This past weekend, I fixed my bike seat (see Broken bike seat and Flexible adhesives).
The idea of the fix was simple: to insert at 15mm diameter aluminum rod (6061 alloy, tempered to T6) that spans the break and the high-stress point on the other side of the clamp, which is also scored and likely to fracture, and glue it in place.
I found the rod I need at McMaster-Carr, with specifications of 15±0.23 mm diameter. I thought it was slightly undersized at 14.7mm, measuring with my calipers, but my calipers are not so precise that I can be sure of that measurement. Measuring with my micrometer gave 14.82±0.01mm, which is within spec. I could have gotten a more precisely specified aluminum rod for an extra $10, but I did not think that it was worth the extra price—I may come to regret that, as the aluminum rod fits loosely, not snugly inside the tubing. I cut the rod to length with a hacksaw (using oil to lubricate the blade) and ground the sharp edges down with a wet wheel, then sanded the rod both to round the ends and to remove most of the oxide coating and provide a rough surface for the epoxy. The rounded ends are to reduce the stress raisers at the ends of the rods, as that is now where the tubing should flex the most.
Aluminum rod: cut, ground, and sanded. The mark indicates where the break in the tubing should be. The rod extends about 15mm past either end of where the clamp holds the tubing. I would have extended it further, but I was worried about bending the aluminum tubing too far to open up the bike seat frame enough to get over the end of the tubing.
I sanded the inside of the tubing on the broken bike seat and the fractured ends to promote adhesion, and I cleaned both the rod and the tubing with rubbing alcohol.
In the comments on Broken bike seat, gflint suggested a silicone adhesive, but I ended up using a flexible epoxy, TotalBoat FlexEpox, that is designed for repairing boats. It is supposed to adhere well to aluminum, have high tensile and flexural strength, and a very slow set time, plus it is fairly cheap at $18 for about 30 times more epoxy than I needed.
I mixed up tiny amounts of the epoxy in old cough-syrup measuring cups:
The cough-syrup cups did not make precision measurement of the resin and hardener easy, as both were quite viscous—I hope I got close to equal quantities. I suppose I should have gotten out the centigram scale and mixed by weight—that would probably have given a more precise ratio.
For the first gluing on Saturday, I coated both the inside of the tubing and the rod with the epoxy and inserted it to the desired depth. I wiped off excess (there was a lot that oozed out) with a paper towel wet with rubbing alcohol, and clamped the rod in place simple by letting the other part of the tubing press against the rod sideways. I used a little scrap of paper towel between the rod and the outside of the tubing to keep them from getting glued together:
The rod glued and clamped in place, with paper towel to keep the rod from sticking to the outside of the other part of the tubing.
On Sunday, I mixed more epoxy and coated the inside of the other piece of tubing and the piece of the aluminum rod sticking out, then lined up the tubing and let it close back together:
Excess glue squeezed out of the joint.
I clamped the joint together by wrapping bungee cords around the frame of the seat, and wiped off the excess glue with a paper towel moistened with rubbing alcohol:
The bungee cords provided a fairly large clamping force, but no more glue oozed out when it was added, so the springiness of the frame alone may have been sufficient.
On Monday, I worked on the clamp on the bike.
I rounded the edges of the clamp to reduce the stress raising that had led to the first failure—the edges had been quite sharp.
In addition to rounding the edges to reduce the stress raising, I also offset the seat by a few millimeters from where it had been, so that the glued joint is now slightly inside the clamp.
Before replacing the seat, I noticed that there was a bad rust spot behind the clamp, so I took the clamp apart to look at the frame:
The frame was quite rusty behind and inside the clamp.
I was going to sand the rust spots, prime with metal primer, and repaint the spots, but my metal primer was no longer any good, so I just did some light sanding and coated all the rusty spots with oil. I will have to buy some more metal primer and repaint in a couple of weeks when I next have time to work on the bike.
I put the bike seat back in the clamp and replaced the mesh seat, which had been washed to remove the road grime.
I plan to let the epoxy cure for another day before riding the bike (they claim a 7–10-hour cure time, 24 hours for high load, but I’ll go a little longer, as it is going to rain tomorrow anyway).
I’m hopeful that this fix, which cost under $50, will let the bike seat be usable for the next 15 years.