Detroit Metro Soap Box Derby

DMSBD Tech Tips
DMSBD Car Layout & Building Prep Clinic, Saturday, March 02, 2002 Written by- Ian Carsten

Although the day started out with a snowstorm, all was cozy inside MSX International in Auburn Hills. The space they provided was quite large and very suitable for our clinic. We very much appreciate their continued support and allowing us to conduct the building clinics in their facility.

Detroit Metro director, Joe Flynn, explained the first steps in the building process as he demonstrated starting construction on a stock car. He began by showing the basic tools that are necessary. They include: two 7/16 inch wrenches, two 1/2 inch wrenches, two 3/8 inch wrenches, a large flat-bladed screwdriver, #2 Phillips screwdriver, pliers, hammer, hacksaw with fine-tooth blade, 5/64 inch Allen wrench, mill file, and an assortment of emery paper in medium to fine grit. At least one of the 7/16 inch and both of the 3/8-inch wrenches must be the open-end type. You may wish to purchase several 5/64-inch Allen wrenches since they are quite small, easily lost, and they are inexpensive. These tools will enable you to do the basic assembly tasks.

Joe recommended supporting the floorboard with a pair of saw horses and noted that it is a good idea to cover the tops with some soft material such as strips of carpeting to avoid damaging the fairly soft bottom. Also, he noted that there are some rather expensive tools that may be necessary to perform some one-time tasks. These will be made available to Detroit Metro racers at this series of clinics and at the track on May 18 during our Tune Up and Practice day, and prior to the start of pre race inspections.

Joe then preceded to demonstrate sanding the floorboard to smooth the surface, which he noted, is especially important for the bottom surface to maximize aerodynamic efficiency. He 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.

He then demonstrated applying the tung oil sealer to the floorboard. This should be applied with a small lint free piece of cloth such as cheesecloth, which should be available from the same hardware store where you obtained the tung oil. The tung oil has to be applied in a thin coat and well rubbed in. You first seal the top and sides. After that has dried and hardened, you then turn the board over and seal the bottom. Additional coats maybe applied after the previous one has dried and hardened. You may wish to apply additional coats to the bottom to obtain a smooth surface that helps your car cut through the air with as little resistance as possible. Joe advised the attendees to wear a pair of thin, disposable rubber or plastic gloves to keep the tung oil off of you hands.

Also, he mentioned that the plans, though not complicated, are in some places worded somewhat awkwardly. You can always contact one of the Detroit Metro folks who will do their best to clarify anything that isn’t clear. He also explained that AASBD rules require us to use only their parts if any additional or replacement parts are needed. Further, he stressed that the cars must be built precisely as shown in the assembly diagrams. If any cars win their local derby or have sufficiently high points in the rally program to race at Akron, they will be scrutinized very closely with respect to construction. And, this includes such seemingly trivial things as how many screws, nuts and washers of each type are used and in what order. To save yourself the bother of trying to reconfigure your car at the last minute, you should strive to build it “Akron Legal” from the start. To this end, we will be assisting our racers at these clinics to get it right the first time.

Next we had a brief talk on spindle polishing, why it is done, how it is accomplished, what tools and materials are needed, and what benefits will result. There were two axles on display with signs attached indicating before and after. The raw axle from Akron had spindles that had the somewhat rough finish typically left by lathe turning and was marked as measuring .497-inch diameter. The one next to it had spindles that had been finely polished and measured .496-inch diameter. The idea is, correctly polished spindles are nearly the same size as before polishing. It is not necessary or desirable to remove much material in polishing. Then a demonstration of spindle polishing followed. A fairly detailed account of spindle polishing can be read and/or copied and printed from www.dmsbd.org/kids under ” Tech Tips” listed under the heading “Construction”. The main idea is you can make your car slightly faster by polishing your spindles. As with most speed enhancements, since most of your competitors are already doing this, if you fail to do so as well, you will be racing at a slight disadvantage.

As at the Start Up Clinic, a drawing was held and several prizes were awarded to the drivers in attendance. They included: several Red Ring and Detroit Tigers ball caps, a Coca Cola clock, a Reese’s Hot Wheel die cast car, and two Detroit Grand Prix 2001 programs.

Our next clinic is the March 16 Steering and Brake Clinic.

Back To Tech Tips
______________________________________