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.
Bt keeping the weight compact, less mass needs to rotate and less energy will be lost overcoming inertia.
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7 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
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