Similar to Alignment, Pinewood Derby® car aerodynamics is another area for speed gains that’s not as well known to many racers.
There are two goals regarding aerodynamics:
Eliminate (or at least minimize) the surfaces that create drag
Efficiently direct the air around the surfaces that can’t be eliminated
The first area to address for aerodynamics is the car body. Consider these two body types:
The thin car is more aerodynamically efficient than the first body. As a result, the thin car is the preferred style for Professional League Racers.
The car needs to be aerodynamically sound on both the top AND bottom. Any holes or cavities should be covered with vinyl, packing, or foil tape to reduce drag created by air catching in these features.
However, the body is not the only source of drag on a car:
The wheels are a significant source of drag. They present a sizable forward profile and create turbulence (especially behind the wheel), further slowing the car. While the wheels can’t be eliminated, if the rules don’t prohibit it, they can be shortened (reduction in diameter) and narrowed (reduction in width), which decreases the amount of air they must move. Additionally, there are methods to move the air around the wheels more efficiently. Reduced wheels are available here!
Fenders
Fenders can help efficiently direct the air around the wheels resulting in more speed. There are a few different types of fenders:
Leading Edge Fenders:
These fenders are placed in front of the wheels and help direct air over them, leading to a faster car.
Leading and Trailing Edge Fenders:
These are the same as the Leading Edge Fenders, with the addition of fenders behind (trailing) the front wheels. These fenders help direct the air over the front of the wheels but also eliminate the turbulence behind the front wheels leading to a faster car.
Full or Plank Fenders:
The front pieces are the same as the previous fenders; however, the middle fender is a solid piece. These fenders direct the air over the front wheels and straight back to the car’s rear. The Full or Plank fender is the fastest design and is the preferred fender by Professional League Racers!
Making Fenders
Making fenders is an involved but rewarding process resulting in a faster and more visually appealing (cooler) car! First-time racers or Race Teams with younger members should opt for either the first (Leading Edge) or second (Leading and Trailing Edge) fender designs outlined in the previous section. The Full/Plank fenders can be challenging to construct, and obtaining the proper gap between the fenders and wheels can be difficult.
Traditionally, fenders are constructed from balsa wood. Balsa is available at most craft and hobby stores. However, it can be fragile, so care must be taken when cutting and sanding. Stock BSA wheels are about .425 inches thick, so look for material around 0.5 inches to use for the fenders.
Supplies needed for fender construction:
• Balsa wood • Rubber band
• Coping saw, scroll saw etc. • Thick Super Glue™ or epoxy
• Sandpaper (80-220 grit) • Craft knife or razor blade
• Spare set of wheels and axles, same diameter etc. as race wheels
Cutting and installing the fenders
STEP 1: Sketch your fender design on your material of choice. Guides or templates can be used to form the basic fender profile (click here for free plans!) Next, cut out the fenders using a coping saw, band saw, scroll saw, etc. Finally, it’s a good idea to sand the wheel well of each fender before gluing it to the car. A spare wheel can be used to ensure the well shape is maintained!
STEP 2: Insert a spare set of wheels and axles into the car body, as the fender-making process may damage them. If utilizing a rail riding setup with a bent front axle, ensure one of the spare axles is bent the same amount. Temporarily add weight to the car (use tape to keep the weight in position) and run it down your tuning board (see the Assembly article). You will want to set the steer near where it will be once the car is complete. Four to six inches over four feet is a good target. You will be using the wheels to help place the fenders on the body; therefore, you want the wheels oriented close to their final race position.
STEP 3: The goal when installing fenders is to get them as close to their respective wheel as possible WITHOUT them rubbing the wheel while racing. Therefore, you’ll want to err on the side of caution when installing the fenders. Too much gap is preferable to too little, which results in the wheels rubbing.
A rubber band wrapped around the wheel helps obtain good gap spacing. A loose rubber band will give a wider wheel gap; a tightly stretched band will provide less.
Make sure to test fit each fender before you glue it to the body; while doing so, mark the fender’s location. Super Glue™ (Cyanoacrylate, or CA) works well due to its quick cure time. Apply glue to the body and then set the fender in place, pushing it against the rubber band/wheel and the body. Wait for the adhesive to set.
