PHYSICS OF SPEED DEMONSTRATION

Transcription

1 Pinewood Derby Pack 418 PHYSICS OF SPEED DEMONSTRATION What: An opportunity for Cub Scouts of Pack 418 and Pack 32 to learn about several methods to make their Pinewood Derby race cars faster through an informative and educational physics demonstration using real cars racing side-by-side on the actual Pack 418 / 32 official track. Where: Cokesbury United Methodist Church - Summit Campus 3300 Summit Blvd. (Corner of Summit Blvd. and Spanish Trial Rd.) McClain Hall (facing Renoir Street) When: Saturday, January 11, 2014 Sessions begins at: 11:00 am There are many legal ways to make a Pinewood Derby car go faster and most involve simple physics. This demonstration will allow the boys to see, first hand, how much faster cars will go down the track when each of these changes is applied. If you are new to the Pinewood Derby, this will be a great opportunity to learn some do s and don ts and pick up some quick and easy tips to add speed to the car. If you re an old pro, you and your Scout will surely enjoy seeing head to head heats between cars that have speed modifications and those that don t. This demonstration is designed with the boys in mind. They will be able to see with their own eyes how each change affects the cars performance on the actual Pinewood Derby track. Approximately ten different methods for speeding up the Derby car will be demonstrated live for everyone to see. They will include polished axles versus unpolished axles, heavy car versus light car, weight placement to alter the COG (center of gravity), long wheelbase car versus short wheelbase car, and more. Each method will be discussed and explained with a brief instruction on how to go about making the change to the car. Then, two identically prepared cars will be placed side by side on the track. One car will have the speed change applied (i.e. polished axles) and one will not (i.e. stock axles out of the box). The result of the physics principles of friction, gravity, inertia, etc. applied to each car will then be evident as the cars race down the track. Please note: Although the methods for making faster cars will be thoroughly discussed and explained, this is not a workshop for building or modifying cars. Opportunities for Scouts to attend free workshops, where tools are provided and they can actually work on their cars, will be announced as they become available. ****Please do not bring cars for trial runs or practice. Pack rules prohibit Derby entrant cars from being on the track prior to the Pinewood Derby.

2 Table of Contents Forward Pinewood Derby Physics Head-to-Head Testing...4 Test Group (A) - Graphite to Reduce Friction...4 Test Group (B) - Aligned Axles vs. Misaligned Axles...4 Test Group (C) - Maximum Allowable Weight vs. Default Weight...5 Test Group (D) - Aerodynamic vs. High Drag Car...5 Test Group (E) - Center of Gravity (C.O.G.) Just Forward of Rear Axle vs. C.O.G in the Center of the Car...6 Test Group (F) Improper Wheel Hub Clearance...6 Test Group (G) - Long Wheelbase Car vs. Stock Wheelbase...7 Test Group (H) - One Raised Wheel...8 Test Group (I) - Polished Axles vs. Stock Axles...9 Test Group (J) - Varied Weight Placement and its Effect on Speed...10 Test Group (K) - Stubby Car Demo...11 Summary - What Does It All Mean? Pinewood Derby Physics Trivia Wheel Rotational Speed Car Speed Potential Energy is What Makes the Cars Go How does adding weight to the car make it faster? Can where the weight is placed make a difference? Top Speed Tips...16 Forward This document was written by (retired) Pack 418 Committee Members Phil Crabtree and Joe Flynn, who obviously had way too much time available. Looking back, it's amazing that a small block of wood, four nails, and four plastic wheels can generate a document of this size, or provoke this much thought. Hopefully, you will find our effort worthwhile.

