Detroit Metro Soap Box Derby

DMSBD Tech Tips
Construction Tips for a First-Time Car Builder
Article 4: Starting Construction - by Ian Carsten

The first three articles in this series covered the preparatory steps in building a soapbox derby car. Just like building a house, you first have to get things ready so that when the actual building begins, you won’t have to go back and take things apart to do something that should have been done earlier. The plans that accompany your kit are reasonably well written and amply illustrated. For the most part, you should be able to build your racer without much trouble. What follows are hints, suggestions to help you make good choices, and recommendations for tightening fasteners.

Getting Organized
To start, place your floorboard on the sawhorses with the topside, marked with the SBD logo, facing up. It is very frustrating to get your car substantially built only to realize you have installed the wrong parts somewhere and now several other things have to be taken apart to get at it in order to make corrections. Make sure you know the difference between the various washers, screws, and nuts. Since some vary by length only, you’ll need a ruler to tell the difference. Also, it is a good idea to sort them into separate containers, such as tin cans, jars or, better still, into a parts organizer such as can be bought at the hardware department at Sears stores. The parts organizer, with labeled compartments, will be very useful later should you have to make a trackside repair or correct an unrecognized rules error. It would be useful to label each container or compartment with its contents. For example, they could be labeled, “screw G 1/4 x 2”, “screw G1 1/4 x 1 3/4”, “screw G3 1/4 x 1 1/2 “, “gold washer R 1/4 x 1 1/4 “, “washer R1 1/4 x 2”, and so forth. This will make your assembly job much easier and faster, and you will be less likely to install the wrong part.

Kingpin Bushing Installation
If you haven’t already done so, you should now install both kingpin bushings. Here is a detailed account of kingpin bushing installation as demonstrated by Detroit Metro Director, Joe Flynn, during our March 2, 2002 Car Layout and Building Preparation Clinic:

Joe prefers installing the kingpin bushings just after sanding the floorboard. He started by roughening the outside diameter of the bushings with sandpaper to promote adhesion of the epoxy. Although not mandatory, Joe explained that epoxying the bushings into the floorboard makes for a more rigid installation, which he feels is an important basis for a competitive car. He chose a specific epoxy that does not harden too fast as he wants to have plenty of time to work it into the hole and clean up the excess from the outer surface of the floorboard before it sets up. He first taped the bottom of the hole to close it off so that the epoxy would not immediately run through. As soon as the epoxy was mixed, he used a small wood splint (a Popsicle stick or tongue depressor works well here) both to mix the adhesive and to guide it into the hole. Also, he recommends allowing the epoxy some time to soak into the pores of the wood before installing the bushing to ensure the best possible bond.

While the adhesive was soaking into the wood, Joe took a 1/4 x 2 1/4-inch fully threaded machine screw, two 1/4-inch fender washers, and a 1/4-inch nut to make a temporary bushing press. He explained that we don’t want to drive the bushing in with a hammer, since that may damage the bushing or drive it into the hole out of square. Also, a hammer blow could easily miss and put a ding into the surface of the board near the bushing. He said it was quite important to get the bushing installed perpendicular to the surface and using the press-in method pretty well guarantees this.

Now, he removed the tape from the bottom of the floorboard under the kingpin hole. Joe first applied a coat of grease (petrolatum may work as well) to the outside of the screw. This acts as a parting agent to prevent the adhesive from bonding to the screw. Additionally, since the screw fits the bushing very closely, the excess grease also coats the hole in the bushing and helps prevents any epoxy that may seep in from sticking inside the bushing. Then he placed a fender washer on the screw. At this point he cleaned the outside of the bushing with a solvent to remove any trace of oily fingerprints from it. It has to be handled with a piece of paper toweling to prevent contaminating it. Or, you may instead wish to wear a pair of thin rubber or plastic gloves. Next, he installed the bushing onto the screw with the starting taper at the bottom, being very careful not to get any grease on to the bushing. Then, he placed the screw through the hole from the top with the tapered end of the bushing resting at the top of the hole. Now he installed the second fender washer and nut on the bottom of the screw. Then he applied some epoxy to the outside of the bushing before pressing it into the board. The next step is to hold the screw head with a 7/16-inch wrench, to prevent it from turning, while using a second wrench to tighten the nut on the bottom. Once the nut applies tension to the assembly, the two fender washers are automatically held parallel to each other. Since the bushing is larger than the hole in the floorboard, the resulting press fit generates a fair amount of resistance. The washer on the bottom is forced into firm contact with the bottom of the board and that holds the screw-washers-bushing-nut assembly perpendicular to the board while pressing the bushing into place. The starting taper on the bottom of the bushing helps guide it smoothly into the hole without shearing the wood.

