Our daughter has been helping a friend learn to ride a bike (at age 15: it’s never too late!) and we’ve been rehabilitating a new-to-her bike in the process. It’s an inexpensive Ross bike, perfect for the task at hand, and is providing a good introduction to machine-shop work.
The fact that it’s much older than she is makes not a whit of difference. Nay, verily, I rode a bike pretty much like this one for hundreds & hundreds of miles back in the day. I got better ones when I could afford them and she will, too; maybe we’ll tempt her into a recumbent bike some day…
Anyhow, the seat tended to spin around even with the clamp cranked dangerously tight. Taking a look down the tube showed that they used welded-seam tubing (it really was an inexpensive bike) and didn’t bother to clean up the internal seam. As a result, the chromed steel seat post rested on maybe three small patches of metal that didn’t provide much friction at all.
I wrapped a neodymium magnet in a rag and stuffed it down the tube to catch the filings, then applied a coarse cylindrical file (a rat-tail would work as well) to the seam. When it was nearly flush, I switched to a finer file to smooth it and the other high spots. The picture shows the improved seam, ready for the seat post. Ugly, but rough is actually a Good Thing in this situation.
The seat tube has a nominal 1-inch OD, so I clamped a random round from the heap in the vise, tapped the clamp around it, and massaged it lightly with a hammer to persuade it into a more cylindrical shape. It’s still not perfect, but at least the bolt lugs engage the seat tube around the slit somewhat better.
With all that in hand, the seat post is now perfectly secure.
On her first “I can ride!” parking-lot outing, she experimentally determined that a bicycle wheel’s lowest-energy state resembles the edge of a potato chip. Fortunately, it was the front wheel and, after a bit more shop derring-do than one might wish, we swapped in another wheel that’s been hanging on the garage wall for a decade, ready for just such an occasion.
Remember how independent your first bike made you feel? It’s working that way for those two, just like it did for us. Life is full of bumps and they’ll get hurt every now and then, but there’s no other way to get through it; they’re just about ready to ride over the horizon.
Happy Independence Day for those of us in the USA!