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### Re: Gravity Vehicle C

Posted: October 10th, 2019, 6:25 am
October 10th, 2019, 5:58 am
2. Yes, I think you're correct in saying that smaller wheels will be beneficial. However, imo it's not for the "accuracy" - smaller wheels usually have less mass and therefore less rotational inertia. The heavier wheels are, the more force is required in order to get them spinning. However, if you think about it, smaller wheels will have to rotate more in order to travel the same distance, which makes the friction between the axles and the bearings/bushings a bigger factor. The accuracy you speak for a smaller wheel is pretty much irrelevant if you calibrate to fractions of a wheel rotation.
windu34 wrote:
September 29th, 2019, 12:37 pm
People are getting way to caught up on the rotational inertia concept. It plays a VERY small role and is negligible for this event. Larger wheels will be smoother and more tolerant to imperfections in the floor - this outweighs those other concern imo. I have always used the 2 7/8", and think they are a good size. I could also see how larger wheels could be better as well.
According to windu rotational inertia is "negligible" so I probably wouldn't worry about that.

### Re: Gravity Vehicle C

Posted: October 10th, 2019, 9:26 am
October 9th, 2019, 7:54 pm
I was wondering what forces could make the car, while going down the ramp, move away from the surface of the ramp in the x-direction (if x was parallel to the ground)? I was thinking it could be the center of gravity of the car in the x-direction. In other words, the CG is too high above the car. Would this be a reason why my car does this?
October 9th, 2019, 7:54 pm
Another question, (physics noob), in my opinion, the inertia of the wheel shouldn't affect the exit velocity of the car at the bottom of the ramp by too much. So I think it would be better to use smaller wheels, which increase accuracy, rather than using the 2 3/8 in wheels. So what would be the advantages of using the 2 3/8 in wheels overusing the smaller like 1.5 in wheels?
The wheels are a small fraction of the car's mass, so you're right that the inertia of the wheels isn't very relevant. For accuracy, I assume you mean less distance per wheel turn, so it's easier to set your brake. I think that's true - but there might also be accuracy disadvantages associated with smaller wheels. I'd guess you could build a good car with either set of wheels, and that wheel choice will not be a dominant contributor to your final score. But that's not much more than a guess.
October 9th, 2019, 7:54 pm
Also, I tried doing some math, and the max time is 1.81 sec (starting at the top of the ramp). Starting .75 m above would cause the time to only decrease by around .281 sec (this is in the ideal world). The effect will probably be more prominent in the real world so let's just say .5 sec. How will making the CG to the front of the car differ the accuracy of the car? Would this only negatively affect skidding (which is positive)?
You're assuming 100% energy transfer to the car's KE, no frictional losses, and timing the car over the 8m timing segment. As you say, this is ideal, and the real world will be slower and have friction, but probably wont slow things by more than a factor of 2 for a reasonable car. So I think you have a good working number there - suppose moving the CM costs you 0.5s, which is 0.5 points of total score. Can you build a car where the score you get for multiple runs is consistent to better than 0.5 points? If not, don't worry about the 0.5 points.

### Re: Gravity Vehicle C

Posted: October 11th, 2019, 9:12 am
Any of you have issue with dowel (less than 1cm from floor) hitting the floor while leaving the ramp. This causes my vehicle to jump and change direction.
Can we place the dowel at the back of the gravity vehicle?

### Re: Gravity Vehicle C

Posted: October 11th, 2019, 9:48 pm
hey guys how far is our vehicle supposed to go? When i read the guidelines it said 9-12 meter and i’m very confused.

