The bullet is being pulled down by gravity at a rate consistent with the falling rate in a vacuum due to the air resistance coming from a different direction. Gravity is gravity. Air resistance's effect on a body falling due to gravity however can change. Weight distribution in an rc car has no effect on flying orientation.

I didn't quite follow your first sentence there. A bullet fired from a gun in air will hit the ground at the same time as a bullet dropped in air from the same height. The force of gravity and upward air resistance is the same in both cases and has the same effect. There is no vacuum involved in this situation, and it's not due to air resistance in another direction. This is a classic Physics 101 experiment. See this link for a nice explanation http://www.lightandmatter.com/html_b...ch06/ch06.html, google bullet fired vs. bullet dropped mythbusters to see it in action.

We may just have to respectfully disagree that weight distribution in an rc car has no effect on it's pitch change while flying through the air.

One more thought to maybe show what I mean...let's think about this at large scale. I drop a 4000 lb car from 50 ft (bullet example shows horizontal velocity has no affect). Will where the weight is distributed affect it's pitch in the air. Absolutely. If the weight is centered it will drop basically flat, if 3000 lbs is in the trunk the net force over the trunk are of the car F=MA (and we know A stays the same due to gravity) will be greater than the net force over the hood of the car which only has 1000 lbs. The rear will be more easily able to overcome the upward air resistance and fall "faster".

Wanderer77 - reference "Forward of center" (F.O.C.) in archery. I think this will strengthen your arguement to make more sense.

Why don't we take a step back a moment. The difference between typical mid motor and rear motor weight distributions is only a few percent of the car's total weight (38%/62% vs 32%/68%).

We're not talking about strapping 3000 lb to a 4000 lb car. We're talking about moving maybe 100g on a 1600g car. Even if it did make a difference (which I doubt it does), it would be so minute as to be insignificant.

HUSH NICK:p! (kidding)

I want to witness a good battle of Wits here!!!! Surely there will be some great physics fallout from all this "banter"! LOL

I understand his physics point that the bullet fired and the bullet dropped fall at the same rate. The falling bullet doesn't have a significant amount of air resistance acting on it in relation to it's mass though. Keep in mind if it starts tumbling, it'll fall slightly slower. A barrel is rifled so a bullet spins and maintains it's trajectory. The effect would be the same in the vertical.

Nick also brings up a good point. Even if it does matter, on the low side you'll have a 60% rear weight bias and on the high side it'll be 70%. If you have a 15" long vehicle, that's a cg shift of only 1.5". How much air resistance would be necessary to visually alter a 4 lb vehicle's pitch over a vertical falling distance of let's say 8 feet with the understanding that at the max height, vertical acceleration is at 0? There's your physics problem.

In addition to center of gravity, a car also has a center of pressure. Basically this is the theoretical center point of the aerodynamic forces on the car. Most cars have CoG a bit rearward of center and the CoP also rearward of center, usually even more rearward than the CoG.

Now what if we made a car with an extreme rear weight bias while also putting some massive front wing on it to get a more forward CoP. This is going to change things. If the car comes off the jump with a nose high angle of attack, the CoP being ahead of the CoG will push the nose up more. If the car comes off the jump with a nose down angle of attack, that big wing up front is going to push the nose down more. This would not be a desirable arrangement since any errors in attitude off the jump will be made worse.

A better arrangement is a CoP behind the CoG. Despite the rearward weight bias of most offroad cars, this is usually what we have thanks to chassis being wider in the rear and big rear wings. This way if the car comes off the jump nose high, that big rear wing and wide chassis will actually create lift behind the CoG which will raise the rear, lower the nose. If it comes off nose down, that wing will push the rear down and raise the nose. This is why arrows and rockets and what not have their fins on the rear. The aero forces need to act behind the CoG for stable flight.

To get more on topic. Moving a few components around a bit is probably not going to move the CoG far enough to make much difference. It's much easier to make a big change in CoP. Just move the wing or slap a bit of lexan on it.

Yup

For the first part there, the barrel rifling has no affect on the bullets vertical drop. Newtonian Laws of physics says horizontal force vector has no affect on the perpendicular vertical forces. See the link I provided.

To the second point of not having much impact. That may open to some debate. I would propose that the affect is larger than expected on this scale. After all, I can move 10 grams of weight from the back of my buggy to the front of my buggy and I'd say the affects on it's handling are quite dramatic. If people are going so far as to say reduce bearing friction my spending $$$ on things like ceramic bearings, then they would be well advised to also pay attention to small things like...weight distribution. Hell some people wonder how aerodynamics even affects these little cars, but it sure does. Just ask any road racer to run a race with no body on his car...and if they do, just check out the slower lap times.

Thanks Red...but I was trying not to get too deep into the weeds of Forward of Center, Center of Pressure, Center of Mass, Center of Gravity and all of the associated funness that comes with that. I didn't really intend this to go as far as it has.

In the end I think we can all agree at this point that there is some effect, whether it is worth considering is up to the readers. I'd just suggest that it's another avenue to explore as one tries to get the fastest/best handling rc car that they can drive. It rarely hurts to experiment.

Um...ok... speechless but whatever.

First off, of course shifting a 10g weight from the rear to the front will affect handling. How many pages of this thread have been devoted to that very topic?

At least you are finally admitting that weight distribution in these rc cars, aka mid motor vs rear motor, 4wd etc, has no discernable effect on vehicle pitch in the air off of jumps. What happens on an immeasurably small scale when rounded out is effectively zero. This is the real world.

This is why I've mostly stayed away from this thread for a while. I've wasted my day.

Well Fred, I am NOT saying that the weight distribution has no discernible affect on pitch change in the air. It does have an affect, but it's exact quantity is unknown. This is the real world.

You can feel free to not reply to my comments if you think they are a waste of your time.

If you really want to get back into hardcore physics (radiation physics is what I do for a living) we can go on for pages and pages, but the result will be the same...there is some affect, and until someone actually does the math we won't know how much that is. I'm just freaking suggesting people experiment and try different things out rather than just listen to some unknown person on a message board, take it as gospel, and not experiment with their setup.

This is exactly why I normally don't even bother with message boards...keyboard cowboys who profess to know more than they really do and are experts at everything.

Sigh...I'm done wasting my time and energy...I've proven my points.

anyone want a beer....on me?!

I do! Odell's IPA please!

I ran into an issue that I need some feedback with. I am building a Durango Dex210v2 and the directions say to use a 2x1.7 in the front with 40wt and a 3x1.7 in the rear with 30wt. In the book there are setup sheets for different conditions and the one I am setting to is almost stock with a few exceptions like the shocks. For my conditions, it says 3x1.6 with 35 front and 3x1.7 with 30wt in the rear.

Considering what I have read in this thread, I am considering just going with the 3x1.7 all the way around (or something that I can use the same on all 4 corners). Would I be silly to do it that way?

Doesn't make sense to use different pistons front to rear AND different wt oil....any thoughts or suggestions?

It does make sense to use different pistons front and rear. There's a lot more oil in the rear shocks and a lot more weight to ram that oil through the holes. I've heard some say that the speeds we are talking about aren't high enough to cause turbulent flow in the shock but I'm assuming that is what "pack" actually is. I think if you went through a technique similar to the spring balancing technique you would find that to get the pack you need while still maintaining balance on the chassis for bump handling and such you would need a different piston. Take this with a grain of salt....I've had three beers and they all say "7.0% Alc. by vol."