3 1.0 Pinewood Derby Physics Throughout this document you are going to hear the terms Potential Energy and Kinetic Energy used over and over again. So, a basic understanding of these terms is desirable if you really want to get the most out of this material. In the Pinewood Derby, energy only comes in two forms: Potential and Kinetic (Tiger Cubs that have eaten too much candy aren't part of this discussion). In the real world, energy actually comes in a lot of other forms but for our discussion we only care about these two. We'll start with the easy one first Potential Energy is just that, the potential energy that something would have if it were turned loose. Potential Energy is calculated using the mass of an object, its height above the ground, and an interesting little physical constant known as the Acceleration due to gravity. However, it's a lot easier to think about it this way: a brick lying in your driveway doesn't have any Potential Energy. It doesn't DO anything it just lays there, so its Potential Energy is effectively zero. Now think of this same brick on the edge of a roof on a two-story house. To someone standing on the ground, this brick has a LOT of Potential Energy. I know that I certainly wouldn't want to be standing directly under it. Now let's see what happens when this brick gets nudged off the edge of that roof. As the brick begins falling, its Potential Energy quickly gets turned into Kinetic Energy. Kinetic Energy is the stuff that hurts you if you get hit by the brick (but it's also the thing that's responsible for pushing the Pinewood Derby cars down the flat section of the track towards the finish line). As the brick continues falling, more and more of its Potential Energy gets converted into Kinetic Energy, until, in the fraction of an instant before the brick strikes the ground, its Potential Energy is reduced to zero, and its Kinetic Energy has reached its maximum. At this point, if it hits something hard, the brick's Kinetic Energy is probably going to be dissipated by smashing the brick into lots of little pieces. In the Pinewood Derby world the results aren't nearly as dramatic. At the bottom of the ramp, the car has exhausted all of its Potential Energy and in the process has acquired all of the Kinetic Energy it's ever going to get. The winner of the race is going to be the car that acquires the most Kinetic Energy and then most effectively prevents that Kinetic Energy from being eroded away by all of the parasitic losses that the car is going to face as it travels down the flat section of the track. In this document we are going to focus on how you can maximize the Potential Energy available to the car, as well as minimize many of the most significant parasitic losses that the car is going to face during its race. For the demonstration phase of this project, we created matched pairs of cars that are used to demonstrate one specific modification. One car in the matched pair has the modification applied, while its twin does not. To make the experiment as rigorous as possible, these cars have no other speed tweaks applied not even graphite is used on any of the cars other than the one car that is specifically being used to demonstrate the effectiveness of graphite.

4 2.0 Head-to-Head Testing Test Group (A) - Graphite to Reduce Friction Car: A1 This car is using the stock axles and NO graphite has been applied Car: A2 This car is using the stock axles with graphite The car using graphite (A2) should be faster Friction between the wheel hubs and the axles, and between the wheel hubs and the car's body are one of the most significant ways that the car loses energy. As the Kinetic Energy that the car gained by rolling out of the starting chute and down the ramp is slowly eroded away by friction and other parasitic effects, the car will begin to slow down. When that energy has been exhausted the car will stop. Applying graphite to any of the car's chafing surfaces is an incredibly easy and inexpensive way to reduce these frictional losses leading to a car that will run much faster. Test Group (B) - Aligned Axles vs. Misaligned Axles Car: B1 This car is using the stock axles that are misaligned Car: B2 This car is using the stock axles that are properly aligned The car with properly aligned axles (B2) should be faster Misaligned axles will lead to a car that can wander, or one in which the wheels are fighting each other to go in different directions. Both of these issues will cause the car to lose energy much more quickly than it would if the axles were correctly aligned. Always try to insure that the axles are properly aligned in both the horizontal (toe angle) and vertical (camber angle) axis. If you are drilling your own axle holes on a drill press, insure that the drill bit is perpendicular to the pine block. Alternatively, you can use a drilling guide (similar to what is shown below) and a power hand drill to drill perfectly aligned axle holes. Pinewood Derby Axle Drilling Guide