Joe continued to turn the nut until the top washer was flush to the board and he could tell from the increased resistance that the two washers were now firmly clamped against the top and bottom of the floorboard. This signifies the bushing is fully seated. He then removed the nut, washers and screw and cleaned up the excess epoxy from both surfaces of the board around the bushing using a rag sparingly saturated with acetone. Acetone is a good solvent for epoxy before it begins to set up. He followed this by wiping the board dry of solvent with a rag. He cautioned that you should do this in a well-ventilated area since breathing the fumes from any volatile solvent, such as acetone, is not too healthy. Fortunately, since you will use so little of it, this will probably not be a problem.

How Tight Should the Fasteners Be?
Once the kingpin bushings have been installed, you can proceed with the steps as given in the plans. You need to be aware that the current floorboard, made of laminated 13/16-inch wide strips of aspen, is rather soft. You can easily apply enough force to the wrench while tightening to crush the wood and cause the washers to sink into the surface. To avoid this, you need to limit the amount of force used to a practical level. When installing the 1/4-inch screws, washers and nuts that are used to fasten nearly all of the hardware to the floorboard, you can use a modest amount of force initially, and then use an inch-pound torque wrench with a 7/16-inch socket to supply the final tightening force. Set the torque wrench to 60 inch-pounds. This will generate about 1,300 pounds of clamping force, which is substantial, but is not enough to damage the board. Further, it is close to the maximum safe torque for these fasteners.

Kingpin Tension
The kingpins are 1/4-inch diameter just like the majority of the other screws on your car. However, they are 28 threads per inch, grade 8 heat-treated and quenched alloy steel and are much stronger than the other 1/4-inch screws. They can, and should be, tightened to a greater tension to perform properly. There is much discussion among racers about how tight they should be. It has been observed that cars with fairly loose kingpin tension are usually not as fast as those with fairly tight ones. Consequently, some racers advocate kingpin tension as high as 185 inch-pounds as giving the greatest speed. However, you should be aware that too much tension could permanently stretch and/or crack them. This could make them suddenly break, which would cause dangerous loss of control if it occurred at speed. So, how tight should they be?

We consulted the engineering book, Fastening & Joining, 2^nd Ed. 1989 McGraw-Hill, New York, by Robert O. Parmley, ISBN 0-07-048522-4, for the answer. On page 1-25 he gives a table of torques for various fasteners. For 1/4-inch, 28 threads per inch, grade 8, he lists a maximum torque of 163 inch-pounds as generating 3,250 pounds of clamping force. That is quite a bit of tension and it appears to be the maximum safe allowable force for this fastener. There is usually some safety factor built in so that exceeding this value by a modest amount should still be safe. With this in mind, you may wish to limit your kingpin tension to no more than 165 inch-pounds. Further, using the 2-inch diameter R1 washers for axle mounting, or the steel plates on the rear axle of a superstock car should spread out the force sufficiently to prevent floorboard damage.

On the other hand, if the tension is too low, when one of your wheels hits a bump or crack, the axle or airfoil may be forced upward far enough to allow it to hit and damage the body. This also seems to slow the car. Additionally, low kingpin tension can cause the washers to dig into the board on the side opposite from the wheel that hit the bump. Eventually, this will damage the floorboard under the washer and leave your axle loose. This doesn’t seem to be a problem with settings of 100 inch-pounds or greater. In view of these two limits, your kingpin tension should probably be between 100 and 165 inch-pounds. Many racers claim they can maximize their car’s speed at a particular track by experimenting with varying amounts of kingpin tension at that track. Of course, that requires you to time your runs with a stopwatch to see what setup works best for your car at that track.

