Gravity Vehicle C

[Balsa Man]

A few thoughts on string braking systems.
Have used on both mousetrap and electric vehicles; one axle and two axle. In both cases, the momentum to be dealt with in braking is a LOT less than in a GV.
Illusionist, I assume you're doing a single-axle system- yes? For a number of reasons a two-axle system for a GV is.....probably not worth taking on. First, the amount of bounce-back you describe is pretty minimal/good. It sounds like it may be due to string stretch. You may already know about/be using Kevlar string- it doesn't stretch....hardly at all. If you're not using it, it may help- you can get it at fishing stores- SpiderWire is one brand- you can get in a range of strength, up to at least 50 pound test. If you are already using it, then the only direction of help I can think of is in the mounting of whatever its spooling off of. You could look at a "friction mount'- where the rod/whatever its spooling off of is mounted so that under the pull when its all spooled out, the whole spool is pulled some distance - an inch, a half an inch, under friction/tension. Like the ends of the spooling rod could be mounted between parallel bars, maybe with rubber layer on the bars, so that it takes a pretty stout pull to move the rod, but it can move some... Only thing i can think of.

[illusionist]

A few thoughts on string braking systems.
Have used on both mousetrap and electric vehicles; one axle and two axle. In both cases, the momentum to be dealt with in braking is a LOT less than in a GV.
Illusionist, I assume you're doing a single-axle system- yes? For a number of reasons a two-axle system for a GV is.....probably not worth taking on. First, the amount of bounce-back you describe is pretty minimal/good. It sounds like it may be due to string stretch. You may already know about/be using Kevlar string- it doesn't stretch....hardly at all. If you're not using it, it may help- you can get it at fishing stores- SpiderWire is one brand- you can get in a range of strength, up to at least 50 pound test. If you are already using it, then the only direction of help I can think of is in the mounting of whatever its spooling off of. You could look at a "friction mount'- where the rod/whatever its spooling off of is mounted so that under the pull when its all spooled out, the whole spool is pulled some distance - an inch, a half an inch, under friction/tension. Like the ends of the spooling rod could be mounted between parallel bars, maybe with rubber layer on the bars, so that it takes a pretty stout pull to move the rod, but it can move some... Only thing i can think of.

Actually, it bounces back about 20cm, but is consistent in the amount it bounces back =P
Yup, it's a one axle system, and I was previously using fishing lline. Yesterday was just a quick run, and after testing fully today, the bounceback adds at least an additional 2 seconds onto the run. The reason I tried the string was to hopefully minimize friction, but the speed didn't make up for it. I've gone back to a wingnut, and it's working preetty well now. Thanks for the tips Balsaman, I'm sure I'll be able to use them some other time.

[Balsa Man]

Cool.
Best of luck!

[illusionist]

Awwww yeah. New wheels, dropped one second at 10 meters

Edit: How are you guys dealing with predicting the time on different floors? I know that our event will be held on gym floor, however the amount of friction of the floor, whether or not it's been waxed recently, or the uneveness of the floor can all effect the time the vehicle takes to travel there. It's possible to estimate the time for the second run based on the first, but even then it's a little iffy. Distance shouldn't be too hard to adjust based on the first run. Actually, do the rules allow for two different time predictions before each run, or just one before both runs (sorry, won't be able to look at my rules for a while)?

[theplad]

Has anyone thought of making their vehicle over 1 meter long, then have a rod on the side of the vehicle(but within the wheel and axle) with an adjustable paperclip so then for state and especially for nationals, if the target point is 821 then the competitor sets the vehicle to stop at 9meters and then adjust the paperclip back 79cm on the rod. I'm pretty sure the increase time will be offset by the increase in accuracy and there would be less testing needed to be done.

[Balsa Man]

With getting two runs, and the best of the two being your scoring basis, you obviously want to be able to adjust after first run. That adjustment can be either setting the brakes to go off at a different distance (with a fixed paper clip point), or setting the brakes to the same distance, and adjusting the position of the paper clip. Which of these aproaches will work best for you depends on how your braking system works (your ability to "dial in" a distance consistently- how close to the same distance can you make two runs). If you can consistently re-set to the same distance, then an adjustable clip positioning set up is the way to go.

The way the points work, 1mm in accuracy gets you the same points as 1/50th of a second, so if you give up 1/10th of a second (5/50ths), you need to pick up better than 5mm for it to be a worthwhile tradeoff.

Doing a vehicle with a wheelbase of a meter or so would be a major give-away, in terms of how much horizontal velocity you come off the ramp with. That velocity is (as discussed at length in this thread) primarily determined by the distance the center of mass falls. Even with a substantial rear weight bias, the CM of a vehicle 1m (100cm) long is going to be significantly lower at start than that of a vehicle with a wheelbase of, say 35 of 40cm.

We're running a wheelbase a bit less than 40cm, with a "grid system" adjustable paper clip mount. Two carbon fiber rods that plug into sockets- one just behind the front wheels, one just ahead of the rear wheels. On/between those rods - perpendicular to them- is a carbon fiber bar. It can be moved left/right (about 7cm each direction). The paperclip can be moved along the bar (taped in position), over a distance of about 35cm. Our wheel circumference is about 18cm, which means an adjustment range of almost 1 rev, plus, and minus.

Our brake system.....increments/is set to a particular number of revs - triggers at the set rve #. A table provides the +/- distance for a particular target distance; for example, 6.5m = 42 revs +8.5cm. So, on first run, the pointer would be set forward from center by 8.5cm. Let's say it ends up 2.5 cm short of the target point, and 1.5cm to the left. On the second run, you'd adjust the clip 1.5cm to the right, and 2.5cm to the front. If it runs the same line on the second run, and the brakes go off at the same distance the clip ends up over the target point....

[NinjaChicken]

It can be moved left/right (about 7cm each direction). The paperclip can be moved along the bar (taped in position), over a distance of about 35cm. Our wheel circumference is about 18cm, which means an adjustment range of almost 1 rev, plus, and minus.

Our brake system.....increments/is set to a particular number of revs - triggers at the set rve #. A table provides the +/- distance for a particular target distance; for example, 6.5m = 42 revs +8.5cm. So, on first run, the pointer would be set forward from center by 8.5cm. Let's say it ends up 2.5 cm short of the target point, and 1.5cm to the left. On the second run, you'd adjust the clip 1.5cm to the right, and 2.5cm to the front. If it runs the same line on the second run, and the brakes go off at the same distance the clip ends up over the target point....

I was under the impression that the paper clip has to be near the sides of the car, but does this mean it can be anywhere in the vehicle within the axles? Such as in the center?

[Balsa Man]

This was discussed at some length a few weeks/pages ago. The rules say, "...on either side between the front and rear axles...and must be easily accessible to the Event Supervisor" - as in to one or the other side (with no distance how far out to the side specified), and as in to the front/to the back movement/positioning constrained by the lines of the axles. Hard to imagine a position in the center would be interpreted as complying.

The grid system I described is mounted on one side of the vehicle.

[illusionist]

With an adjustable paper clip system, how do you know how much to move it? At all the invitationals I have been to, competitors weren't allowed to walk on the track, know the official time, nor measure the paper clip after that run.

[Balsa Man]

Well, the rules in two places refer to the competitors being called (by the Event Supervisor) to retrieve their vehicle. Also to adjusting clip between run. As discussed under various events, Event Supervisors do not always run events in accordance with the rules
At our regionals, when they finished measuring, vehicle was left in-place, competitors allowed to come get it-- do any measuring they wanted, told time, which seems to be in accordancee with the rules.