Pinewood Freak

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Optimum weight location

January 7th, 2006 · 47 Comments

If you’ve read the tips here and elsewhere you know if your weight should go in the front or the back. You know you should keep the weight low. But I’ve seen plenty of cars that meet these criteria but still don’t have the best weight placement.

Lots of people weight their cars with those non-lead weight strips sold in hobby stores. They put the weight toward the back and keep it low by attaching it underneath the car. But these still don’t have the best placement.

When your car moves from the slope the flat portion of the track, it rotates. To minimize rotational inertia weight should be compact, not spread across the body of the car. Spreading the weight across the body increases the amount of mass that must rotate.

Rotational inertia of a car with mass spread across the body

Bt keeping the weight compact, less mass needs to rotate and less energy will be lost overcoming inertia.

Rotational inertia of a car with mass concentrated

Comments

47 comments so far ↓

  • KJD // Jan 29, 2006 at 7:44 pm

    Great TIP. Thanks

  • Larry Phair // Apr 15, 2006 at 5:24 pm

    It is not a question of rotational inertia. It is a question of stored energy in the gravitational field. If your car runs on a straight track (no change of slope) then neither the placement of the weight nor the length of the weight plays any role.

    If your track has a sloped section (at the start) and a flat section (at the end), then you want to put the weight as far back as possible to get the weight highest above the ground. This “stores” more energy in the car (than say a car with weight only in the front) that will be released when you hit the flat section.

  • Adam Kalsey // Apr 17, 2006 at 2:35 pm

    Not exactly.

    The shape of the track doesn’t effect the potential energy (or as you said, stored energy). Potential energy is defined as mass*height. Assuming the amounts of weight are constant, the farther the object falls, the more potential energy there is. So with any track with a sloped starting gate, by putting the weight to the back, you’re also putting it higher up. See http://pinewoodfreak.com/2005/12/14/speed-tip-weight-to-the-back/ for more information. (On tracks without a sloped gate, forward weighting is best. See http://pinewoodfreak.com/2005/12/20/speed-tip-sometimes-weight-goes-in-front/)

    The potential energy is converted to kinetic energy (speed) as soon as the starting gate drops. That’s what gets the car moving in the first place. The energy is released as the car falls, not when the car reaches the bottom of the slope. If the energy was released when the car transitioned to the flat part of the track, you’d see it speed up then, and that obviously doesn’t happen.

    Cars that have a compact weight placement — and therefore a reduced rotational inertia — often appear to speed up as they transition from the slope to the flats. That’s an illusion, as a gravity-powered car can’t gain speed like that. What’s really happening is that the other cars are slowing down so much that the inertia-optimized car suddenly pulls ahead, making it look like it sped up.

  • Dr. B // Oct 5, 2006 at 8:46 pm

    Converting the potential energy of the car is key clearly, but so is stability. All/most of the weight at the back tends to make cars a little wobbly. A real stable car will roll fast also. There needs to be a bit of weight forward or the bulk of the mass (lead, tungsten) needs to be placed to enable energy conversion AND stable roll. My 2 cents.

  • Joe R. // Mar 10, 2008 at 12:58 pm

    On the same factor of” keeping the weight compact, less mass needs to rotate and less energy will be lost overcoming inertia.”
    Would the opposite be true, a spread mass would make the car less likely to turn or wobble against the guide rail?

  • Adam Kalsey // Mar 11, 2008 at 10:51 am

    Dr B:

    That’s why most experts suggest a center of mass of about an inch in front of the rear axles. If you’ve got an exceptionally smooth track, perfectly round wheels, and excellent axle alignment, you can get away with more aggressive weighting, but in no case should you have the center of mass be behind the rear axles.

    Joe R:

    Probably not, although I haven’t done any testing. But the amount of left-right rotation simply isn’t going to be enough for inertia to have an effect. So far, the most effective technique I’ve found to prevent side-to-side movement is rail riding.

