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					SECME Rubber Band Car

Brad Nunn 11/5/2004

Agenda
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Review competition rules Typical rubber band cars Component design Construction considerations Build a propeller based kit Discuss optimization Other designs

Review Rules
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Refer to Go Cart Competition Construction and Operating Rules

Go-Car t Compet it ion
Construction and Operation Rules
1. Each team will consist of one or two students. 2. Each school will enter one Go-Cart of their own design using common construction materials such as, but not limited to: cardboard, dowels, paper clips, rubber bands, etc. All cars must be packed in a container to facilitate handling. 3. On the day of the competition, but prior to the running of the car, an actual operating Go-Cart must be submitted in order to compete. Note: At this time, each entry must pass a visual inspection (see # 4 & 5 below). Entries that fail inspection will be given ONE opportunity to make whatever modifications are needed to pass inspection prior to the beginning of the competition. Then it will be weighed and measured. 4. The Go-Cart must be self-propelled and powered only by rubber bands. 5. The cart must run on a minimum of three wheels. 6. The frame and size of the body, the size of the wheels will depend on the team’s chosen design. It can be as long or as short as desire. 7. The cart will run on a smooth surface. The distance will be measured from the starting line to the stopping point utilizing a straight line between the two points. 8. There will be two runs for each car. The best single performance will be used for final scoring. 9. The formula used to judge the performance of the car will give the best score for the

shortest, lightest, and farthest traveling car.
The formula used to determine the winner is:
2

F =
F D L W = = = =

1 W

D L

Final Score Longest single distance traveled in a straight line. D = 2500 if Go-Cart traveled 2500 centimeters or more. Longest dimension in any direction (length, width, height) in centimeters. Weight of the Go-Cart in grams. 11

Review Scoring
F = 1
*

D L

2

W

•Consider tradeoffs •(D+L squared) •Review last years scores:
TEAM INFO School Name Ben Sheppard Rainbow Park Ernest R. Graham Joella C. Good R. R. Moton Lakeview Hialeah Gardens Phyllis Ruth Miller W. A. Chapman Eugenia B. Thomas SCORING SUMMARY PERATION O 1 g Rank Perf w W 1 1 40.1 607 2 1 47.9 317 3 1 60.5 67 4 1 144.1 38 5 1 71.7 30 6 1 97.6 29 7 1 219.3 23 8 1 50 22 9 1 50.5 11 10 1 161.2 6 cm L 13.5 20.3 39.4 22.5 22.5 30.6 35 18.7 18.5 22.2 ft D1 69.1 72.5 82.4 52.6 22.77 46.23 83.85 20.55 14.08 22.4 ft D2 65.45 84.85 83.82 54.35 34.1 53.5 77.5 15.63 10.47 12.47 cm D1 2106.168 2209.8 2511.552 1603.248 694.0296 1409.09 2555.748 626.364 429.1584 682.752 cm calc D2 Best (D) 1994.916 2106.168 2586.228 2586.228 2554.834 2554.8336 1656.588 1656.588 1039.368 1039.368 1630.68 1630.68 2362.2 2555.748 476.4024 626.364 319.1256 429.1584 380.0856 682.752 calc Max (D) 2106 2500 2500 1657 1039 1631 2500 626 429 683 calc F 607 317 67 38 30 29 23 22 11 6

Terminology
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Potential to kinetic energy transfer Torque Acceleration Speed Momentum Friction

Desired Outcomes
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A small car that travels 2500 cm and doesn’t weigh much A gradual transfer of energy that has just enough torque to establish motion A sustained transfer of energy that delivers sufficient momentum to cover the distance

Typical Rubber Band Cars

Typical Rubber Band Cars

Typical Rubber Band Cars

Typical Rubber Band Cars

Wheel Design
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Wheel diameter

Wheel Design
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Wheel Construction

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Rubber bands around wheels for traction

Axle Design
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Axle diameter and mechanical advantage

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Simple ratio of diameters For distance cars use the smallest axle that provides sufficient mechanical advantage to drive a large wheel Glue or fix at least one drive wheel to axle

Wheel and Axle Design
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Minimize friction loss

Rubber Bands
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All shapes and sizes

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Experiment!

Lubrication
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Graphite powder

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Silicone spray (WD-40 not recommended)

Construction Considerations
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Rotating the axle
• More rotations = farther travel

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Releasing the rubber band for coasting

Construction Techniques
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Use a zip tie or make your own…

Construction Techniques
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Simple, easy to tie knots
• Surgeon’s Loop • Square knot

Propeller Rubber Band Cars

Propeller Rubber Band Cars

Propeller Rubber Band Cars

Propeller Design
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Airscrew Left hand (cw) or right hand (ccw)

Propeller Rubber Band Cars

Iterative Design Approach
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Prototype Calculate performance score Tweak the design (farther, shorter, lighter) Iterate (repeat steps 1-3) Replicate (repeatable results?) Calculate Celebrate

Tools for cutting, drilling, and assembly

Aligning the Frame
• Align the axle holes

• Not the ends of the side rails

Gluing the Frame
• Safety first!

• Use Gel - NOT the Liquid (too watery)

Iterative Design Approach
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Step 1 - Build a prototype
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Establish a working model Hands-on experience and problem solving

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Step 2 – Calculate score Step 3 – Discuss optimization
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Share lessons learned

Problems / Solutions
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What problems were encountered? What solutions were effective? What can be done for further improvement?

Optimization
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Enough torque to get moving? Enough traction to prevent slippage? Were the any friction losses? Did we get all the power there was? Smaller? Farther? Lighter? Multiple rubber bands? Multiple propellers?

Creative thinking
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What advantage is there to winding a rubber band compared to stretching it?

Dixie Roller

Dixie Roller
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Dixie cup Axles CD Wheels Washer, grommet, bead Bearings Drive Lever Driven nail Rubber Band Construction tool – fishing line Tape

Design Challenges
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Little torque so the car must be light Little torque so the friction loss must be kept to a minimum How do you nest the rubber band inside the axle? How do you make one wheel drive and the other one follow? Construction techniques?

Iterative Design
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Prototype Measure and record performance Tweak the design (farther, shorter, lighter) Iterate Replicate Calculate Celebrate

Design optimization
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Frictionless bearing Frictionless unwinding of the rubber band Thick or thin rubber band Length of the lever arm How many wheels Wheel materials for low inertia

Most important
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Have Fun!


				
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