I have an aluminum frameset that has a seat tube that is failing. The clamping mechanism, if you can imagine, is a bolt with angled washers just behind the seatpost. You tighten the bolt, the washers push forward on the seatpost against the foremost wall of the seat tube.
Over the years of overtightening and abuse, the seat tube has begun to stretch and ovalize a bit from this outward force. Recently a few cracks appeared in the tube. One is right at the weld to the top tube. The crack is from the deformation of the seat tube, not stress from an accident or fatigue from hard use. I looked into sending it in for repair, but it’d be like ~$300 to have it re-welded and heat treated, plus shipping, plus a new paint job. It’d probably set me back $600 to do all that. Plus, the shop didn’t wasn’t very comfortable with the repair, and I don’t like the frame enough to spend $600 repairing it!
So I theorized that if I could find a seatpost with a sufficiently thin wall, I could use an industrial or automotive epoxy or adhesive, (similar to those used by companies attaching carbon tubes to metal lugs), and basically reinforce the seat tube. Heck, they are using adhesives to glue airplane wings together, I think it should hold my bike together.
Titanium is the only metal I’ve found that is in the 1mm wall thickness range. The thickness of the seatpost I plan on using is the key though, because an inside diameter of 25.2 would allow me to cut off the top of the titanium seatpost I’m going to epoxy in the seat tube, and insert a standard 25.0mm seatpost. if I ever need to adjust it. I’ll probably just epoxy it in set for my height, and if I ever need to change the height I’ll cut it off about an inch above the frame, add a hole and a slit, use a 31.8 to 27.2 shim and a standard frame seatpost clamp with a 25.0mm post.
This repair job will be significantly cheaper, and while it’s a bit more ‘micky mouse’ than doing it properly, it’ll only set me back ~$200 and it would save the original paint. It’s a time trial frame so it’s not taking a lot of beating, and because of my stubby legs and long upper body, I"m only using about 4 inches of seat post outside the frame anyway. I’ve never heard of a repair like this, but I think it’ll work.
The tricky part will be preparing the titanium and aluminum tubes to accept the adhesive, and choosing an adhesive that will work well with both metals.
This repair could work for just about any seat tube damage. Perhaps your seat tube was crushed in the middle of your frame in an accident? Cut out the bad section and epoxy a seatpost with the top cut off in it’s place as a new tube. Seatposts are designed to be much stronger than a typical frame tube, but the weak link would be the adhesive. I’ve read that they will withstand something like 3000 lbs.
Any thoughts or comments on this process? Feel free to tell me it’s a terrible idea and I should just go buy a new Cervelo.