5 Test Group (C) - Maximum Allowable Weight vs. Default Weight Car: C1 This car is running at its stock weight no weight has been added. Car: C2 Weight has been added to this car to bring it up to the 5.0 oz maximum allowable weight. The heavier car (C2) should be faster The total amount of Potential Energy that the car has available is obtained when it is in the starting chute. A car's Potential Energy is specifically a function of its weight and how high that weight is located above the ground when the race begins. A heavier car has more Potential Energy at the start of the race, so it correspondingly ends up having more Kinetic Energy when it reaches the bottom of the ramp. This Kinetic Energy is what effectively pushes the car down the flat part of the track to the finish line. The more Kinetic Energy that the car has, the more easily it is able to overcome the debilitating effects of friction and drag that are working to slow the car down. Additional Information: See Section 3.3 Potential Energy is What Makes the Cars Go for some fun calculations demonstrating the energy gained from adding weight to the car. Test Group (D) - Aerodynamic vs. High Drag Car Car: D1 This car is using an aerodynamic design. Car: D2 This car has a sail added to its design which presents a large frontal surface area, greatly increasing its aerodynamic drag. The aerodynamic car (D1) should be faster Aerodynamic drag is another of the parasitic losses that erode the Kinetic Energy of the cars causing them to slow down. Thankfully, in the Pinewood Derby world the cars are moving relatively slowly (~8 mph), so the losses due to drag are fairly minimal. Still, the margin of victory in many races is amazingly small, so everything needs to be considered. Additional Information: See Section 3.2 Car Speed for the calculations showing how fast the cars are moving down the track.

6 Test Group (E) - Center of Gravity (C.O.G.) Just Forward of Rear Axle vs. C.O.G in the Center of the Car Car: E1 Weights in this car have been located so as to place its Center of Gravity (C.O.G.) in the center of the car. Car: E2 Weights in this car have been located so as to place its Center of Gravity (C.O.G.) just forward of the rear axle of the car. The car with the C.O.G. farther to the rear (E2) should be faster As we saw earlier, the Potential Energy available to the car at the start of the race is directly related to both the weight of the car and the height of that weight above the ground. Because the car sits in the starting chute at an angle, moving the car's center of gravity rearward also effectively moves this weight higher. This leads to a corresponding increase in the Potential Energy available to the car. More Potential Energy at the start means more Kinetic Energy is created at the bottom of the ramp, and more Kinetic Energy means a faster car at the finish. Of course, it is also possible to move the car's center of gravity too far to the rear of the car. When the center of gravity is brought too close to the rear axle (or at an extreme moved behind the rear axle), the front of the car becomes too light. Small bumps, dips, or rough joints between the track sections can cause the front wheels to bounce, leading to a loss of stability with the car, or can even cause the car to come out of the lane. These stability issues can rapidly erode the car's speed, effectively eliminating any gains that you might have received by moving the weight that far to the rear. An optimal weight placement will put the car's center of gravity in a very narrow range that lies between 0.75 to 1.0 inch in front of the rear axle. Additional Information: See Section Can Where the Weight is Placed Make a Difference for the calculations showing how moving a weight rearward can increase the car's Potential Energy. Test Group (F) Improper Wheel Hub Clearance Car: F1 This car has a wheel that is pressed on too tightly. Car: F2 On this car the wheel hub clearance has been set correctly (~ ). The car with the correct wheel-to-hub clearance (F2) should be faster Wheels that are pressed on too tightly will drag excessively against the body of the car. This can aggressively erode the car's Kinetic Energy causing it to rapidly lose speed. Conversely, if the wheel-to-hub clearance is too large, the wheels can vibrate excessively and the car can wander. The car's wheels are spinning at over 2000 RPM, so on the scale of a Pinewood Derby car this vibration can be quite extreme. Both the vibration and the car's wandering (and the resulting chafing against the lane rail) will rapidly erode the car's speed. For best results, the axles should be pressed on to achieve a wheel-to-hub clearance of approximately Additional Information: See Section 3.1 Wheel Rotational Speed for the calculations showing how fast the car's wheels are spinning.

