A thief attempted to lever off my D-lock with a seat post. In the process they managed to bend the seat tube on the aluminium frame. I don't see any cracks on visual inspection but it doesn't really make me feel a whole lot safer about riding it now. I do intend to replace the frame eventually but I don't have time to strip down the components right now. Does anyone know if aluminium bent like this is still structurally safe to ride?

Sorry to see that .... what jerks .......

It's probably ok. Take some good close up pictures, ride it, and then keep any eye open for cracks appearing. You can refer back to the pictures to see if it's changed at all.

Bummer.

I would keep riding it and inspect regularly for cracks... if a crack starts, saw the frame in half and replace.

Seat tube failure is not generally catastrophic, and you can usually keep riding to safety once it occurs so don't sweat it.

I broke the BB lg at the seat tube many years ago. I noticed the crack about a month before. It broke while I was racing. No big deal, I just pulled over, got off and looked at it, then rode slowly back to the start. New frame time, but no danger at all.

Ben

Thieves aren't too bright.

Well, that sucks. It looks like it was a nice frame.
I wonder if there's any chance the tube could be economically replaced. Probably not, but maybe worth looking into?

Almost definitely not worth repairing. It is a Taiwanese or Chinese made aluminum frame, welded then heat treated as a unit. Replacing a tube would require post-weld heat-treating and re-aligning, then painting. A replacement frame will be much less expensive.

I'm usually in the just ride it camp. But, in this case, I think you not only have a major dent, but the seat tube is also bent.

I'd probably go ahead and ride it, but would also prioritize tracking down a replacement frame. Carbon Fiber?

Sometimes homeowner's policies will cover bicycles.

aluminum frames get stress neutralizing heat treatment after all the welding is done.

maybe you have insurance to file a property damage claim?

Just checked up on my home insurance policy, and bicycles are not included as standard. My own fault really for not covering this when I should have. Oh well, it's just a case of putting up with it for a while, until I can get a new frame sourced. Thanks for all the replies.

I'm living in London and I think a carbon fibre frame would just be a big "please steal me" sign .

Another thing to look at (prompted by CliffordK's post) - if the seatpost is bent, your seat is no longer where you originally set it. Looks like the tube was pushed back. If so, the tube and seatpost will no be at a alightly steeper angle, the seat further forward and the nose down a touch.

In my racing days, I bent a seatpost back (flying down from Stowe, VT's Smuggler's Notch at close to 60 mph, hitting a frost heave I didn't see until it was 20' away. Got kicked in the butt two feet in the air. Everything came down straight. But a week later I raced the very hard state championships and boy, did I pay! That very slight re-position of the seat was a killer!

Ben

I just snagged a Kryptonite NY lock (with a large cage). IT IS HEAVY.

I think it would take a lot more leverage than a seatpost to bend it. Perhaps it would help deter thieves from making the attempt.

I lock my bike a lot, but there are some places where I won't do it. So, I park it at stores, but won't park it at a bus stop (no subway lines around here).

Yes, and yes it is.

Seat tube failures are comparatively rare, and if/when they happen carry a low risk of causing a crash. The most likely result of a seat tube failure this low is inconvenience because you won't be able to put much weight onto the pedals, but the bike can still be ridden gently on level ground.

My personal record for riding a frame with a severed seat tube is about 20 miles in rolling terrain. Rode the flats and descents, but climbing was out of the question. So it crimped my style for a while until I got to where I could buy stuff and improvise a fix.

I did that race in 1980 or so ! Fun one.

It wouldn't be hard to fab a sleeve/splint to augment the ST while a replacement frame is acquired. Trek does have an unwritten crash replacement offer, check it out at the shop you bought the bike from. Andy.

Sent an email to Trek. Response was that I could take the bike to a local Trek dealer and ask them to contact Trek about getting 20% off of a new frame under a customer loyalty scheme.

Which frame tubes are most critical for failure or likely to fail catastrophically?

I ask because I have a steel frame with a dent in the middle of the downtube along its length (1-1/2" long, maybe 1/8" deep) caused when my car was rear-ended and the bike was pushed into the bike rack.

It was replaced by insurance but it's a good frame (Salsa Vaya) and I'd like to build it up again if the risk is minimal.

Any thoughts on which frame components are most problematical?

