Gravity Vehicle C

[Lalaloopsy]

For anyone wondering, I did not consider rolling friction of the vehicle when moving since it has such a little impact on the velocity of the car. I accounted for height of the ramp (not the shape of the ramp, though) IN THE TIME MEASUREMENTS IN ADDITION TO THE HEIGHT SCORE as well as the coefficient of kinetic friction for the tires and the car when the car brakes. The math was checked by the AP Physics teacher at my school and he said that all the math was correct and our conclusion was correct: a short ramp will have the lowest total score barring excessive friction due to the rolling wheels and an imprecise construction technique resulting in extra friction. We don't really have to worry about the construction since our chassis will be printed by a 3D printer, though. The equation that we used, I assure you considered height's affect on both the height score AND the time score.

[iwonder]

Here's a question that I hope is pretty simple... I got my shafting in today, I've had the bearings for a while, I mic'd the shaft, it's round and .1250" in diameter, I got the bearings from a pretty large manufacturer of bearings(sdp/si), they're abec rated, etc, so I trust that they're .125 id, but for some reason the shaft won't fit into the bearing... I've chamfered the ends, and the shaft has no burrs, but the bearing doesn't slide on the shaft like I would expect... any thoughts?

Also, as your AP physics teacher should have pointed out, the ramp shape has very little bearing on the final velocity(as long as it' a smooth transition). Here's my question for you though, does friction actually have an effect on the end calculation, and if it does, what defines 'excessive' friction? A lot of these vehicles only travel 15-20 meters before stopping due to friction. And what's the benefit of a 3-d printer?

[Lalaloopsy]

I realize that the shape has no impact on final velocity but it does hold bearing in the time it takes for the car to exit the ramp. From doing this event last year, friction with the surface it is rolling on does play a role, but I am not sure how major a role it does play in the velocity right before the wingnut hits the bearing versus the velocity right after exiting the ramp. In terms of the 3D printer, it saves us from having axles that are misaligned and only 3/4 wheels touching the ground which impacted the straightness of the path that our car took last year.

[illusionist]

Here's a question that I hope is pretty simple... I got my shafting in today, I've had the bearings for a while, I mic'd the shaft, it's round and .1250" in diameter, I got the bearings from a pretty large manufacturer of bearings(sdp/si), they're abec rated, etc, so I trust that they're .125 id, but for some reason the shaft won't fit into the bearing... I've chamfered the ends, and the shaft has no burrs, but the bearing doesn't slide on the shaft like I would expect... any thoughts?

Also, as your AP physics teacher should have pointed out, the ramp shape has very little bearing on the final velocity(as long as it' a smooth transition). Here's my question for you though, does friction actually have an effect on the end calculation, and if it does, what defines 'excessive' friction? A lot of these vehicles only travel 15-20 meters before stopping due to friction. And what's the benefit of a 3-d printer?

The 3d printer just makes it easier I'm assuming. It's entirely possible and rather easy to cut square corners yourself, but I guess this is easier. Will the printer leave spaces for holes if you choose to include them?

[Lalaloopsy]

Yes we are choosing to have counterbored holes to hold our bearings in place and to build in a loop for our release system out of the ABS plastic. The main advantage is that the bearings will press fit right into the holes and they will be aligned unlike last year.

[iwonder]

Ahh... we're laser cutting ourselves, so I have tab fits drawn in everywhere, same idea, I guess, 3-d printers have always been imprecise in my mind, however. The few objects I've seen don't have smooth curves and holes are near impossible, but that's probably just because I've only seen the cheap ones

Oh, and I did the math for ramp height myself, I got the same results as everyone else it seems(I included friction in there) that an insanely low ramp yields a much lower score... for example, a ramp height of .8m(the lowest height I calculated that would make the distance) and distance of 10m would give a score of about 190, whereas a 2m ramp and 10m distance would yield 822

[bearasauras]

The 2 meter limit is an upper limit just so that things will fit through the doorway and I highly doubt the winning team to have a starting height of 2 meters. We did the math too when writing the rules; so yes, starting your vehicle too high will end up a really high score. I think the most effective way to do this event is really to build your vehicle and test out different starting heights to figure out what will give you the best score for a given target distance. Your calculation will give you a good estimate but depending the approximations and assumptions you make, your calculated optimum starting height might not be too accurate.

[Lalaloopsy]

Ahh... we're laser cutting ourselves, so I have tab fits drawn in everywhere, same idea, I guess, 3-d printers have always been imprecise in my mind, however. The few objects I've seen don't have smooth curves and holes are near impossible, but that's probably just because I've only seen the cheap ones

Oh, and I did the math for ramp height myself, I got the same results as everyone else it seems(I included friction in there) that an insanely low ramp yields a much lower score... for example, a ramp height of .8m(the lowest height I calculated that would make the distance) and distance of 10m would give a score of about 190, whereas a 2m ramp and 10m distance would yield 822

I don't know how the lowest height to go 10m was .8m tall since I tested our car from last year and it was successful in going 10m from a height of only approx. .3m and our car had a lot of rolling friction before braking.

[retired1]

[quote="iwonder"]Here's a question that I hope is pretty simple... I got my shafting in today, I've had the bearings for a while, I mic'd the shaft, it's round and .1250" in diameter, I got the bearings from a pretty large manufacturer of bearings(sdp/si), they're abec rated, etc, so I trust that they're .125 id, but for some reason the shaft won't fit into the bearing... I've chamfered the ends, and the shaft has no burrs, but the bearing doesn't slide on the shaft like I would expect... any thoughts?

The problem is that both are extremely close to 0.1250. It can not fit, not even a press fit. Machine shop basics are the shaft needs to be 0.003" smaller than the bearing for a press fit if you have a good method of doing it so that you do not ruin the bearing. 0.004 gets closer to reality for most, me included. This is from memory, most large bearing Mfg's have a help section for better guidance.

[bearasauras]

Ahh... we're laser cutting ourselves, so I have tab fits drawn in everywhere, same idea, I guess, 3-d printers have always been imprecise in my mind, however. The few objects I've seen don't have smooth curves and holes are near impossible, but that's probably just because I've only seen the cheap ones

Oh, and I did the math for ramp height myself, I got the same results as everyone else it seems(I included friction in there) that an insanely low ramp yields a much lower score... for example, a ramp height of .8m(the lowest height I calculated that would make the distance) and distance of 10m would give a score of about 190, whereas a 2m ramp and 10m distance would yield 822

I don't know how the lowest height to go 10m was .8m tall since I tested our car from last year and it was successful in going 10m from a height of only approx. .3m and our car had a lot of rolling friction before braking.

Yes, but how was the score based on the time? As you start at a lower height, you travel slower and so the time is longer. I think this is one where your actual testing will be really important.