By on March 29, 2019

1988 Mercury Cougar Window Run Channel, Image: Sajeev MehtaTony writes:


This automotive design element has perplexed me for years. What is the small, usually black bump or protrusion on the front window glass run channel of certain cars? The 2010 Nissan Versa has the most obvious one that I have seen recently. Thanks!

Sajeev writes:

Son, this bothered me ever since my 10-year-old self ran his eyes across his father’s brand new 1988 Mercury Cougar XR-7! It’s high time we got to the bottom of this!

I went to the garage, took the above photo, rolled the Cougar’s window down and took a long, hard look at that stupid rubber stump on the glass run channel.

There’s no structural connection; it’s not an external cover for a metal clip to the door. It doesn’t provide significant structure to the channel, and the odds of it helping a closing window seal perfectly is unlikely. Similarly, if warpage is an issue, this ain’t gonna help.  And since I have no manufacturer PR contacts (somewhat proud of this), we can’t get a scrubbed answer from someone in the car design business.

I reckon these reduce wind noise as air transitions from the windshield to the A-pillar, much like the looney bits attached to modern lights (Nissan Leaf) and side view mirrors (Toyota Camry).

There’s a fancy SAE paper suggesting a metric ton of fluid mechanics analysis at this corner of automobile bodies, so let’s ask the Best and Brightest: what do you think the stump on a window glass run channel is for?

[Image: © 2019 Sajeev Mehta/The Truth About Cars]

Get the latest TTAC e-Newsletter!

30 Comments on “Piston Slap: Stumped by Stumpy Glass Run Channels!...”

  • avatar

    Obviously its there to keep you “grounded to the ground” :)

  • avatar

    I’ve always “guessed” it was something that aided in the installation process, being left intact to befuddle the best of us.

    • 0 avatar

      My thought was similar – it is something used to pull the part out of a mold… maybe? A tab for a worker to grab during installation? It almost seems to small to help in aerodynamics. However a high frequency whistle generated by wind noise would only need a tiny surface change to shift the frequency to something less annoying. So I am going with that.

  • avatar
    Arthur Dailey

    Look at that roof design and it leads you to a far more important question and one that I have posed to Sajeev. What happened to the ‘gutters that used to either be added to our included in the roofline of vehicles? Open that Versa after a snowstorm and the driver’s seat and the controls on the inside of the door become covered in snow.

    Why was this design element eliminated?
    And why haven’t designers created a new element to prevent this bothersome issue?

    • 0 avatar

      “Why was this design element eliminated?”

      bad for aero

    • 0 avatar
      Roberto Esponja

      “Why was this design element eliminated?”

      I’ve been asking the same thing about the darker tint on the upper part of windshields. Wish cars still had this.

      • 0 avatar

        ^This! +infinity! (Infiniti?!)

        That used to be a step-up from something like a Civic into the Accord. Now, even Bentley and Rolls-Royce leave this out!

        I’d also like to know who started the current industry obsession with rimless inside mirrors with weird, inverted-trapezoidal, “smiley” shapes that remind me of the facial expression of a demonic, demented clown, and how that was allowed to proliferate! Besides being fugly and reflecting glare off the edges, those mirrors have less visibility than the old-style, “rimmed” mirrors! (See the new Silverado and RAM pickups for examples of this!)

  • avatar

    Isn’t it used to redirect water coming off of the windshield? I have driven older cars both with and without the little bump.

    Those without the bump usually have water covering the entire side window while driving in the rain. Those with the bump seem to direct it into a small stream instead.

  • avatar

    On GM cars, it is used to encourage the glass to remain in the channel when opened and closed at high speeds. Try opening and closing the window at 100 mph and you will see what I mean.

  • avatar

    I am an engineer, but I am not an automotive engineer.

    I always thought it had to do with clearing the front side window of water drops. With modern cars having windshields that are recessed relative to the A-pillar and relatively flush side windows the high pressure area ahead of the windshield would cause flow separation down the upper sides of the car. These nubs act somewhat like vortex generators to keep flow attached to the side. This has the side benefit of reducing noise, but I think its primary purpose is to clear water. You can usually see the pattern of water clearing start at this piece and diverge toward the rear of the car.

  • avatar

    I’m pretty sure it’s Federally mandated and weighs about 275 pounds per side.

  • avatar

    If they item’s purposes include tweaking the car’s aerodynamics, then it’s called a “vortex generator”:

    This introduces a little bit of turbulence in order to make the air flow (and separate) more smoothly. Like the divits in a golf ball.

    Keep in mind that it may have more than one purpose.

  • avatar

    They even have a name: Blow-out tab.

    • 0 avatar

      I googled “Blow-out tab” and I got everything from a song by Radiohead to a “unique” service offered by some young ladies while waiting for your oil change. Nothing about widow bowing, but it made for some interesting reading/listening ;-)

  • avatar

    I saw on the Interweb that these are RDF transmitters placed overnight in the factory on the order of the Illuminati.

    Please keep this to yourself, we don’t want panic.

    • 0 avatar

      Well, that makes another of about a half-dozen transmitters already on or near my car. So, when someone says to “Get lost”, you can honestly say “that’s impossible”

  • avatar

    My 2003 Maxima is a smooth, aerodynamic, attractive car and at 40kph these things start whistling.

  • avatar

    Great answer to a car design enigma. Has someone covered the reason why the side view mirrors were moved from the A pillar to the door side on most cars? Some like Hondas went from A pillar to door then back to A pillar again. There must be a good reason for this.

    Another element of modern car design I’ve been thinking about is that piece of sheet metal that used to go under the tail lights. All new cars don’t have these anymore. The bumper cover goes all the up to the tail lights now. Used to be all cars had this piece welded on then the seam covered with seam sealer. I suppose this cuts costs. Makes me wonder who came up this because the whole industry has adopted it and why only now. They could have done this years ago but I only began noticing it in the 2000s.

Read all comments

Back to TopLeave a Reply

You must be logged in to post a comment.

Recent Comments

  • mcs: @toolguy: I have a proprietary lock ring wrench on the printer now. ABS + carbon fiber. I have the real aluminum...
  • deanst: Is hoping for a hybrid wagon manual expecting too much?
  • Lie2me: No kidding, I call those old Jeeps “ball-busters”
  • Corey Lewis: Both those are good choices.
  • wolfwagen: TUI/TWI – Typing under the influence/Typing while intoxicated or Drowsy typing?

New Car Research

Get a Free Dealer Quote

Who We Are

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