Symmetry can be found everywhere. It can be found in man-made design and in nature. Sketchup has a very easy way to help you model symmetrical. This is perfect if you are looking to model your Pinewood Derby car in Sketchup. If not, these same principles can be used in many different modeling applications.
Think of a butterfly. Each wing is symmetrical to the other. This means that they are both shaped exactly the same way, but mirrored to each other. So when the butterfly folds it’s wings together, they match perfectly. This is called reflection symmetry, or bilateral symmetry.
As the wings of a butterfly can be thought of as a flat, 2D shape, reflection symmetry can apply to a 3D shape as well. Most cars are symmetrical. If you were to split a car down the middle from front to back, each half would be symmetrical to each other.
Reflection symmetry can be easily achieved using reflected Components within Sketchup.
Pinewood Derby Racing
If you are involved in Cub Scouts, I’m sure you’re familiar with the Pinewood Derby. It’s an annual race, where each Cub Scout is challenged to build his own wooden race car and then competes with his fellow scouts in a series of races to see who built the fastest car. Typically, four cars race at a time. They are set up on a track that has a ramp at the top and a straight track at the bottom. Powered by gravity alone, the cars race to the end of the track. Races continue in an elimination style tournament until the final four cars are left.
I used to be a Cub Scout (and eventually made it to Eagle Scout), and I loved the Pinewood Derby. My father and I were really into it. We even built our own track right in the house so we could test out different configurations. Yeah, we went a little overboard, lol. But we found little changes in design that made big differences on the track. Unfortunately, we didn’t have Sketchup back then, so we had to do all the design work manually.
We became so good at designing cars, we won first place two years in a row. We went on to the regional competitions, where we won 1st place one year, and missed first place by a mere 1/1000th of a second on the next year. (I can remember trying to comprehend how fast a 1/1000 of a second was) Those were some really fun times as a kid!
Sketchup is a great tool to design your Pinewood Derby cars. You can draw the car using simple tools, and try out different designs to see how they look. You can even print out templates of your car in full size scale to help you cut out your car from your block of wood. But what makes Sketchup really useful in designing a car is that it has the capability to design symmetrically.
If you’d like to download a model of a Pinewood Derby Kit, as opposed to modeling it from scratch, you can download my model for free from the 3D warehouse. Using that model as a base, you can design your car in Sketchup and print out templates.
Use Components for Symmetry
The challenge with creating symmetry within a model, is that whatever you do to one half of the model, you have to replicate it exactly on the other half, but in the opposite direction. This would be very time consuming task if done manually, and you run the risk of making mistakes.
By using Components, you can create an exact copy of a collection of Sketchup entities. Any changes made to one component will be reflected in every copy of that component within your model.
REMEMBER: The difference between Groups and Components, is that while you can create multiple copies of a group, any changes made to an instance of that group will NOT be reflected in any copies of that group. Each group is independent of each other. It’s a good idea to use groups for organization, as well as for making copies of things you know are similar but are not exactly the same.
The trick to configuring a set of components to be symmetrical is knowing how to flip, or scale one of the components inside out.
Create a component
So the first step is to create one half of your car. The regulation size of a Pinewood Derby car is 7″L x 1 1/4″H x 1 3/4W”. So we want to build a box half that Width (7/8″W). Use the rectangle tool and the push/pull tool to create a cube.
Select the Rectangle Tool (R), and draw a rectangle. Type in
7", 7/8" ENTER to specify the dimensions in the VCB. Then use the Push/Pull Tool (P) to create a height of 1 1/4″. (Remember, you can pull a face up close to where you want it, then type the actual dimension on your keyboard after.)
Now that we have HALF of the car block made, let’s turn it into a component. Just triple click (Hit
SPACEBAR to make sure the Select Tool is active before you click) to select all connected faces and edges, then right click and select “Make Component”.
Copy and flip the component
With half of the car made, the last step is to make a copy of the component, move it beside the other component, and mirror it. There are a couple ways to mirror a component, I like to use the Scale Tool. The other way is to right click the component, and select
Flip Along, and pick whichever axis you need it to flip about. Depending on how you created your model, it should be either the green axis or the red axis. The following steps will show you how to do it using the Scale Tool.
With the component made, select it and create a copy of it using the Move Tool (M). As you are moving it, hit
CTRL on your keyboard to make a copy. The next step is the most important in order to create symmetry.
Select the component you just moved and select the Scale Tool (S). Grab one of the handles in the center of the long face. Scale the component to a value of -1.00.
Finally, move the component side to side with the other one. You can now open up one of the components and make edits, and the other one will be changed as well. Only it will be mirrored, giving you a symmetrical model.
Complete your model
You are now set up to model your car. All you do is open up either one of the components, and make your changes. They will be mirrored on the other side. Once you have completed your model, you can set it up to be printed out to scale so you can use it as a template for carving out your car. But that will have to be a different post! Have fun and be creative!