Also, in the interest of minimizing aerodynamic drag, many derby racers orient the hexagon heads of all the screws on the bottom of the board with a point, rather than a flat, facing forward. The idea is, at speed, the air will flow past each screw head with slightly less resistance this way

Where the Plans Cause Confusion
The following things from the plans often cause confusion. For example, look at “Step 2: Installation of Parts”. You will notice that the drawing shows stacks of three 1/4-inch “V” nuts on the 1/4-inch “H1” screws to form axle stops to limit the amount of front axle pivot. The nuts are drawn with their hexagonal surfaces perfectly aligned. This is for illustration only. Each nut has to be firmly tightened to the next one to prevent their loosening. Consequently, their hexagonal surfaces will be oriented randomly on the screw.

Also, in figure 3.1, note how the setscrews in the brake cable clamps, “T”, are drawn facing horizontally outward. This was drawn this way for the convenience of the illustrator. If you were successful in aligning them this way, with the setscrew bearing against one part of the cable, which in turn clamped against the other, you would have a very unsafe setup. The slightest vibration could cause one part of the cable to roll off the other. This would result in the end of the setscrew not bearing against either cable. Then it could slip out the eyebolt and your brake would not work. Since the same type clamps are used to secure the steering cables, if this happened to a steering cable clamp, this would result in sudden loss of steering. In order to work properly, the setscrew must bear against both parts of the cable across their widths as illustrated below.

Since the brake cable loops around eyebolt, “E1”, horizontally, the clamps must be oriented with the setscrews facing 90 degrees to the loop of the cable. Since you need to access them for tightening, they should face up rather than down.

Further, next to figure 5.3 B, is a comment about the use of the awning pulley and its support bushing. It claims this will improve pedal leverage. It won’t do any such thing. Once the slack has been taken out of the cable, each one inch of forward travel of the brake pedal eyebolt will result in the same one inch of downward travel of the brake plunger. This is the same for each allowable brake configuration.

Which Option is Best?
When building your car, you will notice there are a few areas where you are allowed to use one of two or more hardware mounting options. What follows are some suggestions for the best choice. However, you may have some reasons, such as a smaller than average driver, that requires you to use a different option than suggested.

In the rear axle mounting for the superstock car you are allowed two options diagramed as figure 2.2 and 2.3. Figure 2.2, showing the use of the optional upper and lower steel axle-mounting plates is a stronger and more rigid setup. The plates are much larger in area than the washers, which allow you to tighten the rear mounting screws more firmly without crushing the wood. This is the better choice of the two allowable configurations.

In the stock plans, figure 5.3 shows three variations for the floor-mounted pulley of the brake cable. A cable should never bear against the flanged edge of the pulley; it is not designed to do so. In figure 5.3, the cable is forced to bear against the flange of the pulley on the right side of the brake/steering assembly (this setup is shown in figure 6.5 of the superstock plans). This makes for a rough action as the cable alternately grabs then slips from the flange. In figure 5.3A of the stock plans and figure 5.3 of the superstock plans, the cable is forced to bear against the flange of the floor-mounted pulley as it angles upward towards the brake pedal eyebolt. In addition to giving a rough action on the forward stroke of the pedal, this can cause the cable to get pinched between the flange of the pulley and the wood cable retainer block when the brake is released after application. If this happens, then the brake pedal and plunger may only retract halfway when the driver releases the brake pedal. However, in the awning pulley option, diagramed directly below figure 5.3 in the stock plans and as figure 5.3B in the superstock plans, the cable loops about the awning pulley with little or no interference with the flange. This produces a much smoother action and is very unlikely to cause problems. The awning pulley option is usually the best choice of the three.

