Steering judder - left or right lock

[markdav]

Backing out this morning, whenever I get past half a turn on the steering wheel, the front wheels seem to be jumping slightly - like a big alignment issue. the more I turn, the more it happens

Stering whell feels normal, normal driving seems ok.

I will go for an alignment check.

Before I do, anything obvious to check? No work has been performed on the car and no accidents - can something like an allignment error just happen overnight?

Thanks, Mark.

[markdav]

Sorry - Boxster 2000, 2.7, US, Manual, 65K miles...

Backing out this morning, whenever I get past half a turn on the steering wheel, the front wheels seem to be jumping slightly - like a big alignment issue. the more I turn, the more it happens

Stering whell feels normal, normal driving seems ok.

I will go for an alignment check.

Before I do, anything obvious to check? No work has been performed on the car and no accidents - can something like an allignment error just happen overnight?

Thanks, Mark.

[RFM]

This behavior is caused by Ackerman effect and specific for Porsche sport cars front alignment, this phenomenon becom exacerbated by colder outside and therefore colder tyre temperatures.

[dougdearmond]

Sorry - Boxster 2000, 2.7, US, Manual, 65K miles...

Make sure it isnt something simple like the belt driving the power steering pump slipping. It will be worse when cold in the morning. Have you ever inspected or replaced the belt?

Edited December 11, 2009 by dougdearmond

[Glyn]

I have heard of this so many times but on neither of my cars have i experienced it.

Many replies have indicated the Ackerman effect - and whilst i understand the Ackerman principal i cannot see how that would create the condition that this post and others identify.

To clarify - Ackerman is simply a mathematical formula that is used in steering systems to ensure that the steering wheels are at different angles during cornering - to understand this - with the steering wheels on full lock the inner wheel has a smaller circle to travel than the outer wheel - therefore the angle of the wheel has to be slightly different - that calculation differer from car to car dependant on a number of factors including the distance between the steering wheels and the turning circle of the car.

Ackerman principal would not cause the condition described - well not in my mind, i think it is more likely to be the camber angle.

[stephen wilson]

I agree with RFM, my 987 has been doing this more with the colder weather. It's Ackerman combined with the camber gain effect of Caster/SAI. The tires ride more on one edge, and the radius of the inside vs. outside tire paths don't quite match, so the tire with the least grip has to "scrub" across the pavement to match the path of vehicle travel.

Steve

[jworden]

I agree with RFM, my 987 has been doing this more with the colder weather. It's Ackerman combined with the camber gain effect of Caster/SAI. The tires ride more on one edge, and the radius of the inside vs. outside tire paths don't quite match, so the tire with the least grip has to "scrub" across the pavement to match the path of vehicle travel.

Steve

If you think about it even a single wide tire cannot scribe an arc without scrubbing because the path followed by the inside of the tire has a smaller radius than the path followed by the outside of the tire. Yet the entire tire rotates at the same speed. Something has to give. The reason the car jumps is because one side will encounter better traction than the other and cause the opposing wheel to slide. This is very noticeable in winter conditions when backing up.

Porsche probably doesn't care too much about steering geometry while in reverse!