Piston Slap: Stumped by Stumpy Glass Run Channels!

Sajeev Mehta
by Sajeev Mehta
Tony 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]

Sajeev Mehta
Sajeev Mehta

More by Sajeev Mehta

Join the conversation
9 of 30 comments
  • VWGTI VWGTI on Mar 29, 2019

    They even have a name: Blow-out tab.

    • Lie2me Lie2me on Mar 30, 2019

      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 ;-)

  • Zipper69 Zipper69 on Mar 29, 2019

    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.

    • Lie2me Lie2me on Mar 31, 2019

      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"

  • Lightspeed Lightspeed on Mar 29, 2019

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

  • Ferd Ferd on Mar 30, 2019

    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.