For the stock car, figure 6.2 gives two kingpin assembly options. Option one uses a small diameter “N” washer between the upper R1 washer and the axle, while option two uses the larger “R” washers above and below the “R1” washers clamped to the floorboard. Option two is a more rigid setup and helps spread out the clamping force on the board. It is probably the better setup of the two.

It is important to thoroughly read all of the text and compare it to the diagrams. For example, in the superstock plans, there are two illustrations, figures 6.2 and 6.2A, showing examples of washer use on the kingpins. If you failed to read the text at the left, you might be misled to believe these are the only configurations allowed. By reading Step 6, Item 2, we read that we can use whatever washer setup we choose, provided we use at least two washers between the axle and floorboard, and that “R1” washers are not permitted on the rear axle. Since the R1 washers have about 2.6 times the surface area of the R washers, the clamping force is spread out more and is much less likely to crush the board. Also, the R1’s are quite a bit thicker. Therefore, they are not likely to have their centers dished into the wood under high kingpin tension, as the R washers are prone to do. The best configuration for the superstock front axle is not shown. It is: one “R1” washer below the board. And above the board, in the following order: one “R1” washer, one “R” washer, the axle, two “N” washers, and two “W” nylon insert stop nuts, as shown in the following diagram.

Front Axle Washer Setup For Superstock Car

The allowable pulley configurations for the steering cable are given in figures 6.5, 6.5A, and 6.5B. Setup 6.5 allows the steering cable to rub the corners of the square tubing that forms the guide tube for the brake plunger. This is especially a problem for a longer-limbed driver for whom the brake/steering assembly must be moved forward to accommodate. Both setups 6.5A and 6.5B route the cable around additional pulleys at the corners of the assembly, preventing the cable from rubbing the square tubing. But the more pulleys we introduce, the more flex and backlash in the cable. Of the three configurations, 6.5B is the best alternative. Please note, although the awning pulley configuration makes the floor-mounted wood cable-keeper block irrelevant, many racers at Akron in 2001 were unpleasantly surprised to find that it was still required. Unless the AASBD relents, even with the awning pulley option, the cable-keeper block is still mandatory as of March 2002.

A Few Hints
Sometimes the wood around the screw holes contracts enough to make pushing the screws through difficult. If any of the 1/4-inch diameter screws can’t be pushed through the floorboard, use a power drill to run a 1/4-inch drill bit through each hole. Likewise, you’d use a 5/16-inch drill if any of the weight bolts won’t go through. To install the 5/16-inch “O” t-nuts, put a 5/16-inch "K2" washer onto a “K1” weight bolt and push the bolt through the pre-drilled hole from the bottom. Use your other hand to thread on the t-nut from the top until no slack remains. Now simply tighten the bolt with a 1/2-inch wrench. That will force the prongs of the t-nut into the board. Stop tightening when the top of the t-nut is flush with the top of the board.

The plans advise you to solder the part of the cable you plan to cut when trimming excess cable length to prevent the cable from fraying. Silver solder, with the correct flux, is the material to use. Be careful not to overheat the cable. For example, if you use a propane torch it is very easy to anneal the cable by overheating it. That would weaken it at that point. Also, some batches of cable seem difficult or impossible to solder. If it doesn’t work–forget it. Some wire rope cutters, such as a current model sold by Sears stores, have straight jaws. But these tend to flatten the cable as the cut is made, splaying out the individual wires that make up the cable. To cut off excess cable length, try to borrow a wire rope cutter with curved jaws. Many bicycle enthusiasts own such tools for trimming brake and shifter cables. The Park CN-4 works well. However, the Swiss-made Felco C7 is the best tool of this type. It can be ordered online at www.biketoolsetc.com. These tools keep the cable nearly circular as the cut is being made. Consequently, the cut is crisp and not frayed.

You must install the front airfoils before the putting on the steering eyebolts because the clearance holes in the front airfoils for the eyebolts, lock washers, and nuts have to be completed before the airfoil can be installed.

