I'm not too sure but i think that this have not been discussed.

I made a simple test ( and so can you)
Choose XRT. On Blackwood(regular) get to the long straight ( after the first chicane), maintain speed of (about) 150 km/h when passing the first checkpoint and change gear to neutral position. After 100-200 m the track gets some negative elevation ( say about 10 degrees) and amazingly our car, right on that spot, doesn't lose any speed(!). In my case the car was going for this short time with speed of 145 without loosing a single kilometer per hour.

In real car/in real life

On my way home I go by my car home through the road which on some point is elevated at about -20 or -30 degrees. Being on "top" of the road, (on the very beginning of that slope) having speed of 100 km/h I switch my gearbox to neutral position.
Guess what ? My car LOSES speed there. YES ! even though the speed is lower then the speed mentioned in LFS case above air resistance is so big it slows down my car to about 75-80 km/h

ohh my car is a sport one, doesn't have worse air resistance factor then xrt for sure.
Anyway , im pretty sure that air resistance in LFS is far to low.
What do U think ?

It's a well known fact that the aerodynamics in LFS are not well modeled yet. The devs are working on it.

i just tried what you described and could not replicate the behaviour
could you attach a replay of this ?

Quote from Dupson :

On my way home I go by my car home through the road which on some point is elevated at about -20 or -30 degrees. Being on "top" of the road, (on the very beginning of that slope) having speed of 100 km/h I switch my gearbox to neutral position.
Guess what ? My car LOSES speed there. YES ! even though the speed is lower then the speed mentioned in LFS case above air resistance is so big it slows down my car to about 75-80 km/h

I doubt that. you sure know that 20° mean about 40% on the warning signs? There is a road, where you can go 100 and then go done this steep? Get some real figures and we can talk again.

Quote from Dupson :

I made a simple test ( and so can you)
Choose XRT. On Blackwood(regular) get to the long straight ( after the first chicane), maintain speed of (about) 150 km/h when passing the first checkpoint and change gear to neutral position. After 100-200 m the track gets some negative elevation ( say about 10 degrees) and amazingly our car, right on that spot, doesn't lose any speed(!). In my case the car was going for this short time with speed of 145 without loosing a single kilometer per hour.

To be honest with you there mate, yes, LFS has a few problems with aerodynamics and resistance physics, but I can assure you that the Blackwood straight has no elevation changes, except for the hill after the split that you should notice. If anything, you should be gaining speed, not loosing here.

Quote from Dupson :

On my way home I go by my car home through the road which on some point is elevated at about -20 or -30 degrees. Being on "top" of the road, (on the very beginning of that slope) having speed of 100 km/h I switch my gearbox to neutral position.
Guess what ? My car LOSES speed there. YES ! even though the speed is lower then the speed mentioned in LFS case above air resistance is so big it slows down my car to about 75-80 km/h

A 2000 lb car with a frontal area of 20 ft^2, a 0.36 drag coefficient on a 20 degree downgrade would reach terminal velocity in neutral at 194 mph (318kph) (ignoring rolling resistance losses, which are pretty miniscule at this speed). The hill you're talking about can't possibly be that steep (The steepest street in San Francisco is 17 degrees according to a very quick search.)

http://en.wikipedia.org/wiki/Drag_equation

Others can check my math here. I did this quickly so might have made a mistake:

F = 0.5 * air_density * drag_coef * frontal_area * vel^2

air_density = 0.00233 slug/ft^3
In our case, we need F = sin(20) * 2000lb = 684 lb drag to balance the car and keep it at a constant speed.

Setting F = 684:

684 = 0.5 * 0.00233 * v^2 * 7.2
v^2 = 684/0.5/0.00233/7.2
v= (684/0.5/0.00233/7.2)^0.5 (square root)
285 ft/sec = 194 mph

Seems reasonable as if you take the car in free fall with F = 2000lb then you get 323 mph (530 kph).

Anyway, mistake or no mistake, I don't think those slopes are anywhere near 20 degrees. The point is, there is of course some speed at some slope where the car is balanced at a constant speed due to aero drag and rolling resistance. If you are going faster than this when you shift to neutral, the car will slow down. If you're going slower, it will speed up. If you're going just the right speed, it will stay right at that speed, which is probably what you were doing at 145kph in LFS.

Nothing is probably wrong here at all

Quote :

Anyway , im pretty sure that air resistance in LFS is far to low.
What do U think ?

I doubt it. Air resistance is modelled very accurately using the drag equation in the above link. All you need is air density (easy to calculate), a drag coefficient (make one up for your fictional car), and the speed it's moving. Presto. 99.99% perfect aero drag model

I ll provide you with a photo of this slope soon.

This one is 19 degrees

http://en.wikipedia.org/wiki/Baldwin_Street,_Dunedin

imagine there is a 20 degree slope, a normal production car can have an acceleration of a bugatti veyron man!! if you know how to change the gears fast. HAhahaha!

Yes , I have to agree with you guys , the slope I have written is about 10 degree and car slows down when on it to 90 - 95 kmh. So i was wrong there

But i wasn't when talking bout air resistance in LFS cause on the same elevation, like the one I mentioned, car in lfs would nt actually slowdown to 90-100 so my point is still true.. I believe.

Although as I said I had exaggerated that elevation - sorry for false alarm.

so, to make it clear - you guys think that forces like air resistance and friction are properly modeled ?

I think the air resistance is about right if I let the Focus I drive slow down with the engine clutched and my foot off the throttle pedal it slows down slower than the TBO/LRF cars in LFS, natural I suppose because of the smaller engine and engine braking effect.

Quote from Dupson :

so, to make it clear - you guys think that forces like air resistance and friction are properly modeled ?

Yes, I think it's right. If it were too low the top speed of the cars would also be too high.

Quote from Dupson :

Yes , I have to agree with you guys , the slope I have written is about 10 degree and car slows down when on it to 90 - 95 kmh. So i was wrong there

I even doubt it's 10°. It seems like you have no idea, when you talk about geometrics... 10° is still 15%, meaning 15 meters drop on a way of 100 meters. I wanna see pics of the road that drops that steep and where you go 100km/h that easy. And I promise you, that on such a road, a car does not loose speed in that region of about 100km/h. It'll accelerate.

I just want to give you an idea on what you see and think you experience.

I agree. In most of the world it would be rare to find a road with ~100km/h speeds and more than 6 percent grade. Even at that grade, you are likely to find warning/info signs.

I know of a road that is 12 or 14% on which I normally do 70-80 km/h, and my aerodynamical brick manages to accelerate even against the engine braking (of 4th gear)...

Dupson - take a glass of water and a protracor and take a measurement of the slope. I think only then will it make sense to continue the discussion. Cheers!