Forming tapered tubes for bicycle frames
Mon, October 10, 2022 I was recently asked, “How are tapered tubes formed for steel bicycle frames, like chainstays, seatstays, and fork blades?”
When I had my frame building shop in Worcester, England, I was only 25 miles from the Reynolds tube factory in Birmingham, and I would often visit.
I have witnessed first-hand the various processes of tube manipulation, and the machinery used for that process. Also, I was a Machine Tool Engineer prior to building bicycle frames, so I understood the workings of these special purpose machines.
I could not find a video of the actual Reynolds machines, but I did find the one above of a tapered tube being formed. Not a bicyle tube, but the principle is the same. As you can see the process is done “Cold.”
The tube walls are relatively thin and flexible enough to be manipulated without heat, using the required force, usually applied by mechanical means or hydraulic oil pressure. The process, (If one thinks about it.) is not unlike a potter forming pottery from soft clay on a wheel.
You will also notice the tube is held in the machine at both ends, so it cannot grow in length as its diameter is squeezed smaller. That material must go somewhere, therefore, the wall thickness of the tube becomes gradually thicker towards the end, as the tube becomes smaller in diameter.
This is desirable, as the finished tube has a uniform strength throughout its length. Also, this cold manipulation “work hardens” the material, making it stronger by compressing the molecules or particles of the steel, tighter together.
These machines, once set run automatically. A long length of plain tube is fed into the machine, the taper is formed at one end and is then cut off automatically, and the tube auto-feeds in again. When the length of tube is used another again automatically takes up its place.
You will also notice that there is a liquid coolant running over the tube being worked during this process. This is usually in the form of a mixture of soluble oil and water.
Without it the pressure and the friction generated by this process would create a great amount of heat. This would not be good for the machine, or the product being produced.
I suggest you watch the video in “Full Screen” mode to fully appreciate the mechanical process involved. Here is the YouTube link if needed. https://www.youtube.com/watch?v=ZAnzRRjKPpc&t=3s
Reader Comments (4)
It is more common to do this forming with rotary swaging.
I have 2, 4, 6, and 8 segment die machines.
The OD of the tube will end up whatever profile the die has.
If you also want to change the ID you do this over a solid mandrel bar.
https://www.youtube.com/watch?v=HCqIKKL-qq0
https://www.youtube.com/watch?v=LFly3VRqjiE
You did not mention that Reynolds made their high end fork blades from taper gauge tubing. This had thinner walls at the bottom, so the blade did not get as stiff after it was tapered..
So Bates Cantiflex and Diandrant tubes are the result of some
Reynolds tubing boffins having few too many pints at lunch ?
Oi, Alf, what if we run the tube swagger backwards ?
A few experiments latter.....
Oh my what are we going to do with these cocked up tubes ?
Ahh, we can sell them to that Bates chap, he can keep a secret.
And then of course there is hydroforming.
And the old favorite, let's use a press and make this an oval.
The one thing that you really have to watch is what any of these forming methods does to the mechanical properties.
All metals will strengthen as you work them. Some a little (plain steels), some a lot (most Stainless steels), and some dislike being cold worked (Ti). In general when they get stronger, they lose ductility.
If you want to do very sever forming you often need to re-anneal partway through the process.