    With rail riding, you intentionally steer one of your front wheels slightly inward — enough to make the car drift about an inch over the course of two feet. The car will hold that front wheel lightly against the rail the whole ride down, virtually eliminating wobble.

    It sounds counter-intuitive, but I’ve raced the same car configured as a rail rider and configured to run straight and the rail rider was faster by over a car length.

  • Kevin Butler // Apr 2, 2008 at 8:37 pm

    Adam,

    The shape of the track does affect the potential energy.

    If you have an even slope, then weight placement doesn’t matter, because the front and back of the car both drop the same vertical distance by the time the front of the car passes the finish line.

    If the track starts out sloped, then levels off, however, the back of the car drops an extra inch or two of vertical height to get to the flat section, so moving the weight backward gives that much extra energy to convert to velocity. It isn’t a “released when the car transitioned to the flat part” but instead it allows the car to keep accelerating just a little longer as the center of mass drops that last inch or two.

    kb

  • Hypertech Pine // Jan 26, 2009 at 1:49 pm

    The center of gravity should be as high as possible. Your overlooking the total distance traveled by the actual mass like a pendulum. shorter distance by a greater weight. Our cars have finished in the top 2 our of a 100 plus cars 11 times and we always place the weight rear and high. The few times we didn’t win outright the same weight placements were used. The location of the balance point depends more on the axle placements. Shoot for 1-1/4″ to 1-1/2″ from the rear of the car. This works for full spread axels as well as slot placed axels (hope you aren’t actually using the slots though, flip it over and cut new slots if you have to use them)

  • Richard J. Hanson, Sr. // Jan 28, 2009 at 12:38 pm

    Amusing.

    I would simple note that the axis of rotation of a pwd car is the point equal distance between the car’s axles and not as indicated by the nifty little arrows in the diagrams, at least if the car stays on the track. Of the two cars depicted, the car with the mass concentrated in the back actually has the greater rotational inertia as more of the car’s mass is concentrated farther away from the car’s axis of rotation.

    I = integral r**2 dm

    As we concentrate the mass of a car in the back in an effort to maximize potential energy, we also increase a cars rotational inertia. If we desire to decrease a cards rotational inertia, then the mass should be concentrated at the cars axis of rotation and not at the rear.

  • E Peterson // Feb 9, 2009 at 12:25 am

    If our sons only knew all the thinking that went into this….

    Before jumping into the fray let me admit that its been a while since I worked in my trained field (aerospace engineering) but I think that some folks are a bit off relative to the weight placement issue. While it is true that potential energy is indeed a function of the distance that a given mass travels; i.e. the change in the elevation of the track from the start to the finish, the truth is that moving the center of the gravity of the car higher will have no effect on the potential energy because the change will affect both the starting and ending position of the center of gravity.

    To better understand, assume that the difference in the elevation of the track at its beginning and end was 3 feet, and the center of gravity of the car was 3 inches above the track. Then the change in elevation of the center of gravity (which determines potential energy) would be 36+3-3=36 inches. Raising the center of gravity to 5 inches would mean the change in elevation would be 36+5-5=36 inches.

    This is not to say that putting the weight high on the car wouldn’t make it faster, but I suspect the reason is that doing so (presuming it was located behind the rear axle) would create a moment around the rear axle and unload the front wheels, reducing friction.

    As far as the angular momentum issue, I am going to have to dust of my trusty HP calc and run the numbers, but I suspect the difference is not going to be material compared with other factors (drag and friction to be more specific).

  • Chris Davis // Mar 11, 2009 at 10:42 pm

    E Peterson:

    It seems that your logic would make sense if the cars started out level, then went down a slope. But since they begin tipped down at an angle and end up flat wouldn’t placing the weight farther back indeed increase the change in elevation of the center of mass?

    What you are saying does make sense when considering the placement of the center of mass in the vertical direction of the car (meaning how high above the plane of the axles it should go). If you’re right, then it doesn’t matter from an energy standpoint, though like you say there may be other reasons for high placement.