Fit The Car To The Driver
Setting up your car to fit the driver is one of the most important tasks you must do. It has a major influence on how competitive the car and driver can be. This has to be figured out before installing the cables, since once they’ve been trimmed, you would probably have to order replacements to reposition the brake/steering assembly or brake pedal. Also, this must be done before making your ballast weights since it may alter front/rear weight distribution, and it may determine whether or not you use all or only the required front piece of cockpit foam.

In order to custom fit the car to your driver, you must first understand the position he/she must assume. For a stock or superstock driver, correct racing posture is seemingly awkward. The driver must first lean forward and then tuck the upper body down as far as possible so that he/she can just barely see above the front edge of the body shell. Then the driver must scoot back until the back is in contact with the rear of the cockpit opening. This is standard competitive racing position. It is in this position that the steering wheel, brake pedal and footrest must fit the driver.

Here is why it is done. When the driver leans forward, there is a natural tendency for the back to move away from the rear of the cockpit opening. At speed, the air flows along the driver’s back and is directed down into the cockpit opening. Then it is forced to sharply loop back upward generating a great deal of turbulence. It takes a meaningful amount of energy to do so and that comes from the forward motion of the car. Consequently, the car would be somewhat slower as a result. Scooting back to close off this gap allows the air to flow along the driver’s back and make a smooth transition to the car body. This reduces drag and consequently allows the car to attain slightly higher speed. Also, it is important to get the upper body as low as possible for maximum aerodynamic efficiency and to lower the center of mass of the car/driver system. The driver must be able to comfortably steer, brace against the footrest, and operate the brake. Consequently, if the footrest and brake are too close, then it may be impossible for the driver to get down low enough to cut through the air with the least drag and lowest center of mass. Obviously, the body shell must be installed on the floorboard to check the driver’s position and relationship to the controls and footrest. You could probably use only four body mount screws at this point, say, two at the front and two at the rear to determine this.

And don’t forget to check this again next year, a month or so before the start of the racing season. Your driver may very well have outgrown the previous setup. If so, don’t panic. You just have to reposition the controls to suit. However, you’ll probably have to order new cables to accommodate. Also, that may require modifying the weights somewhat to attain balance

Don’t Get Them Too Tight
There are a few small screws and nuts that are easy to damage by over tightening and are not practical to check with a torque wrench. Also, some items, if over tightened, can spoil you car’s performance. When tightening the steering cable turnbuckles, you need to remove any slack in the cable. However, don’t tighten them too much. If they are over tightened, the axle will bow rearward, causing your front wheels to toe-out. Then the tires will scuff the pavement at a slight angle, slowing your car. Likewise, don’t over tighten the stabilizer bar turnbuckles on a stock car. That would bow the rear axle forward, causing your rear wheels to toe-in, scuffing the tires and slowing your car. If you suspect either the steering cables and/or stabilizer bars to be too tight, you can check the spindles for deflection with a spindle gage. Also, when tightening the turnbuckle locknuts, don’t overdo it. They are only 3/16-inch diameter and made of fairly soft steel. Also, the nuts are thin and don’t have much thread to begin with. It is easy to crush or strip the threads

Airfoil Mounting
Installing the airfoils is a pretty straightforward job. However, for the superstock car only, you have to either trim the end of the front airfoils to clear the body, or you may open up the axle openings in the body to admit the ends of the airfoils. Most of us feel pretty uncomfortable with the idea of enlarging the front axle openings in the shell enough for the airfoils. Therefore, trimming them is probably the better choice. If you want to see about how much to take off before actually cutting the airfoils, you can purchase a piece of rough 1 x 2 lumber, sometimes called firring strip, from a lumberyard and make some stand-ins. This helps prevent spoiling a set of airfoils by cutting too much off. These strips are cheap and similar in dimension to your airfoils. Cut a piece the same length as the airfoil and estimate how much to cut off and at what angle. First mark off the saw cut with a soft pencil. You may want a compound angle. That is an angle, such that you cut off more wood at the rear than at the front and one that leaves more wood at the top than the bottom to follow the contour of the shell. After cutting the strip along the layout line, place it in position with its end flush to the end of the square stock to see if it fits the way you want. When you are satisfied with the fit, duplicate the layout on the airfoil and cut it. However you choose to do it, don’t try to do any significant rounding of the edges at the ends of the airfoil as that would probably be construed as an illegal modification under 2002 rules.

