Soap Box Derby Design: How To Build A Soap Box Derby Car

Soap Box Derby Cars are to be built by the drivers, here are a few simple steps to building one.

Building a soapbox derby car according to rules drafted by the National Control Board and adopted by the Board of Trustees of the All-American Soap Box Derby is possible following a few simple steps, keeping the rules in mind. These steps include building the floorboard, axletrees, brake holes, suspension, steering, brake and finishing.

Layout the by marking a rectangle 79 inches long, next measure the sitting driver to determine the width of the car and mark on the paper. The floorboard will keep the car within the legal limits when the body is finished. Cut out the paper along the lines of the rectangle and fold. Now fold the paper again in the opposite direction, the pattern should now have four folds. Start 11" from the corner of the first fold; draw an even curve to the opposite corner of the paper. Cut all four layers along the curve, then unfold the pattern and lay it on the board designated for floorboard. Trace the pattern outline onto the board and cut out. Use a wood plane to get the floorboard to the exact size and to round off the sides and ends.

Next build the axletrees, which are the wood coverings used to reinforce the axle rods and keep them from bending under the weight of the car and driver. The best axletrees built create as little wind resistance as possible, and are usually very thin. Do not use softwood to make the axletrees, as they will have the tendency to break or split. Make two axletrees, 11/2"x51/2"x28". Locate the grooves for the trees by drawing a line parallel to the front edge of the board, 2 ½" from it and another line 5/8" behind the first. Now cut the grooves for the rods 5/8" deep and 5/8" wide by using a saw or chisel and hammer. Work the trees down to streamline shape using a plane and finish them with sandpaper to a smooth, even surface. Bolt the rods to the axletrees by using one 3 1/16" stove bolt at each end of the rod drilled 3" from the square end. Make a bushing for the Kingpin by using a 3" length of 0/8" pipe. Put the pipe in a vise and use a hacksaw to split the pipe for a distance of 1 ½". To keep from crushing the pipe place an old bolt in the part of the pipe that is between the jaws of the vise. Flatten the two split pipe halves being careful not to squeeze the remainder of the pipe out of round. Drill a 3 1/16" hole in each flat piece. The king pin is now ready to install into the front axletree using a carriage bolt, 0/8" in diameter. Drill a 9/16" hole in the exact center of the axletree lengthwise, in front of, but close to the axle rod. Insert the bushing with the flat pieces on top of the tree and using wood screws, screw it into place. Make sure the Kingpin is the same distance from each end of the axle rod, allowing the wheels line up.



Brake location is very specific in the rules. Cut a hole in the floorboard to allow the drag to operate while making certain the measurements are correct. The rules state that the car with the driver must have the necessary clearance of three inches, if the floor is thicker than two inches, cut out places in the floor to set the trees to meet the rules. Draw an outline of the brake hole, 18" x 31/2", on the rear end of the floorboard, exactly between the sides of the floorboard to keep the car from swinging to one side when the brake is applied. Smooth all the edges. Do not forget that the front wheels go behind the nose of the car and do not place the axletree so close to the front that the rims are as forward as the nose of the car. To allow the front tires to turn, the channel for them should be wide. Cut across the floorboard along the drawn lines. Make the cuts only as deep as necessary to give the bottom of the car the 3" clearance. Using a hammer and chisel, clear out the waste wood between the cuts then use a planer and sand paper and finish the wood.

Cars with suspension handle better, ride better and are faster than cars without, even though suspensions are not necessary. Derby rules allow the use of nearly any kind of spring suspension as long as the action of the springs does not interfere with proper control of the car. Flat auto springs are the safest, easiest to get and install. Derby rules allow manipulated springs to suit the car. Two old auto springs measuring ¼"x 1 ½ '" x 18" for the rear and one ¼" x 1 ½" x 12" are used for suspension and are usually found in any auto shop or spring company. These businesses will usually drill and shape them to fit the car. Now drill a 3/8" hole, exactly on the centerline of the floorboard in the exact spot where the front axle rod will be 8 ½" from the front point of the floorboard. Insert the Kingpin up through the floorboard, through the bushing in the front tree and through the 3/8" hole in the front end of the front spring. Make sure this spring is directly over and in line with the Kingpin hole in the floor. Carefully bolt the leaf on the floor. Measure the distance from the Kingpin to the cotter pin holes in each end of the front axle rods. The distance from each hole to the Kingpin should be the same. The next step is to attach the rear axletree by clamping the tree with the rod in its place onto the car floor. Do this where the centers of the front and the rear axle rods are 63" apart. For proper alignment, measure the distance from each cotter pin hole in the rear axle to the center of the Kingpin, this distance should be the same on each side. Now bolt the two springs to the rear axletree. Bolt the other ends of the springs to the floorboard, making sure the tree does not slip out of alignment. Put all four wheels on the car and make another alignment check. To do this place a long, perfectly straight piece of wood against the left front and rear wheels, touching both the front and the rear rims of both of the wheels at the same time. Use a similar straight edge; repeat the test on the other side of the car. To limit movement of the front axle so the tires will not turn more than 2" either way, make a stop block out of two pieces of wood, 1 ½" x 1 ¾" x 3 ½". Install the blocks by aligning the axle in the straightforward steering position and putting a mark on the floorboard edge just 5/8" from the rear edge of the tree. Turn the axle so the edge of the tree meets the mark. Install the block and repeat the procedure with the other block.

