By on December 19, 2017


BMW X2, Image: BMW Group

With reader feedback always on my mind, perhaps an overview of commonly used terms in the car design trade is needed. Let’s define the Daylight Opening (DLO) and dig into one of the more confusing terms in a car designer’s handbook.

Daylight Opening likely originated from architecture, as the first designers (a.k.a. coachbuilders) applied such techniques to create bespoke passenger cabins elegantly resting atop a rolling chassis. While most coachbuilt efforts lacked the brilliant integration of the 1937 Delage D8 120 S Pourtout Aero Coupe (pictured above), the marching of time ensured even the entry-level Geely HQ has an integrated form: therefore our DLO definition needed a re-think.

Simply stated, the Automotive Definition of Daylight Opening is the collection of glass in a car body, discussed as a single element from the front, back or side.

The US DOT provides mind-numbingly complex more precise definitions for body side and the front/back DLO, but we focus on the side as it’s a far more expressive place for a designer’s touch. Well, 1996-1999 Ford Taurus notwithstanding, the body side is usually the most expressive.

So today, DLO is commonly referenced when describing the large swath of glass down the body side, between the first and the last roof pillar. An appealing DLO makes your eye move quickly from front to back, complementing other styling elements, and visually speeding up the roof so it completes a vehicle’s sleek styling.

In the case of the BMW X2, the DLO does a fantastic job transitioning the muscular body side into its short rear glass and chiseled posterior.BMW X2 Side View, Image: BMWToo bad the X2 has a black plastic cheater panel at the end of the DLO, giving the false appearance of a longer, sleeker glass element. (My not-so-loving nickname for such phenomena, DLO FAIL, requires a separate post.)

That said, never forget that doing the opposite, making the DLO more upright and architectural often projects the right image for a design, with no need for cheater panels: take the Land Rover Discovery.

The Disco’s double window treatment between the C and D-pillars is certainly an extreme example, not to mention recent changes to roof-crush standards (and head-curtain airbags) permanently altered our landscape: it’s likely stomped out a fair bit of DLO creativity.

Take the sedan’s DLO: Rolls Royce Phantom aside, most sedans now sport fastback rooflines worthy of Steve McQueen and their DLOs are forced to match, even when the aforementioned cheater panels are needed to make it work. Perhaps that will change over time!

[Images: BMW]

Get the latest TTAC e-Newsletter!

22 Comments on “Vellum Venom: Daylight Opening (DLO), Defined...”

  • avatar
    Arthur Dailey

    Stone me as a heretic, but I prefer the clean lines of the pictured Taurus to those of the X2 and believe that they will age much better.

    The Delage D8 120 S Pourtout Aero Coupe is however a thing of beauty. We laypeople do not generally give adequate acknowledgement to the fantastic work of the pre-War French automakers.

    • 0 avatar

      Totally in agreement with you about the Taurus v the X2, Arthur Dailey. More generally, I find DLO that doesn’t provide much opening, to be ugly. When the window looks too small to provide a decent view, it looks ugly to me. All the fortress-like cars of the current decade look ugly.

  • avatar

    I had to chuckle, clicking on “read more” and going from that beautiful classic to the a** end of a Taurus.

    I appreciate these stories about design. I understand mechanics, but how they actually plan these things out has always been a mystery to me. Thank you.

    • 0 avatar
      Menar Fromarz

      Thanks for the article, however in truth, I have always been as much appreciative of an actually useful DLO from the INSIDE, looking out. A clean and well crafted design that shows few DLO “fails” from the exterior view can be (and usually is) nullified and rendered unusable if the vehicle is a rolling bomb shelter of visibility, when trying to look out. You know what I’m talking about, not just intrusive pillars, but more frequently that, PLUS glass masking that renders what appears to be a generous amount of greenhouse useless, or negatively impacted by these ” masks” You see them everywhere, rear windows, side lights, even from windscreens are marred by them. I personally have more issue with these than a outward design DLO ‘fail”. Its one thing the full size trucks have still: reasonable visibility from within. Another reason for most “car” segments decline, IMHO.
      Another reason to bring back true hardtops, dammit!

  • avatar

    I responded too late for most people to see my comment on your dash-to-axle article, and I don’t want to steal your thunder on your “DLO Fail” article; this is just my segway between the two:

    Both the 1937 Delage D8 120 S Pourtout Aero Coupe AND the Land Rover Discovery have a generous dash-to-axle ratio AND adequate space between their door and the rear fender. So, no filler panels needed on the front or back of the DLO opening.

    The Chrysler “cab forward” design first came out while the Taurus team was working on the 1996-1999 Ford Taurus. But, fortunately, they were still using the same platform as the previous two generations of the Ford Taurus, so the firewall, or dash-to-axle ratio, was fixed in place. So, they tried to achieve the “cab forward” look with a more sweeping windshield, but with the result that there was no plastic panel needed to move the front of the door back enough so that it would go down all the way.

