Sand bending is not a practicable method of bending chassis tubes, as the constant heating and re-heating of the tube during the process would almost certainly lead to failure of the tube if used in a stressed situation such as a chassis.
However, exhaust tube is often very thin walled - 1.2mm-1.6mm (.0469"-.062"), and virtually impossible to bend cleanly without the use of very expensive mandrel bending equipment, and sand-bending is a perfectly suited technique for forming un-stressed exhaust systems. It is invaluable where the exhaust tube must twist and turn in configurations not possible with moderately priced tube benders.

I have covered the making of sectional exhaust systems in Make Your Own Exhaust System, but I offer this article as an alternative method of forming tubes for the more adventurous, or for those who prefer the looks of single-piece “pipes”.

The process basically involves stuffing sand tightly into a tube, sealing both ends, and then applying heat to the tube until it becomes pliable. The sand prevents the tube from collapsing, and in most instances, retains the concentricity of the tube throughout the bend.

There are a number of safety issues concerned with applying pressure to (hot) plasticised metal that must be observed. Any latent pressure within the sealed tube could rupture the tube wall, particularly if it were red hot, and shower hot or molten metal about the place. Therefore, the sand used for stuffing into the tube must be bone dry to avoid the production of steam and consequently, internal pressure.
The best sand to use is a coarse sand such as washed builder’s sand, but it must be thoroughly dried overnight on a shallow tray placed in a low oven.

Before any bending begins, you’ll need to make a “squash pipe”. The squash pipe is a very simple tubular device, which is stouter than the thin-wall exhaust tube you’ll be using, and must be securely welded to the tube you wish to bend. Being thick-walled, the squash pipe can be held tightly in a vice while you work on the attached thin-wall exhaust tube.

The squash pipe incorporates a mechanical tamping device for ensuring the sand is soundly packed into the tube to be bent. It sounds highly technical and complicated, but consists of nothing more than a large nut welded over the open end of the pipe into which a bolt is inserted. Tightening the bolt further compacts the sand inside the tube to be bent.

Most motorbike headers you’ll come across will be 38.1mm (1½") OD so the best size of pipe to use for the squash pipe is DN 32 x 3.2mm wall CHS (1¼" x .125" NB pipe) which is the standard black pipe available at every steel yard. You’ll need a piece approximately 250mm (10") long.

The only other items required are a 1¼" UNC nut and bolt, and a length of 32mm (1¼") wooden dowel or curtain pole to use as a ram rod.
 

Putting It All Into Practice. 

• Using a set square, make sure the ends of the length of pipe are perfectly square.
• Place the nut face down on a flat surface and carefully centre the piece of pipe on top of it.
• Make a few tack welds to hold the nut onto the pipe. Screw the bolt into the nut and check that it can enter the pipe without fouling it. If all is well, finish welding the nut to the pipe.
• To form your first bend, cut a length of exhaust tube slightly longer than the proposed finished tube.
• Flatten one end of the tube with a hammer or in the vice, and weld the flattened end to seal it.
• Carefully weld the other end of the tube to the open end of the squash pipe.
• Start pouring the dried sand into the tube through the large nut.
• Thump the welded end of the tube onto the floor as you pour to help settle and compact the sand. The ram rod can also be used to help compact the sand.
• Continue filling with sand until it reaches the bottom of the nut, and then give the squash pipe a couple of taps to clear any sand from the nut's threads.
• At this point, the bolt is inserted into the nut and tightened down onto the sand.
• Secure the squash pipe in the vice and then taking a piece of softwood, start tapping lightly (so as not to dent the thin wall tube) up and down the length of the tube while tightening the bolt.
• When the bolt won’t tighten any further using reasonable force, the tube is ready for forming.
• If possible, place the whole thing into a hot oven to pre-heat the tube and, more importantly, the sand. The sand can really suck heat away when using the Oxy set to heat the tube.
  Make sure you wear heavy leather gloves when handling the heated tube.

• Using a rosebud tip (not a welding tip), uniformly heat the area of the tube to be bent to a dull red colour.
  
   

• Slowly start bending the tube, pulling it as you bend it to help avoid any sharp kinks.
  
   

• Regularly check the progress of the bend against any wire or sheet patterns you may have made.

• If making multiple bends in the tube in close proximity, make only one bend at a time, allowing the tube to cool between bends.

• When the bending is completed, and the whole tube and pipe has cooled down, remove the bolt from the squash pipe and empty out the sand.

• Cut the squash pipe from the tube and clean off the welds on the grinder ready for the next time you need it.

• If the freshly bent tube has a lot of scale in the heated areas, it can be cleaned off with some emery ribbon wrapped around the tube and pulled to and fro.