  • J Brunner // Mar 17, 2009 at 7:46 am

    Hypertech Pine, E Peterson, Chris Davis, et al:

    How far back the center of gravity is and how high the center of gravity (on a level car) is are two different considerations that are often confused.

    The goal isn’t to maximize the height of the initial center of gravity, it is to maximize the vertical change in the center of gravity. A lot of people misinterpret this to mean that the center of gravity should be as high above the floor as possible and place the weight high in the car.

    E Peterson got that part right, but then incorrectly stated that placing the weight higher in the car made no difference in that regard.

    If you look at the profile of a track and do some simple math you can see that weight should be as low as possible, independent of the fore and aft positioning.

    The reason is that the track is sloped. Assume that you raise the weight from its initial low position to a position y above that (on a level surface).
    When you raise the car to its starting position on the sloped track, the high position is now (y cos theta) above the low position, where theta is the slope of the track. This value is less than y.

    This increase in height at the starting position is less than the increase in height on the flat section of the track.

    The distance that the weight falls is lower when the weight is placed high than when it is placed low.

  • Hyper-Tech Pine // Dec 14, 2009 at 10:03 am

    The reason the weight is placed back end of the car is due to the transitional portion of the track from downhill to the flat. The weight placed in the far back translates to continued “push” through the entire transition (i.e. highest potential to kenetic)
    The car appears to other layouts as accelerating through the curved section due to the extension of “push” time all the way through the transition.
    The reason to place it high is due to the pendulum effect through this same area.

  • Shane Pekny // Jan 3, 2010 at 4:53 pm

    Haven’t built a derby car for 20 years, but this is how I recall our weight-placement strategy:

    We pushed the center of mass rearward so that, when the car sits on the slope, the amount of friction in the front axles and wheels would equal the amount of friction in the rear axles and wheels.

    Our goal was to keep the friction equal between front and back because we didn’t want either do drag.

    I even remember a vector diagram that we used. The exact location of our center of mass depended on the slope of the track.

    Notably, our rearward placement was very similar to the placement described here — and our car was among the fastest every year. But I guess I recall a different explanation of the physics that what I am seeing here.

    Does this make sense to anyone?

    Shane, Omaha

  • Dad // Jan 20, 2010 at 7:52 am

    You know fellas, it sounds to me like your sons are not the ones building these cars. I’m putting my 8 yr old completely in charge and we’ll see how he does. That’s how HE will learn.

  • Adam Kalsey // Jan 20, 2010 at 8:42 am

    Sounds like a plan, Dad. Give him no instruction and see how he fares. Like most 8 year olds, I suspect he’ll give up in frustration.

    We don’t teach our children math by handing them a sheet of math problems and expect them to figure it out. We don’t coach sports by putting them out on the field and letting them struggle through it.

    You certainly want to encourage creativity and ensure your child does the work himself. But you also want to teach them what makes a car successful. Show him how to straighten the wheels and discuss and experiement with weight placement.

    Pinewood Derby is a wonderful way to get kids introduced to physics and woodworking. It’s a shame so many adults don’t take that opportunity. The goal of this site is to explain to inquisitive kids how to do things that Dad when show them.

  • Michael Blue // Jan 28, 2010 at 1:52 pm

    Keep it compact, as far back as possible without adversely affecting stability. Generally, low and just before the rear axles (which should be as far back as possible) is the best placement.

