A Perfect Bowed Cross Spar

How To Make It From Wooden Dowel And Kite Line

Cross Spar - finished dowel spar


This page shows you how to make a bowed cross spar for an MBK Dowel kite. Of course, these instructions could also be applied to a range of other spar materials and situations.

The photo up there is the finished horizontal spar for my first Dowel bowed kite, the Dowel Diamond. In the instructions below, the letters DL stand for Dowel Length. For example, 0.5DL means 'half the length of the dowel'.



Attaching The Cross Spar Bow Line

Here is the procedure I used to make a bowed cross spar, after the dowel had been cut to length and the tips rounded off with a wood file. Also, a mark had been drawn all around the spar at the exact mid-point, which is needed to balance the finished spar.

  1. By eye, see if there is already a slight bow in the spar. If so, make a small mark exactly 0.05DL from one tip, on the convex side of the dowel. If not, just make the mark anywhere, 0.5DL from the tip.
  2. Make another similar mark at the other end of the dowel, ensuring that the 2 marks line up. For example, if one mark is exactly facing the floor, so should the other one.
  3. Using the edge of your wood file, put a notch into the wood at one of the marks. Strength is not an issue out near the tip of the spar, so you can make the notch fairly deep.. Up to a 1/4 of the diameter of the dowel is OK.
  4. Do exactly the same for the other end of the dowel. Like you did for the marks, make sure the 2 notches are facing the same direction.
  5. Take a length of flying line 0.6DL long and tie a Simple Overhand Loop into one end, making sure there is ample room for a short piece of dowel to fit through the loop.
  6. Tie the other end of the line around the dowel, seating the line in one of the notches. Use 3 Half-Hitches. Slide them down tight, positioning the knot on the opposite side of the dowel from the notch. The loop should just reach the exact center of the spar.
  7. Take another length of flying line, also about 0.6DL long. Take a 0.02DL length of dowel, notch it in the middle with the file and tie the line to it. Just like before, use Half-Hitches. Let's call this the Toggle.
  8. Thread the Toggle through the overhand loop of the first line. Bend the spar until both ends are a fraction more than 0.1DL above the table, or whatever the recommended amount of bow is. Loop the line through the remaining notch, wind it round 4 times and tie off with 3 half-hitches. It took me several attempts to succeed - so persevere. This is definitely the hardest part! It's like stringing a bow.
  9. Double-check that the required amount of bow remains in the dowel, by putting the spar next to a straight line and measuring from the line to each tip. The edge of a table works well, with the bow line parallel to the edge.

    10. A non-symmetrical bow in the cross spar will show up sooner or later as wind strength increases. The kite will tend to lean in one direction or the other, severely limiting how high it can fly. Very annoying! Balance is not quite so important, but it also has an effect. Why not take a few minutes to eliminate both these potential pit-falls before the kite has its first flight...



    Checking Cross Spar Curvature

    Cross Spar - curvature diagram


    1. While the spar is still bowed, lay it down on a smooth flat surface. Concrete or cement is ideal.
    2. Using a piece of colored chalk, trace around the tips and all along the convex side of the spar. Keep the spar firmly in place with your foot in the middle. Trace lightly so nothing moves, and don't worry about tracing the area under your foot.
    3. Swap the tips around, carefully lining them up with the chalk marks at each end. The curvature of the spar should follow the chalk line you drew.
    4. Take another piece of chalk, of a different color. Again, except for the bit under your foot, trace along the entire convex side of the spar.
    5. Move the spar away from the chalk lines and look carefully. If one side is bending even a little more than the other, it will be clear! Parts of the first chalk line will show, above and below the second line, as in the diagram above.
    6. Figure out which side of the spar is bending less than the other, from what you have just done.
    7. While the spar is still under tension, file up and down the concave side of the dowel, taking off a small amount of wood from the stiffer side. Do the chalk test all over again. Repeat until the 2 chalk lines line up much better. They will never line up exactly, since the correction is being done very crudely, but you should be able to get a big improvement!

    Note: If no cement is handy, a table-top and colored pencils could be used. That is, if you don't mind erasing pencil marks from the table afterwards!



    Checking Cross Spar Balance

    1. While the spar is still bowed, place the concave side on a sharp edge, right over the center mark. That mark had better be dead-center! Double check it if you like, with the spar out straight of course. A cutlery knife is sharp enough to do this balance check.
    2. If one end of the spar droops a bit, get your wood file and remove some wood from the heavier tip. File on the concave side of the dowel, between the bow-line attachment and the tip.
    3. Keep checking the balance until it gets hard to tell which end of the spar is heavier. That will be quite accurate enough!

    Note: Just adding a little insulation tape on the lighter tip might be easier, but why add weight un-necessarily? All other things being equal, the lighter the kite, the better!



    The Final Touch

    Remove the Toggle from its loop, and check that every Half Hitch is tight, in all 3 sets. That's one at each end of the cross spar, plus the Toggle. With the dowel now straight again, put a generous drop of glue on every set of Half-Hitches to secure them permanently. It's a good idea to slip a piece of paper under each knot to catch any excess glue.

    Hope you found these instructions on making a bowed cross spar handy!


    Last updated: 22 Jan 2009

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