7 Test Group (G) - Long Wheelbase Car vs. Stock Wheelbase Car: G1 This car is using the stock wheelbase. Car: G2 The axle locations on this car have been stretched to move them as far forward and rearward as possible. The long wheelbase car (G2) should be faster In the automotive world a longer wheelbase adds stability to the vehicle. In the Pinewood Derby world, stability isn't as significant of a concern since the cars (generally) aren't performing any dynamic maneuvers. The real benefit of this modification in Pinewood Derby cars comes from the fact that extending the wheelbase allows you to position your weights even farther to the rear while still keeping the car's center of gravity just forward of the rear axle. As we saw earlier, moving the center of mass rearward increases the Potential Energy that the car has available at the start. As always, more Potential Energy means more Kinetic Energy is created, which means the car can go faster for a longer period of time. In the photo below, the axle holes for the extended wheelbase have been moved out as far as possible, and are placed from each end of the chassis. This gives you substantially more area between the axles that can be used to optimize the weight placement. Additional Information: See Section Can Where the Weight is Placed Make a Difference for the calculations showing how moving a weight rearward can increase the car's Potential Energy. Stock Axle Slots vs. Axle Holes for an Extended Wheelbase

8 Test Group (H) - One Raised Wheel Car: H1 This car is running with all four wheels in contact with the track. Car: H2 This car is running with one front wheel raised off of the track. The car with the raised wheel (H2) should be faster For each of the car's wheels that are in contact with the track, there are frictional losses associated with the effort of turning those wheels as the car progresses down the track. These frictional losses erode the car's Kinetic Energy and are one of the major factors causing it to slow down. Lifting one of the wheels off of the track can effectively eliminate one-fourth of these frictional losses whenever that wheel is not in contact with the track. Less frictional loss means more Kinetic Energy is preserved, yielding a faster car. Of course, there can also potentially be a downside to using this technique. In most cases it is recommend that one of the front wheels be raised. If the car tends to track fairly straight down the track then this can be an effective speed enhancement technique. However, if the car is prone to wandering, or if the track is bumpy, raising one of the front wheels may exacerbate the car's stability problems. If you do opt to attempt this technique, the use of a drilling guide to drill the hole for the raised axle is highly recommended. The guide shown below will raise the wheel approximately above the track surface. Axle Drilling Guide that Provides an Option for a Raised Axle

9 Test Group (I) - Polished Axles vs. Stock Axles Car: I1 This car is using stock axles. Car: I2 This car is using polished axles. The car using the polished axles (I2) should be faster As shown in the below photo, the stock Pinewood Derby axles generally have fabrication stamping marks on the nail shank and can also have two small burrs protruding from the underside of the nail head. All of these present an abrasive surface that significantly increases each wheel's frictional losses. This friction in turn rapidly erodes the car's Kinetic Energy and thereby its speed. Removing these machining marks and polishing the axle surfaces will insure that these frictional losses are reduced as much as possible. Stock Pinewood Derby axle (left) compared with a properly prepared axle (right) that has been filed and sanded to remove irregularities. The arrows in the left photo highlight the stamping marks that can occur during fabrication. The best way to prepare the axles is using a drill press or, if you don't have a drill press you can mount a hand drill in a vise to accomplish the same thing. Chuck the pointed end of the axle into the drill press and spin it at about 800 rpm. Then, using a flat needle file held lightly against the axle shank, you can file away the fabrication burrs. Be certain to smooth the underside of the nail head as well. Remember you only want to file enough material away to smooth out the fabrication burrs. You don't want to aggressively grind the axle shaft because reducing its outside diameter too much will result in excessive slop between the wheel hub and the axle. If the fit is too sloppy the wheel can vibrate excessively, robbing the car of speed. Once you have filed the burrs away, it's time to begin the axle polishing process. Working with a succession of grits of Wet-N-Dry sandpaper begin with 400grit, then 600 grit, then 1000grit cut 1/2 wide strips of sandpaper and loop it half-way around the axle shank. Spin the axle in the drill press again at 800 rpm while working the sandpaper strips slowly back and forth to polish the axle shank and the underside of the head. Repeat this process with each of the finer-grit sandpaper strips to achieve a highly polished finish. When the sanding is complete, place some graphite onto a paper towel and then polish the axle with it.

