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Hammer Inserts
You can buy hammer inserts at the major track & field suppliers, or you can make your own out of plywood. These are the filler pieces that you place inside the discus rim to reduce the radius for a hammer throw competition.
There are two basic configurations. The first requires twice the amount of plywood (2 pieces instead of one), but requires less measuring. The second requires more measuring but less plywood.
Design#1:
You will need 2 pieces of pressure-treated (outdoor grade) plywood, 4'x8'x3/4" (more precisely, the plywood thickness is 23/32"). Alternatively, you can use indoor wood, and treat all surfaces and edges with a wood preservative.
First mark the midpoint (4') along one long edge of each piece. This will be the center of your radius.
Now, you should check with your discus circle to make sure your discus ring is circular. Sometimes circles can be oval or have some irregularity. This will effect the cut of your outside radius.
Assuming your circle is perfect, Draw a 1.25 meter (4'1 1/4") radius arc cutting off both opposite corners of the plywood. Because the arc is about an inch more than the length of the plywood, the arc will go off the plywood at the sides.
Cut along this line with a jigsaw.
Now try to fit the two semicircles into the discus ring. Right now, the hammer circle is not cut out--this keeps the insert from bending during fitting so that we can get the best fit.
Ideally, the pieces should fit with the seam running sideways so that the seam can also act as a foul line. If it doesn't quite fit, use a hand sander to sand the arc down. Do not sand the uncut straight piece.
(Note: if you are laying out a new discus ring, this piece can be used as a guide to layout the ring perfectly circular.)
Do not round the edges. Sand only at a 90-degree angle from the face of the insert.
Now measure another arc 3'6" from the same radius reference point. This will be a continuous arc that will be 7 1/4" inside the other arc. Cut with a jigsaw. Sand smooth, but do not round edges.
Option 1: connect the two pieces together with hinges, so you can fold them together. This is helpful if you do not have a discus rim to keep the hammer insert in place.
Option 2: for easier storage and transportation, cut each piece in equal halves to produce 4 equal pieces.
Design #2:
One piece of plywood.
Draw a reference line down the center of the plywood dividing it lengthwise into two 2'x8' pieces (green in figure). On this line, mark 6 points (red dots) at the following locations: 1", 10", 19", 28", 37", and 46", from one end. From each of these reference points you will draw 2 arcs that define the outer and inner curved edges of the insert pieces. The radius of the outside edge is 49 1/4", and the radius of the inside edge is 42".
From the 46" reference point, the outside radius should run to within 3/4" of the opposite plywood edge. The reference points are the pivot points for your tape measure or jig.
Mark all inside and outside radii of all six inserts first, then mark the sides of the six inserts. The sides are exactly 42" wide at the inside arc. Therefore, draw two more lines parallel to the reference center line, 3 inches in from both 8' sides of the plywood (blue lines). Connect the intersection of the 3" lines and the inner arc with the corresponding reference point (magenta lines). With a straight edge, extend this line across toward the outer arc. Note that this line will go off the edge of the plywood before crossing the outer arc.
Now cut out the inserts starting with the one furthest out. Cut the outer radius first, then make the two side cuts, then make a relief cut on each side to get rid of the extra triangle outside the sides, then finally cut the inside radius. You can use a jig or tie your saw to a string attached to the pivot point to help you guide your saw (although it will still have to be guided by hand control).
If you did it right, you should have 6 identical insert pieces with a small triangular area uncovered at each corner of the outside radius.
Before you sand smooth the edges, take the inserts to the discus circle for a fitting. If they don't fit, in a pinch, you can use 5/6 inserts. Put 2 together along the back of the circle, and 3 together along the front of the circle. There wil be gaps at the sides, but nobody uses this part of the circle anyway.
To fit the arcs: Fit all 6 in together snug, with one overlap. Measure the radius. If it is 7', you want to sand only the straight edges between each arc piece--especially adajacent to gaps due to wobbly fit. If it is less than 7', then you want to sand everywhere that you see a bad fit--the inside arc, the outside arc, and the straight sides of each insert piece. If the 6 pieces fit without overlap, but the inner diameter is still less than 7', sand only the inner arc to increase the diameter.
Care: Use all 6 arcs for competition. Lock up 4, but leave 2 out for hammer practice, telling throwers to use the arcs at the back of the circle to protect the discus rim from dings by the hammer head.
Tell your throwers NOT to leave the inserts in the discus circle or else they will get waterlogged when the circle fills with rainwater.
Additional use: you can drill and bolt 4 or 5 stacked inserts together directly onto your indoor plywood practice circle to make a practice toeboard. Countersink the bolt heads or tails for safety.