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Indoor portable throwing facilities

This page describes the construction of plywood-based throwing facilities. Typically used indoors, but they could also be used outdoors in fair (nonrainy) conditions. I suppose they could be constructed with pressure-treated wood if there is a possibility of outdoor use in the rain. The sections are arranged from easiest/least expensive to hardest/most expensive. Of course, for a permanent circle, one can always construct an outdoor-style concrete circle in an indoor facility. In this case, refer to the Outdoor circle construction page.

Simplest: a single sheet of plywood

Budget: $100, Estimated cost: $50.
Materials: 1 x 1/2" (or 15/32") thick, 4'x8' plywood sheet; 1 tube of heavy duty Liquid Nails brand or similar adhesive; Something to spread adhesive: 2" wide Plastic sawtooth edge grout knife 17 linear feet of 21" to 2' wide noslip foam rubber carpet padding Staple gun and staples

The simplest practice circle is a sheet of plywood laid on the floor. Use a thickness of 1/2" (or 15/32"). Get one with one sanded side (paint quality).

One sheet of plywood is 4'x 8'. This is long enough to take care of the front and back of the circle. You will be missing 1 1/2 feet of circle on each side, but you never need to use this part of the circle anyway. You can even get away with a 4'x8' piece of plywood for discus practice. The edges of the circle will overlap only at the four corners of the plywood sheet.

The main concern is keeping the plywood from slipping during the throw. On most surfaces, this is solved by an application of the no-slip carpet padding. This comes in 2' wide rolls, is available at Home Depot, sold by the linear foot. Buy 17' and cut in half to make 2 8'6" lengths. If you only have a smaller Home Depot store, the pads are only available as precut 2'x4' sizes. You will need to buy 4 of these.

Place the plywood on newspapers with the good side down. Spray adhesives do not work--you need a good construction adhesive such as Liquid Nails. Apply beads of the caulk with a caulking gun, then use a plastic grout knife to spread the beads to cover the entire surface. Contact cement out of a can may also work, but I haven't tried that.

Have two people hold each end of the noslip padding. Carefully place the padding so that 3" extends over each end and about 1/2" hangs over the long edge. Do the other side. If done right, you should have a gap down the middle.

With a staple gun, pull taut and staple the noslip padding around the side edge of the plywood. Do NOT staple on the bottom--this may cause scratches on good floors (e.g. a basketball court). Trim off the excess on each end and discard.

If you are using the precut 2'x4' pieces, these pieces are cut to fit 2'x4' area rugs without showing, so they are actually about 1'10"x3'10". Place each pad in each corner, starting about 1" from the edge. Trace the outline of the pads with a pencil onto the plywood. Remove the padding and apply the adhesive just inside the outlined rectangles. You will have a cross pattern of a few inches thick uncovered.

Turn over and place on newspaper over flat surface. Place weights on the plywood to facilitate bonding. The adhesive may need 48 hours to dry completely. Since the noslip padding is porous, do not place on good floor for 2 days without newspaper.

The noslip padding works well to both protect good floors and to keep the circle from slipping. To insure the best performance, make sure the floor is dust-free before laying down the circle. If it still slips, you then have to resort to placing weight plates on the corners of the circle. This is not recommended because it can cause a tripping hazard for fouling throwers.

If your circle is going to be placed on top of artificial turf, treading may be more effective than the noslip padding. Treading is done with a 1/2" diameter square routing bit, set at 1/8" depth. This produces very sharp edges which the grain of the turf grabs onto and prevents slippage. The treads should be straight and both parallel and perpendicular to the grain/weave of the artificial turf. See below for a recommended tread pattern.

Marking the indoor circle

The options are to either 1) draw the circle on the plywood or 2) add a rim and/or toeboard. The first step is just drawing a circle on the plywood with a marker. Later, you can upgrade by adding a rim.

I recommend drawing the circle in the exact center so that you have exactly 6" on both the front and back, and that the center is exactly 2' from both sides.

If you are not going to get a rim for a while, you can paint the circle edge using flat white paint. I recommend all circle surfaces use flat (not gloss or semi-gloss) paint. If you use spray paint, make sure it is flat finish.

On the practice circles, you can paint practice footwork lines within the circle. On the shot and discus practice circle, outline a 16" diameter circle right in the middle of the circle. This is the target for placing the pivot foot in the center of the circle for the shot and discus.

