fishman100 wrote:questionguy wrote:For the braking system, are ball bearings going to have a much larger effect than simply using washers and bolts? Also, are CD wheels capable of providing enough speed for the vehicle? I've asked previous Mousetrap participants at my school, but I wanted other opinions.
I've used CDs (for this event) and obtained good results.
Thoughts on CDs as wheels first:
CDs are a viable choice for Mousetrap Vehicles (vehicle weight on the order of 200-500grams, speed on the order of 1 meter/second). Gravity Vehicles are a very different critter (vehicle weight 2.5kg, speed on the order of 2-3 m/sec) , and there are a number of reasons why CDs would probably not be a good choice:
-As discussed before, you want the weight of a GV to be as close to the max 2.5kg as you can. You want it to be coming off the ramp as fast as you can. Together these factors will put a lot more load/stress on the wheels.
-Being thin, CDs will tend to flex under load- particularly in the transition zone at the bottom of the ramp, and to a lesser degree rolling along the floor. That flexing will tend to kick the vehicle off a straight path, and that will vary every run.
-Being thin, even with rubber around the edges (like from a balloon), CDs will provide a very small contact area for braking- with 2.5kg moving at a couple m/sec, significant skidding. Flexing will make the direction that skidding takes.....less than predictable.
-Both wingnut and string winding braking systems work off counting and setting up the brakes based on number of revolutions. Larger wheel diameter means distance travelled per revolution (i.e., the circumference of the wheel) is greater. The resolution, or precision of braking is going to be some fraction of the distance travelled per revolution; larger circumference = larger linear error.
-Last, as discussed before, to maximize velocity off the ramp you want the center of mass to come down from as high above the floor as you can (and to go down as close to the floor as you can). Big wheels create issues at both ends of the ramp. Remember, no part of the vehicle, in launch position, can extend above a plane that is 1m above the floor. To see the big wheel problem, near the top of a piece of paper, draw a horizontal line. From the right end of that line, draw an angled line down toward the left (at the angle of the ramp; roughly 0.75 horizontal to 1 vertical). Take a CD- line its edge up on the top line, slide it over till its also touching the ramp line. Trace the outline, and mark the center (where the rear axle will be). Now take a smaller circle; position it so it also touches the top line and ramp line, and mark its center point. How much further up the ramp is your rear axle location? Obviously, there are limits, but the smaller the wheel, the closer to the top of the ramp the rear of the vehicle will be. When you look at the bottom (where you want the center of mass as low/close to the ground as you can) you have a similar problem. With the chassis plate about the radius of your wheels above the ground, your mass is further off the ground than it would be with smaller wheels.
Thoughts on ball bearings and effect on the braking system:
The effects on braking are going to be limited to minimizing the amount of slop/play in your axles. With ball bearings, the ability of either end of the axles to move under braking loads is going to be very small. With ‘washers and bolts’ its going to be significantly more. If you set them up to be nice and tight (minimizing slop), then the friction gos way up; if you set them up looser to minimize friction, then slop goes up. Axle movement will cause direction changes (not good). Where ball bearings are going to have a major effect (compared to ‘washers and bolts’) is before your brakes come into play -in your time score; significantly more rolling friction means vehicle momentum gets gobbled up faster; means vehicle slows down faster, means it takes more time to get to a given distance.
