Motor Trend explains Prius' "blockish" front corners / wheel arch interaction

[Frank Lee]

Yes, it causes all kinds of speculation.

[Bicycle Bob]

On average. . . wrong. A hemisphere is a bit too abrupt, but not by much. A zeppelin or a classic airliner nose is about optimum. More length increases skin friction. The abrupt shape just processes the air faster, in a place where the pressure differential makes it easy to maintain control. At the back, we must be gentle. Getting the air to part is like pushing a pen through a sweater, but getting it back into a small wake is like pushing string into a hole without a needle.

I respect minds which question everything, but not if they hear nothing. I won't try to reply to all trolls on this topic, but might post original metaphors from time to time.

[hypermiler01]

Seems on this point, you will get different answers from different people. 10, 20, 30 years ago, it was "common knowledge" and taken for granted that low and pointy was better, and everything from the EV1 to the Honda Accord to the Ferrari F40 reflected that.



There are some who focus strictly on Cd and seem to ignore air pressure and lift, which are also essential to a well handling car.



As you said, the shape of the nose does directly affect the pressure of the air in front of the car, and this pressure is multiplied by the surface of the nose to determine how many pounds of force is being applied to the nose. Decreasing the vertical surface of the nose has the double effect of lowering the air pressure in front and also decreasing the area of the body which is exposed to the highest pressure anywhere on the car's body.



The two pictures above illustrate how the shape and airflow around the car affect the air pressure. The red zones are above atmospheric pressure and the purple zones are below.

The shape of the nose also does directly affect how far ahead of the car the air is disturbed, and in which direction the air moves out of the car's path.

A very low stagnation point will direct more of the air over the top, greatly reduce the air pressure and turbulence beneath the nose, and increase the pressure above the nose, which together have a dramatic effect on downforce and stability. The lower nose will also push less air ahead of the car.

Many of the new cars are also tapering the nose much more in the overhead view, angling the headlights sharply back to direct more air out to the sides. This also reduces the surface of the nose which is perpendicular to the airflow, and at the same time decreases the turbulence at the sides of the car.

It is true that lengthening the car increases friction, but friction is a quite small fraction of the total air resistance.

[NeilBlanchard]

Hi,

What I hear being said is that the nose can be blunt, or it can be more pointed -- and the results are very similar. If you get too pointed (even the GM car above has a blunted leading edge), or too long a nose, and it starts to add drag. (What is the Cd of this car?)

After the nose is work well, it's the trailing edge that is harder and more exacting to do.

Our intuition is not always correct: what car has a lower Cd: a 80's Corvette (pointy) or a Durango (blunt)? A: the Corvette has a Cd of 0.45 and the Durango is 0.39.

[Edit: the EV-1 is in the blunt camp, I think; as is the Aptera.]

[MetroMPG]

Originally Posted by Bicycle Bob

Getting the air to part is like pushing a pen through a sweater

Yes, but are we talking about a pointy pen tip or a blunt one?

Originally Posted by hypermiler01

There are some who focus strictly on Cd and seem to ignore air pressure

Could you please explain what you mean by this a bit more?

The measure of air resistance ("Cd") is inherently about pressure - pressure drag comprises the largest fraction of drag a road vehicle encounters.

So a vehicle with a low Cd is better at "managing" air pressure than one with a high Cd. It isn't possible to talk about drag coefficients of cars and "ignore" air pressure.

[Cd]

I'm really dense, because I'm not understanding what you guys are saying about how that air pressure affects the Cd of a car. ( If at all )

If you have two cars with an identical Cd*, but one has a blunt nose, how does the larger area of high pressure at the front of the nose affect the car ?

From what I gather, it has an effect on handling, but what negative effects are there ?

( *for example the LS400 and the new Corvette, both at .26 Cd if I remember correctly - nevermind the larger fontal area of the LS400, I'm just talking of Cd here. )

[Cd]

Originally Posted by Bicycle Bob

I respect minds which question everything, but not if they hear nothing.

O.T. ( sorry ) Did you just make that up ? What a clever quote !

[MetroMPG]

Originally Posted by Cd

If you have two cars with an identical Cd*, but one has a blunt nose, how does the larger area of high pressure at the front of the nose affect the car ?

What do you mean by "affect the car"?

If you're asking about resistance to moving through the air, different shapes with identical Cd are subject to the same drag force (assuming identical A, etc.). That's what Cd indicates.

[hypermiler01]

Originally Posted by NeilBlanchard

Our intuition is not always correct: what car has a lower Cd: a 80's Corvette (pointy) or a Durango (blunt)? A: the Corvette has a Cd of 0.45 and the Durango is 0.39.

Faulty conclusion, which would only be credible if 100% of the Cd were determined by the first six inches of the vehicle length. The Corvette has a noticeable vertical surface area in front, many sudden, air-catching transitions along its length, a lack of proper radiusing of edges, and a HUGE wake.

The new Corvette has had those areas addressed, and has a Cd about half of the old Corvette, even though it is a similar overall size and shape.

But the old Corvette still has a lower CdA than the Durango.

Originally Posted by NeilBlanchard

[Edit: the EV-1 is in the blunt camp, I think; as is the Aptera.]

Some of you guys make it sound like blunt=blunt=blunt. "Blunt" isn't a measurement. There are varying degrees of "bluntness". If the Aptera were any less "blunt", it would be a public safety hazard.

I swear, I could show you a razor blade and some of you would get out your electron microscope and say "See there, it's BLUNT!!!"

[hypermiler01]

If two cars are exactly the same except for the nose, and one has a low nose and one has a high nose, then they are NOT going to have the same Cd. The one with the low nose will have a lower Cd.


A more valid comparison is this: