For a tire jump that can be changed as quickly as the jump poles, don't use springs to tension the tire. Instead use two vertical lengths of chain or aircraft cable that are attached to the top and bottom horizontal frame members. These run down through two PVC tubes that go through the sidewalls of the tire. It is suspended by only one chain at the top of the tire. It also works very well for teaching the tire as it can be raised in increments as small as one inch as fast as you can say "tire jump". (Greg Ruhe)
Here's a good design: A single rope runs through eyelets on the tire and frame. There are 4 eyelets on the tire, roughly at the 1,5,7, and 11 o'clock positions. 8 more eyelets are on the frame, two each on the top and bottom near the corners, and 2 on each side. So the rope goes thru the tire's top eyelets, down one side of the frame, thru the tire's bottom eyelets and up the other side of the frame. A pull-through sailboat cleat (V-channel are cheaper but the cam-type work too) is located on each side between the frame side eyelets so that the rope runs thru the cleats. The height is adjusted simply by changing the point where the rope runs thru the cleats. You use marks on the frame and rope to tell where to cleat the rope. The rope also does a great job of suspending the tire, holding it in place but with some give in case a dog hits the tire hard.
This type of tire is easier to adjust than the single-chain supported tires. The cleat is much faster to use than a hook and chain (that's why they use them on sailboats) and while you adjust one side, the other side holds the tire's weight. The biggest problem with the single chain support is you often need both hands to rehook the chain because the chain is supporting all the weight of the tire.
A good single-chain supported style is the one that uses 2 vertical wires passing through the tire. The tire can only slide up and down with the chain holding it in place. You might opt to replace the chain with a length of rope and a single side-mounted cleat for this.
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