Gravity Vehicle is an event that returned for the competition season.
In this event, competitors design and build a vehicle powered solely by gravitational potential energy transferred to the vehicle by the ramp. A ramp is used to direct the energy into propelling your vehicle forward. Although it is not necessary, some teams may find it beneficial to construct an adjustable ramp. One way to do this is by hinging your ramp at the front end the area that touches the floor and using supports of various heights to raise the back end, resulting in different slopes.
Also available is the system that is used on pool chairs, although it may be more complex to build. The surface of your ramp must provide enough traction to avoid wheel slippage without being so tacky that the wheels stick to the ramp and provide drag.
In order to have the best surface, you want any material that is smooth enough to not damage your wheels but has enough to friction to not let them slip. Some materials to keep in mind are:.
However, if the surface of the ramp is too slippery and the wheels of the vehicle lack grip on it, then the vehicle will have problems with the straightness of its path. Ramp shape will affect your performance to some extent. A flat ramp without any curve is the easiest to construct, but a vehicle may lose some forward momentum, depending on the angle at which the ramp meets the floor.
The direction of travel might also be adjusted if the vehicle meets the floor with a jolt. Another possible shape would be the Brachistochrone Curve.
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A ramp in this shape could be difficult to construct, but it may provide the best transfer from ramp to floor and the best accuracy. Below is an example of a vehicle on a ramp in the shape of a Brachistochrone curve, courtesy of the Scrambler Wiki.
Note that the egg on the vehicle's front is not part of this event:.
During the construction of the vehicle, attention must be paid to where the mass will be concentrated in order to gain most gravitational potential energy. However, consideration must also be paid to the location of the mass relative to the braking mechanism so as to avoid unnecessary wheel slippage during braking, which could impact the accuracy of the vehicle.
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Regardless of what aspect, your vehicle will need to be light but not so light that it doesn't have any traction. Then again, a light vehicle is not a competitive vehicle because the center of mass is probably too low. Construction of the vehicle will be similar to "vehicle" events in the past, like Wheeled Vehicle, Electric Vehicle, Mousetrap Vehicle, and most notably, Scrambler. The chassis of your vehicle will need to be rigid and well built. Unlike years past, this year's event calls for a propulsion method that will place a great deal of stress on the frame.
Therefore it is crucial to build a solid structure.
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A flimsy chassis will result in an unpredictable path, leading to a loss of points. The chassis can be built either as four walls, or simply just one flat board. Some materials for the chassis are:. Some materials will work better than others, a combination might work best. When building your chassis, ensure that you have right angles and that your axles will be situated perpendicular to the direction of travel.
This is crucial.
Also, an adjustable steering system might be of use if you cannot build your chassis as wished. From the Mousetrap Vehicle Wiki, "If you cannot build the axles to be exactly parallel, then there are some other options. One commonly used solution is to build the car in two halves.
The front axle will be part of the front half, and the rear axle will be attached to the back half. The two halves of the vehicle are connected together with a bolt that runs to the middle. To adjust the path of the vehicle, simply loosen the bolt, turn the front half, and lock it in place by tightening the bolt again.
Now you can adjust how straight or arched the path of your vehicle is! Axles connect your wheels to your chassis. They will need to be non-flexible and stiff. A crooked axle will greatly affect the path of your vehicle. Materials for the axles include:.
When attaching your axles to the chassis, you have multiple options. One common method is to simply drill a hole in the walls of your chassis, and simply insert the axle into these holes.
Another popular method is to use some sort of sleeve, like a straw, attach the sleeves to the chassis, and then place the axle inside these tubes. One thing to note though, is that with this event, the vehicle will endure more stress and will carry more weight than in years past.
Thus, it will be most beneficial to use ball bearings this year. You can find them in skateboards and in-line roller blades, or you can purchase them individually.
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Quality bearings will improve your time score and can also help in ensuring your vehicle follows a straight path. The wheels will need to be straight as well as light-weight for the front and strong for the back. There are multiple things to consider when choosing wheels. For example, thick or thin? Large or small? The answer will depend on your ramp design and vehicle. Common choices for wheels are:. The last option is not recommended unless you have the proper tools and abilities to make perfectly circular wheels.
Friction is your enemy when it comes to axles. A necessary option is to use ball bearings and they are not difficult to find. The pay-off is incredible because the negligible increase in weight of the bearings does nothing negative while your time score will improve drastically. Another idea might be to use some sort of sleeve to hold your axle, and apply either liquid or graphite lubricant to the area not recommended. Be careful not to overdo the lube, or else you'll end up with a sticky mess that will attract large amounts of dirt.
A great idea is to cut a strip of latex balloon, and wrap it around the circumference of your CD wheels. It will provide a nice amount of traction. If you are using wheels in the front, they need no balloons to operate optimally.
A good braking system will stop your vehicle on the mark, with precision and accuracy.
Skid is a factor that affects the braking ability and stopping distance. A simple fix to skidding would be to use better wheels. Many braking systems have appeared over the years, and three of the most common are the ones listed in this excerpt from the Scrambler Wiki:. There are two commonly used braking systems. Here they are listed in order of difficulty of construction.
The first braking design is made by running a string from one axle to another.
As the car travels, the string from one axle unwinds and wraps around the second axle. Once all of the string has fed through from one axle to the another the axles lock and the car stops.
The distance the car travels can be controlled by the amount of string wrapped around each axle.
This design tends to have poor accuracy for several reasons. First, the string used will often stretch in an irregular way. Second, the string will not always wrap in exactly the same manner, meaning there is a slight variation in the amount of travel allowed before stopping the car.
Third, while the taut string will prevent the car from moving forwards, nothing prevents the car from moving backwards. So, you will get some amount of backlash. This system relies on the braking power of the rims of the wheels, which may lead to skid, as this design does not allow for gradual locking of the wheels.
That being said, this is the easiest system to implement.
In order to be even remotely competitive, you need to use this system. This system is very popular among competitors. While only slightly more complicated than the string method, it is more consistent.
The basic concept of this design is using a threaded rod for the axle, placing a nut usually a wing nut is used on the axle. As the wheels rotate their rotary motion is transferred into linear motion in the form of a wing nut moving along the axle.
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When the wing nut reaches a barrier, it will no longer be able to move, and thus stops the axle from turning. The distance is set by setting how far the wing nut starts from the barrier and is usually measured in rotations of the wheels. While as described the system is still vulnerable to skid, it can be minimized by carefully choosing a material for the barrier that the wing nut engages during the stopping motion.
By choosing something rubbery many teams achieved a gradual locking of the wheels, which effectively eliminated any skid inherent to the system. The wing nut also adds some friction to the axle, thus the car may not roll as smoothly or as far.
The above section is from the Scrambler Wiki page.
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The wing nut system is the most popular method, due to its simplicity and effectiveness. This video is an example of the wing nut system.
The easiest method of reducing skid is to reduce the mass of the vehicle. This will reduce your vehicle's inertia, and therefore will reduce the amount of skid it experiences. And having wheels with good traction also helps. If the wheels hop when the brake engages, the easiest fix is to add mass near the axle that has the braking system. There are functions for time and distance and the inaccurate times are highlighted in order to be ignored during LSRS calculation.