A truck is designed by making a 'master,' and using this master to make the mold for casting (follow the Metal link below for details). Before computers, the master was made by hand using soft metals, body filler, wood, and so on. Plaster was then used to make copies of the master, and molds were made from those copies. A handfull of trucks would be made and tested, and then the master would be changed to fix any problems. The process repeated many times until the final trucks were just right. Because all steps in this process are done by human hands, there are many places where errors can pop up. First, no matter how hard we try, humans aren't perfect. The master itself could be made slighty asymmetric or lopsided, resulting in trucks that don't turn smoothly or pull to one side. Errors can also be introduced in the master copying stage, so even if the master is nearly perfect, the copies may not be. In that case some trucks could be ok and others could be bad.
Something you see a lot with trucks made using this process is that the hanger and baseplate don't exactly line up like they should. This can pinch the bushings with uneven pressure, making the trucks feel like they are rocking back and forth when they turn. Sadly, though, because this process of hand-made masters and molds is the cheapest and easiest way to make skate trucks, a lot of truck manufacturers still make their trucks this way.
Today sophisticated computer-aided drafting (CAD) can be used to make digital 3D models of masters that are perfectly symmetric. These 3D models can be input to an expensive computer-driven machine that can make the molds directly, removing human error from the mold-making process and resulting in trucks that ride and turn extremely well. If the truck design needs to be changed, the digital 3D model is changed and then computers take over to make the new molds.
Sure, computers are great, but have you ever heard the saying 'garbage in, garbage out?' It means that no matter how fancy your computer modeling and mold making system is, if your digital 3D model of the master is crap, your trucks will be crap. What you see on a computer screen isn't always representative of what you'll get in the physical world. At Alpha we combined the best of both worlds--human skill and computer precision--to design our trucks. We commissioned the skill of talented skateboarders, pattern makers, artists, and engineers to design and create our initial masters by hand. These masters were then digitized and 3D-modeled to remove any imperfections and asymmetries. Once we had the digital 3D models of the hanger and baseplate masters, we used computer modeling to optimize the truck geometry, including the kingpin angle, pivot stem angle, action angle, and so on to make the best turning and most stable truck possible. By doing this we were able to guarantee that the hanger and baseplate fit together perfectly at the bushings and the pivot stem. Our molds are made directly from the digital 3D models to ensure 100% accuracy. When we wanted to make changes to the truck design during the testing phase, we modified the digital 3D models and used fancy stereolithography machines to '3D-print' physical copies of the digital models, ensuring that the changes we made in the computer were correct in the physical world.
Some truck manufacturers have stopped making masters and molds by hand because they realize the computer-aided process is better. But, beware of a truck manufacturer that brags that they do the entire process digitally, because this can be a mistake too. It is important to maintain the link between computer modeling and the physical world when designing a truck (or anything else for that matter). Physical models are necessary to understand how the truck will really perform, for example how the truck behaves with different bushings, in the real world.