  • Don (known as PAPA) // Feb 12, 2010 at 12:37 am

    The wood and wheels need to weigh 2.5 ounces and the weight needs to weigh 2.5 ounces. Buy one of flat boy scout weights at a hobby store that is long and flat and comes with little screws. Carve out the bottom of the car making a little box from the rear of the car to the end of the weight. Cut out only enough wood for it to fit. Then cut your design of car. Sand and paint it. Make sure all ripples and any rough parts are filed off the nail or axil. They must be shinny smooth. Also check the tires. If there is any extra plastic, sand it down carefully. Use powdered gravite and work it in the wheels and nails. Saw off or use a dremel to cut the back nails (axils) to fit the dimensions of the weight you have to put in the bottom of the car. The wheels need to spin at least for 30 seconds each. Make sure all four wheels are perfectly flat on the car. It does not make that much differance using on three. I have beet that kind with using these instructions. Your car will get 2.600 seconds on the track. You can get 2.576 by modifying the nail (axil) by carefully taking out a bit of the nail inside where the tire spins on the nail. Leave only 1/32 of space and each end so wheel can spin. You take out 60% friction doing this. It leaves a section also where gravite is constantly lubing the wheels. This car will win you races. Be carefull and have fun. PAPA

  • Hyper-Tech Pine // Feb 24, 2010 at 10:32 am

    Dont place all 4 wheels flat. Big mistake. Lift one wheel on the front and shift weight to the rear.
    As to prior statements concerning stability, the higher weight could cause stability issues… if there were turns, but I haven’t seen that track yet.

  • Gogo // Feb 25, 2010 at 5:20 am

    My son and I have been doing the Pinewood Derby for only two years; the first year, we won…did our homwrok but I think is was also luck…this years we finished second by .001 seconds. My question to the group: is there a different optimum weight location for a circular arc ramp compare to the incline plane ramp?

  • Gogo // Feb 25, 2010 at 5:32 am

    Wow, too many typos…well, it is 5:00 am!!

  • Stoney // Feb 28, 2010 at 11:24 pm

    Okay guys,
    In a real automobile you put weights on the wheel to balance them out I have decided to approach our derby the same way. After reading and re-reading the rules there is nothing on adding weight inside the wheel wells. My thinking is that the car itself will use inertia as a second gear when it hits the flat part of the track. The spinning weighted wheels will carry more force of forward movement. The result is less energy loss as well as, less chance of wheel hop or wobble at the lower part of the track. Combined with the other tips here this could be the thing that puts the car over the finish line first.

  • Adam Kalsey // Feb 28, 2010 at 11:33 pm

    Generally on a pinewood car, we try and reduce the weight of the wheels where possible. Increasing the wheel’s mass will help maintain inertia of the wheel as it spins, but also results in a slower start (harder to get the car moving). Since the wheels tend to bump the track guide and slow down and spin back up as the car heads down the track, a lower inertia on the wheel is beneficial.

    A third issue with weighted wheels is the gyroscopic effect. If you don’t have perfectly straight wheels, energy will be lost as the wheels act as a gyroscope, resisting the car’s natural path down the track.

    There’s been a few experiments in the past where people have tried weighted wheels, hoping for a flywheel effect. In the end, builders always return to the lighter wheels.

  • Stoney // Mar 7, 2010 at 1:38 am

    The weighted wheel idea sounded great in theory. In reality it sucked, it was a momentum killer and overcoming the energy loss was impossible. Now I know for myself this is not the way to go. At least we won the best in show hands down.

  • Hypertech Pine // Jun 8, 2010 at 9:11 am

    Increased mass in the (rotating) wheels will only help in very long flats. Calculations bear out the break point of a an ideal car at around 112 LF of flat. Also results from an open race in a SW district several years ago on a very long track where stock wheels actually beat out extreme modified wheels to everyone’s surprise.

    Interesting side point the new BSA wheels (2009) are about 0.2 grams lighter then the old ones as well as considerable truer and rounder. Last race event had both types new and old and there was a noticeable difference from the top end to the flat between the two types. Newer wheels were faster on the slope and the older where faster on the flat, but not enough to make up the difference on a 45′ overall track.
    You can spot the difference from the molding marks are 3 even points on the inside vertical face of the wheel. Normally these wheels are very superior to the older wheels with mold marks on the tread surface due to weight and camber, roundness, balance right out of the box.
    Also any wheels that are colored are the newer type.