### Re: Gravity Vehicle C

Posted: October 11th, 2019, 11:19 pm
builder2703 wrote:
October 11th, 2019, 9:12 am
Any of you have issue with dowel (less than 1cm from floor) hitting the floor while leaving the ramp. This causes my vehicle to jump and change direction.
Can we place the dowel at the back of the gravity vehicle?
No. The rules explicitly state that the dowel must be at the front of the vehicle. Other than wheels, no part of your vehicle can be more than 0.5 cm in front of you dowel.
095067 wrote:
October 11th, 2019, 9:48 pm
hey guys how far is our vehicle supposed to go? When i read the guidelines it said 9-12 meter and i’m very confused.
So essentially, on the day of the competition, they'll give you the exact distance needed. At regionals, for instance, per 6.c., the target distance may be 9m, 9.5m, 10m, 10.5m, 11m, 11.5m, or 12m. At state, the interval is 0.25m (as in 9 m, 9.25m, 9.5m, 9.75m ... ), and at nationals, its 0.05m. You find this distance out after you impound the car on the day of. Therefore, you need to be make a braking mechanism that can be easy adjustable. The only way practical and effective way to do this (that I know of) is to to use a wingnut braking mechanism (like this: https://www.youtube.com/watch?v=RJwFtkFSuJM).

Hope I could be of hope!

### Re: Gravity Vehicle C

Posted: October 14th, 2019, 7:34 am
builder2703 wrote:
October 11th, 2019, 9:12 am
Any of you have issue with dowel (less than 1cm from floor) hitting the floor while leaving the ramp. This causes my vehicle to jump and change direction.
Can we place the dowel at the back of the gravity vehicle?
The dowel has to be at the front of the car, and only the wheels can be beyond 0.5 cm in front of the dowel. If you are having trouble with the dowel hitting the ramp, I would suggest replacing the front wheels with larger ones, positioning the front axle closer to the dowel, and/or raising the dowel as much as you can without pushing the limit.

Just to explain why the dowel has to be where it is: if the dowel is in the back, and the car is 60 cm long, the second photogate won't be tripped if the car has to travel 9 meters, as the dowel will fall 10 cm short of the laser, so timing won't stop.

### Re: Gravity Vehicle C

Posted: October 15th, 2019, 8:13 pm
Car lengths. What would be ideal? Assuming one has a full sized ramp, and knowing that timing doesn't start until 0.5 m after we are off of the ramp, would it be better to have a car that is exactly 0.5 m from the dowel to the bottom of the back wheels, or to maximize car length on the ramp making it about a meter long. The way I see it the advantage of the 0.5 m car would be that you cross the 1st photogate at maximum speed, because the end of the car just left the ramp and will only start decelerating therein. On the other hand, for the maximum length car, you will cross the 1st photogate while the back of the car is still on the ramp, meaning you will continue to accelerate into the timed section of the run, but this design could also shift your center of gravity forwards depending on how heavy your car is. Which one would yield the better time?

### Re: Gravity Vehicle C

Posted: October 16th, 2019, 1:03 am
builder2703 wrote:
October 11th, 2019, 9:12 am
Any of you have issue with dowel (less than 1cm from floor) hitting the floor while leaving the ramp. This causes my vehicle to jump and change direction.
Can we place the dowel at the back of the gravity vehicle?
Moving the dowel closer to the front axle will decrease how much it moves from the ready-to-run position to "final" moving position.

### Re: Gravity Vehicle C

Posted: October 16th, 2019, 1:09 am
Lorant wrote:
October 15th, 2019, 8:13 pm
Car lengths. What would be ideal? Assuming one has a full sized ramp, and knowing that timing doesn't start until 0.5 m after we are off of the ramp, would it be better to have a car that is exactly 0.5 m from the dowel to the bottom of the back wheels, or to maximize car length on the ramp making it about a meter long. The way I see it the advantage of the 0.5 m car would be that you cross the 1st photogate at maximum speed, because the end of the car just left the ramp and will only start decelerating therein. On the other hand, for the maximum length car, you will cross the 1st photogate while the back of the car is still on the ramp, meaning you will continue to accelerate into the timed section of the run, but this design could also shift your center of gravity forwards depending on how heavy your car is. Which one would yield the better time?
You are correct, you would have a faster time with a shorter device that you described because you would hit the peak velocity earlier. However, the difference in time will not be worth the loss of accuracy you will experience. Larger wheelbases will result in less deviation from the intended path of travel and will be more forgiving in terms of accuracy. In my opinion, you want to maximize wheelbase because the savings in time arent large enough to be worth it. Time is simply not worth nearly as much as accuracy when you consider the tradeoffs.

### Re: Gravity Vehicle C

Posted: October 16th, 2019, 10:15 am
A track diagram for Gravity Vehicle has been posted to the national website.