The (aero tubed) aluminium seat tube on my Fort Road Aero frame has developed a hairline crack on the front (so 180 degs. from the binder bolt), about 11mm long. It starts at the end of the ST and runs downwards.

There is unfortunately AFAICS no way to lop off a section of the seat tube and use a seat post collar because of the oval section of the seat tube.

My plan is to drill through with a 5mm metal bit at the bottom of the hole to stop the crack, then epoxy in a cheap 27.2mm seatpost at my preferred riding height, and forget about it.

Is there any particular epoxy I shoud use or will any old stuff do the job?

Does anyone have any thoughts in general about my master plan?

I know pics would be helpful but at this stage I donīt have any..... maybe tomorrow if I can find the mini USB cable that the kids have hidden somewhere

Thanks all
Simon

Photo of the seat post. Crack is on the front, so RH side of the post as you look at it.

The cure is welding, not epoxy. drill the crack stopper hole in the meantime ,
then seek out an aluminum welder who will repair your cracked place,
welding in aluminum, to join the metal back into a solid .

I suspect over-tightening the seat binder bolt. the stress that started the crack ..

maybe post fit a bit undersized.. maybe not enough post in the frame ...

Maybe epoxy in a 25.0 to 27.2 shim. long enough to go well into the seat tube
then get a 25.0 seatpost ..,. USE UK. makes good long ones in 7075.

I personally suspect a previous owner used too small a seatpost. Although there might be a bit of, ahem, īcurrent user errorīin using too short a seat post (cough cough).

There is not enough value in the frame to make it worth seeking out a specialist aluminium welder (although I might ask the guys at work anyway).

What I am after is a safe, ghetto solution. I will be the last owner of the bike anyway as I would never sell on a frame with this kind of fault so if I end up with a non-adjustable post, then no problemo.

Hmmm, he's talking about epoxying the seatpost into the seat-tube so as to not require any tension from the binder-bolt. However, that will still not solve the geometry problem. Which is that the weight-loads on the forward-positioned seat creates a torque on the post which bends it forward and causes A LOT of stress in a direction not in-line with the tubing. That bending-forward post is what most likely caused the crack over time. The ultimate solution is to have a bike with +80 degree seat-tube to allow the centre of the seat to be directly above the hole in the seat-tube. In which case, there won't be any bending moments.

For how, you want to get a STIFF seatpost (alloy or steel) that fits VERY tightly and far DOWN inside the seat-tube. So something with 4mm+ wall-thickness, like an SR Laprade or similarly beefy post. Then make sure it's at least 6" down into seat-tube. With that much contact surface-area, any 2-part epoxy will hold easily.

Good advice. Many thanks.
I had originally set up the bike as a TT/road convertible, and it worked well, but maybe a combination of the post being a bit short and the forward angle overstressed that part of the frame.

If I proceed with the repair i will have the bike exclusively set up for road cycling i.e. no forward post, with my weight directly over the post.

I had done an upgrade on a Trek 5000 frame with a solution that may work for you. We wanted to use more forward position for the seat, but the Dura-ace seatpost was the only one available with internal-expander. And its clamp didn't allow the rotation-adjustment needed to reverse it. So I took a 150mm long section of thick steel seat-tube tubing leftover from a frame-building project and did the following:
Basically I added a steel seat-tube with regular binder-bolt to the top of the Trek 5000 frame to allow me to use an off-the-shelf seatpost with the clamp reversed to move the seat forward.

Well, an aluminum welder is not that big a deal. Almost every good welding shop has aluminum capability. IF it can be fixed by the welder (I have no idea if it can or not), you're probably talking their minimum charge - whatever that happens to be. It's not like you're looking for an underwater, upside down pipeline welder (now THAT'S an expensive specialty). You're just looking for an aluminum welder. It's a skilled occupation, yes, and I have a lot of respect for a good aluminum welder, but they're not rare and the job you're talking about sounds pretty simple. It might be 1/4 hour at their shop rate, at most 1/2 hour, whatever their minimum happens to be.

Not to say your ghetto solution isn't reasonable, just don't be too awed by aluminum welding. Sounds like if it is possible (if), it would be a very minor deal.

Thanks all for the advice. Very much appreciated.

I will check with the stage builders at work and see if they can weld aluminium too (I know they can do steel).

It looks like you can create a collar reinforcement under your current seat binder. Bend a piece of metal around the seat tube under the collar, and attach it with it's own bolt under the current binder bolt. No welding necessary.

That's what I would think as well. The part of the seat tube extending above the top tube is not structural. One concern would be to prevent the crack from propagating further down into the weld area, which is structural. Drill the end of the crack to help prevent this, and keep a close eye on it to detect any tendency to extend the crack.

Simon, If you remove the paint ~1 cm on either side of the crack, then use a dremel or a V shaped file to create a valley to puddle in the weld , a welder with equipment to weld Al can fix the crack for about $20-$30. I've had similar repairs made on suspension pieces for my autocross car and welders like the parts prepped and ready to weld.

Brad

+1 with our TX friend, best way to fix the crack,

... is to fix the crack, with aluminum, welding up, of said crack.

If you decide to go down the glueing route I'd recommend a polyurethane glue instead. You spray a bit of water on the glued surface before bringing the parts together. This causes the glue to foam a bit, which helps fill all the gaps.
A situation like this, a huge area to cover and nicely loaded in shear too, pretty much any old glue with a tensile strength better than chewing gum will do the trick.
And if you allowed it to dry properly I wouldn't even count the gum out....

LocTite breaks down when heated, another reversible bonding /thread lock

several grades made . color coded..

Give some thought to fiberglassing the seat tube. You can get a kit from an auto parts store and do it your self. It's not as good a repair as welding, but it might be worth a try.