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**Rectangular steel tubing bend procedure.**

The thread Mongoose started struck a cord with me as I have often done this sort of thing in engineering; architectural; motorcycles; and artistic, where with the help of my cousin, a quite accomplished structural artist and fabricator, we first figured out this procedure.

**Getting started**

See illustration below.

The first thing you need to do is draft up an accurate template of the stock frame and wheel setup, particularly the width of the wheel assembly including the brake assembly and how the braking loads are fed to the frame or swingarm, the width of the frame mount points, the relative distance of the frame pivot to the axle with the chain adjusters in mid range and the anticipated travel of the adjusters. With all that information it is relatively simple to knock out the basic layout of the main arms, and most importantly the necessary angle, desired radius, and location of the bends. You knew I would get there eventually, right? Once there the fun starts.

**Initial Layout**

See illustration below.

When drafting the bend cutouts with CAD or a drafting board (does anyone still have one of those?) it is easiest if you use the center of the bend as a vertical origin and the center of the bend radius as the horizontal (x) origin, and the profile a “V” shape with both sides half of the desired final angle. For ease of illustration I’m using 2 x 3 inch tubing with a wall thickness of 0.083 inch, a radius of ten inches, and a bend angle of twenty degrees. Draft two lines (Red) of a length of ten inches from the origin at two hundred sixty and two hundred eighty degrees respectively. A curve of ten inches minus the stock wall thickness including the total width of the chamfer drafted at the origin to the two lines is the radius of the cuts you need to make (Green). Now draft a second radius of ten inches to twenty degrees centered on the origin (Blue). This is the outside of the bend. Draft two straight lines ten inches long tangent to the arc you just drafted to establish the outer constraint (Magenta), then two lines two inches long to the ends of the outer constraint lines at a right angle (Cyan). Next, draft two lines parallel to the outer constraint until they meet at y = 0 (Orange), generating the inner constraint. Just one more line from the junction of the two Orange lines to the center of the Green arc (Black). When flattened out this is the path for the center cuts.

**Finishing the cutting pattern**

See illustration below.

In order to make a useful pattern it is necessary to flatten out the draft you just did. This is where the elementary math mentioned before comes into play. Using the formula Length = Radius x Angle x ( / 180), determine the actual length of one of the Green arcs generated previously. Plugging the values into the formula thus: 9.834 x 10 x .01745329 = 1.716357 inches. Double that and you have the length of the straight lines replacing the arcs previously drafted. Rotate the Magenta lines on the left side down ten degrees, and the Brown lines on the right down ten degrees as well. Join the sides with the calculated length lines and the pattern is ready to be printed and cut out. Using heavy photo paper print two on the same sheet, one for the top and the other for the bottom. When cutting out leave a wide section of the center of the “V” connected. Once they are both cut out invert one (printed side down), lay them on top of each other, carefully align the cutout patterns, then trim the ends so both are exactly the same length. This facilitates aligning the patterns when applying them to the stock.

**Applying patterns to the stock**

See illustration below.

First, clean and degrease the stock in the area where the patterns are to be applied. Use acetone for a final wipe.

It is of paramount importance to apply the patterns accurately to insure the alignment is as exact as possible, and to that end I use two rectangular pieces of 1 x 6 wood cut exactly to right angles clamped to the three inch sides of the stock and carefully aligned to each other across the stock, but also an exact as possible right angle to the horizontal surface, a two three four block works well for this, then c-clamp them to the stock. This serves as an accurate location when placing the patterns. To apply the patterns I use a 3M product called Spra-Ment: a mild spray adhesive commonly available at Michael’s, Hobby Lobby, and pretty much any craft store worth its salt. Spray an even layer on the non printed side of the pattern, allow to set for the time specified in the directions, then place a 6 inch steel scale against the edges of the boards and butt the edge of the pattern against the scale, then roll it down until it is firmly adhered to the stock. No need to adhere it to the chamfer. Repeat on the other side. When the adhesive is cured remove the wood and cut out the piece of paper inside the “V” on both sides using a scalpel and straightedge but do not peel them off yet. Place a straightedge on the long side of the stock and line it up with the edges of the top and bottom “V” cuts, then scribe across the stock. Clean up any residual adhesive. You are now ready to mark the stock. Using a good sharp scribe score the stock all the way around the cutout pattern. It is not possible to accurately scribe across the chamfer, so don‘t bother. Once that is done spray paint the top and bottom avoiding the scribed marks on the wide side.

Elaboration on the chain adjusters

If you noticed the eccentric chain adjusters in the first illustration look a bit different, they are. The one problem with eccentric adjusters is while adjusting the chain you are also changing the ride height. In an effort to overcome this I came up with double eccentric adjusters which allow adjustments in both chain tension and ride height independently. Actually, it was originally designed for a custom GSXR frame with the adjusters mounted to the pivot which allowed fine adjustments to the pivot / countershaft angle and distance to optimize anti squat and mechanical traction. Worked like a charm. That, and it was fun to draft it up.

That's it for now.

Rob