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24 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.

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