  • Hypertech Pine // Jun 8, 2010 at 9:28 am

    Regarding grooving of axles, studies have shown its detrimental. Reduction in surface area of the axle results in increased loading per available contact area with the wheel. Secondly it introduces instability as the wheel moves side to side slightly, thus moving the leading edge of the groove around on the hub contact area. Thirdly, any extra new graphite introduced during a run into the mated surfaces now has to be ground down, resulting in a loss of additional energy. The actual thickness of the graphite lubrication area between the surface areas is a few microns. Unconditioned graphite is much larger hence throwing lumps into the works, not optimal.

  • KC // Dec 31, 2011 at 3:15 pm

    Firstly, I’d like to express my gratitude to those who have shared on this forum. It has been a fun and enlightening read over the course of my son’s “racing career”.

    I’d like to share a bit of info that might be useful to some. Last year, we thought we had a winner. Car wood was minimized and replaced with dense material in a focused position very high and rearward. Our idea was to maximize rotational energy at the very tip of the lever. Long story short, the car flopped. My theory is that the massive weight behind the rear axle acted as a brake, as the car was forced to “tow” the weight as the track flattened. This was a great disappointment after some first place wins in previous years. The lesson learned is that any weight distribution tactic should be used judiciously, taking advantage of the car’s fulcrum (which, as someone else commented, is always half the distance between the two axles), while also being careful not to focus too much weight in any one place. If the front of the car is too light, it will bounce and rattle all the way down the track.

    Also, placing the weight low in the car makes more sense, because height is only a factor if the rear wheels have a starting position off the track (not the case), and because a low center of gravity minimizes energy loss as the wheels strike the rail of the track.

    Good luck this year, gents! And remember that this is an opportunity to teach your boys about physics, craftsmanship, and sportsmanship. Not everyone is a winner because they entered a competition. The winners are those who successfully apply knowledge, take pride in their work, and share their experience with others!

  • Ernie Gonzalez // Jan 19, 2012 at 12:05 am

    OK, I gotta add my 2 cents…

    I weight as far back and low on centerline as I can. I figure there are 6 things I need to optimize and in this order:

    1.) Rear wheels exactly as far back as they can go.
    2.) CG at 1″ in front of the rear wheels.
    3.) Weight as low on the car as possible.
    4.) Front of the car as light as possible.
    5.) Car as short and long as possible.
    6.) Weight as centered L-R as possible.

    OK, here is the logic. #1 is a prerequisite for #2. #2 will allow us to place our weight as far back as possible to store the most potential energy without the front end chattering (tail wagging the dog). #3 is a finer detail of #2. The CG is in three dimensions. Your weight will end lower if it starts lower - yes the CB will fall exactly the same distance, but it also changes orientation. Consider if the car started at 90 degrees and the CG was 1 inch from the track surface (high) versus 1/2 inch from the track surface (low). After the rotation to the flat the low CG will have fallen 1/2 inch lower. On 40 degree track it is very small but it makes a difference in the last round of the event. #4, #5, and #6 all address stability and steering losses. If you could center all of the 5oz weight at the CG the car would turn left and right with the least energy. The correcting effect of the steering wheels will lose less momentum if the weight is centered and low (engineers: think - low polar moment of inertia). A light and long front reduces the correcting force (parasitic loss) of bumping the guide rail by mechanical advantage and reduced angular momentum. The benefits for low CG and stability is that a low CG will make it more difficult for the car to tip off left or right when it hits track joints and bumps. (Considering #4, #5, and #6: polishing your axles is likely a better use of time, but it costs nothing to build it in from the start).
    The first year it is difficult to communicate this to your child. You build your car with meticulous attention to the physics – and win. Next year your child will pay very close attention to your guidance when they build their car. And one other thing: make it as beautiful as possible. Pretty cars are just faster!

  • Alex // Jan 19, 2012 at 11:53 am

    Thanks for all the education! My son and I just finished his car but we added weight behind the rear axle, and sure enough the front ends lifts up on the straight-away. (Melted weight embedded in the body, it ain’t coming out). Any thoughts for last minute corrections? Add a little weight to the front? Or is this a good thing? Less friction perhaps?