Also outline 16" diameter circles centered 30" apart at both ends of the circle. This will mark the starting point of the spin in the back as well as the block foot in the power position in the front.

Finally, connect all 4 end circles with the center circle by 2" lines. These represent rough extensions of the sector, and serve as the drive lines from the South African position. Figure 5 illustrates the pattern of the finished practice circles.

Practice circle with rim/toeboard Budget: $100 w/ rim only, $250 w/ toeboard.
Estimated cost: $60 w rim only, $210 w/ toeboard.

Even if you have no budget for another toeboard and are not willing to, or can't, remove the outdoor toeboard; a rim really gives the thrower a better feel for the circle and greatly adds to the throwers' comfort level when going into competition.

There are two options: adding a piece of old bolt-on rim or cutting a rim out of plywood. If you happen to have some old rim that comes in 4 sections, you can bolt on one section if the bolt holes on one piece are less than 4' apart. If they are more than 4' apart, you can't bolt it onto a 4' wide piece of plywood.

If you can bolt it on, place the bolt head on the bottom and the nut on the top. Countersink for the bolt head on the underside at most 3/16" depth, preferably 1/8". You have to weigh the possibility of scratching the floor with the bolt heads against the thickness of the plywood. Keep in mind the bolt heads will pull in as the plywood compresses during tightening.

I recommend making a plywood rim. You need two remnant/scrap pieces of plywood that are 23/32" (or 3/4") thick. The pieces must be exactly 4' wide, at least 12" long, and have one original, square end. Alternatively, you can buy a piece of 2'x4' plywood and cut it lengthwise to form two pieces of 1'x4'.

Draw a 3'6" radius arc centered on the plywood sides, with the edge 6" from the end. Cut the arc with a table band saw, a hand jigsaw, or a reciprocating saw. Cut it 1/16" short and sand smooth with a drum sander or a cyllander sanding bit on a drill. When cutting and sanding, keep in mind you want to keep the cut square and sharp (no rounded edges).

On a practice circle, I recommend attaching the rim to the circle with drywall screws only, not using any glue. This is in case you want to remove a rim later in order to attach a 4" toeboard. It is preferable just to place a 3 1/4" toeboard on top of the rim, but often times the old outdoor toeboard becomes the one used for the indoor practice circle, so not gluing leaves this option open. Gluing does not add any structural integrity to the practice circle because, unlike the competition circles, below, it is all one piece.

Use a #6 1" drywall or deck screw. These can be hard to find. Often the shortest available is 1 1/4", which are definitely too long. The point of the screw will penetrate the bottom, causing the possibility of scratching the floor.

Finally, I highly recommend rounding the outer edges of the rim plywood with a 1/2" diameter round router bit. I mean the outer rectangle--NOT the inner circle, and not the side edge of the full plywood sheet. This is for safety.

Competition circles:

Budget: $200 w/ rim only, $400 w/ toeboard.
Estimated cost: $130 w rim only, $280 w/ toeboard.

Each indoor throwing circle requires four sheets of 8'x4' unfinished plywood 15/32" (1/2") thickness, two with one sanded (paint-quality, unknotted) side.

Assemble the plywood to produce an 8'x8' surface. It will be assembled upside-down. Select the best sheet of plywood and lay it on the floor with the good side down. Select the second best sheet of plywood and cut it lengthwise so that you produce two 2'x8' pieces. Lay these two pieces on either side of the best piece, with the factory ends facing each other, best side down (the cut edges are on the outside). Now you have an 8'x8' area, with one 4'x8' piece centered, so that there are no seams through the middle of the circle.

Then take the lower-quality plywood sheets and lay them side by side across the first layer. You don't have to cut one of these lengthwise. They should overlap so that each piece runs across all three pieces of the first layer. Check the seams for fit; you might want to turn one or more pieces over or rotate 180 degrees until everything fits perfectly. This is the bottom layer, so either side can be facing up. The layers and configuration of plywood sheets are illustrated in figure 1. Click on the image to enlarge it.

The two layers will be held together with yellow carpenter's wood glue or construction adhesive for wood. Take off the top layer, remembering the orientation. Apply glue to the first layer. Apply 1/4" to 1/2" thick beads starting 3 inches from the edges, every 9 inches. Bring the second layer on top. The beads can be put on in a zig-zag pattern. To hold the two layers together while the glue is drying, you will use drywall or wood screws starting 3" from all edges and seams (top and bottom layer) and then one every 21 inches. Screws will be drilled from the second layer down only. The screws serve only as clamps while the glue is drying although they are not removed. The screws will have to be 1/8" shorter than the thickness of two pieces of plywood. In addition to the screws, you may want to stand on all parts of the circle and then leave shots or weight plates on top overnight.