Repeat these steps for the remaining fenders/wheels. Optionally, a centerpiece can be inserted between the trailing and rear fenders. Otherwise, these edges can be sanded flush with the body.
Shaping the fenders
When shaping the fenders, three areas need to be carefully addressed:
Fender bottoms - The bottoms of the fenders need to be shaped or notched so they do not rub the track.
Forward profile - The fenders should be shaped to cover the wheels’ forward profile. Some clearance on the bottom and top is ok.
Trailing Edge - Great care should be taken when shaping the trailing edge fenders; they need to be more narrow than the wheels, tucking in behind the front wheels to ensure that no part can stick out past the wheel.
Failure to properly shape the fenders will result in a significant increase in drag/friction, negating the benefit of the fenders. In many cases, a poorly done fender car will be SLOWER than a similar car with no fenders.
STEP 1: Using a pencil, mark the underside of the fenders (see photo). Cut or carve the excess material using a craft knife or similar. Race Teams may find it better (and safer) to use sandpaper from the start. You’ll want to cut further in around the front steer wheel if running a rail rider setup. The front steer wheel is where the fenders will most likely rub the rail. The purpose of cutting around the other fenders is in the event the car wiggles; this will keep the fenders from rubbing, further slowing the car. Cut the notch to the bottom of the car.
Don’t cut or carve all the way to your lines, give yourself a buffer so you have room to sand the fenders to their proper size/thickness.
STEP 2: You want the fenders wide enough to cover the wheel’s forward profile. Use the edges of the wheels to make your marks and cut the fender down to the body.
STEP 3: For the trailing edge fenders, cut in a bit more behind the front wheel than the front, leading edge fenders. You want to ensure that NO PART of the trailing fender can stick out past the wheel.
STEP 4: It is time to sand the fenders to bring them to their final shape. It is a good idea to leave the spare wheels on the car while sanding; this helps ensure you’re molding the fenders to the correct form. 80-grit sandpaper can be used; however, be aware that it removes balsa quite quickly, so be careful not to over-sand. Once close to the final shape, start using higher grits (220) of paper to smooth the balsa.
PRO RACER TIP FROM Castoro Racing: If you accidentally remove too much material, glue a thin piece of balsa in place and sand/blend to shape!
STEP 5: After sanding, inspect the fenders closely. You want to ensure they are not rubbing the wheels in any location. You’ll also want to ensure the fenders will not contact the track. The most popular Pinewood Derby® track (BestTrack®) has an inner rail 1.625 inches wide and 0.25 inches tall. Mock up a track center rail with these dimensions (or the track you will be racing on) and place your car over it. If running a rail rider setup, place your front steer wheel against it with the rear wheels centered over the mocked-up rail. Again, check to see if the fenders can rub at any location.
PRO RACER TIP FROM DWS Racing: Hold the car sideways against good lighting where you can see the light between the wheel and fender well. Rotate the wheel with light finger pressure and try to make the wheel contact the wheel well. Use the slop/play between the axle and wheel bore to tilt the wheel in and out while rotating.
STEP 6: To strengthen the fenders rub them with a coat of thin Super Glue™ (CA). Small, inexpensive art brushes or rubber-gloved fingers are good options for applying glue. The fumes from CA glue can be strong, therefore use in a well-ventilated area. After the glue cures, give them a light sanding with high grit sandpaper.
STEP 7: Some league racers use Sharpie™ pens to color their fenders for ease of use and minimal weight addition. It’s easy to apply, especially for younger Team members, and is available in various colors. The fenders can also be painted, though staying away from water-based paints (unless using a sealer first) is recommended, as they can cause the fenders or the car body to swell or warp. Additionally, it is wise to avoid mixing different types of paint.
Due to its challenging nature, fender construction may not be feasible for all Race Teams. Alternatively, lightweight plastic fenders that need little to no sanding or shaping are an excellent alternative to making fenders. Some of the fastest professional racers in the country use these fenders!
Air Shields/Guides:
Another source of drag is air entering the back or inner cavity of the wheels.
Air redirected by the wheels or fenders can flow into the rear cavity or inside of the wheels. This airflow creates drag and turbulence. However, there is a solution to keep air out of the wheel cavities known as air shields available here.
PRO RACER TIP FROM HurriCrane Racing: The higher the level of competition, the more aerodynamics matter!
Articles in this series (click to read):