic than real in the commercial world. Broken bike frames do happen but sudden, non-accident induced failures are very rare. Al is cheap, light, easy to fabricate and the welding techniques are well known. It's fatigue life is finite but plenty long.

As an aside, do you use steel for all of your bike parts? Handlebars, stems, cranks, etc. are almost exclusively Al (or carbon) these days and your safety is actually more dependent on these items than the frame in case of sudden breakage.

Aluminum alloy handlebars and stems are not subject to the same repetitive stresses as frames and are therefore not as likely to fail, and unless you you're standing on your pedals a crankset failure is not that much of a safety problem; you just slow down and stop.

Aluminum's fine for me, but not when it shows any signs of damage in a high-stress location. If anybody buys that damaged frame, I fear for their safety.

If an aluminum tube is flexed, it will break after a certain number of repetitions. How many repetitions depends on the type of aluminum, and the amount of flex. For example, a handlebar that flexes about 1/100th of an inch every time the rider goes over a bump or pothole is going to last a lot longer than a bar that flexes 1/8th of an inch each time it is stressed.

The "better" bike parts designers, such as Easton, have developed sophisticated testing procedures to verify that their aluminum bars have essentially zero flex under normal use. And, zero flex means the part will last forever under normal use. Some of the "Brand X" component companies like to brag about how light their bars, and other components are. But, if that ultra-light weight results in flexing of the part, that part is a trip to the ER, waiting to happen.

The obsession with the weight of a bike makes sense for Pro cyclists. It makes little sense for folks who are riding to lose weight, get in shape, relax, or who are riding to work, the grocery store, or to school. "Joe Average" bike rider would be better off riding a bike made with high quality steel for the frame, fork, stem, bars, and seat post. Yes, that bike might weigh 26 pounds or 28 pounds. But, a 28 pound bike that is can take twenty or thirty years of use and abuse without any problems is a better bike for NON-racing than a 19 pound bike that MIGHT fail when you hit the next pothole.

If you can see the crack, it's not a fatigue regime you have to worry about. It's not going to take 10,000 or a 100,000 cycles to failure. We will be dicussing numbers in the low hundreds down to one. You enter a failure regime controlled by the KIC fracture toughness of which aluminium has a lower fraction of steel or titanium's value, than it does a comparison of their strength or elongation properties.

It's not even junk. As a general rule, five pounds of scrap won't just kill someone. It's a disaster zone waiting to happen.

Please link this ebay auction. I'd like to know who the heck is going to buy this.

I'm the OP and I'm the one who is interested in this frame. I'm not very good at processing links, but here is a picture of the frame head with the crack.
EDIT: In case it is difficult to find the crack, it is located just above the top tube weld line.

definitely do not buy this frame under any circumstance

another word: why?

ed rader

All metal, and aluminum especially, has this nasty habit of propogating cracks over time until the structure breaks. The most vulnerable part of the bike frame is at the welds. The crack will not stop at the welds, it will, rather, reroute itself to follow the weld line. Eventually, the weld will fail.

Keep away from a cracked aluminum frame. If the frame has a crack, IT WILL FAIL IN TIME. This is not opinion; this is fact.

PS. I just looked at the picture. It is junk. The crack will simply propogate around the weld until the frame fails.

no..... 2 words: ULTRA JUNK