  • Adam Kalsey // Jan 19, 2012 at 1:09 pm

    You’ll have exactly the same amount of friction, it would just be all in the back wheels than distributed across the front. See the page about running on three wheels - especially the discussion in the comments.

    Having a light front-end will cause it to bounce. Bouncing is bad. Bouncing will slow your car to the point it can be several car lengths ahead at the bottom of the ramp, and finish several car lengths behind.

    Drill out some of that weight (if your son is old enough, this is a great time to teach him about using power tools), and migrate some forward. Read here about where you want the balance point.

    And finally, if the weight you added is lead, please wear a respirator and gloves when handling the weight, both the melting step and the drilling out. A fast pinewood car isn’t worth getting lead poisoning over.

  • Ernie Gonzalez // Jan 19, 2012 at 4:28 pm

    Placing the CB at one inch in front of the Rear wheels IS the optimum “fix” for the front end bounce.

  • Ernie Gonzalez // Jan 19, 2012 at 4:35 pm

    Regarding the friction, Adam is right on the rolling resistance it will be mostly concentrated at the back. The light and long front end works to make your weight function as if it is compact like the diagrams in the opening article above.

  • Alex // Jan 19, 2012 at 5:18 pm

    Thanks (yes a good vent and a respirator, absolutely!) Unfortunately the shape we went with will not allow for much weight directly in front of the axle. I’ll place some in the front end and hope for the best. Wish I found this website before we cut! Live and learn.

  • tom // Feb 4, 2012 at 1:35 am

    Best weight place is near the front axle. Three years in a row my son won his Den’s pinewood derby. We used more real “wood” than most other cars. The wheels were sanded and axles polished - that’s it.

    The most time spent building was in the painting. A simple design was achieved by a single cut to rake the front at a 45 degree angle and a similar, smaller rake on the rear. This was done to show a race car profile.

    Paint, a few weights placed on top near the front completed the car and a Three-Peat!

  • Adam Kalsey // Feb 4, 2012 at 11:07 pm

    Tom, I’m afraid science and actual testing don’t back your theories up. For tracks that have a sloped starting gate (the most common sort of track), rear weighted cars are faster.

    http://pinewoodfreak.com/2005/12/14/speed-tip-weight-to-the-back/ describes the physics behind this.

  • Bill Klingler // Feb 11, 2012 at 10:37 am

    Thank you for both the truth and some nonsense. If you are in BSA a lot of this is not applicable - changing wheelbase, bending axes, rail riders, etc. BSA rules are usually fairly restrictive.
    Regarding where to have the COM (Center of Mass) should be 1″ in front of the rear axle slot. Use Tungsten. Make sure you use the long end of the block for the front of the car and cut off the back and move it up front. Cut off the back right behind the rear wheels because your car probably can’t be more than 7″.
    Here’s the reason: No matter where your car starts on the Circular Arc Ramp the COM will reach the horizontal run at the same time. This is to be expected because of the similarity of the Circular Arc Ramp to the Cycloid of Constant time. The higher the COM at the start, the more benefit you get from the “Front End Extension” effect. You also will have a high linear velocity. Remember, it is the front of the car that sets off the light. Thank you Dr. John Jobe, author of the “Physics of the Pinewood Derby” Page 40. Remember, if you are going to argue with me, you’re going to be arguing with Dr. Jobe and my 27 years of racing, and still racing. I welcome your comments. You folks are lots of fun.

  • Bill Klingler // Feb 13, 2012 at 6:16 am

    To Gogo. No, the weight location stays the same. You simply benefit more from the “Front End Extension” effect and Higher Velocity on the Circular Arc Ramp than the Incline Plane Ramp. If your track is a constant incline it doesn’t matter where you put the weight.

  • Bill Klingler // Feb 13, 2012 at 6:48 am

    To Hypertech Pine. Someone is teasing you. The 2009 wheel weighs 2.6 grams and the 1999 wheel weighs 3.6 grams which makes a 1 gram difference, not .2 gram difference.

