New RX-8 slows down going downhill in neutral
 

I have a new RX-8 with 700 km.

Today I was going downhill in neutral. At the top of the hill I was going 120 Km/h and at the bottom I was going at 98 km/h.

Why so much drag ?

For comparison when I did the same in my Toyota Corolla,if I started at 120 km/h at the bottom I was going at 123 km/h. an acceleration insted of slowing down.

Was the wind blowing?

no wind

maybe your parking brake was on :)

I hate that it's up at an angle even when not enabled.

no parking brake either

...its something called wind resistance

I would expect that a sports car would have less wind drag than a Corolla. Furthermore, the RX-8 is heavier and it should have more inertia.

Maybe it's rolling resistence... 225mm Potenzas vs Corolla pizza cutters... ?

It will loosen up quite bit....bearing are tight when new

You can get an answer to your problem very easily ... drive a few miles an hour on level ground and see if you car slows down as if the brake is on . .. if it does then bring it in for servivce and find out why is the brake still engaged

the Drunk gets it in 1!:D

I take it you are not a big fan of Galileo...

Galileo Fan, Inertia
 

More mass does mean more potential energy stored when you are are at the top of the hill.

Galileo's famous test demonstrated that in free fall the gravitational acceleration is the same no matter the mass. That is different than inertia, but inertia really isn't the key here but rather potential energy.

In a vacuum Galileo's experiment holds true, but if he had had enough distance the "real world" effects of drag would have confused the experiment.

Imagine dropping two aerodynamically equal balls from a good distance, say from an airplane at 30,000 feet. One ball is filled with lead, and the other styrofoam. Would they hit the ground at the same time? No, of course not. The denser, heavier ball would get to ground quicker because it has more potential energy at the same height, (and once it gets moving, more inertia) to overcome the drag of the air with less loss of speed than the lighter, yet aerodynamically equal ball. See? Even though the acceleration is the same on both balls.

A fully ladened semi going down a mountain pass has to brake more to control its speed than a lightly loaded one.

The RX-8 is newer, tighter, and has big fat tires, and likely more overall aero drag. The tires add a lot to the overall area for aero drag.

Don't worry, you are not the only one to over-interpret that famous experiment of his..

Hans

Light and Heavy things fall at the same rate of acceleration my friend, weight has nothing to do with it (says Galileo the Medici Scientist). Was the Corolla an Automatic? In notice that my Solara goes downhill on Neutral faster than my RX-8 on nuetral. But when I rolled downhill with my RX-8 on neutral plus I pressed on the clutch, it actually went downhill as fast as the Solara.

I presume your Corolla was an Automatic.

Gravitational acceleration is indeed the same for all objects, but the sum of the forces equals zero in all cases as well.

F(gravity) = mass * gravitational acceleration

So for a heavier object gravity is exerting more force in order to achieve the constant rate of acceleration.

F(drag) = something like 1/2 * air density * drag coefficient * frontal area * air velocity^2

My fluid texts are all at my office so I can't look up the exact equation. But the net effect of these two is that two objects with equal drag coefficients and frontal areas falling through the air will experience the same drag force assuming the same speed regardless of mass. However the acceleration force due to gravity will be higher for the object with greater mass.

Take a cannon ball and a styrofoam ball (coated with bondo of course to smooth the surface :D ) and drop them from an airplane and the cannon ball will reach a higher terminal velocity (the speed at which drag force exactly cancels out the accelerative force of gravity). Its just that at low speeds where the velocity^2 does not cause much drag force yet light and heavy objects will appear to fall at the same rate since the gravitational force will be so much greater than the drag force and gravitational acceleration is after all equal for all objects.

Take your heavy object and light object and drop them in a vacume where there is no drag and they would indeed fall at the same rate.

You weren't dragging a dead llama carcus behind your car, were you? I've notice this slows my car down considerably.:D :D

Fine.

You all want scientific proof, well here is the results of my statistical analysis:

During my commute I encounter two hills, one going each direction. On said hills, for the last 12 years I have driven the following to determine their relationship to drag --

Chevy Tahoe, Dodge Ram Pick-Up, Nissan Frontier Pick-Up, 64 Corvette Stingray, 66 Chevy Bel-Air Station Wagon, Mazda 626, Jeep Cherokee, VW New Beetle, Ford Taurus, Buick Lesabre, Olds Delta 88, Buick Century, and Ford F250 Diesel Pick-Up.

The vehicle with the slowest speed at the bottom of these hills, with the tranny in neutral, was the heaviest, the Ford F250. The fastest is the RX-8, which just this week reached 92 mph.

I had a 96 Z28 on 275/45-17's and in neutral on any down hill surface i would at least maintain the speed i started at. If it was a steeper hill i would ALWAYS pick up speed.

That's because our Z's are shaped like arrows and have a little known amazingly low drag coefficent. :)

You don't say what year Toyota Corolla you tested. In 1999 version, the drag coefficient was identical to today's RX-8 at .31; However, in 2004, the Corolla got their drag coefficient down to .30

There are too many environmental factors that likely changed between the test runs to make an informed judgement on why the RX-8 speed was lower. It would be interesting to do the test side-by-side.

Stop arguing over what IS NOT affecting him!!! You guys need to just throw the science part out the window and use plain old common sense. At the speeds he is going at, a difference in drag coefficient would have to be huge to make any difference. I don't care how wide the tires are or what the drag coefficient is. A brick wall would accelerate down a hill at the speeds he is going. He isn't moving that fast so wind resistance is irrelavant. The RX-8 has a better drag coefficient than a Corolla anyways. That argument is irrelevant. I'll bet Gallileo would look to the obvious before he'd theorize the unlikely. Something is causing him to slow down. Get back to the obvious. The simplest explanation would be that somehow the brakes are dragging or a bearing is sticking. Another option is that there is an awful lot of loss through the differential and driveshaft somehow. Weight and drag are not going to affect him in THIS scenario. The wind will only do it if he happens to be driving in a tornado at that moment. To me if there is a serious drag somewhere in the drivetrain or braking system, it explains alot about why the car could get lower gas mileage than rotaries are capable of and appear to have less horsepower than it should. Ding, ding, ding!!! Are the bells going off yet? That correlation is much more obvious than whether or not the wind is blowing, the drag is too great or the tires are so wide that they are a hinderance. Get off the science lesson and get on topic about what REALLY is going on. Keep it simple. Science is good but we are looking for a mechanical explanation here and that is what it will be. I wouldn't fix an alignment problem by putting the car in a wind tunnel and judging straight as when the wind resistance becomes even on both sides. That would be ludicrous. Just like where this thread has gone. Get the picture? You guys are weird. This is why we have mechanics. It's because scientists can't fix cars!

:D :D :D ROTFLMAO :D :D :D
You have all been b-slapped.

nope rotaryGOD-SMACKED!! :D

It has to do with the rotation of earth. If you are going against the earth’s rotation you will appear to go faster. However if you go down a hill in the same direction you will seem like you are going slower since you are moving in the same direction as the rotation of earth. :)

LOL, occasionally a snappy retort catches my attention and I laugh while scrolling through the back and forth jibberish in some of these threads.

Thanks, MM.