Is my (slightly crimped) aluminum bike frame a write-off?

[scottydog]

I made a big mistake with my bike 2 weeks ago: I installed a used Yepp Maxi child seat on my 2005 aluminum performance hybrid (Rocky Mountain RC70).  It was fantastic and fun... until I realized that the child seat bracket, which clamps to the seat tube, has permanently squished my bike's seat tube.  I didn't take a photo because I didn't think it would show up well, but it's clearly visible as a 4" length with an oval cross-section that used to be round.

Google just gave me horror stories about aluminum frames failing catastrophically.  Maybe that's a sign, but if possible I'd like to hear from some real people.  Do any of you have thoughts/experience with aluminum frame damage? 

It's an old bike, with many good memories, but I've been on the fence about selling it many times in the past 3-4 years, and now I'm not sure that I can trust it.  I've moved the child seat to my (steel) winter bike, and am pondering my next move with the RC70...

TIA!

[GuitarStv]

Can you still get your seat post into the seat tube?  Are there any visible cracks anywhere?

[scottydog]

Can you still get your seat post into the seat tube?

Yes.  In fact, I can lower the seat post down through the oval portion fairly easily.  Normally the seat post sits up higher, well above the crimped portion but there's still more than the minimum length of seat post in the seat tube.

Are there any visible cracks anywhere?

No.

[scottydog]

I took it to two LBSes this morning...  At one, which I like but it's a smaller shop and the owner readily admits to not knowing a ton about how aluminum frames fail, the feeling was that it's just cosmetic.  At the other shop, which is close to home but I feel like they're always trying to rip me off, the mechanic told me that he wouldn't ride it but it's my choice and my life.

I understand the argument that the low probability but potentially catastrophic failure probably outweighs the cost of a new bike frame, but it seems like such a waste.  Maybe I'll swing by my local bicycle recycler to see what they say.

[GuitarStv]

Personally, I'd keep riding it.  It sounds like cosmetic damage to me.  I've got an aluminum framed bike and was hit by a car a couple years ago.  It put a ding in the frame on the seat post, but has been fine ever since.

A seat post failure isn't like a front wheel blow out or a handlebar failure.  Worst case scenario you'll have to lift your butt up and stand on the pedals.  It's hard for me to imagine that being a fatal problem.

That said, it's your call.

[Clean Shaven]

Can you still get your seat post into the seat tube?

Yes.  In fact, I can lower the seat post down through the oval portion fairly easily.  Normally the seat post sits up higher, well above the crimped portion but there's still more than the minimum length of seat post in the seat tube.

Are there any visible cracks anywhere?

No.

How long is the seat post that you have on the bike currently?  Since you said you can lower the post down past the crimped section --

If you have a short(ish) post (say 250-300mm), you could buy a longer post (400mm) intended for mountain bikes, if the longer post section would insert down past the crimp.  Might add some peace of mind...  easy enough to measure and see if it would work.

[No Name Guy]

If you choose to keep riding this bike (not generally recommended), know this:

By imposing a permanent deformation on the aluminum, you've over stressed it.

From a fatigue damage perspective, this is one of the most damaging things you can do (think bending a paper clip back and forth - it doesn't take too many times to cause it to break, compared to just gently flexing it without permanently bending, it takes thousands and thousands of cycles to cause it to crack and break).  And recall, aluminum isn't as good as steel in fatigue (in the general sense - no nitpicking from other engineers, this is for a lay audience).

If you are going to ride this bike, consider, as suggested, having a post long enough to go below the damage.  That way, if it breaks, the worst likely outcome is the seat drops instead of totally falling off.

Do a close visual inspection of the damaged area frequently - probably before and after each ride, to see if you can see any cracks in the area.  Run a finger nail over the area feeling for a crack.  If you detect a crack, scrap it if a competent welder can't repair the area.

Or just look at this as an opportunity to upgrade.......

[scottydog]

Thanks everyone!

I'm thinking it shouldn't break... but if it does, the seat tube would collapse and then the rear triangle would almost certainly detach, somewhere, from the front triangle.

I just took some photos to show it a bit more clearly.  The squished part is just above the screws for the (absent) water bottle cage.  In these views, the seat tube seems to get ever so slightly thinner; from the front or back, the seat tube looks fatter.

That's a neat idea to look for an extra-long seat post to try reinforcing it from the inside!  I'm going to look into that.  I can't imagine how the seat tube would buckle if there's an extra pipe inside it.

However, since I'm aiming for many rides out in the wilderness while camping with the family, I'm leaning more and more towards upgrading.  In the future I'll stick with steel because I know myself and that will give me more leeway for the next time I try to hack something onto my bike.

[Reynolds531]

I worked in a bike shop for eight years in HS and university. I would closely watch other joints in the frame. I agree it's significantly weaker. No loaded touring, off-road, etc. It should be OK for a while, then you can get a frame for not a lot of money from price point etc.