Gravity Vehicle

Gravity Vehicle is an event that returned for the 2020 competition season. In this event, competitors design and build a vehicle powered solely by gravitational potential energy transferred to the vehicle by the ramp.

Construction

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:

• Plastic (flimsy, very low level of friction)
• Metal (more rigid than plastic, offers similar level of friction)
• Wood (smooth wood is rigid, and decently smooth. May also be easier to work *with)
• Plexiglas (flimsy if using thin sheets, similar friction level as plastic)

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. 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.

The Vehicle

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

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. 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:

• Wood
• Metal
• Plexiglas
• Plastic
• Carbon Fiber+3D Printed Parts

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

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:

• Wooden dowels
• Brass/Aluminum rods
• Plastic dowels

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. Quality bearings will improve your time score and can also help in ensuring your vehicle follows a straight path.

Wheels

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? 3 or 4? The answer will depend on your ramp design and vehicle. Common choices for wheels are:

• CDs
• Hobby airplane wheels (like these)
• Wheels from companies like BaneBots are common within very competitive teams, but are relatively expensive.
• Home-made wheels

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.

Brakes

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. Skid can be fixed by using wheels with more traction. Other ways to reduce skid include lightening the vehicle and lowering the vehicle's starting point on the ramp. Reducing the mass of the vehicle causes it to have less momentum, which then requires less traction to stop the vehicle. Lowering the starting point on the ramp reduces the speed of the vehicle, which also reduces the momentum of the vehicle.

Many braking systems have appeared over the years, and two of the most common are the ones listed in the following excerpt from the Scrambler Wiki:

Braking Systems

There are two commonly used braking systems. Here they are listed in order of difficulty of construction.

String Type

Pros	Cons
Easy to build	Poor Accuracy
	Backlash
	Skid

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. So, if you need a quick solution or don’t plan on putting much time in your system, this is certainly an option.

Threaded Rod Type

Pros	Cons
High accuracy	Somewhat complicated to build
Consistency	Skid
Little to no backlash	Added Friction to the axles

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. 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. 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.

General Tips

• Most of your time should be spent making and modifying your vehicle. It is crucial that you test your vehicle, but it is even more important that you are not wasting time testing an inferior device. Ensure that it is consistent and accurate.
  • A great idea I've used in the past is to test on many surfaces. You will not be 100% sure about the competition surface, so it is best to test your vehicle on many surfaces such as gym floor and hallway tiles.
• Clean the floor before your run at competition, and while you practice. This will allow you to keep dirt and dust from affecting the stopping ability of your vehicle. Note: this will decrease the chance of skidding, but may cause your car's braking wheels to hop a few times when the brake engages, decreasing accuracy.
• You can use mathematical calculations to determine the amount of turns that will be needed for the wingnut if you are using the "Threaded Rod" system. Measure the distance from the starting point of your vehicle to the finish mark. Divide this by the circumference of your wheel, and then you have the number of turns that are needed. However theoretical calculations are no substitute for hands-on testing.
• As you test on each surface, keep data in the form of a chart. This allows you to accurately replicate your vehicle's performance for the day of competition.

There are functions for time and distance and the inaccurate times are highlighted in order to be ignored during LSRS calculation.

Internal Links

Elements from many other building events in the past can be used to aid you with your design. Below are some links with valuable information regarding construction and testing:

• Scrambler (This is very complex and extremely detailed, however very useful as well. So read carefully.)
• Electric Vehicle
• Battery Buggy
• Mousetrap Vehicle

Not all information will pertain to this event, but the construction aspects regarding chassis and wheels especially will be beneficial. Template:Vehicle Events