10 Test Group (J) - Varied Weight Placement and its Effect on Speed The pair of cars in this demo session are designed to show the effects of moving a large chunk of added weight - a 3.25oz tungsten round - to various positions on the cars. The weight can be moved to the front, middle, or rear locations on each of the cars. 1. The cars will begin the session with the weight placed in the middle of each car. The cars will be assigned specific lanes on the test track and will run in these same lanes for the duration of the test session. Two or three runs will be made with this setup to determine which car is the faster car. For reference purposes during the remainder of this test, the faster car will be designated J1-FAST, while the slower car will be designated J2-SLOW. 2. At this point, J2-SLOW will have its weight moved to the front position, while on J1-FAST the weight will stay in the middle. With the cars using the same lanes as in the first test, another series of test runs will be conducted with this setup. Has the performance of J2-SLOW improved, or gotten worse? 3. Now the weight on J2-SLOW will be moved from the front position to the rear position, while on J1-FAST the weight will stay in the middle. With the cars using the same lanes as in the first test, another series of test runs will be conducted with this setup. Has the performance of J2-SLOW improved, or gotten worse? 4. Move the weight on J1-FAST to the rear position, while also retaining the weight on J2-SLOW in the rear position as well. With the cars using the same lanes as in the first test, another series of test runs will be conducted with this setup. Has the performance of J1-FAST improved, or gotten worse? 5. Allow the audience to make suggestions regarding possible weight placements on the cars and perform test runs to see the result. The placement of the weight on the cars can have a significant effect on a car's performance. Additional Information: See Section Can Where the Weight is Placed Make a Difference for the calculations showing how moving a weight rearward can increase the car's Potential Energy. Demo car supporting multiple weight placement locations

11 Test Group (K) - Stubby Car Demo The inspiration for the stubby car demo arose from an interesting trend that we had noticed over the years at our local Pinewood Derby events. Every year there is at least one entry that uses a radically shortened wheelbase i.e. a stubby design. Many times the stubby cars would crash especially during the transition from the ramp to the flat straightaway. The question we wanted to investigate was whether this design was fundamentally unstable sort of a Pinewood Derby equivalent of a mid-1960's Corvair automobile, which critics had stridently declared unsafe at any speed, or whether something else causing these problems. For our demo model we created a shortened car chassis that had an overall length of 3⅛, yielding a wheelbase of only 1⅞. A 3.25oz tungsten weight was recessed into the bottom of the car to bring its overall weight up to 5oz, and this weight was positioned so that car's center of gravity was effectively placed at the midpoint of the car's longitudinal axis. Testing with this demo model shows no real issues with the car's stability. The car never crashed and it doesn't appear to be unstable during its runs. What then could be the causing the issues that we so often see in the Pinewood Derby races with cars that are using this design? The physics of the car can give us the answer to this question. By this point in our demonstrations we have used the term center of gravity (COG) quite frequently. We have talked about the benefits and drawbacks of moving the COG towards either the front or rear ends of the car but we haven't yet talked about moving the COG up or down in the car's vertical plane. Since we live in a 3-dimensional world, the center of gravity in the Pinewood Derby cars will exist in three dimensions as well. Here's an example, in the real world it's a challenge to balance a large weight on the top of a tall, narrow shaft - this layout is inherently unstable. However, flipping everything upside-down and mounting the large weight to the base of a tall, narrow shaft, makes the entire system significantly more stable. In the Pinewood Derby world, the cars behave the same way. On a typical Derby car, the wheelbase is generally at least 4⅜ long and 1¾ wide. The top of a typical car is approximately 2 above the track surface. The top surface of the car also seems to be the favored placement location whenever weight is added to the car. However, adding weight to the top of the car effectively moves the car's center of gravity up as well. A normal Derby car has a fairly long footprint, so even though adding weight to the top of the car moves its COG up, the overall impact isn't too bad because the elevated COG is still balanced over this large area. However, a stubby car has a significantly smaller footprint. Elevating the COG too far above the axle center line on this type of design can make it very unstable. Even if the car doesn't crash, our other tests have showed us that an unstable car is often a slower car. Why did our car work? In the design of our stubby demo car we added a significant chunk of weight (3.25oz) into the bottom of the car. This effectively keeps the vertical component of the car's COG right along the axle center line, which helps to keep the car stable. The takeaway lesson from this demonstration is that in ANY car design, not just on a stubby car, it is more advantageous to keep the weight of the car as low as possible, since this will make the car more stable.