The steering assembly is one of the most important parts of the car. Often the car that runs the straightest will win a race. According to the rules, there will be no mechanical steering. There can be, however, a vertical or horizontal type steering assembly. Either one of these types will work well for a small driver, but a larger driver will probably need to install a horizontal type steering, giving them more legroom. The following instructions are for the horizontal type steering. First, construct two body bulkheads, one smaller than the other and held together by flat steel or corner braces. The smaller bulkhead consists of 3 pieces of hardwood, 2 pieces are ¾" x 1 1/2" x 11" and the third piece is ¾" x 1 1/2" x 12". This third piece attaches on top of the other two, creating a horseshoe shape. Slightly arch the top piece of wood with the beginning of the arch 1" lower than the center of the arch. Build the second bulkhead the same way, but with measurements of ¾" x 1 1/2" x 13". Drill a 1 1/16" hole through each bulkhead for the steering shaft. Install the smaller bulkhead in the front of the floorboard. Next, the steering shaft is constructed from a piece of ¾" pipe, 15 ½" long with a hole drilled 10" from one end of the pipe. Weld a ¾" floor flange to the end of the pipe closest to the drilled hole. The rules allow for many types of steering, even ones made out of ¾" plywood with slots cut out for the fingers. Bolt the steering wheel to the welded flange. Now place the steering through the holes in the two bulkheads, running from the large bulkhead to the small one. To prevent endplay, place washers on each side of the large bulkhead and fasten them with cotter pins through holes drilled in the shaft. Use two pieces of ¾" strap iron 6" long, shaped and drilled, and attached to the front axletree with bolts. Attach 3/16" turnbuckles to each piece of strap iron. Connect steering and braking cables by bringing the cable through the eye of the turnbuckles, then make one or two knots around the cable and clamp or wire the end of the cable to the cable proper. Now bolt two ¾" awning pulleys to the floorboard under the steering shaft, one about 1 ½" in front of the other. Start rigging by fastening one end of the length of 1/8" cable to the eye of the turnbuckle on the right hand end of the front axletree. Thread the cable through the back pulley, then up and over the left side of the shaft. Make four complete turns around the shaft and thread the cable through the hole in the shaft. Make 4 more turns the same direction, then bring the cable down to and through the other pulley and anchor it to the turnbuckle on the left end of the axletree. Adjust the turnbuckles to take the slack out of the cable.

The rules only allow a single drag brake. The rules are also very specific on where the brake is located, size of the brake and where it is facing on the drag. Inspectors check all brakes before each race, so make sure they are right. To begin construction of the brake, make a pedal out of ¾" x 8" x 10" hard wood then attach the eyebolt to the top of the pedal 1" from the edge. Below this eyebolt cut out a 3/8" x 2" notch. Now a piece of hardwood cut 7/8" x 3" x 8" bolted to the floor, at 54 ¾" from one end, with a hothouse pulley for the brake cable to run through is bolted to the center of the board. Cut another hardwood board 2" x ¾" x 17 ½" with a diagonal cut across one end. Cut an old tire tread 2" x ¾" x 17 ½" bolt it in place with at least 9 square inches of tire thread touching the road. Now attach the brake to the underside of the floorboard using bolts and a 3" strap hinge and make four brackets out of 1"x 1" angle iron. Put an 8 ½" length of 3/8" pipe in a vise. Use a hacksaw to split one end for two inches and widen it so a piece of 3/8" pipe will fit in it. Flatten the other end with a hammer and drill a ¼" hole for a bolt. Drill holes in the split top end. Now flatten both ends of a 12 ½" length of 3/8" pip and drill ¼" holes in them. Drill another ¼" hole in the pipe 5 ½" from one of the holes in the ends. Bolt the brackets to the pieces of pipe by bolting one on the end of the pipe with two flat ends and the other pipe with the forked end is bolted 6" from the bracket. The other bracket is bolted to the end of the forked pipe and placed on the floorboard 6 ¾" from the edge. Attach a length of screen door spring to the top the pipe with two flat ends to keep the drag up. Now bolt two 3" hinges to the brake pedal and to the floor. Anchor a length of 1/8" cable through the hole in the top of the 12" pipe, and then thread the cable through the pulley, through a 20" length of ¼" copper tubing attached to the floor and anchor the cable to the eyebolt in the brake pedal.

The shape of the floor determines the body's shape. The body should match the pointed front and rear of the car. Be sure not to leave any exposed rough or sharp edges. Make a third bulkhead similar to the first two only having dimensions of ¾" x 1 1/2" x 10", cut 2 and cut 1 of ¾" x 1 1/2" x 13". The longer piece is the top piece, arched like the other bulkheads. Install this new bulkhead like the other two and place it 54" from the nose of the car. The bulkheads should be placed at 18", 30" and 54" with the smallest one at 18" and the last one at 54". Now make the front and rear triangular shaped pieces by cutting 3 sides per piece measuring 5" x 11" each. Build the front piece by nailing together 3 sides. The rear piece also has 3 sides nailed together but has a cutout in one that is located 1 ¾" from a point where the sides come together and is cutout 2" from the bottom of the piece. Screw both pieces to the floorboards with the points matching the floorboards. Attach strips of wood, ¼" thick x 1 ½" wide, to the rear of the front block and to the first and second bulkheads. Attach a strip of wood to the rear block and the end bulkhead. Finally, attach a third strip of wood to the second and third bulkheads, below the strips already attached. Cut out coverings for the sides and top and make sure the patterns match the existing pieces. Attach the side covering to the side rails and to the floorboard for the full length on both sides, except for the cutouts over the axletree. Now attach the top. To shield the car from damage, make metal covers, bent to cover the front and rear pieces. No seat is installed, as this would cause the driver to sit higher and cause more wind resistance. Installing a seat back is permitted as long as it is hinged or removable for inspection of the back of the car.

© High Speed Ventures 2011