Wheels "knock" on full lock?

[SGoody1]

Hi,
Another question.
I have noticed when my I put my wheels on full lock - either reversing or going forwards, I seem to get a knocking or thudding, and it almost feels like I am hitting something like a curb - but I'm not.
Any thoughts on what that could be? Should I get it seen to? Is it likely covered by warranty? Thanks

 

[pwebb]

Hi, a well known effect of wide tyres skipping on the road surface on full lock, worse in cold weather on cold tyres. It’s known as the ‘Ackermann effect’

 

[Lancerlot]

I assume this happens during low speed manoeuvring. As Peter says its quite normal and most 911's do it. The Ackermann Angle built into the front geometry mitigates the different radii the front tyres must follow during cornering.

However, if it is happening at normal speed on road journeys, you should have it checked out ASAP since it may indicate a loose, damaged or defective chassis component.

Regards,

Clive

 

[SGoody1]

Hi, a well known effect of wide tyres skipping on the road surface on full lock, worse in cold weather on cold tyres. It’s known as the ‘Ackermann effect’

Thanks - it worried me - quite dramatic but only at very slow speeds, eg reverse parking / 3 point turns etc. All fine at speed, no issues, but am never at full lock at any speed anyway. And it has seemed worse the colder it gets. But thank you for letting me know - I was about to book it in!!

 

[SGoody1]

I assume this happens during low speed manoeuvring. As Peter says its quite normal and most 911's do it. The Ackermann Angle built into the front geometry mitigates the different radii the front tyres must follow during cornering.

However, if it is happening at normal speed on road journeys, you should have it checked out ASAP since it may indicate a loose, damaged or defective chassis component.

Regards,

Clive

Thanks - also my car has that option where the rear tyres also turn - maybe that makes the Ackermann thing worse? It has never happened at any speed over about 3 miles an hour.

 

[Timrice]

I don’t think RWS makes much difference, but cold weather and worn tyres do (i.e.when the tread is stiffer).

 

[SGoody1]

I don’t think RWS makes much difference, but cold weather and worn tyres do (i.e.when the tread is stiffer).

The tyres are actually almost new (Michelins) - only put them on a month ago or so - but maybe they are stiffer because of that?

 

[Scrounger]

It's just the Ackermann effect as mentioned above. It doesn't matter how old the tyres are its just the way the cars geo is set up. I wouldn't worry about it too much. It's not only 911s that do it, my 718 does it as well.

Dan

 

[ianthow]

Thanks - it worried me - quite dramatic but only at very slow speeds, eg reverse parking / 3 point turns etc. All fine at speed, no issues, but am never at full lock at any speed anyway. And it has seemed worse the colder it gets. But thank you for letting me know - I was about to book it in!!

As others have said, they all do it when moving off with the wheels turned, not necessarily on full lock. It is most pronounced in cold weather when the tyre is stiffer. Once warmed up, you will still feel it a little . Part of the cause is the rear drive wheels are trying to push the car forward while the fronts are not rolling so they jump a bit until they are rotating. On full lock, the Ackerman geometry makes it worse for the 'inside' wheel. Wide tyres add to the problem. Front wheel drive and cars with narrow tyres don't have an issue. This is not limited to Porsche, my 2005 C6 Corvette on 245/40 18s, bought in Texas, never showed this problem until I brought it back to Aberdeen.

 

[GM_991_2_GTS]

As others have said above it's the Ackerman effect. I picked up a 2018 GTS in August and barely noticed it. It's much more noticiable as the temps have dropped. I have also had my suspension/chassis inspected by Porshce and independent suspension company whilst getting a goe carried out and it's in great shape. Incidentally I didn't get it inspected becuase the noise, I recognised it from previous cars as being part of 911 ownership but it is much more noticable on my 991.2 compared to my 997.2, which I had immediately prior to the 991.2. I run Michelin Pilot Sport 4 S's and apparently it's noticable on some tyres than others due to compound and grip levels etc.