12 Summary - What Does It All Mean? Pinewood Derby District Championship - Final Race The above photo is a finish line shot of the final race in the Webelos-II Division of a Pinewood Derby District Championship. The outcome of this race literally decided the championship. As you can see, races are won by fractions of an inch and thousandths of a second, so every little thing just might end up being something important that makes the difference.

13 3.0 Pinewood Derby Physics Trivia Wheel Rotational Speed Wheel RPM = ((total distance traveled)/(wheel circumference)) x (60/(travel time in seconds)) BSA wheel diameter = 1.185" Distance traveled per wheel rev = x = " Pack 418's track is 37 feet in length (444") A fast car on the track runs 3.16 seconds or better Wheel RPM = (444/3.7228) x (60/3.16) = 2264 rpm or faster Car Speed Car Speed = (total distance traveled)/(travel time in seconds) Pack 418's track is 37 feet in length A fast car on the track runs 3.16 seconds or better Car speed = 37/3.16 = feet/sec or [(37/3.16) x (3600/5280)] = 7.98 mph or faster

14 3.3 - Potential Energy is What Makes the Cars Go The Pinewood Derby races are all about maximizing the Potential Energy that the cars have available at the start of the race (using the max allowable weight and insuring optimum weight placement) and minimizing the energy losses from friction and drag - that the car will see throughout the race (by using graphite, aligned/polished axles, proper wheel hub clearance, aerodynamics, etc). The car that does both of these things the best will probably be the winner. The Potential Energy of a car is a function of the car's weight and the height that that weight is positioned above the ground at the start of the race. A heavier car has more Potential Energy at the start of the race, so it correspondingly ends up having more Kinetic Energy when it reaches the bottom of the ramp. This Kinetic Energy is what effectively pushes the car down the flat part of the track to the finish line. The more Kinetic Energy that the car has the more easily it is able to overcome the debilitating effects of friction and drag that are working to slow the car down How does adding weight to the car make it faster? Increasing the weight of the car increases its Potential Energy. We can use the Potential Energy equation illustrated below to show how much Potential Energy can be gained by simply adding 0.1oz to the weight of the car. Potential Energy (joules) = mass (kg) x g (acceleration due to gravity) x height (meters) Both cars will be on the starting ramp at a height of 48" = mm. Acceleration Due to Gravity = 9.8m/s² Test Weight1 = 4.8oz ( grams) Test Weight2 = 4.9oz ( grams) Total Potential Energy available using Weight1: ( /1000) x 9.8 x (1219.2/1000) = joules Total Potential Energy available using Weight2: ( /1000) x 9.8 x (1219.2/1000) = joules That's about a 2.08% increase in Potential Energy available to the car at the start of the race.