Head tube separating from down- or top-tube would be most scary for me.

I have had chainstays break and I keep control of the bike but in some instances is not really ridable.

I have seen others with broken or separated seatstays and keep riding, but my old road frame was developing cracks under the seat cluster and I retired it because I didn't want to worry about it when doing 80 km/h.

Riding with a damaged frame is always a bit of a crap shoot - usually you can recover from a broken frame, but the potential for loss of control or a crash is real.

As for your specific description of your dent, I can't really comment, except to say that failure in the middle of the downtube is very rare, unless you are riding on extreme terrain or suffer a bad crash. Give the dent a regular inspection to ensure no cracks are forming and use your best judgement. It's your arse on the line here should something bad happen.

Yet another option: If you can find a steel pipe or rod of a diameter close to the inside diameter of the seat tube, you could pre-lube it and gently tap it down to where the dent is. Carefully push out the dent and check for cracks. If the tube is slightly bent, this method could even straighten it. You have little to lose. Aluminum is more malleable than some people think.

I actually own a large steel drift that would work. No idea where it came from, but there must be others online in the $10-20 range. You would want to be sure you could draw it out again, so pre-greasing is important.

This is a steel frame, with an impact dent in one of the least consequential places possible. The very best course of action is to ignore it and get on with your life. Any attempt to fix it won't change it mechanically except possibly for the worse, and will do nothing other than cost you dough.

OTOH if it really bothers you, consider using body filler and a decal to hide it, and soon enough it'll be out of sight---out of mind.

Edit -- in the intervening time, I lost track. It's an aluminum frame, not steel, but the rest of my answer still holds. Except for the possible negative consequences of trying to fix it. Those will be worse with aluminum vs. steel.

So, either turn it in for a discount on a new bike, or ride it until it dies or you decide to replace it anyway whichever comes first.

Aluminum doesn't like getting bent back & forth much. I think it might do more harm than good to try pushing the dent back out. It will still look dented, and not really solve anything.

Or the lower half of a long aluminium seatpost. Pound it in, and leave it. Of course, if you don't get it positioned right, you might not get it out again and might make the frame unusable. If you could get it in (maybe lube with epoxy???) then it could permanently reinforce the frame. Perhaps first use a long steel rod to reshape the frame, then pound and epoxy the seatpost in place.

Were you responding to the OP's question (aluminum frame), or asmac's question (steel frame)?

Different tubes, different materials.

In asmac's case, the downtube is under tension (some framebuilders simply use a cable rather than a tube), although it will experience some lateral torsion. Assuming a rather mild dent, and not bent tube, then FBinNY's advice to leave alone would be appropriate. Perhaps check the frame alignment before investing a lot of money in the project.

I feel like a broken record sometimes, but consider an ugly/cheap bicycle if you have to leave it outside a lot in theft prone areas. It can ride every bit as good as a fancy expensive bike without looking the part. Winter road conditions and the nature of daily commuting wreck my commuter bikes enough that I see no point in starting out with something new and shiny, even if they never get stolen.

Bad advice. Bend it once, and it might be ok. Try to bend it twice (back into the original shape) and it will only make things worse, with zero benefit.

I agree there's some risk involved.

Looking at the photo, it appears (to me, at least) that the tube is not only dented but also slightly bent. That introduces the possibility, however unlikely, of the frame collapsing when the OP hits a few bad bumps, since the SP is a vertically stressed member. As others have said, a seat-post failure is unlikely to be dangerous, but I can never leave well enough alone - I'd try the method I described. But that's just me.

I'd have a pipe wrench handy to twist out the drift, pipe, whatever, rather than leave it in where it adds weight.

You might even get away with using an appropriately sized hardwood dowel instead of a steel rod. Whichever you use, you should file any sharp edges off the end.

My experience with aluminum is that it fatigues and breaks more easily than steel, but it's definitely not brittle. It should survive the operation. The frame should then be stronger than it is now, although it can never be good as new. Body filler and paint would go a long way toward making it LOOK as good as new.

Incorrect. Look up plastic deformation. Trying to get it back to its original shape will only make it WEAKER than it is now unless a proper heat treatment process is performed on the entire frame afterward. These engineering properties of aluminum are the reason you aren't supposed to "cold set" the rear triangle of aluminum frames to a different width.