Gravity Vehicle C

[sj]

Another thing that may have happend is that the kid who timed our run may have stopeed the watch earlier than he should have.

Keep in mind that typical human reaction time is in the tenths of a second range, which is why the rules insist on 3 independent timers being used for each run. I've seen a lot of good suggestions about trying to replicate flooring and such during practice runs, but haven't seen anyone discuss the human timer aspect of it, which can also have a big impact.

Actually I was saying that the person who was timing our run was not familiar with our vehicle, which has a tendency to skid quite a bit before stopping, and therefore might have stopped the timer when the brake engaged rather than when it stopped skidding. However with 3 timers who know to wait until the vehicle has stopped completely this problem should be negated.

[illusionist]

Talking about skidding, I've heard that putting a somewhat weak spring on the end of your threaded axle (if you're using the wingnut breaking system), will prevent skidding and allow for a smoother stop. Does anyone have experience with this?

[sj]

Talking about skidding, I've heard that putting a somewhat weak spring on the end of your threaded axle (if you're using the wingnut breaking system), will prevent skidding and allow for a smoother stop. Does anyone have experience with this?

We've experimented with springs, sponges, and foam. We found that the softer the material the less skid you would have. However the big issue that we had was that it severely impacted our time score as well as our accuracy. For the time it turned out that the material took a lot of time to dampen the significant amount of kinetic energy of the car, smoothly. And the accuracy was because that it is hard to find a consistant reference point of how compressed the material should be when you start to wind back the wing nut. It is definitely a worthwhile experiment though and if you can make it work it could have huge benefits.

[illusionist]

Slight digression, the rules say that the vehicle must have a point on either side, between the front and rear axles. They also state that there is a Targe Point, rather than a Target Line. So, doesn't this mean that the vehicle will have to travel a crooked/curved path, assuming that it starts on the center of the ramp and the ramp is perpendicular to the track? :/

[chalker7]

Slight digression, the rules say that the vehicle must have a point on either side, between the front and rear axles. They also state that there is a Targe Point, rather than a Target Line. So, doesn't this mean that the vehicle will have to travel a crooked/curved path, assuming that it starts on the center of the ramp and the ramp is perpendicular to the track? :/

Or you set the vehicle up slightly off center on your ramp...
This rule was to prevent the fixed point from hitting the ramp or floor (which could occur if it was on the front of the vehicle and a team had a sharply curved ramp).

[illusionist]

Slight digression, the rules say that the vehicle must have a point on either side, between the front and rear axles. They also state that there is a Targe Point, rather than a Target Line. So, doesn't this mean that the vehicle will have to travel a crooked/curved path, assuming that it starts on the center of the ramp and the ramp is perpendicular to the track? :/

Or you set the vehicle up slightly off center on your ramp...
This rule was to prevent the fixed point from hitting the ramp or floor (which could occur if it was on the front of the vehicle and a team had a sharply curved ramp).

Yeah, I know that's a solution, but unfortunately I overlooked that bit when constructing our ramp.

[sj]

You could move your ramp to the left or the right along the start line.

[blazer]

Slight digression, the rules say that the vehicle must have a point on either side, between the front and rear axles. They also state that there is a Targe Point, rather than a Target Line. So, doesn't this mean that the vehicle will have to travel a crooked/curved path, assuming that it starts on the center of the ramp and the ramp is perpendicular to the track? :/

What is the significance of either side? Can the paper clip be in the middle of the car, or slightly off to the side, or does it have to be on the far left or far right?

[Balsa Man]

Its of average mass not super-light not super-massive but the interesting thing is the speed of the car is not constant it takes about 4.5 seconds to go to 10 meters so im thinking it may not be quite as astounding as it may have initially seemed, though in my opinion the most remarkable thing was the accuracy to the point.

SJ – That 11mm distance result is very, very good; downright impressive. If you can do that consistently, you’re going to do very well this year. I’m curious, of course, on how consistently you can hit that range...?. Getting predicted time within a couple tenths- that’s into the range where the likely payoff for additional work is small, and probably not worth the time spent; you’re very close to the range of human error by the timers. Run time to 7.5m is solid; improving it may be easier than you think.

