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450knotOffice's Avatar
 
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Question Cowl Shake. Can somebody please define it for me.

Sorry guys. I've done a few searches on this within this website and with Google and have gotten nothing that gives me a real definition.

Everybody talks about "Cowl Shake", but what the heck is it, really? How would you define it. How do I feel it when I'm driving? I know my driver's side door will rattle a little when I hit a sharp edged bump and I know that's due to some body flex but is that basically the only way to sense it, or does it manifest itself in others ways as well (that are noticeable)?

I read all the time about convertibles of all kinds having cowl shake and the efforts engineers put into diminishing it. But writers always seem to assume the reader knows exactly what they're talking about.

Myself...I have a vague notion of what it is but I would like a much better understanding of the term.

Thanks guys.

Old 10-13-2005, 11:45 AM
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Hi Scott,

It is when the body twists under suspension loading. Without the "triangulation" (of sorts) of a roof structure, the entire body is more prone to flexing when placed under torsional loads such as driveways, uneven pavement, or aggressive cornering (esp track). Manufacturers know this and reinforce the floorpan and add'l structures to minimize the flex, and in so doing also add weight to the car. Think of each end and corner of the car not being "fastened" as tightly together, and you'll get the idea. This is also why many opt for less-than-stiff suspension upgrades when upgrading their cabs and targas, and why the preference goes to coupes for track appliances.

Edward
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Old 10-13-2005, 11:59 AM
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I think he may have been looking for something more practical. Like, what does it sound/feel like when you are driving the car. How do you know you have it, etc...
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Old 10-13-2005, 01:43 PM
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To put a finer point on what Ed posted, it's undampened oscillations of the chassis from torsional flex.

In modern open cars, the hydraulic engine and trans mounts do a great job of damening these oscillations. In the '83-'89 911 Cabriolets, they used a special engine mount with a center pivot point, and a dampening strut to accomplish the same thing.


From the driver's seat it will feel like the steering wheel, which is attached to the front of the chassis, is wiggling back and forth opposite of the driver's seat, which is attached to the center of the car. It's subtle, but noticeable if you drive a cab or Targa back to back with a coupe over a bumpy, potholed road.
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Old 10-13-2005, 01:49 PM
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Quote:
Originally posted by Tyson Schmidt
To put a finer point on what Ed posted, it's undampened oscillations of the chassis from torsional flex.
That's just what I was going to say! (ok, I was going to say "undamped vibrations")

Quote:
Originally posted by Tyson Schmidt
In modern open cars, the hydraulic engine and trans mounts do a great job of damening these oscillations. In the '83-'89 911 Cabriolets, they used a special engine mount with a center pivot point, and a dampening strut to accomplish the same thing.
Does the (OEM) 911 cab have less "cowl shake" than the targa? If not, I guess it implies that the targa is a little stiffer than the cab since the factory chose to use this mount system on the cab.
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Old 10-13-2005, 01:59 PM
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Thanks for the definitions guys. Tyson, that was a nice description and picture you painted for me.

Since I've never driven a coupe, I have no basis for comparison, however my Cab's flex doesn't bother me at all when I'm pushing hard (on the street). I don't really feel it except when I hit sharp edged bumps and the car shudders a little bit. Maybe I'd notice it more on the track though.
Old 10-13-2005, 06:41 PM
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COW Shake

That when cows have mad cow disease and they start shaking before they drop dead
Old 10-13-2005, 06:57 PM
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He'll be hea all ze veek...
Old 10-13-2005, 07:03 PM
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Scott, I can't notice it on track with a Targa with stock suspension, though I do notice it on sharp edged bumps on the street as you said. I haven't driven a targa with big torsion bars on the track to see if I could notice it there...
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Old 10-13-2005, 08:13 PM
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The two vibrations are...

1. "The Wiggle" when one of the front wheels hits a pothole and the chassis twists and shakes it off like a wet dog.

2. "The Hoppity Horse" when you go over uneven pavement like highway concrete and you bounce up and down in your seat ever so slightly like a little kid on his Hoppity Horse.

I prefer to think of these not as design problems but rather as built in entertainment.

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Old 10-13-2005, 10:22 PM
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