For a school competition I am costructing a rubber band driven car. My design has three wheels to reduce weight, and it has a gear system with a ratio of 3:1. Would it be better to have a lightweight car or a heavier one with gears? Any other hints?
Are you allowed to use Vex materials?
I would suggest a lightweight car since its being driven by a rubber band.
How long is your rubber band?
Yes, Vex materials are allowed in the competition, and the rubber band is 3.5 iches long and 1/4 of an inch wide. I am trying to make it as light as possible too, but nothing is working and i am running out of ideas.
if you have the car to light, the wheels will sping because of not enough traction, so i would go with medium weight
MRClark’s right, but I think that with vex pieces you won’t be able to make it too light. I would suggest the simplest fram possible with the two wheels in back and one wheel in front. I don’t know how you plan to use the rubber band but this is my suggestion. Attach the band to a fixed point (cut the rubber band if you can so it is one long length) the take the other end and wrap it around the axle. Keep wrapping it till it gets to tight. Then set the car down and let her rip. You can permanetly attach the rubber band to the axle but then once it has unwrapped it will start to re-wrap and then it will slow the car down. Have fun!
You haven’t stated the goal.
Different goals have different solutions.
If goal is to go the farthest, you need a high gear ratio.
If goal is highest top speed, maybe also a high gear ratio.
If goal is fastest sprint for 5 feet, a low gear ratio would be better.
If the distance is long, or the speeds fast, an aerodynamic car will help.
If the track is not flat, then weight is important, one way or another.
Too low a gear ratio (too much torque) for the available wheel traction will spin the wheels (bad).
Lighter wheels will accelerate faster.
Using a larger wheel is like using a higher gear ratio.
A heavier robot with heavier wheels will coast longer after rubber band is done, so might be good for longest distance.
There is a tradeoff of traction vs rolling resistance.
To win, without getting just getting lucky, you’ll need to iterate, iterate, iterate, using guidelines similar to above to see what will give improvement.
Other “out of the box” ideas:
- A three wheel robot is lighter than 4, but two might be lighter yet (but zero length wheelbase will limit your torque that can be applied.
- The lowest rolling resistance is from the small wheels without tires, use two front bare wheels and one wheel with tire as rear wheel pusher?
- For a faster start, you need more torque, long run needs low torque; how to get variable torque? wrap rubber band on a cone shape!
- Does twisting the rubber like a rubber band powered plane work better than stretching it?
- Is it better to stretch the rubber band, and use a string wound on axle, or just to wind band itself on axle? (or on axle collar)
- Get a whole bag of rubber bands, and characterize them to cherry pick the best one.
- See also google search for mousetrap powered cars for other ideas.
- The tread on 4" wheels can be cut off except for one concentric rim, to save rolling resistance.
it sounds like Jgraber knows what hes talking about and I would think you would want a light car made of something like balsa wood?
If i were you i wouldnt use balsa because it is very fragil and when working with rubber bands, it will break and when balsa wood breaks there are pieces everywere.
Maybe you could use a swall steel rod to reinforce the structure that the rubber band is mounted to would be sufficent