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physicsphan wrote:

Balsa Man wrote:Last thought is on what might be done next year. Precise timing being impractical, how else might you more precisely measure/score the “speed” factor? It comes from and is dictated by how well you do two things; a) maximize the velocity off the ramp, and b) minimize the friction loss rate through the run. Those same two factors happen to determine a precisely measurable value – how far the vehicle can roll. Out of curiosity, we did this test on our vehicles last weekend- T1 goes a bit over 30m; T2 is a bit over 20m. If (for space practicality), the ramp height were reduced a bit, and maybe the weight reduced some (less momentum = less distance capability), a ‘total distance capability’ factor could be used as a practical and precise scoring factor. Just a thought….

I like the idea of the distance you are trying to optimize for scoring purposes. Maybe having a variable mass or initial height that the vehicle had to be release at to add the challenge that the variable distance did this year.

Maybe not the variable height, as it might make some release mechanisms more complicated, but a weight that would change and had to be used to propel the vehicle would be cool. But since this is a new event and we're just getting started, it would be nice to not see to many major changes next year

Also, I'm kinda confused with it, but whats the benifit of small diameter axels? I've heard people mention rotational inertia, but I just can't seem to understand how that effects speed that much. And from a building standpoint... How did other teams use bearings with the threaded rod? We ended up turning our own axel on a lathe, it seems out of reach for most teams(in fact, it took us a couple months to find someone who would help us turn it)

What's a material that can be used for covering CD's to give them grip. I've tried balloons and latex gloves, but the issue is that they rip and break down due to the friction of braking. Any other suggestions?

Balsaman-So does your release system release the car and the weights, or just the weights, which jolt the car to a start? It seems that the first would be the best option, but it also seems like it would be much harder to get a system that release the weights and the car at the same time.

Twototwenty, first trigger (activated by a pencil) releases the weight. The weight trips a second trigger on the underside of the vehicle, releasing it from the ramp.

Iwonder- it is small wheel size that reduces rotational inertia. That can help.... a fair amount. Energy goes into "spinning up the wheels"- more mass, more radius, more energy goes into it Small axle diameter just means smaller bearings- less friction than larger ones. We''re running 1/8th i.d, 1/4 o.d. For the threaded rod axle on our Team 2, we just had to sand down a few thousandths to get a nice fit.

Illusionist, that's why we went to wheels that would take O-rings. I really can't think of anything to slip on a CD that will hold up....

illusionist wrote:What's a material that can be used for covering CD's to give them grip. I've tried balloons and latex gloves, but the issue is that they rip and break down due to the friction of braking. Any other suggestions?

I have had great luck using rubber bands, they tend to last a good period of time and add enough grip to help.

Balsa Man wrote:Twototwenty, first trigger (activated by a pencil) releases the weight. The weight trips a second trigger on the underside of the vehicle, releasing it from the ramp.

Iwonder- it is small wheel size that reduces rotational inertia. That can help.... a fair amount. Energy goes into "spinning up the wheels"- more mass, more radius, more energy goes into it Small axle diameter just means smaller bearings- less friction than larger ones. We''re running 1/8th i.d, 1/4 o.d. For the threaded rod axle on our Team 2, we just had to sand down a few thousandths to get a nice fit.

Illusionist, that's why we went to wheels that would take O-rings. I really can't think of anything to slip on a CD that will hold up....

Thanks...that is quite impressive, compared to my school especially. Is your ramp anything special, or is it just a well made curve that's as tall as possible?

Thank you. It's fun when you have a team that wants to ... really push it in an event.

Our ramps are pretty straightforward. Surface is a piece of formica sheet. Glued to a piece of pressboard shelf, The bottom, oh 18", of the sheet hangs off the end of the shelf board, so the upper part is flat/straight; the bottom, where the formica extends past the board is curved. That transition curve is at a radius of about 30cm. Base board has a wooden frame to hold the shelfboard. When you put it down (with a concrete block on the baseboard), the floor pushes the last part of the formica into the transition curve. One very useful/functional thing it has is a guide rail. Its a 1/4 x 1/2 cross section strip of HDPE plastic, screwed onto the ramp (can't glue HDPE). Vehicles have finger units front and back- they carry a pair if 1/8 nylon rod pieces that engage the sides of the rail. Circular cross section of the rods means very small contact area. Two/three thousandths clearance; nylon to HDPE friction is very low. With the rail, vehicles come off the ramp on a VERY consistent line. If the ramp doesn't move, they'll run the same line within a centimeter at 10m.

One of our coaches got video at State yesterday- YouTube at:
http://www.youtube.com/watch?v=nHhQAAdU ... e=youtu.be
I'll try to get a few pictures together for better close detail.

It was a good day; everything worked. 7.8m, 3.5cm, 3.1 sec. Our timers had it at 2.8 sec; 2.7 on the first run. Got a bit of brake rebound on the second run that added time.

As a matter of principle, simpler is generally better. But in this case, a fairly complicated set of things to optimize a number of performance parameters.....worked like it was designed to. Hard work and careful testing and practice paid off

Balsa Man wrote:One of our coaches got video at State yesterday- YouTube at:
http://www.youtube.com/watch?v=nHhQAAdU ... e=youtu.be
I'll try to get a few pictures together for better close detail.

It was a good day; everything worked. 7.8m, 3.5cm, 3.1 sec. Our timers had it at 2.8 sec; 2.7 on the first run. Got a bit of brake rebound on the second run that added time.

As a matter of principle, simpler is generally better. But in this case, a fairly complicated set of things to optimize a number of performance parameters.....worked like it was designed to. Hard work and careful testing and practice paid off

Congrats on the gold! Your vehicle looked nothing like I imagined it, but I'd love to see some close up pictures of the individual assemblies.

Thanks.

One thing that surprised me at our State tournament, that has relevency for those going to Nationals; almost everyone was running the paper clip in a fixed position. Rule 5(g) explicitely allows adjustment between runs.
If you run it in a fixed position, to improve on your distance score on the second run, you have to adjust both braking distance and L/R alignment. Braking distance is/should be pretty straightforward, easy, and pretty precise. Alignment is a much more challenging thing to do consistently/reliably.
With an adjustable clip positioner, the problem becomes much easier.

For this to work/help, you do have to get to where it rolls close to the same line each time; this can for sure be done with a guide rail (we were seeing within a cm L/R at 10m); I suspect with a good solid/stiff chassis, good tightly positioned bearings/wheels you could get pretty good linear repeatability with wheel positioning/alignment blocks on the ramp. You have to be able to do your first run alignment to get within the L/R range of your adjustment "frame." In the video of ours, you can see that frame sticking off the left side. Its about 15cm wide, and 30cm long. Two carbon fiber rods sticking out, with a bar that can be slid L/R along them; the clip is mounted on that bar, and cam be adjusted fore and aft. With good linear consistency, how you adjust for the second run depends on how close the first is. Let's say first run, the clip is 6cm to the left of the mark, and 7cm past it (a bit over 9cm fm the mark). You adjust the clip right 6cm, and back 7cm, set the brakes the same. If it runs the same line, and the brakes go off at the same distance, the clip ends up right over the point. If the braking distance is off more than your frame allows, you have to adjust number of revs- let's say 30cm off - long. Wheel circumference on ours is about 18cm. Take off 1 rev (-18cm), adjust clip back another 12cm, and you've adjusted to -30cm