On tube bending

A local company is trying to bend some 3/4" 4130 tubing to use as stays on BMX frames. I hope to help them make this happen.

I did a little research on this and found this page about bending tubing for recumbents:

Apparently, bends become difficult or impossible when
( tube diameter / wall thickness )
exceeds 20 : 1.

The .75" x .035" wall tube they've been experimenting with has a ratio of 21.4 : 1, which has led to serious crinkling.

So I will recommend either decreasing tube diameter, like to 5/8":
5/8" (.625") x .035" wall = 17.9 : 1

or, better yet, increasing wall thickness, like to .049":
.75" x .049" wall = 15.3 : 1

Of course, changing this tube spec will affect the frame design. Larger-diameter, thinner-walled tubing is generally stiffer (up to a point) than smaller-diameter, thicker-walled tubing.

Furthermore, bends become difficult or impossible when
( bending diameter / tubing diameter )
is below 8 : 1.

Hence .75" tubing ought to be bent on at least a 3" radius mandrel, which is pretty reasonable. A fork blade bent around a 6" radius is considered "tight".

And of course, the bending rig should accommodate the tubing as described in the link above: good rigidity, mandrels shaped to fit the tube closely, etc.

No comments: