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Old 06-08-2020, 07:10 PM   #1
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Question 986 Stability Control system

I experience too much yaw after hitting, say, a shallow pot hole and such. Is it that my PSM is not functioning right or is it the system it "governs"? Drives great otherwise so I think it's the PSM.

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Old 06-08-2020, 08:14 PM   #2
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Originally Posted by SamPedro View Post
I experience too much yaw after hitting, say, a shallow pot hole and such. Is it that my PSM is not functioning right or is it the system it "governs"? Drives great otherwise so I think it's the PSM.
Yaw?


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Old 06-09-2020, 05:16 AM   #3
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Yaw?


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https://www.aceable.com/safe-driving/what-is-yaw/

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Old 06-09-2020, 08:32 AM   #4
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Buys a Ferrari and forgets Google exists.

https://www.aceable.com/safe-driving/what-is-yaw/

ha! yer killin' me here, smalls!

I know what "yaw" means (at least I think I do). But my definition of yaw (as well as te definition you've linked us to) doesn't seem to fit what the O.P. is describing. IE: hitting a pot-hole isn't likely to make you "spin around its center of gravity".

I wanted to hear from SamPedro what he feels like "yaw" is. (MY heritage is Scottish & Welsh... don't get them started on what "yaw" is. hahaha)
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Old 06-09-2020, 09:11 AM   #5
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could be that he is referring to a worn out lower control arm..?
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Old 06-09-2020, 10:01 AM   #6
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Probably toe adjustment is off. Have your alignment checked.

Or what Gilles said. Loose suspension components can do that too.
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Old 06-09-2020, 05:06 PM   #7
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Yaw

I can sense a 5 or so degrees away from the cars linear tracking. No spin just a yaw of the nose and tail of the car about the center of gravity (the mass right behind me ie. the engine/tranny is nominally the "center of gravity"). This yaw is controlled, among the car's other motions, by the Porsche Stability Management system when sensing imminent danger such as skidding etc.

On good pavement there is no steering pull, suspension clunking, rattling nor squeaking; My lil roadster drives great [on good pavement]. So, is this yaw I sense happening because the PSM is not functioning correctly? Or, is there something about the car's suspension I'm not seeing. Hense; this post.
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Old 06-10-2020, 04:10 AM   #8
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That aceable.com description is perhaps the worst definition of yaw I've ever read. The wikipedia definition is much more accurate: https://en.wikipedia.org/wiki/Yaw_(rotation)

As SamPedro describes in his 2nd post, yaw isn't always a spin, it's just a turning motion -- anytime the vehicle rotates at all about the z (vertical) axis, that's yaw. If you drive in a complete circle, you've completed 360 degrees of yaw, regardless of the radius of the turn -- a freeway ramp, a parking lot, or a donut.

Anyway, I would suspect the alignment is a likely cause, either too much toe-in or toe-out on the wheel hitting the bump or pothole. If the PSM were doing anything to cause or correct the yaw, I think you'd get a flashing light on the dash during the PSM activation.
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Old 06-10-2020, 10:44 AM   #9
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PSM shouldn't engage from hitting a regular shallow pot hole in the road because the car shouldn't rotate very much at all (I don't want to say zero but the amount of rotation due to a pot hole should be close to zero). The suspension should soak up that shallow pot hole and the car should continue forward in a straight line. PSM shouldn't engage until the yaw angle and tire rotation rates indicate that a slide is imminent. I don't think that you're getting close to the point of sliding the car so PSM shouldn't engage.

If you can feel some rotation after hitting a pot hole, it more likely means that the car is wallowing. That doesn't mean that its bouncing up and down like a stanced Honda Civic. What I am mean is much more nuanced and is characterized by a feeling of uncertainty in the forward direction of the car due to what should have been a vertical motion in the suspension being transferred into a diagonal chassis motion. You might feel this as induced yaw.

My 911SC did this. When I hit a bump or even a rise in the road at >80mph, the car felt like it was going in several directions at all once. In my case, new shocks and a proper alignment fixed it.

If this is the case, then the likely causes are worn suspension components, worn shock absorbers, wrong tire pressures, poor alignment, and worn tires.
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Last edited by thstone; 06-10-2020 at 10:51 AM.
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Old 06-10-2020, 01:44 PM   #10
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Thanks for that PMS and suspension info. I'll just have to have all four corners checked out by me (I'm 76 & the task is taxing) or my local German Cars mechanic; which might cost some bucks but well worth doing.

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