[ferraris]

Hi, new owner here and I couldn't see anything similar asked before in this forum, my previous BMW had 19" wheels my 718 Cayman S has 20" wheels so I'm not sure if it's the different profile but on turning at slow speeds the types seem to judder or skip across the road, particularly when reversing.

Is this a common attribute of 718s and possibly combined with the rear wheels pulling or pushing the fronts?

Thanks! Simon

 

[DriveInHouston]

Google Ackerman effect... Ferraris do it too...

 

[ferraris]

Thank you, not this one though.

 

[jimmuller]

High performance tires that are still cold. My Michelin PS4S do that too. Not the Michelin Alpins winter tires on the car now.

 

[aeronca65tl]

Yes...as noted above, it's Ackerman effect.
The 718s seem to do it at slow speed more than most cars.
Probably due to tire width combined with wheelbase/track ratio on these cars. It's harmless.

 

[Keirik]

Yep they all/most do that.

Very disconcerting at first

 

[roundtail]

Why is this being referred to as the “Ackermann effect”? Ackermann’s linkage addresses the problem, it doesn’t cause it. For our cars, my layman’s understanding is that the large cross-section of the tire-wheel set in the wheel well limits the linkage’s travel, resulting in a loss of correction at larger steering inputs. Hence, the uncompensated judder results.

 

[aeronca65tl]

"Ackermann" is a generic term used to describe tire geometry and related scrubbing in certain situations.

Engineers base the appropriate amount of geometry on use of the vehicle, but steering geometry is always a compromise to some extent. So there is no perfect solution and sometime tire scrubbing occurs because there is no single, ideal Ackermann setting. And backing up in a car can make tire scrubbing worse because most steering geometry is biased toward forward direction travel.

There is "pure" Ackerman (for horse drawn wagons), reduced or modified Ackermann and even reverse Ackermann.

Here's a good link with more details for anyone who is interested:

Ackermann Angle - Super 7th Heaven

Ackermann angle steering geometry solves the problem of wheels on the inside and outside of a turn needing to scribe circles of different radii.

www.super7thheaven.co.uk

 

[jimmuller]

Engineers base the appropriate about of geometry on use of the vehicle, but steering geometry is always a compromise to some extent.

Indeed. Ackermann steering choses a steering angle for each front wheel so that the axle lines of both front wheels intersect the rear wheel axle line at the same point. The idea is to create a single pivot point about which all four wheel run in concentric circles. however it doesn't take into account the effect of slip angle between the direction a tire is pointing and the direction it is actually moving. Modern cars use a modified Ackermann but the slower you are moving the smaller the slip angles, and thus the closr to true Ackermann it should be.

 

[sevencyclist]

This thread is very interesting and helpful! Thank you for showing the science behind this phenomenon.

I also notice much more of this when the tires are cold. Is this because less lateral flex of the rubber so more sudden skips when the tension is exceeded?

 

[HooosierDaddy]

If you think the 718's are bad... you should try a Corvette.

 

[jimmuller]

I also notice much more of this when the tires are cold. Is this because less lateral flex of the rubber so more sudden skips when the tension is exceeded?

Apparently. I have no juddering on my winter tires, FWIW.

 

[HooosierDaddy]

Apparently. I have no juddering on my winter tires, FWIW.

maybe because of the softer compound they would have the ability to flex with the binding created in a full lock turn?

 

[jimmuller]

maybe because of the softer compound they would have the ability to flex with the binding created in a full lock turn?

Well, of course. Then too the winter wheels are 19" instead of 20".

 

[SpinDoctor Mario]

Freaked me out first time I backed out on a cold morning and turned the wheel. In fact, I can replicated it even on warmer days. Porsche dealer said it was perfectly normal after double-checking that nothing was rubbing. I'm betting that and the condensation in the tail lights freak out new owners fairly often.

 

[jdstein11]

We had a whole thread on this last year if you want the details on tire scrubbing, including a link to Porsche North America’s technical guidance: Front Tire Scrubbing

 

[Peter]

It's even worse on a highly polished surface like my garage floor. Positively screeches as it judders around.