    However, the rear fender cuts a big chunk out of the door; on the previous generations of the Taurus, you could not roll the rear window down all the way as a result. On this generation, they added that fixed panel so you could; but the C pillar is still upright enough that no plastic panel was needed to make the window opening flow into the roof line.

    The A pillar is far enough back on the BMW X2 that no plastic triangle or fixed window glass is needed on the front door. But the combination of the door cutting into the fender AND the sweeping roof line results in both the fixed pane of glass in the rear door and the plastic insert in the C pillar.

    I was just looking at pictures of the Chrysler Concorde — despite the cab forward design, the A pillar is still far enough away from the front fender that the door cut line is vertical from the A pillar, so no filler panel was needed. But, the rear wheels were set far enough back from the rear doors that no fixed window glass nor plastic panels were needed. It has a very clean DLO opening.

    But, reducing the dash-to-axle ratio as well as making the greenhouse more fastback or coupe shaped are both causing designers to make DLO failures trying to tie the design together within the constraints of space.

    (I also think the Taurus ages better than the BMW X2.)

  • avatar
    Sam Hell Jr

    A question, maybe one better-suited to a different article: in any given vehicle segment, what’s the relative price point at which there’s enough money for clean DLO? Or from a different angle: at what level of vehicle do consumers “demand” no DLO fail, even if they aren’t specifically familiar with the concept?

    • 0 avatar

      I think a clean DLO is more a function of car length, then a price point; see my comment above. The BMW X2 is not a cheap car; but it’s compact design forced the designers to use the plastic panel to make the DLO flow with the roofline.

      On the other hand, long cars like the older Taurus and the Condcorde had enough space between the axles and an upright enough C pillar that no plastic panels or fixed glass was needed to make the doors and windows work with the roof line.

      Likewise, the Disco was not long, but the greenhouse was upright, so again, no panels or fixed glass was needed.

  • avatar

    I’d love to understand the “why” behind these DLO fail triangle design decisions. Many vehicles have them in areas that to me -as a non car engineer, but as a graphic designer- don’t make much sense. Especially up front where a slight change in sheet metal, a different cut line, an angled cowl, better mirror location, etc… could get rid of them. I realize in many cases the windshield rake is the cause, thus for safety and aero reasons you are forced to work with certain angles. Most fails occur at a 3 way junction between the windshield, hood and door. Some add the mirror for a 4 way combo. And honestly if you can’t hide something using 4 elements then you have clearly FAILED. I saw a newer mini van or CUV yesterday that despite having all 4 elements still had a plastic triangle!

    The rear cheater panels make more sense, the rear door and window combination is often limited by the wheel arch and if you want windows that roll down in the back you have to split the glass. Attempting to give the door a sharp crease at this point is just asking for trouble so I can see why a small bit of plastic gets inserted there. But up front? Sorry but someone needs to explain why this happens so often because I don’t get it.

    • 0 avatar

      I’m no designer but I think it is a case of “good is the enemy of great” – the designs being put out are “good enough” because most consumers don’t notice or care. Plus much of this work is being done on fairly low profit margin vehicles like sedans and if that plastic triangle saves a penny then so be it.

      • 0 avatar

        Yup. Design (for anything) is always a balance between style and cost. DLO FAILs are excusable on cheaper cars, it is when they pop up on incredibly expensive machines that it becomes most egregious.

        • 0 avatar

          If cost really is the reason, then how come the VW Jetta can pull it off? It’s inexpensive but it has perfect DLO at the front and back despite the low cost and FWD proportions.

          The cheaper VW Polo has a plastic triangle at the front, but then it’s cheaper than the Jetta. Most other VW cars (all of them?) have perfect DLO. It’s most likely a design philosophy issue rather than cost.

    • 0 avatar

      Someone pointed out in the dash-to-axle ratio article that in times past, if you drew a line following the A pillar, it would intersect the front axle. Paying a casual glance at the Fords in my display case, that appears to be the case in most instances.

      However, look at the trio of the current generation of Ford Focus/Fiesta/Ecosport. In all three cases, the line of the windshield in fact runs in front of the front axle; roughly to the air dam.

      The front door cut line has to be nearly vertical because of the door hinge. So, when you run a vertical line up to the windshield; you end up at a point behind the front tip of the DLO. Hence, you end up with a plastic triangle ahead of the door.

      Same story with our 2007 Dodge Durango — draw a line from the A pillar, and it ends up going through the air dam instead of the front axle. It too has a big plastic triangle. So moving the windshield forward, and sloping it back to where it no longer is in line with the front axle results in a front triangle because the cut of the door has to remain vertical (unless we are talking about gull wing front doors.)

      I think door mirror and hood cut place less of a role. Most designers place the mirror on the front triangle because it looks and fits well there; but wind tunnel studies show that mounting the mirror below the hood line on the door is quieter, and results in less drag.

      (Going back to the 1996-1999 Taurus, some designers noticed this and wanted to mount the mirrors on the doors; but they were ruled out by those who wanted the conventional mount on the front triangle. Both the clamshell door lips and the mirrors are major sources of wind noise on both my 95 Taurus and 07 Durango.)