After screwing, there are two options to help prevent the circle from sliding. First, if the circle is to be placed on artificial turf, the bottom of the circle may be treaded with a router. Using a 1/2" SQUARE bit set at 1/8" deep, Cut 2' straight lines beginning 6" from the edges, every 12". The edges of the treads must be sharp to be effective, not rounded. So use a square bit, not a rounding bit. Stagger the lines, and alternate them, so that there are treads running in both directions. Figure 2 illustrates the recommended pattern of glue beads (green), screws (blue), and tread lines (red). The seams from both layers are indicated as brown lines.

If the circle is to be placed on a shiny hard surface, like a wooden basketball court, use the noslip padding. Refer to the practice circle section, above.

Now turn the whole assembly over and wipe off any excess glue. Next, assemble the rings and/or toeboard. The toeboard should be placed on the center 4'x8' piece of plywood, so that the seams run parallel to the direction of the throw (figure 3, left). The circle should be centered on the 8'x8' surface, so that you have 6" at each side. Fasten a toeboard and bolt-on surface ring purchased from a catalog.

Alternatively, apply a plywood rim and bolt on a 3 1/4" toeboard rim on top of the plywood rim. This is a good option for a combined shot-weight circle.

For a separate weight circle, either fasten a bolt-on rim from a catalog, or make a rim out of two additional sheets of plywood, 23/32" or 3/4" thick. Cut the circle out of this sheet and glue and screw it onto the top, with screws from the top surface down (figure 4). These screws can be longer because you have 3 layers of plywood.

Round the edges of the outer square perimeter of the platform (NOT the inside circle rim) with a 1/2" radius round (router) bit. While outside platform edge is rounded for safety, the inside rim has to be sharp and square.

Finishing: The toeboard, the sides of the rim, and the top surface of the rim out to 6mm (1/4") should be painted white to facilitate the judge to observe foot fouls. The surface of the circle should be unpainted, or painted a contrasting color: grey or a school color. You might also paint the outer edge of the platform orange or a bright color for visibility so that one does not trip over it.

All paint should be flat finish. Gloss or semi-gloss is too slippery when wet or dusty. Do not mix sand, grit, or other abrasive material into the paint to aid traction. Sand will only (1) keep your foot from turning/pivoting, (2) twist your knee ligaments, and (3) wear out your expensive throwing shoes. The outside platform surface should also be a flat finish to prevent slipping on fouls.

A flat finish on bare wood provides an excellent throwing surface. You can even use a primer paint as your finishing coat; primers are flat finish. A flat finish will attract dirt; a dark color is recommended for the platform and circle. This will also contrast well with the white rim. The white rim and toeboard should be repainted at least once a year to cover dirt and shoe scuffs. It also makes the circle look sharper and aids in officiating.

Finally, I do not recommend attaching handles to the edges or sides of the circle because they can cause a tripping hazard. I think it is as easy just to get four people, one on each corner, lift the circle from underneath. Just work the corner onto your foot before slipping your fingers under the edge. Reverse the process when placing the circle down.


Idea for a portable concrete circle: This is an untried idea. It may be possible to construct a portable concrete circle by pouring a layer of Quik-rete or other hydraulic cement onto a subsurface. I would recommend a bottom layer of plywood, then a second layer of Durarock or similar stiff tile subsurface. The durarock is a rigid surface, about 1/4" thick, used to prevent cracking of tile floors (Plywood subflooring flexes too much, which would result in tile and grout cracking.)

Make a plywood ring equal to the thickness of the Quikrete--1/4"? Then pour the Quikrete and smooth flush to this ring.

Finally, attach a bolt-on iron ring on top of the plywood or attach an additional 3/4" plywood ring on top of the thinner ring.

I have never tried this because I don't know how thick the cement and/or Durarock/plywood combination has to be to prevent the cement from cracking if you lift it. If it had to be very thick, then it might very well become too heavy to move eaasily with four people. I would also be worried about the whole cement disc dropping out if the circle was turned upside down. I would be very interested if any reader has tried this.

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