Gravity Vehicle C

[fishman100]

Sounds like the string braking system; either that or they invented another braking mechanism. Did they win?

[illusionist]

Sounds like the string braking system; either that or they invented another braking mechanism. Did they win?

The whole "lunging forward" part reminds of a string brake system as well. It could be placed easily inside a small tube.

[Balsa Man]

Yup, that does sound like a string-based system; the bounce-back suggests an elastic element (which spreads braking over a bit of time/distance, avoiding skidding).

On Henry's gear question, yes; on our Team 1 rig, we're using a gear pair to drive a threaded rod, and running a low friction thread-chaser on that, that then trips the brakes.

On posting scores, to have any meaning, give a way of comparing performance, need to know the run distance. For instance, 2.8 sec to 5m is....decent; at 10m, it would mean some sort of warp-drive at work. So, run distance, time, distance from target, est time

At Regionals yesterday, our Team 2 car ran 6.5m at 2.61 sec (130.5 pts), off target about 2cm (20pts) (didn't get official #- just from watching the measurement); predicted time 2.60 (1pt)-tot= 151.5. They won. Running a very conventional wingnut system. Team 1, with the cool, complicated brake system missed set-up by 3 revs first run (>600 pts - ). Run time was faster than T2. Pushed a bit for time on their second run, the brake setup failed to trigger. Passed right over the target point (<1cm off), but kept going at high velocity right on to the wall about 10m further on; major crash! Broke both front axle carrier bars off the chassis plate, so a bit of repair, and then some intensive work to set and use the system consistently and reliably.

While we'd seen the timing issue in testing, (0.2 to 0.3 sec delta in different timers), the importance hadn't sunk in. Going to the State level, and even more so at Nationals, the (real, but small) differences among the top few are going to be certainly within 0.2, probably within 0.1 sec, even out at 10m. Timers near the ramp can catch the start accurately; those near the target can catch the stop accurately, but catching both accurately, no way. Averaging/using midpoint of all timers seems the only practical way to approach it, but it will be an approximation. One tenth over 3 seconds suggests that a 3% wild card factor is an inherent part of the event. One could argue that averaging multiple times gets you a consistent management of inherent error - and again, I can't think of a practical better alternative - but I don't think it can. get you an accurate time with a precision inside 1/10th of a second

[illusionist]

Balsa Man, based on your description of the braking system (for Team 1), it looks like you're using two axles for braking- a round one for the wheels and a separate axle with the braking mechanism on it. Doesn't it increase friction/waste energy to have to transfer through gears and then to another axle??

[Balsa Man]

Balsa Man, based on your description of the braking system (for Team 1), it looks like you're using two axles for braking- a round one for the wheels and a separate axle with the braking mechanism on it. Doesn't it increase friction/waste energy to have to transfer through gears and then to another axle??

It's not two axles. Nylon gear on the front axle; same sized (1:1) gear-also nylon on a threaded nylon rod that rides, on bearings, parallel to the axle. Thread chaser rides the nylon rod. Very small contact area between the chaser and threaded rod; very, very light load on the chaser, steel to nylon friction very low. A little magic gear lube and precise set on the gear mesh also helps. Yes, there is a loss (<10%) in the gears, but it is more than overcome by the low friction in the thread chaser (compared to a conventional wingnut system, where you have multiple thread lays in 360 degree contact). In the 7m range, the Team 1 vehicle is 0.2+ sec faster than T2's wingnut.

[illusionist]

Balsa Man, based on your description of the braking system (for Team 1), it looks like you're using two axles for braking- a round one for the wheels and a separate axle with the braking mechanism on it. Doesn't it increase friction/waste energy to have to transfer through gears and then to another axle??

It's not two axles. Nylon gear on the front axle; same sized (1:1) gear-also nylon on a threaded nylon rod that rides, on bearings, parallel to the axle. Thread chaser rides the nylon rod. Very small contact area between the chaser and threaded rod; very, very light load on the chaser, steel to nylon friction very low. A little magic gear lube and precise set on the gear mesh also helps. Yes, there is a loss (<10%) in the gears, but it is more than overcome by the low friction in the thread chaser (compared to a conventional wingnut system, where you have multiple thread lays in 360 degree contact). In the 7m range, the Team 1 vehicle is 0.2+ sec faster than T2's wingnut.

Ah, okay. Thanks for the explanation. Any idea where I can get a plastic/nylon wingnut for a threaded rod? In the Image Gallery, there's a picture of a mousetrap vehicle, in which the team used a metal threaded axle and a cylindrical plastic thing-y (not sure what to call it) as the wing nut. http://gallery.scioly.org/data/media/40 ... G_4109.jpg (the rear axle)

[Balsa Man]

That looks like either a nylon bushing with threads tapped, or a nylon rod drilled and tapped. Bushing with the center hole of the right size already there would be easier- drilling an axially-aligned center hole would be challenging w/o a lathe setup. Nylon is strong, and low friction, but the wingnut in a wingnut brake sees a lot of force; at some point of axle size and fineness of threads, you'll run into thread distortion from braking forces....

The best source for nylon hardware bits I've found is Ace Hardware. Home Depot has some, but a much more limited selection.

[_HenryHscioly_]

for time, how can u determine the minimum time?
if 2.8 wud be "impossible" for 10m, what is the, fastest ideal time?

[GoldenKnight1]

While we'd seen the timing issue in testing, (0.2 to 0.3 sec delta in different timers), the importance hadn't sunk in. Going to the State level, and even more so at Nationals, the (real, but small) differences among the top few are going to be certainly within 0.2, probably within 0.1 sec, even out at 10m. Timers near the ramp can catch the start accurately; those near the target can catch the stop accurately, but catching both accurately, no way. Averaging/using midpoint of all timers seems the only practical way to approach it, but it will be an approximation. One tenth over 3 seconds suggests that a 3% wild card factor is an inherent part of the event. One could argue that averaging multiple times gets you a consistent management of inherent error - and again, I can't think of a practical better alternative - but I don't think it can. get you an accurate time with a precision inside 1/10th of a second

I think it would make a lot more sense for the timers to be at the target distance since I feel it is easier to see when the motion starts then when the motion stops, especially if you have a vehicle that has a string system that causes it to sway back and forth when it is coming to a stop. Also it is easy to believe that even with everything perfect that the timing could be off by at 2/10th of a second.

[hmcginny]

for time, how can u determine the minimum time?
if 2.8 wud be "impossible" for 10m, what is the, fastest ideal time?

There is nothing saying that 2.8 for 10m is actually impossible, it would just require so little friction that it seems nearly impossible. I wouldn't be that surprised if, by the end of the year, we see cars going that fast. Ideally, a car would be able to travel ~4.429 m/s, meaning a time of ~2.26 seconds for 10 m. However that assumes there is a perfect transfer of potential energy to kinetic energy, which very few ramps will offer, as well as a center of mass at exactly 1 m and absolutely no friction or air resistance or any other dissipative force. So the theoretical fastest time within these restrictions is 2.26 seconds, but no one will come too close to that because of the above. At this point in the year, I haven't seen anything come close that close to 2.8 s, but i've only been to two invitationals so far.