  • Bill Klingler // Feb 13, 2012 at 6:57 am

    To Ernie Gonzalez. The Center of Mass should be 1″ in front of the rear axle slot, not the rear wheels. If you move the COM to 1″ in front of the rear wheels you will be giving up 9/16″ benefit of that Cycloid Thingy.

  • Bill Klingler // Feb 13, 2012 at 7:35 am

    To Don (known as PAPA)
    The zinc plate is too light and makes it nearly impossible to get the balance point. Use tungsten cubes.

    You don’t “work in” graphite. That’s a Myth. It is a simple film on the wheel hub. Many believe, as I do, the nail really doesn’t hold graphite but go ahead and put some on anyway. We polish the nail to the highest shine possible which gives the most benefit.

    Cutting nails is probably against BSA rules.

    NO flat wheels. Too much chance for imperfections. Wheel is concave from BSA from the cooling of the wheel. It can work to your advantage.

    A 3 wheeler will beat a 4 wheeler by about 3 1/2″. It comes from the 25% saving of Rotational Energy, not a reduction in Friction. Friction reduction is another Pinewood Derby Myth. Run it on 3 wheels if your rules allow.

    You do not reduce friction by taking a section out of the nail. You simply move the friction to a smaller area.

    You don’t want graphite to constantly lube the wheels. It might even scratch your polished surface. It will cause clumping and work against you. You want a “film” of graphite on the wheel hub. No loose flakes flying around.

    I hope this helps.

  • Bill Klingler // Feb 17, 2012 at 4:47 am

    Ernie Gonzales. I have some questions.
    Are you racing in the Cub Scouts?
    Do they allow you to move your wheelbase? If the answer is “Yes”, why aren’t you moving the front axle slot as far forward as it can go?
    Do you have an over all length limit of 7″?
    Please explain your Number 5 to me.
    Thank you.

  • Don Hetzler // Feb 27, 2012 at 6:15 am

    i just ran a pwd this weekend. my son and i built 3 cars and tested them at a shortened track at the dan beard council hq track. basically, we had 3 designs. one tested our theory of whether contacting the gate high would give us a jump at the start, the other two had different shapes.
    when we ran our test, the car with the high contact point on the starting post won the majority of the races, so we used that one in the actual pwd.
    the track we ran on, most fast times were 2.61 to 2.63 seconds, so i don’t know how long it was. the test track we used was barely 2 seconds, if that, so we didn’t really get to see how the car performed way down the track
    so anyway, our fastest car during testing got us a 3rd place within the wolf den, and about 6th in the pack. when we ran the family races, my daughter and i ran the cars we thought were the slower ones, and turns out the car with the most wood, and the cog about an inch and a half in front of the rear axle ran a 2.59, and blew everyone away. i was suprised. i don’t know if it has do with it being a little more stable down the track or what, but it wasn’t even close.
    what i noticed about the track, which might have screwed up our two cars with the weight more rearward, is the joints in the track were not filed down, so you could hear the cars that were riding the rails really bang through those sections. with a lighter front end, it may have been upsetting the car more, i don’t know.
    i didn’t really extensively test how much the cars drifted, so it’s possible that the two cars with the cog more towards the back didn’t go quite as straight as the fastest car, but i need to check it out.
    also, all three cars were basically 3 wheelers. our rule is that the third wheel can’t be more than a driver’s license off the ground, which they all qualified, but the fastest one was probably the highest off the ground.

  • Don Hetzler // Feb 27, 2012 at 7:44 am

    actually, my previous post should say that the 4th wheel needs to be within a driver’s lisence.

  • Adam Kalsey // Feb 27, 2012 at 8:54 am

    Don, a raised front end doesn’t provide a benefit on most tracks. See Do Raised Front Ends Work?

