- From: "Rick Horwitz" <firstname.lastname@example.org>
Tube bending is based on the diameter and thickness of
the tubing. In addition, the radius of the bend is also
important. I have discovered that most aluminium tubing
can be bent using a standard conduit bender. In practice
I have formed 7/8" with .048 wall, 1" with .058
wall, 1.25" .065 wall with great success. Thinner
tube can be formed but the radius must be larger. Muffler
shops normally have special tube bending machines that
allow even a tighter radius. For an example of tube
bending methodology visit http://www.ihpva.org/com/PracticalInnovations/seat_fabrication.htm
and now his new Tube Bending page.
- From: Glenn <email@example.com>
A large tool can be built from plywood that mimics the
action of the tubing benders used for 1/4 inch copper
tubing. The key is that the mandrill and the roller have
to be concave to match the tubing diameter. Sand may or
may not help. Alternately you could go down to the
hardware store and buy a conduit bender. Typically
referred to as a "Hickey". Artful usage allows
you to bend any radius greater than the minimum the tool
is designed for. I haven't used one for some time, my
best guess is that the minimum radius is around a foot.
- A few other notes on bending tubing-
Wrinkles are caused by the tubing not being fully
supported on the compression side (inside of bend). While
anything you can do to better support the tube (shaping a
nice groove in the die, filling with sand or cerro bend)
will help, it's just plain hard to bend larger diameters
of thin wall tubing without wrinkles.
In 4130, anything with a diameter:wall thickness ratio
over 20 is getting into tough territory. I would plan on
wrinkling a few tubes before getting things right. I
wouldn't attempt anything with a ratio over 30 without a
very fancy mandrel bender.
Professional tubing benders are expensive because they
control so many aspects of the bending process at once
(tubing support, clamping, smooth, steady bending action,
accurate & consistent die alignment). If you're doing
very many bends, though, they are worth every penny.
Here's a checklist of things to try when bending tubing:
1. Anything you can do to make your bending jig more
rigid. 4130 is very strong. If the jig breaks or shifts,
the bend won't be even and will be more likely to
2. Support the tubing fully and evenly (nice groove in
bending die, no gaps between clamps/braces and die, no
sudden or sharp edges touching tube).
3. Ensure the tubing is well clamped and doesn't slide or
slip. This is an important secret. Tubing doesn't want to
bend evenly. It wants to crumple, focussing all the
bending at one point. If it can't slip, it's harder for
all the bending to happen in one place (one end or other
of the tubing has to slide to do this). Imagine holding a
firm tension on the tube as you bend it.
4. Fill with sand (preferably tamped wet sand; you'll
have to do this 1 or 2 inches at a time). I've never been
able to find low temperature metal alloy bending material
(cerro bend?), but it sounds like good stuff.
5. Arrange enough leverage to enable you to bend slowly
and steadily. Repeated lunging and grunting tends to
crumple rather than bend.
6. Large radius bends are a lot easier to do without
wrinkling than small radius bends. Figure anything with a
bend diameter:tubing diameter ratio under 8 or so will be
- Mark Stonich
Minnesota Human Powered
Subject: Re: Lowracer drawing/Large Tube Joint Design
Warren's photo and Felix's drawing show large diameter
tubes that appear to have been butt welded at an angle.
Given the difficulty of getting large diameter, thin wall
tube bent this makes sense. However, unless you have
inserted internal diaphragms to prevent the resulting
elliptical joint from distorting, this could lead to
I learned about this from Peter Ross the British builder
of Trice trikes and Ross Recumbents including the
Festinal Low Racer. He was answering a question about
bending large, thinwall tubes.
Bending large diameter tubes with thin wall thickness is
a very specialised job, and in the United Kingdom there
are not many companies that will do it.
The company must have a machine which draws an internal
mandrel down the tube at the exact point where it is
For many years we could not find anyone to do it, and our
Trice was built using straight tube with an angle. This
is OK but it does not look as good. If you do it with
straight tube you MUST put a diaphragm inside the joint,
otherwise the tube will 'lozenge' at the joint when it
bends and this will rapidly lead to fatigue failure.
A diaphragm can be made of very thin sheet, and is just
an elliptical shaped piece of metal which fills the tube
at the joint (like a wall across the tube.)
I tack with gas weld, and take the tube to my local TIG
welder who makes sure his weld penetrates the tube and
creates a sound joint. TIG welding also looks neater than
most gas welding.
You have also to ensure that the steel you are using has
properties which allow welding without creating cracks.
I asked him;
Is the diaphragm slightly larger or smaller than the
The diaphragm is very slightly larger to allow metal to
be melted back to the original diameter. You could make
it the same size and use more filler rod when you weld.
The important thing is to ensure that all three
components are fused together.
With a pro welder, the larger diaphragm would certainly
save time. Given my limited welding skill, I'd opt for a
smaller one and filler rod to ensure penetration.
BTW the tubes Peter was joining this way were 50mm
diameter and 1mm (0.040 inches) wall. My guess is that
they were Reynolds Chrome Manganese instead of the U.S.
4130 Chrome Molybdenum.
Whether or not you can get away without such a diaphragm
would depend on the angle that the tubes meet at, and the
- From: Robert Parker
- Tube bending is an expensive venture if you must have it
sent out to be done. Fortunately you can build your own
tube bender that will NOT kink your tube. Lindsay publications
has a book out called How to Build a Pipe Bending
Machine. I'm in the process of building one. It even
shows you how to make your own hardwood dyes. A very
impressive design that any home shop could have built in
a couple of weekends.
- From: Marten Gerritsen
Factors influencing the formation of wrinkles are the
tube thickness to diameter ratio and the curve of the
bend. Tight bends in thin-wall tubing (< 20:1) are
therefore more difficult to obtain than bends with large
radii. I've seen mild tubing being bent over a flat
circular form (for Dutch roadster ladybikes), this is
basically the same process as used in bending forkblades.
For higher quality tubing I suppose it is more usual to
bend the tube between three rollers. For tight bends use
a mandrell bender.
Freebending with heat (fill the tube with -dry-blasting
shot, weld the ends shut and use the biggest torch you
can find) is OK for exhausts, but I wouldn't recommend it
for bicycle frames.
Thursday, 29 January 2009