Gravity Vehicle C

[questionguy]

Yes, depending on what you mean by shape.

If you’re thinking about aerodynamic drag, a streamlined, dragster-like shape would have less than a .....chunky block of wood. Shape, and cross-section (frontal area) would determine that. Would that be a “significant” difference – more than a fraction of a second? Possibly, depending on how streamlined dragster-like, or how chunky block of wood-like. The effect; the difference in run time, would be significantly less than the difference in bearings/no bearings.

If you’re thinking about wheelbase length and where the center of mass is, yes, it could make a significant difference. There has already been pretty extensive discussion of how and why in this thread- worth re-reading. In this context, dragster implies, to me, long wheelbase, weight low (as in close to the axle line), block of wood implies shorter wheelbase, weight higher (part of the “block” well above the axle line).

As has been said many times, the energy going into the vehicle, which primarily determines its horizontal velocity, depends on how far the center of mass falls – “h”; height over which gravity acts. You want to maximize h – center of mass as high up when you start, and low down to the ground as you come off the ramp as you can..

If the weight is evenly distributed throughout the chassis, a long wheelbase is going to have less h than a short one- its going to take h off at the top. If its, say 50 cm long, then the center of mass is going to start 25cm down the ramp (the midpoint of the wheelbase). If its 25cm long, the center of mass is going to start 12.5 cm down the ramp; you loose 12.5cm of h. However, if you take the wheelbase too short, you are going to loose linear stability- it will tend to randomly kick off the intended ‘run line.”

If the weight is not evenly distributed, then it depends on how you’ve done that. Again, its center of mass that drives things.

If you have a long, or short wheelbase, and you concentrate the weight toward the back (as in build light, and put a chunk of mass back toward the back- a piece of metal, a rock, whatever), you gain h. How much gain depends on how far back, how much mass, and on how much the parts of the chassis in front weigh, compared to the chunk of mass. The tradeoff here is, if you concentrate the mass too far back – make the center of mass really close to the rear axle, you again are inviting linear instability. Maximizing h at the bottom says you want the center of mass as close to the ground (once the vehicle is on the floor) as you can; at, preferably below the axle line.

This helped so much, thank you!

[ChrisYim]

has anyone tried using two wingnuts on both axles in order to make the vehicle stop better?
(personally i think it will just slow it down and add more weight)

[chalker7]

has anyone tried using two wingnuts on both axles in order to make the vehicle stop better?
(personally i think it will just slow it down and add more weight)

I've heard of a variety of people testing it out over the years, but the biggest problem is coordinating the axles. It is extremely difficult (if not impossible) to set the wing nuts perfectly on both sets of axles such that they will both engage at precisely the same time and even being off by a couple mm will nullify the benefits of having a second brake.

[illusionist]

Okay, another question, relating to the rules. It says that the release mechanism must be a pencil (line 3.h), but it does not state unlike last year's mousetrap rules that the pencil must be used in a vertical manner. So based on that line of the rules, I can use the pencil to pull out a release mechanism right? I just want to confirm it. And I know this isn't the place for official clarifications, these are solely opinions, etc.

[bookluvr-yoyo39]

i have a very general question.... concerning the wheels and minimizing friction, are we allowed to put lubricant on the wheels and/or ramp?

[elmono]

What are some good materials to make a ramp out of? I was thinking of making a curved ramp over a flat ramp for obvious reasons but creating a ramp out of a bendable material is the tricky part. Would aluminum channels glued onto a wooden base be efficient?

[illusionist]

i have a very general question.... concerning the wheels and minimizing friction, are we allowed to put lubricant on the wheels and/or ramp?

Technically, yes you are. I would put some lube on the wheels, but I'm not sure why you would like to put it on the ramp.

[chalker7]

i have a very general question.... concerning the wheels and minimizing friction, are we allowed to put lubricant on the wheels and/or ramp?

Technically, yes you are. I would put some lube on the wheels, but I'm not sure why you would like to put it on the ramp.

Also, why put them on the wheels? That would make braking quite difficult....

[ChrisYim]

i have a very general question.... concerning the wheels and minimizing friction, are we allowed to put lubricant on the wheels and/or ramp?

Technically, yes you are. I would put some lube on the wheels, but I'm not sure why you would like to put it on the ramp.

Also, why put them on the wheels? That would make braking quite difficult....

and also, it wouldn't help the car go faster, since the wheels wont spin as much. it would just slide down and go slower on the track...
put lubricant on your axles instead

[illusionist]

Technically, yes you are. I would put some lube on the wheels, but I'm not sure why you would like to put it on the ramp.

Also, why put them on the wheels? That would make braking quite difficult....

and also, it wouldn't help the car go faster, since the wheels wont spin as much. it would just slide down and go slower on the track...
put lubricant on your axles instead

Yeah, that's what I meant. Put lube on the axles/bushings/bearings. Any points with friction. However you don't want any on the surface of the wheels nor the track. Btw, can someone please answer my question from the bottom of the previous page?