Story of the weird Prius roof ?

[bbald123]

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

[PriusSport]

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.

[donee]

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.

[Rokeby]

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.)

[hiremichaelreid]

Originally Posted by donee

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 "

Yes, I've gathered that these trade-offs were made. I and my family DO like the feel and reality of interior space, and the hatchback space is wonderful for a family of 4. I was originally considering a station wagon, so this is a plus for me.

That said, I think it would be nice to see a future "Prius family" model that is lower, more aero and is more of a 2 door sports coupe. I'm not sure I'd buy one, but for single people and the higher efficiency seekers, I think this would be wonderful.

[hiremichaelreid]

Originally Posted by Rokeby

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)

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.

Interesting. Among "Style, Stiffness, Aerodynamics", I'd favor stiffness and aero, rather than style. My reasoning is that I haven't seen any Toyota photos or videos that show this feature, and I have not seen it mentioned anywhere. IMO, if it was a style feature, it would be trumpeted in the ads or specs.

Since they don't trumpet it, I feel they may be almost hiding this feature, perhaps due to concern some might find it weird and off-putting.

Yes, bending metal instead of leaving it more flat increases stiffness AFAIK. Perhaps this helps explain it.

I'm most willing to believe it was engineered for aero, and the roof rack experience (extent of MPG loss) I think may help to indirectly confirm that hypothesis. I.E. there are subtle effects that can be destroyed with "un-approved" modifications to the Prius silhouette.


I'm still not ready to discount any beneficial aero from the rear spoiler. In fact doesn't Toyota claim this improves aero ?

Yes, rear spoilers on most cars are for looks only, and where useful for downforce, only help at very extra-legal speeds from 120 MPH up say.

[RhythmDoctor]

Originally Posted by hiremichaelreid

I'm still not ready to discount any beneficial aero from the rear spoiler. In fact doesn't Toyota claim this improves aero ?

Yes, rear spoilers on most cars are for looks only, and where useful for downforce, only help at very extra-legal speeds from 120 MPH up say.

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?

[Rokeby]

Originally Posted by hiremichaelreid

Interesting. Among "Style, Stiffness, Aerodynamics", I'd favor stiffness and aero...

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.

[patsparks]

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.

[hiremichaelreid]

Originally Posted by patsparks

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.

That reason makes so much sense IMO, I'll believe it. Just personal opinion or did you hear that somewhere ?

So these are like mini head bubbles... If the bubbles were more pronounced and transparent they'd look very 1950-1960s retro-future, like the Batmobile or various concept cars of the era.


Yes, I'm still disappointed with the CD of 0.26. Hopefully auto manus will improve on this and find ways to blend style with aero. I don't recall where I saw a list of low production and experimental cars, but some were in the low 10s, like 0.12 or so.

According to Drag coefficient - Wikipedia, the free encyclopedia a "smooth sphere" will give us 0.10, so why not more spherical cars ? Or how about an "F-4 Phantom II" shaped vehicle with a CD of 0.021 ? How much mileage could we get at 80 MPH with that CD ?