As Chalker7 said a while back (and I totally agree with,, “...the real optimization for this event will come from friction reduction during the run (from high quality bearings/wheels) and vehicle adjustment (being able to travel perfectly straight and pinpoint stopping accuracy)...”

That it takes about the time it took to go the first 7.5m to go the next 2.5m is, indeed interesting, because it sheds some quantitative light on the factors that are in play in optimization. As is so often the case in an engineering problem, the way the factors “play together” leads to tradeoffs; optimizing one factor de-optimizes one or more other factors; the trick is balancing those factors; in this case to maximize the score given the scoring system rules. The run you reported does a very good job of that. A few thoughts, then. They center around a basic physics equation; P (momentum) =m (mass) x v(velocity), and a number of earlier posts indicating the importance of getting vehicle mass close to the maximum allowed.

The vehicle comes off the bottom end of the ramp at some v. That v is determined by (as has been discussed at great length earlier in this forum), the vertical distance (the height over which) the center of mass of the vehicle falls. Were it not for friction, you could extend the competition floor to the edge of the universe and the vehicle would continue at the v it had at the bottom of the ramp. Friction does exist, and is at play, and v changes over time.

Friction, coming from various sources, absorbs energy, reducing momentum as the vehicle rolls along. The mass stays constant; the velocity slows. The loss/absorption of energy happens at some rate; that rate is not constant. The primary source/type of friction is rolling friction, which is a combination of a) the wheel/axle (i.e., bearing) friction, and b) the rolling friction of the wheel (energy absorbed into the wheels, either in deformation of an elastic “tire”, or in the case of a “hard”/inelastic wheel, the energy of repeatedly raising the vehicle as it goes over little bumps. There is also some aerodynamic friction. With a reasonably small frontal area, the amount of aerodynamic friction is significantly less than rolling friction, so I’m going to ignore it in the following analysis.

The rate at which rolling friction absorbs energy; reduces the momentum, hence v of the vehicle is not constant; it is higher at higher velocity. However, if we just look at part of the overall run, say from 5 to 10m, it is for all practical purposes, constant. Does that then mean that the rate at which v changes is constant (i.e., there’s a linear relationship between time and distance travelled)? No. For any short increment of time, the rolling friction is going to ‘suck” some amount of energy, hence momentum, hence (since m is constant) velocity, out of the vehicle. Let’s consider two of these short increments of time, in succession. Let’s set mass at 1 arbitrary unit. At increment 1, momentum (hence velocity) is 100 (arbitrary units); loss is 10 (arbitrary units), so momentum & velocity at the end of that increment is 90; a 10% reduction in v. In increment 2, loss is another 10 units, so momentum/velocity is 80; reduced by 11.1%. The percent reduction increases with each increment. So, if you were to plot v versus d (distance), you’d have a curve, not a straight line.

Now, let’s consider two different vehicles; the one we just talked about with m=1, and another with m=2, or 3. If we were to do the same v versus d plot, the plot line for the heavier vehicle would lie above that of the lighter vehicle; at any given d, v would be higher, which means the time to get to any given d would be less..... So, SJ, since it sounds like your vehicle is not up to the maximum allowable weight of 2.5kg, does that suggest an experiment? Like adding mass and measuring time to a given d?? Adding mass will increase rolling friction (hence the amount of lost energy/momentum/velocity in a given time increment) somewhat, but with ball bearings and.....decent wheels, not by very much...a few percent at worst. If you can increase mass by....25, 50, 100% (can’t tell from your comment where you’re at vs max allowable weight), you will gain significant time, especially at longer distances.

The other thing just to think about is how the friction (loss) in the bearings compares to the friction loss in a wingnut riding along a threaded axle section. With decent bearings, its probably close to, or may be more than the bearing friction loss. What that suggests is the importance of anything you can do to minimize the friction of the wingnut.

This is a really fun event, with lots of really cool angles to work on.

[sj]

does that suggest an experiment? Like adding mass and measuring time to a given d??

Yes it was definitely an experiment that we tried and we found that as we added mass to the car its momentum caused so much skidding that the accuracy was destroyed in exchange for less travel time. However after regionals and our first invitational (in 2 weeks) we will be doing more experiments with regard to adding mass.