A good trebuchet counterweight is compact, heavy, and cheap. A heavy counterweight lets you throw bigger projectiles. A compact counterweight makes for a more visually pleasing design. And, the cost is the most critical factor in any project.
While designing the Pennypult, we experimented with steel, lead, and tungsten. Ultimately, we decided to use US pennies for the counterweight. This article will explain why.
The first prototypes used the "best" material we could think of at the time. Tungsten is a dense metal used in lightbulb filaments. It is 2.5x denser than steel and, in this case, comes in the form of cylindrical pellets. In terms of weight and compactness, Tungsten is one of the best materials you can find. Unfortunately, it's an exotic material and very expensive. This is only suitable for small scale trebuchets.
Lead is another popular material for making things heavy. It works well in fishing applications, but it's poisonous and leaves residue on your hands. For those reasons, it wasn't suitable for use in a toy. That aside, it is cheap and denser than steel. If you are building an enclosed counterweight, it may be a good option - even for larger trebuchets.
Stainless steel is also a viable counterweight material on a small scale. Nuts and washers from the local hardware are easy to find and come in many shapes and sizes. Steel is the least dense of the materials listed. As a result, designs using steel may have bulky counterweights.
Cost is what led us to use pennies for the Pennypult counterweight. Pennies are a blend of zinc and copper with a density close to steel. By using something common and cheap, we did not have to ship these materials in the kit. This allowed us to bring down the price to a more acceptable level. Aesthetically, the copper looks fantastic when the pennies have a shiny copper finish.