  • Bill Klingler // Feb 28, 2012 at 6:07 pm

    A Super fast car is an engineering masterpiece. It is probably no more than a quarter inch thick, perhaps using tungsten squares for weight, correct wheel and axle prep, the oil/wax system on wheels if allowed, 3 wheel rail rider, and of course, it has to be painted red. Everyone knows red cars are the fastest. There is no one thing that will make a car win. If a High Nose car has any advantage at the start line I would guess it would be off set by numerous other things going down the track. When some of these racers search through wheels to get the smallest bore diameter and then machine the wheels better than what you can buy online, they become very hard, if not impossible, to beat. How many racers perfectly balance their wheels? Not many, I suspect. How many racers know what a Rail Rider is, let alone set one up properly. How many folks have read Doc Jobe’s big green book? Probably Mr. Kalsey. My point is, if you are going for speed then read the true Physics of the Pinewood Derby and start there. Stay there and fine tune the car every year and learn as much as you can. It’s lots of fun. It’s up to your scout if he wants his car to look like the Batmobile or if he wants to win the race.

  • Jeff // Feb 28, 2012 at 7:21 pm

    We compete this weekend. My friend has asked me to help him and his son with their car about 2 weeks ago and as a physics major in college I’ve been obsessed ever since! The boy wanted a “truck” which turned out to look like a chevy El Camino. The primary weight is a 3 oz lead sinker has been hollowed out to 2.4 oz. We velcrowed the bed and the bottom of the sinker to allow for easy movement of the weight and it sticks up in the bed of that El Camino like a big highway cone. It’s completely BSA legal. We tested it at the local large hobby shop in Greensboro, NC and it ran the fastest time any of the employees have seen there at a 2.5491. It seems to me that the torque created as the track goes from inclined to flat due to the high center of gravity makes the car faster down the flat section. If you take that to the nth degree by getting the center of gravity at the rear and as high as possible you could potentially make the car flip over forward. (This would be in the event you had perfect test conditions, a very light weight support “mast” and a super dense, small ball on top) Anyway, if you’re a beginner, and you want to be at least competitive….build an El Camino so you can hollow out the front and “mess” with the weight mass in the rear. The other kids might laugh at first….but when your kid hauls home the hardware you’ll be teaching him a lesson that “it ain’t how ya drive….it’s how ya arrive.”

  • Bill Klingler // Feb 29, 2012 at 6:18 am

    Jeff,
    There is no advantage in weight placement when the 2 “Perfect” cars are traveling down the incline. The advantage comes when the cars go from incline to flat. The CG (center of gravity) will reach the horizontal run at the same time. (Cycloid of Constant time) Picture a pendulum. The front weighted car will have its body sticking out behind it and the rear weighted car will have its body sticking out in front of it and it will be going faster because its CG will have been higher up the arc. This keeps getting worse and worse for the front weighted car all the way to the finish line. This is called the FEE (Front End Extension effect) Page 40, Doc Jobe’s book, The Physics of the Pinewood Derby. You pretty much got that part correct. You do NOT want the weight high in the air out of the car. You want all the weight as low as possible. If your friend is in the Cub Scouts and the District has somewhat lenient rules, here’s what will happen. Some Scout is going to enter his car and it will be this: A car that is 1/4″ thick, 3 1/2 ounces of tungsten with a CG of 5/8″ to 3/4″, a 3 wheel rail rider, wheels lathe turned and balanced, axles lathed and prepped, oil and wax for lube if allowed and if not, Hodges graphite or Hob-e-lube graphite and prepped properly. 5/16″ cut off the back and moved to the front to pick up another 5/16″ on the Front End Extension effect. You did use the Long end of the block for the front, I hope. The scout taught how to line up his car properly. This is what he will be lining up against.

    On the Positive side, you might have designed your El Camino to look like a 1970 model. That year the 454 CID engine was available and being the “sand bagger” I think you are, this is probably what you are concealing.

    The part I like the best is you gave the scout what he asked for. If we have to fight the Laws of Physics, so be it. Best of luck to you and your scout. Let us know how you do.

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