15 3.3.2 Can where the weight is placed make a difference? As we saw above, the Potential Energy of a car is a function of BOTH the car's weight and the height that that weight is positioned above the ground at the start of the race. Therefore, moving the weight farther to the rear of the car - and thus placing it higher in the starting chute is a simple way to also increase the Potential Energy available to the car. Think of it this way, the higher a weight is at the start of the race, the longer it gets to fall before it reaches the bottom of the ramp. Effectively, this weight gets to push on the car for a longer period of time before reaching the bottom and therefore imparts more energy to the car. For our test case, we will use a car is sitting on the ramp in the gate at a 45 degree angle. For every 1" farther to the rear that you move a "chunk" of weight you gain an approximate 1.5% increase in the Potential Energy available from that chunk of weight. Potential Energy (joules) = mass (kg) x g (acceleration due to gravity) x height (meters) Movable Test Weight: 3.21oz tungsten round = 91.0grams Test Position1 = Tungsten weight at height of 48" = mm Test Position2 = 48" + sin(45)x(distance_moved) = 48" +.707(1") = " = mm Total Potential Energy available at Position1: (91/1000) x 9.8 x (1219.2/1000) = joules Total Potential Energy available at Position2: (91/1000) x 9.8 x (1237.2/1000) = joules (~ +1.5% increase)

16 4.0 Top Speed Tips (...or, I don't want to read the whole document, I just want to know what we can do to make the car run faster!) The tips below are listed in their recommended order of importance. However, you need to insure that anything that you do complies with your local Pack's Pinewood Derby rules. For instance, some Packs require you to use the stock axle slots that are pre-cut into the wood blocks. So, racers in these Packs don't have the option of drilling axle holes or extending the car's wheelbase. 1. Use Graphite. Use graphite on all chafing surfaces. Even polish the wheel treads with a paper towel coated with graphite. Remember, unlike the wheels on your automobile, these wheels don't need to produce traction. Instead we want them to slip if possible. Most importantly, before pressing the axle/wheel assemblies fully onto the cars, treat them liberally with graphite. With the axles inserted into the wheels, puff graphite into the wheel hubs and then spin the wheels to work it in. Repeat this process several times for each wheel, puffing graphite into the wheel bore from both the inside edge of the wheel and the outside edge of the wheel. It's also recommended that you don't paint the wood block immediately around the axle holes instead, keep this patch of wood bare. Before mounting the wheels onto the car, liberally polish these patches of bare wood with a coating of graphite remember, this is where the wheel hub is going to rub against the car, so the graphite will help reduce that friction. 2. Bring the car's weight up to 4.9oz. Hmmm, the rules allow 5.0oz, so why don't I recommend a target weight of 5.0oz? Basically, because I have seen too many cars damaged on race day by parents frantically trying to bring the car's weight down during the weigh-in just prior to the race. A target weight of 4.9oz gives you enough of a cushion to avoid weigh-in problems. If you do want to target the 5.0oz max weight, then I recommend that you have an easy way to remove at least 0.05oz if you end up overweight. For instance, you could use an easily removed small weight such as tungsten putty, or grind a 1/8oz tungsten weight down to about half its size and glue it onto the car. Either can be quickly (and for the car, painlessly) removed if you end up overweight. 3. Drill axle holes and insure the axles are properly aligned. Properly drilled axle holes will generally yield wheels that are truer than those using the stock axle slots. Always insure that the axles are properly aligned - this is especially important when you are using the stock axle slots. 4. Clearance between the wheel hubs and the car body set to Pressing the wheel assemblies onto the car is pretty much the final step in its assembly. Try to take an extra minute to insure that you set the wheel clearance properly. You can use an automotive feeler gauge, or any of numerous Pinewood Derby tools to insure that the wheels are installed correctly. 5. Weight placement optimized to place the car's center-of-gravity 0.75 to 1.0 in front of rear axle. If the car's design allows it, adjust the weight placement so that the car's center of gravity ends up located within this range. 6. Polish the axles. The photos of the polished and unpolished axles shown earlier in the Test Group(I) portion of Section 2.0 Head-to-Head Testing tell the whole story of why this is important - the stock axles are really rough! 7. Extend the car's wheelbase. Moving the front and rear axles farther toward the ends of the wood block will improve the car's longitudinal stability and also gives you the opportunity to move any added weight farther rearward while still keeping the car's center of gravity forward of the rear axle. 8. Raise one of the front wheels. Slightly raising one of the front wheels can potentially lower the car's rolling resistance.