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    hiremichaelreid New Member

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    It wasn't until a few days after I picked up my new Prius last week, that I noticed there are some interesting angles in the top of the roof. :eek:

    Are they really that hard to notice ? Anyone else ?

    Is there any known story behind the weird Prius roof ? Like aerodynamics or structural engineering issues ?
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    Patrick Wong DIY Enthusiast

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    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.

    Attached Files:

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    donee New Member

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    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.
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    PriusSport senior member

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

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    Wow. This car never ceases to amaze me.
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    tkil New Member

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    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...)
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    Patrick Wong DIY Enthusiast

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    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."
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    hiremichaelreid New Member

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    Thanks for the PDFs. Love the line: "Together with the crystal-like taillights, turn signal lights, and backup lights, they provide a futuristic and cool look." :cool:

    Yes, they don't specifically address those curves. I just found it so funny I didn't notice until I had the car a few days. And I obsessed over pictures for a month before, but these curves don't seem to show clearly in most pictures. Perhaps my black paint helps me see them with the bent light reflections ?

    I just looked more closely and there are 2 main humps, one on left and one on right. BUT, they are connected by a longer, much more shallow middle hump... And it appears that the middle hump changes in width from front to back. Very unusual... :nod:

    Yes, CD 0.26 is impressive for a mainstream passenger vehicle (Prius is in top 5 or so ?), but I was disappointed when I saw the CDs some of the exotics have. Like in the 0.1X vicinity. Still much room for improvement IMO.
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    hiremichaelreid New Member

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    Has anyone figured how much the larger spoiler on the Touring impacts CD or MPG or downforce ? Or is it just a visual enhancement ?
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    Bill Merchant absit invidia

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    The larger spoiler on the Touring model has about the same impact as the wheel trim rings on the Standard version. Zip.
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    bbald123 Thermodynamics Law Enforcement

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

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    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.
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    donee New Member

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    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.
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    Rokeby Member

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    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.)
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    hiremichaelreid New Member

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    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.
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    hiremichaelreid New Member

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    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.
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    RhythmDoctor New Member

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    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?
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    Rokeby Member

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    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.
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    patsparks An Aussie perspective

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    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.
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    hiremichaelreid New Member

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    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 ?

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