Just a very quick rundown:

With free form hydroforming (pressure washer style, not die forming), the smoother the outside radius the better. A pipe with lots of twists and turns isn't going to be best for free form hydroforming but there are some cheats around that I'll discuss later.

Mgty3whlr, you basically hit the nail on the head. The only thing I have to add is to not skimp on the cutting method, hand shears are going to make a more jagged edge which is going to make it more likely to buckle during inflation.

I used a woodward fab rotary shear:

It works like a gigantic can opener, rolling the metal between a smooth roller and a serrated roller. It's throatless and the work side is the one rolled against the smooth roller so it's smooth. The leave has a serrated edge on it but it's not a huge deal.

This was going to be an OOF pipe for a drag trike so shape was, for the most part, unimportant but I can tell you I shaped and reshaped the inside and outside radii several times to make it easier to cut and pump up.

There are several things to keep in mind here. When you pump it up the difference in inside radius and outside radius means that the pipe will "contract" in on itself. I'm sure there are some very complicated mathematics that can quantify the amount of change but I left passed trig a while back and have no desire to revisit that crap....my advice? Just wing it, the more change between the inside and outside, the more it's going to contract.

The other thing that changes is the tuned length based on the cone angles. The pipe designing program does NOT take into account the changes so I had to do them offline. I did some real quick and dirty math to figure out the difference, pythagorean theorum states that the square of the hypotenuse is equal to the square of the other two sides. We know the other two sides of each cone, Length and Radius (radius being half of the given Diameter) so I figured out how much longer the hypotenuse (the outside metal of the pipe) needed to be roughly and added the extra metal to the sheet before I cut it. Now I say "roughly" because unlike a rolled multi stage cone, the cones on this pipe are smooth transitioning (not a real difference in power, just a difference in the "start" and "stop" of the right angles). For the purpose of this discussion lets assume that the calculations were perfect and lay out a table of "corrected" Length and Diameter for the given pipe.

Now, we still can't start cutting.... we don't actually need Diameter now, do we? The pipe we're drawing needs to be half of the circumference wide and the "corrected" length, long. Recalculate (promise, this is about all of the math which, so far, has only been marginally harder than balancing your checkbook) the table by multiplying given Diameter by Pi and deivide by 2. Then record the new pipe design table as Length and Circumference (really, half circumference but I need something to call it that makes sense!).

Start off with a full sheet of 18-20 gauge steel. 18 is easier to weld, 20 is easier to pump. I had a BOSS 11hp honda pressure washer at my disposal so I went with 18 gauge sheet steel so I wouldn't have to fret about the weldling as much.

Draw the "center" of the pipe in a shape which you think you can make your pipe work with. Remember that we're making a 2D "ballon" so 3D turns will have to be sliced and turned later to make the pipe fit into tight areas. Just go into the drawing stage with an idea of what the 2D balloon would need to look like to include the turns needed. Make the center line the "corrected length" long and put a mark at each of the Length measurements as you go.

At each Length measurement, using a ruler, measure out from the center line at a right angle to the Circumference measurements and make marks. This is where a well laid out pipe design will shine and the first time you make one you'll realize how far down the rabbit hole you actually are Alice... If everything is the correct shape, you should be able to connect the Circumference lines by drawing a nice smooth transitional line between each mark. What you end up with should look, more or less, like this:

Obviously that's a VERY simplified pipe and I've already plasma cut it out from the main sheet but you can sort of get the idea.

A few notes: Leave a little "tail" at the tip of the tail cone for the puckering that will happen. The tail cone won't be perfectly round (more pictures in a bit) and leave a "spade" at the headpipe end for forming around the fitting and drawing in that will happen as a result. You can see the spade left in the picture above but I didn't leave enough of a tail on the tailcone for this pipe.

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