      That is a benefit of the Focus/Fiesta/Ecosport design; the high hood line results in a triangle that is low enough that it is below the hood line; so you get the benefits of the low mirror installation while still mounting it on the triangle. Many modern designs are the same way.

      I think the hood cut line plays a lesser role; I think there are many good designs where the hoot cut line just run straight up to the windshield; and that is it.

  • avatar

    DLO is, sort of, the negative space between pillars.

    Sometimes, designers start with the DLO and work the pillars to fit. Other times, designers start with the pillars, and try to make the best of the DLO.

    Typically, for a classic look, the C-pillar should have some relation to the rear wheels. Either the visual center of mass of the pillar should be over the wheel, or the edges of the pillar should be tangent to the wheel. A rare car manages both, like a 76 Seville. That can drive DLO shapes.

    Other cars like the 96 Taurus, the DLO was penciled in first, then the C-pillar shapes left to fend for themselves.

    Other cars, like a 71 Chevrolet Impala, manage to get the DLO shapes done very nicely, and the pillars as well. Especially when the glass is down, even on 4 door hardtops, these cars combine a strikingly handsome DLO with classic A & C pillars.

    Thee are few NHTSA rules about visibility (I would argue that there should be more, but I’ve lost that argument every time) other than wiper coverage, so blaming DLO, and windshield shapes on the government is just an excuse for poor design.

    The same for aero. Aero has little or no effect on CAFE, which is the only fuel economy that car makers care about. In real life, other things, like grill shutters, make more of a difference than windshield angle.

    Plastic triangles are sometimes a design call, and sometimes an engineering necessity to get eh glass to drop. More steeply raked windshields make it more difficult to get the glass into the door, hence bigger triangles. There are pros and cons for putting the mirrors on the door or the plastic triangle. The whale-bodied Chevy Caprices had them both ways. Neither way helped out that car much.

    I’ve worked in styling studios, and engineering groups, and mid level management. Believe me, styling calls the shots for appearances, and engineering makes it work, somehow. So, when a car’s design fails, it’s the designers, and no one else, who are at fault. I think designers, especially younger ones, get a pass despite the fact that far too many of them are just not that good. The fact that they all came from the same few schools doesn’t help, either.

    • 0 avatar

      Which dovetails into my above comments. When the A pillar and C pillars were tangent to the front and rear axle respectively, and the DLO opening followed the roofline; you end up with a clean DLO. The Chrysler Concorde example I gave earlier was an example.

      But, as they sloped the front and back glass more and more to where they are more in tangent with the front airdam and trunk opening, in order for the DLO to follow the roof outline, you had to have a triangle on the front and back.

      • 0 avatar

        I already pointed out above how when designing the 96-99 Taurus, the engineers wanted to mount the mirrors on the doors because it was a quieter design; but the stylist insisted on mounting them on the triangle instead. I have noticed that most cars now nowadays have the mirrors mounted on a stalk that is below the hoodline, so it is not in the flow rolling off the cowl, and thus cutting down on wind noise.

  • avatar

    Until recently, I owned a 91 Toyota Cressida, and that car’s DLO was so expansive it felt like you were driving convertible, especially with the sunroof open. It was one of those cars you could easily rest your arm on the window-sill, literally a greenhouse. Replaced that car with a 2003 Nissan Maxima, which is nearly as bright and airy feeling. I can’t even sit in a new Camaro without getting claustrophobia.

  • avatar
    punkybrewstershubby aka Troy D.

    I find it interesting that cost was brought up as a cause of DLO fail. My ‘17 Versa S has exactly ZERO plastic panels anywhere in the DLO area, and it’s the least expensive automobile sold in the United States.

    • 0 avatar

      I am glad an actual Versa owner mentioned this so I didn’t have to. The ’06 Versa/Tilda proves that you don’t need to spend a lot of money to have a DLO free of Fail. The latest Versa sedan (not the Note) has a little plastic bump on the A-pillar, but even that’s commendable considering the price.

  • avatar

    Thanks for the writeup – I always enjoy your postings.

    Another oft discussed design element would be the Hofmeister Kick: Does the term only apply for BMW’s? When is a Kick a kick? Are there angle restrictions. Do only RWD sedans apply? Mud is thrown almost as much as guesses.

Read all comments

Back to TopLeave a Reply

You must be logged in to post a comment.

Recent Comments

  • 28-Cars-Later: Ignorance must be bliss.
  • slavuta: “why should we support Putin?” Why do you feel that you support Putin? We’re free country...
  • slavuta: I think, a lot would be different. sure. Presidents must stop listening to the press.
  • slavuta: “most Americans are not that anxious to be part of a dictatorship” I came to America to be a...
  • slavuta: Jeff, I know only enough to imagine what is going on. But let me throw this at you from Q: Why...

New Car Research

Get a Free Dealer Quote

Who We Are

  • Adam Tonge
  • Bozi Tatarevic
  • Corey Lewis
  • Jo Borras
  • Mark Baruth
  • Ronnie Schreiber