Gravity Vehicle

From Science Olympiad Student Center Wiki

Gravity Vehicle
Engineering & Build Event
Forum Threads 2013 2012
There are no tests available for this event
Images Image Gallery
There are no question marathons for this event
This event was not held last year in Division B
Division C Champion Brookwood High School
Gravity Vehicle is an new event for 2012 competition season. It is similar to the old Scrambler event, except that Gravity Vehicle does not involve an egg. The energy for travel comes solely from gravitational potential energy. It would be wise to understand the basics of it. For even more information about construction, visit the links the in the "Internal Links" section below.

Contents

Construction

The Ramp

A ramp is used to direct the energy into propelling your vehicle forward. Ideally, it would be best if the ramp was adjustable. 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, you can use the system that is used on pool chairs, although it may be more complex to build.

The surface of your ramp must be smooth as possible, in order to reduce friction, a waste of energy. (Rolling friction? Nope) 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, also realize that if the surface of the ramp is too slippery, and the wheels of your vehicle also lack grip, you will have problems with the straightness of your vehicle's path.

Ramp shape will affect your performance. Using a straight ramp instead of a curved one will change the speed of your vehicle (no it won't.) The simplest design would be a flat ramp, without any curve. Another possible shape would be the Brachistochrone Curve. Below is an example from the Scrambler Wiki:

SC-ramptype.gif

Consider where you must concentrate the mass of your vehicle in order to gain most gravitational potential energy. This may impact your braking method, so be sure to plan with that in mind.

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 like this. Some materials for the chassis are:

  • Wood
  • Metal
  • Plexiglas
  • Plastic

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. This is an example of a steering system, possibly the simplest method.

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. Here is a modified example of this method. 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)
  • 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 over do 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. 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:

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


SC-brake1diag.gif SC-brake1.gif

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


SC-brake2diag.gif SC-brake2.gif

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 the rotating motion is transferred in to horizontal motion of a wing nut moving it 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 to reducing skid is to add more weight to the end that is skidding. This will greatly reduce the amount of skid your vehicle will experience. And having wheels with good traction also helps.

General Tips

  • Most of your time should be spent making and modifying your. 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 increase your car's skid.
  • 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 excuse 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.
  • Here is an example of a good chart: [1]

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:

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