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Front End "Bump" Feeling In Reverse

 
Old 11-02-2013, 04:42 PM
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CardiffDweller
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Default Front End "Bump" Feeling In Reverse

I'm a long term lurker, new poster. I've enjoyed the forum. I'm driving a 991 C2S Cab. I've noticed a strange little bump coming from the front axles/wheels/tires when reversing and turning out of my driveway or basically almost any parking spot. Sometimes it is more noticeable than others. But it manifests most when reversing and making a more or less right angle turn. It does not occur when reversing in a straight line. The best way to describe the feeling is of running over a rolled up newspaper with one tire at the front end.

Has anybody else experienced this effect? I know it's happening and not a figment of my imagination but I wonder if it is just a quirk of the car and the new electric steering.
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Old 11-02-2013, 04:51 PM
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mi650
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I've noticed this on other vehicles, but not my C2S yet. Always seemed to me like the tires 'skipping' when fully turned.
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Old 11-02-2013, 04:52 PM
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rpilot
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Welcome to the forum !

I am not clear what you are trying to describe. Are you by any chance talking about the tire skipping when the wheel is turned fully?
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Old 11-02-2013, 05:07 PM
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Same with my c2s cab
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Old 11-02-2013, 05:20 PM
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LexVan
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That's normal I forget what it's called and what causes it though.
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Old 11-02-2013, 05:56 PM
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CardiffDweller
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Yes, it does some times feel like a skip. It sounds like this is normal. I assume this is caused by the forces generated in the steering arms when you are near lock making a sharp turn, combined with the lighter weight at the front end and the big tire size. I wonder why it doesn't happen in a forward gear, though.
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Old 11-02-2013, 06:49 PM
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dodo1porsche
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that is Pirelli summer tires operating at low temp (~45F). there is no bump in warm months.
I mounted the winter set and it went away completely.
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Old 11-02-2013, 06:50 PM
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This is very common on 981's with cold tires, at/near lock in reverse, just as you describe. I experienced it myself in my 981 but leaned to pull out of my drive different in my 991 to avoid it.

Something about tire angle and tires skipping, it's well documented and wouldn't worry about it.
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Old 11-02-2013, 07:01 PM
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This is an unavoidable and normal characteristic that arises from the combination of wide tires and suspension alignment, notably caster angle. http://en.wikipedia.org/wiki/Caster_angle
Brings up the subject yet again of trade-offs. Setting up a car to handle the way 991 drivers expect necessarily entails tread-offs, some of them (like your tire hop) completely unexpected by the average guy who just wanted a fast car, hold the physics.
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Old 11-02-2013, 07:02 PM
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chuck911
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Originally Posted by j080808 View Post
I experienced it myself in my 981 but leaned to pull out of my drive different in my 991 to avoid it.
Its completely harmless, by the way.
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Old 11-02-2013, 11:18 PM
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I notice this in my car as well...think it is exacerbated by the AWD?
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Old 11-03-2013, 01:28 AM
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Originally Posted by dodo1porsche View Post
that is Pirelli summer tires operating at low temp (~45F). there is no bump in warm months.
I mounted the winter set and it went away completely.
+1

I treat this as a reminder to myself to slow down! Tires are not as sticky anymore when cold.
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Old 11-03-2013, 01:36 AM
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MarcusG
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I like LexVan's answer.

He say's it's normal and doesn't know why or how but for some reason I believe him not to worry about it!

So I won't.

Maybe it clunks because race car.
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Old 11-03-2013, 03:42 AM
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chuck911
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Originally Posted by Hammer911 View Post
I notice this in my car as well...think it is exacerbated by the AWD?
Its completely due to suspension geometry, awd would only be a factor if Porsche uses different settings for the awd cars. This might even be the case because with power going to the front then the effect of caster angle on torque steer becomes a factor. But even then it would have nothing to do with the front differential or any of the awd components. It would still be entirely due to suspension goemetry- caster, toe, camber.

Some awd cars and trucks will act similar but due to the differential. Turning on a tight radius the outside wheel travels further, the differential restricts the extra rotation causing the outside wheel to hop, because essentially its skidding. In that situation it is caused by awd. What's happening with the 991 - 2wd or awd- is different.

Hard to explain in a post but I was able to find this video which is pretty good. Notice at about 1 min he's showing a tire patch and talking about what happens with the car going forward. The tighter the turn the greater this offset will be and the more the steering will try to self-correct back to straight. I'm sure everyone has experienced the squealing tires when turning in a parking garage even at slow speed. Its caused by this effect. On street pavement you'll feel the outside front feathering or vibrating slightly. Its actually sliding just a little bit, and if you study the diagram I think you'll be able to understand why.

So going forward in a tight turn caster puts the tire patch way outside the caster (geometric angle of steering rotation, the red dot) which causes the outside tire to really work. Going backwards its much worse. Now instead of trying to self-correct the steering straight ahead, this force is trying to increase steering angle. Look at the diagram, see how the tire patch is outside of the red steering axis (caster) dot. He only draws one tire, but if that one tire is the right and you imagine another left tire drawn beside it, the whole thing going backwards, it should be easy to understand that the tire patch is going to be scraping along an arc not rolling smoothly. Its this scraping or skidding action that causes the hopping, and you can probably tell by the diagram that the wider the tire the more noticeable the effect will be.


Last edited by chuck911; 11-03-2013 at 03:46 AM. Reason: L/R, R/L, whatever
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Old 11-03-2013, 08:29 AM
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Originally Posted by CardiffDweller View Post
...

The best way to describe the feeling is of running over a rolled up newspaper with one tire at the front end.
...
Good description of the feeling. As others have posted, this is normal. I had the same thing happen with my previous 996 and my current 997.2.

BTW, it does make it more difficult to read the newspaper though.
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