I have an aluminum bike frame that has a hair-line crack in it. The frame isn't a very expensive one, so welding it is out of the question.
I thought to use epoxy to repair it. What do you think? Any suggestions/advice you may have would be helpful.
Thanks

Twenty to one it was caused by a seatpost not inserted deep enough into the frame.

The minimum insertion marks reference the minimum insertion for the seat post not to cam out of the seat tube. But that's only one half of the equation. On designs like this one where the seat tube extends beyond the top the post must extend to a depth of about one inch below the bottom of the top tube, which is usually beyond the seat post's mark.

A post inserted only to the 2.5" minimum mark will end above the top tube, or very close to that creating a major stress concentration right at the weld, which is the worst possible scenario.

Your good luck is you spotted it before it let go completely with seri-ass consequences.

Step one if you want to keep the frame, get a seat post that extends at least 3" below the crack, so it carries your weight well below the damaged area and transfers the load to the frame. Next use JB weld, or epoxy and a piece of fiberglass to reinforce the crack and prevent it's spreading and you should be OK, but keep an eye on it. It doesn't have to be a super strong repair since now the extended tube's only function is to support the clamp.

Good thing you didn't put your money down.

The seat-post is almost bottomed-out in the tube. Only 1/4'' or less space remains at the bottom of the tube by the seat-post. So that isn't what caused the crack.
I don't know what caused the crack, but it's there.
Your suggestion to reinforce it with fiberglass, was something I hadn't thought of.
I'll try this repair out and see how it goes. Then post the results.
Thanks for your suggestions/advice...

PS, I was born and raised in NYC, I know what your saying and agree.

This sort of crack is also in a spot where the frame sees a lot of tension in the main triangle. If it's a mountain bike that has seen a lot of jumping between the tension that results in G'ing out during a landing and the possible flex from the rider landing on the saddle at the same time repeated enough times could possibly produce such a crack.

In any event there isn't an epoxy (and JBWeld is JUST another epoxy) around that would do more than maybe keep out the water. None of them would make a tinker's damn worth of difference to the damage or propogation of the crack further down and around that weld.

If it were mine, and considering that I'm also a cheap bugger, I'd just sigh regretfully and saw it up and put it into the metal recycling bin.

The nature and direction of the crack indicates it was caused by the extended mast being pulled back too hard or too often, or a combination, leading to a stress crack.

If a correct-fitting seatpost bridged this with at least 3" below, it shouldn't have cracked. So you have to consider possibilities because unless you identify and eliminate the cause of excess stress there's no way your repair will hold.

Possibly the seatpost is undersized or is too flexible, allowing movement, especially with a heavy rider or rough service in mtb or crappy roads. Check by painting brittle nail polish across the crack at the front, installing the post and levering it backward. If you can crack the nail polish your repair will also fail. If necessary buy a more rigid seatpost and try again.

Then to get any kind of reliable repair you'll have to do figure 8 wraps of the glass encompassing the top tube to get any kind of structure.

The frame is a mountain bike type frame.
I don't ride of road and have never preformed jumps as described. Non the less I really don't see much hope in my fixing this frame aside from welding it. Welding an alum frame is costly, not to mention the heat treatment the frame will need in order to do the welding.
I've monitored the crack and it hasn't gotten any worse over a 3 month span of riding everyday. The crack seems to have found it's bottom. Also, I don't believe the crack will result in total failure of the frame. For me to trash the frame seems like a radical suggestion to me, but not one that I haven't thought of.
I respect your opinion, but I'm opting for the latter and will try to repair it with fiberglass and epoxy.
Another thing is that if I had the money, I'd trash the frame and purchase a new one and this post would be nonexistent.

I'd probably keep riding it gingerly and save my spare change 'til it added up to $100 and get this frame from Nashbar.

You got the seatpost almost slammed, huh? Is it hitting bottle cage bosses or Bottom bracket (wow!)??

Good suggestion... I'll probably end up getting a new frame anyway, but for now I'm going to try the repair.

The seat-post is about 1/4'' or so near the bottom of the tube. That's the way I installed it from the beginning. It's fine, the bottom of the seat post is inside the tube and cant be seen.

A bike with a long set back even when the seat post is inserted as far as yours puts stress on the seatpost/top tube junction. Add a large rider and a poorly heat treated frame and it's pretty easy to crack a cheap frame. Once cracked, however, you shouldn't trust this frame no matter how long it's been since the crack elongated. I don't think any bandaid of epoxy or fiberglass is going to stop the process of eventual failure. All fiberglass or epoxy will do is cover the crack. That will just make you feel safe but it won't make you safe.

Sorry to say but the frame is a zombie. It's just waiting to eat your brains.

Scrap it....... or just ride it cause you seem to think it's OK....

Thanks, I hear you, but can't just ditch the frame right now. I need the bike for transportation.
I guess that's an even better reason to just trash it and get a new one. Moneys tight now, can't buy a new frame at the moment. That's why I thought to try and repair it (temporarily) with fiberglass and epoxy. Until I can afford a new frame.

OK what brand is the frame and did you buy it new? any chance there is a warranty?

The frame is an old specialized, no warranty that I can prove. I spoke to them I need proof of purchase.

You could always cruise Craigslist for an entire replacement bike. Last year I saw a SWEET GT Karakoram and a KILLER Rocky Mountain Blizzard for $50 and $75 respectively.

Already scoped out Craigslist. All the other part of the bike were expensive up-grades I made and don't want to trash them. I should have changed the frame a long time ago. Hindsight is 20/20...

Don't ride it! Aluminum fails catastrophically: you are running the risk of the frame shattering. The crack means that there's stress that overcame the metal. That stress is making to other parts of the frame, which now are experiencing metal fatigue and want to crack more than they did before. You run a risk of going to the ground in close proximity to jagged metal at full riding speed when you hit a pothole or something.

I understand that it's your transportation and that money's tight - I just got my transportation bike back into working order after a month of misadventures and waiting for parts, and I live on a grad student's stipend. Frames aren't all that expensive; cf. the $100 aluminum frame above. Frankly, I'd revolve a few hundred bucks of credit card debt before riding an aluminum frame with a crack in it.

my allez cracked in the same place. specialized replaced and no mention of repairing if it was possible. looked like an easy fix to me but when they offered a new frame i jumped at it. i know you are not original owner (no warranty) but i would think not a good idea to keep riding it.

Thanks for all the suggestions/advice, but I have to keep this bike on the road regardless of it's crack.
I try to keep my rear-end off the saddle, so not to create down force that might increase the cracks size.
I have to use this bike on a daily basis, so I have to for go the risk. I really don't think it will have a catastrophic failure any time soon.
A new frame is what's really in order here and I think you all have convinced me of that.
Any repair I do that doesn't involve wielding is futile.
Thanks all...

PS, I am the original owner, but have modified the bike that there are no longer any original parts left, except for the frame...

@FBinNY,
I need 50 posts in order to reply to your massages... I under stand what your saying.

@FBinNY

I understand, but can't reply until I have 50+ posts...

The frame isn't a traditional type frame, which might explain it's failure. The tube that the seat-post is inserted into is only, at best 6-7'' in length. Not much room for adjustments there.

This brilliant design requires that anyone wanting to lower the saddle beyond a certain point has to hacksaw excess post sticking out of the bottom of the seat tube.

I removed only about an inch of the seat post shaft, very little at best.

Oh, yeah. That's a bit of a weak point in frames like that. Without seatstays behind the seatcluster the seatcluster is a major failure point, especially when you're rockin a massive setback seatpost.

Your seatpost has what, 3" setback? That's kinda bad.

I suggest getting a frame size one notch larger and a zero setback seatpost.

The current configuration with the massive setback seatpost puts WAY TOO MUCH stress on the seat cluster.

Replace frame with one that can work for you with a seatpost with little or no setback.

The bike is nothing of what it was when I purchased it. I don't have the original bill of sale, so even with all the modifications/upgrades there is no warranty what so ever.