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I noticed that we could graph both our times and brake turns v. distance on a linear graph. Is this true for anyone else? I just thought it a bit weird that a linear equation could be found for both variables.

Also, has anyone noticed significant differences in time or distance accuracy on different surfaces?

BalsaMan: Is the kickback that occurs when your vehicle brakes consistent?

Hmmm.... shouldn't be linear. We got the following curve:
y = 0.0325x^2 - 0.1367x + 1.7166, with an R² = 0.9988

Yes, the kickback is consistent.

All our testing and both regionals & State on a wooden gym floor - didn't notice any difference

Balsa Man wrote:Hmmm.... shouldn't be linear. We got the following curve:
y = 0.0325x^2 - 0.1367x + 1.7166, with an R² = 0.9988

Really? The linear model fits ours to within .05 turns at each distance... Could it be that the skidding decreases at the longer distances, so less turns are needed? I'd rather not post our equation, since a lot of other teams are probably using CD wheels with the same cirumference.

For us the distance vs turns graph is linear with a RMSE(like standard deviation) of .037m, which makes sense, each turn is a certain distance and we tried very hard to minimize skidding, but the time vs turns graph is quadratic because of the physics of the friction involved. However, if you do have skidding/kickback I would think it would be a quadratic fit for distance(with a very small squared term) because of the slight change in energy adsorbed in the braking system at different distances.

Also, we haven't seen a significant change in numbers with the floor surface, we test on industrial carpet(the gyms here are always in use...) however at state we ran on a wooden gym floor and still were within 8 cm and human error in timing(.1s ).

Next question... has anyone given thought to how to accurately time the vehicle? The only thing I can think of is a high speed camera to time the motion of the vehicle, but this seems slightly extreme for any competition, even nats.

illusionist wrote:I noticed that we could graph both our times and brake turns v. distance on a linear graph. Is this true for anyone else? I just thought it a bit weird that a linear equation could be found for both variables.

Also, has anyone noticed significant differences in time or distance accuracy on different surfaces?

BalsaMan: Is the kickback that occurs when your vehicle brakes consistent?

We were able to use a linear equation too with little error. Theoretically, if you were able to eliminate all skid, could you use a linear equation?

Hi guys. My partner and I are noobs at this event and we're having a great amount of trouble getting our car to even pass the 5 meter mark, much less go the entire distance. We're desperate for any advice - our car is around .5 k (maybe less), around 40 cm long and 20 cm wide, wheel diameter 7 cm. we've tried adding weight to the back, to the front, and to the entire thing. Adding weight just makes it slow down or improve by a little. We've also tried shortening the vehicle and using smaller wheels... Do you guys have any tips for how to get the car to go all the way to 10 m? Any help would be much appreciated.

The short answer is read back through this thread. Maximize speed off the ramp, minimize friction and energy loss.

There is a quick summary of the basic factors 2 pages back. There is detailed information and discussion on all the factors - what and how - as you go back through this thread. A lot of this discussion is around speed, and how you get it; faster = will roll further

If, after you have read through this, you have more specific questions, please feel free to ask, and I'm sure you'll get helpful answers.

With difficulty getting to 5m, your problems are.....pretty fundamental. If you have a ramp that's close to a meter high, a reasonably stiff chassis, wheels/axles reasonably lined up and if you use ball bearings, and your wheels don't flop around, well over 5m is cake- it will happen. With those things and a mass approaching a kilo, it will go 10+m. As I mentioned a few days ago, get it all pretty much right, it'll go 30m

illusionist wrote:

Balsa Man wrote:Hmmm.... shouldn't be linear. We got the following curve:
y = 0.0325x^2 - 0.1367x + 1.7166, with an R² = 0.9988

Really? The linear model fits ours to within .05 turns at each distance... Could it be that the skidding decreases at the longer distances, so less turns are needed? I'd rather not post our equation, since a lot of other teams are probably using CD wheels with the same cirumference.

Ah, clarification- that curve I gave the formula for is for time vs distance. Wheel revs per distance IS linear; the wheel circumference is constant; each rev is a constant distance.

From the basis physics at work, t vs d is not linear. Velocity (v) off the ramp gives you an initial momentum; combined energy/momentum consumption factors (bearing friction, rolling resistance, aerodynamic drag) eat into that momentum as it rolls along. For each increment of time the energy loss is some % of momentum; that % changes as momentum (and v) is lost, hence the change in momentum/v changes. The combined energy loss factor/amount also changes with time; not a lot, but some. The resultant plot is not a straight line. With some combinations of design factors, it will be flatter than others, but it will be a curve.

iwonder wrote:For us the distance vs turns graph is linear with a RMSE(like standard deviation) of .037m, which makes sense, each turn is a certain distance and we tried very hard to minimize skidding, but the time vs turns graph is quadratic because of the physics of the friction involved. However, if you do have skidding/kickback I would think it would be a quadratic fit for distance(with a very small squared term) because of the slight change in energy adsorbed in the braking system at different distances.

Also, we haven't seen a significant change in numbers with the floor surface, we test on industrial carpet(the gyms here are always in use...) however at state we ran on a wooden gym floor and still were within 8 cm and human error in timing(.1s ).

Next question... has anyone given thought to how to accurately time the vehicle? The only thing I can think of is a high speed camera to time the motion of the vehicle, but this seems slightly extreme for any competition, even nats.

For maglev, photogates are used frequently without, to my knowledge, too many issues. However, they would need to be at the end of the ramp, and would start the time after the vehicle reaches that point, so using them would require a slight alteration of the rules.

twototwenty wrote: For maglev, photogates are used frequently without, to my knowledge, too many issues. However, they would need to be at the end of the ramp, and would start the time after the vehicle reaches that point, so using them would require a slight alteration of the rules.

Photogates are wonderful for the teams that travel on a perfectly straight line, however they pose a huge challenge for teams that have any curve or unpredictability. Also, it is somewhat challenging (but not impossible) to time when something comes to a complete stop with photogates. They're mostly useful for timing when something passes by on a track (like in maglev.)