Story of the weird Prius roof ?

Hi Michael,

Now you can see why a factory sunroof is not offered.

I attached a couple of excerpts from the 2004 New Car Features Manual that may be of interest. However they don't specifically address the curves found in the roof panel.

 

Hi Michael,

Yep, the roof shape is there to help maintain laminar air flow over the top of the roof. The depressed center keeps air from tumbling over the sides of the car as energy is lost in the flow due to friction.

 

The drag coeficient of .26 is lower than a Porsche (.29) and most other sports cars.
The car is incredibly well-engineered.

 

Wow. This car never ceases to amaze me.

 

Note that this is intentional (on the part of both Toyota and Porsche engineers). Sports cars regularly trade higher drag for increased downforce (and hence better grip for acceleration and cornering).

F1 cars have stupidly high drag, but it's also said that they generate enough downforce to stick to the ceiling (at sufficient speed, of course).

Also, the coefficient of drag isn't the entire story; you have to multiply it by the frontal area to the the "area of drag". I don't know of a good free web source of that data, but one of the dead tree car mags publishes it. The Corvette actually has lower Ad than the Prius (and, unsurprisingly, it gets about 30mpg when driven gently on the freeway).

Not that I don't think the current-gen Prius is amazingly well-engineered; it's just that Cd is not the whole story.

(Remember, the Insight had even lower Cd...)

 

Regarding the exterior2 file, note:

• the color list on the third page is not correct - perhaps this list was intended for Japan or Europe markets
• regarding the description of the front view on the first page, I particularly like this:
  "The headlights consist of a simple, vertical two-level box shape, which gives the vehicle an intelligent and refreshing look."​

 

The larger spoiler on the Touring model has about the same impact as the wheel trim rings on the Standard version. Zip.

 

As do most wings/spoilers on street cars. They are generally too small or too poorly placed to have any effect whatsoever.

 

The Prius height counters its low drag. The Boxster, for example, is 4-5 inches lower, though it's drag coeficient is higher .29. The Corvette, as well, along with most sports cars. That gives them better road hugging ability. And excellent gas mileage on the highway. The Boxster will do in the mid 30s mpg on the highway.

On the opposite end, you have those high, boxy SUVs, not designed for economical highway travel. Not good highway gas mileage.

 

Hi All,

The design thought goes something like this: "We need to put the battery inside the car now (their testing showed that radiated heat from the roadway was frying batteries). Ok, but that will use up the trunk. Ok, how about we sit the seats more upright, and put the battery in a little nook between the rear seat and the hatch area? Ok, but then we will have a tall car with high cross section area. Ok, what can our aerodynamics and chasis people due to help that? -> .26 Cd "

So, if the battery was back in the hatch, they could cant the seat back like other cars, and lower the roof line. But then the cargo space would be much less. Kinda like the Camry hybrid. Which I hear all the time around here that people are rejecting the Camry hybrid because of the big box in the trunk. To which I reply - go get a Prius instead, it was designed to avoid that problem.

 

The weird depression in the Prius' roof was also discussed on ecomodder.com,
in March '08. Check it out.

Strange OEM Aerodynamic Shape (longitudinal roof indents)

The discussion sort of petered out without there being any real firm
conclusions as to whether the shape really is part of the aerodynamic
package.

If it is, even in the world of sub-sonic aerodynamics, it is very subtle.
And then again, if it is, that might explain why MPGs take such a vicious hit
when a roof rack is installed.

(Some years back, I had a Dodge long-body minivan with an OEM mileage
gauge. According to the gauge, it got better mileage with a DIY roof rack
with symmetrical airfoil shaped cross bars. MPGs went from ~20 to ~24 on
the highway, IIRC. Woo-Hoo! The racks stayed on 24-7 for eight years.)

 

My theory on the purpose of the spoiler

I have not been around here long enough to see any discussions on the reasons for the spoiler, but as highly optimized as the Prius is, I would expect it is something more than just looks. Why would you destroy the rear view like that unless you had a good reason for it?

I seriously doubt that the spoiler does anything to increase tire grip.

Here is my guess - the purpose of the spoiler is to reduce wind noise. Without the spoiler, there is likely to be turbulence and chaotic eddies at the rear stagnation point that causes wind noise. The spoiler forces a the air flowing over the car to separate cleanly and non-chaotically, cutting down on eddies that could cause noise.

Has anyone seen anything from Toyota to contradict this theory?

 

The ecomodder.com thread has a brief mention of NASCAR roof edge dams.
I've read about these, but can't find a cite/site right now. The topic came up
in a thread about some folks feeling that the Prius gets squirrely in a cross
wind. As I recall, the NASCAR dams prevent, or at least reduce the
likelihood that, the cars will go airborne should they get going crabwise,
sideways, at high speeds. The dams are there to disrupt crosswise flow, not
channel front-to-back flow.

Maybe, just maybe, the ridges act like those dams in a subtle way. I was
thinking abut the roof bumps today, and at one point I was parked next to a
Matrix, which to my eye has a Prius-like shape. It does not have the bumps.
But it does have rather pronounced ridges, say ~1-1/2 inches tall, ~3/4 inch
wide, at the roof edges. So what's that all about?

Back in March when I first read the ecomodder thread, I made it a point to
check out other car brand roofs. I did find the bumps on another make of
car. It wasn't exotic or anything. Sadly, I don't remember what it was.
Maybe a Nissan Versa? I'll have to start looking again.

 

The reason the roof dips in the middle is to minimise frontal area while giving maximum head room on either side where the driver and passenger's heads are.

The Cd of the Prius is 0.26, several people have found cars with matching Cd figures. Now has anyone found a current 4 door, 4 or 5 seat car with as low a Cd? I bet you can't.
I found one some time back, I believe it was made in the 1950s.