How long will this alum frame last?

Question

I picked up this very cool 96-97 GT rage from Craigslist with the intention of restoration- a few rusty pinch bolts, spider eggs in the brake hoods, grinding bearings and pitted races, and all the good stuff that comes to a bike living outside for 5 years.

I was excited to get my first aluminum frame for the project as I knew it wouldn't be rusted like steel would have. (It spent a lot of time in the rain.) It wasn't for another day until I found hairline cracks forming under the seat post clamp and around the GT rear stamp. I suspect the clamp was over tightened at one point, but it was very normal when I got to it and the little slit cut into the back of the tube isn't squished or malformed which I've seen happen on steel frames that were over tightened.

How long will the frame actually last? It's a GT so it's got the triple triangle so I'm not worried about my seat stays coming loose or my seat post falling into the tube. Should I be? I know fatigue cracks in some places means it's time to trash the frame, but I'm not a pro racer and I ride pretty lightly. I'm interested in other people's experiences with aging aluminim and possibly hearing thoughts aside from putting it on the curb for trash day.

I can feel the cracks with my fingernail but they're too small to actually fit my nail inside.

I've read 7005 aluminim cannot be repaired since it requires heat treatment. I love this bike because I love 90s and GT (and its so hard to find tall frames that aren't 70s gas pipe builds) but I'm not sure it's worth being a money pit. Are there other repair options? Does drilling cracks actually work? Can I permanently install the seat tube?

Answer 1

A crack developing there, is most likely too much seat post out of the frame, or in actual fact, not enough seat post being in the frame.

The biggest problem with a crack in this location, is that it then causes most force to be placed on the join between the top tube and seat tube. You also have to think about what would happen if the frame were to suffer a critical failure in this position - it will most likely be landing from a drop or, more likely, while sitting and hitting a hole/bump; neither great options and the more likely of the two being much more painful!

Drilling a crack is to minimise crack propagation. And there are few repair options with this style of frame (for sensible money).

This can be welded, but you would risk this just getting worse. If I remember correctly 7005 using artificial ageing rather than a traditional heat treatment - a filler material could be used between the post and frame to bond them and strengthen the seat post. The sensible money "option" if you really want to keep the frame is to get a very long seat post (ensure that there is a lot in the frame, around 20cm+) and keep checking for further crack propagation. The best thing to do here would be to contact a specialist aluminium frame builder and ask their opinion.

To answer the question in the title - aluminium work hardens quicker than other materials, and tends to crack due to fatigue much quicker than say carbon, steel or titanium.

Unfortunately I think it could be time to consider a new frame.

Answer 2

I have the same kind of issues - needing a tall bike for long legs.

In the past I have cracked my steel MTB in the same way, which was due to having a long seat post, up high, with the bare minimum inserted into the frame. Plus my 100 KG mass on top was too much leverage.

So you should buy and fit a longer seat post straight away. I personally reinforced my seat post with a second steel tube up the inside, and it now extends 8" down into the seat tube.

As for the crack, watch it and if it grows, discard/recycle the frame. My steel bike came 3/4 off before I noticed, but steel fails slower than aluminium. It could go from its current state all the way to fully broken in one ride, or even in one pothole.

Answer The crack is not good, so watch it, and look out for a replacement.

---

UPDATE I found cracks in my aluminium 7005 road bike, in the same general area. After much thought and talking to aluminium welders, I scrapped the bike and took all the usable bits off for later projects. Heartbreaking but frame failure can be sudden and deadly.

You could make a funky wall art bike out of this, but I'd not ride it again.

Answer 3

I've seen this kind of crack on a few frames - not just alloy. So it is not uncommon. If it were steel it would be easier to repair - I've not heard of anybody who has repaired their alloy frame either - so probably not such an economical repair to have done.

How long it will last will depend on a variety of factors - so its difficult to say. I certainly wouldn't go mountain biking on it. Personally - as said before - its probably best to find another frame.

Answer 4

Those cracks are not terrible but not good. The upper is terminated and the lower is less than 1/4 around (from what I can tell).

There is only one crack to drill and that is questionable. Do you have a drill press. For sure not something to try with a drill.

With a long quality seat post and light use the chance of a catastrophic failure is very very low. You might get 5 years out of it or 5 minutes but I would give 5 years a much better chance.

Thing is this is a restore. Not worth putting money in it. You can find good frames for $300. If you have a lot of the component already then maybe.

Answer 5

I would not worry too much about that crack. As long as the seat-post is deep enough in the seat-tube and the bolt on the clamp is tightened to the required torque. What could be of some help (if the clamp is removable) is a double post-clamp where the lower part is around the seat-tube and the upper part of the clamp holds the seat-post thus distributing the pressure and putting vertical load on the seat-tube.

Answer 6

Alu has a limited lifespan due to work hardening. Work hardening is where the metal has become brittle after many vibrations and the only way to repair work hardening is to completely melt the piece down and start again. So where you see a crack, its quite likely the entire area is kaput. While its unlikely that a failure of the top of the seat tube will cause a fatal accident, and a longer seat post will probably delay the inevitable for a while, this frame has had its day.