You can center the airfoil vertically to the axle by sandwiching the axle and airfoil between two thin pieces of wood such as shims, one above and the other below the axle. Then use two small c-clamps to hold everything together while you install the screws. You place the foot and swivel of the clamps about half over the axle and half over the airfoil. Make sure the airfoil is flush to the end of the axle square stock before tightening the clamps. Then place an A2 washer over a 2-inch A2 screw, place it through the pre-drilled hole, and screw it into the airfoil with a #2 Phillips screwdriver until tight. You don’t need to pilot the holes since the airfoils are soft wood. Put the second screw in and you are done.

However, the front airfoils require a clearance hole for the steering eyebolts, lock washers, and nuts. With the airfoil screwed into position, put a 3/16-inch drill bit into handheld power drill. Since the clearance hole in the axle for the eyebolt is 3/16-inch, the drill fits the hole nicely. With the motor off, push the drill through until it stops against the airfoil. Switch on the motor and drill about 1/2-inch into the airfoil. Now remove the airfoil from the axle. You will have to fasten it down to a bench top with a c-clamp to drill the clearance hole. Or you can use a vise if you have one.

Since the airfoil is rather soft, you should wrap it between something like an old magazine to prevent damaging the wood while it is clamped for drilling. You’ll need a drill bit of about 7/16-inch (.438-inch) diameter or as large as 1/2-inch to clear the 3/8-inch nut, which is about .433-inch across the corners. Make sure you drill perpendicularly into the mounting surface. The 3/16-inch hole you transferred from the axle should guide the larger drill perfectly into location. The full diameter of the drill should cut into the airfoil about 3/8-inch deep. Now install the steering eyebolt and mount the airfoil on the axle to be sure it clears properly. Since the pre-drilled holes in your axles are not uniformly located from axle to axle, each airfoil is essentially custom fit. With that in mind, it is a good idea to mark the mounting surface of each airfoil with its location such as RF, LF, RR, LR, so they can be quickly reinstalled if you should need to remove them for some reason.

The Body Must Clear The Axles, Stabilizer Bars, and Cables
Once you’re satisfied with the position of the mounted body, you must check for proper clearance between the body and the axles, cables, and stabilizer bars (on a stock car). Swing the front axle through its full operating arc. The body should clear the axles, stabilizer bars and cables by at least 1/8-inch from front to rear. Also, these should clear vertically by at least 1/4-inch. When the wheels hit bumps, the axles and stabilizer bars spring forcefully upward quite a bit more than most folks realize. Further, the lower the kingpin tension, the greater they will deflect. Cars whose axles and stabilizer bars strike the body shell will not be as fast as ones with proper clearance. Additionally, the axles sometimes strike the shell forcefully enough to crack or otherwise damage the shell. It takes energy for the axles and stabilizer bars to pound the shell and that comes from the forward motion of your car. Naturally, that will slow you down. This is a pretty good argument for keeping your kingpins fairly tight.

If your shell has any clearance problems, you are allowed to file excess shell material to gain the needed clearance. According to the checklist in the plans for the stock and superstock cars, “Shell openings may be notched to permit free movement of the cables, radius rods, and axles.” (The AASBD uses the terms “radius rods” and “stabilizer bars” interchangeably.) So, if you find you need clearance, mark the area with a pencil or marker to about where you think the new outline should be. Remove the body from the board and file out the excess. Then replace the body and check it. Repeat as necessary until you are satisfied you have proper clearance. Also, examine the area of the body adjacent to the axles, and stabilizer bars after you have made a few runs down your hill. You may be surprised to find marks in the shell where the axles or stabilizer bars have hit. If so, then you need additional clearance. Since you will generally not be allowed to make any changes after inspection on race day, it is best to find this out during a practice session so that you may make corrections prior to racing.

This Concludes Article 4.

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