987 with a lumpy ride when on hard lock

[poobarr]

Hello all

I have a Boxster 3.4S (2007) which I bought in Dec 2010 which i love apart from the following:
the car has an issue with when parking.

Basically it feels as though your going over a bumps when your have the steering on hard lock. This happens going forward and backwards. The car has the 19" turbo wheels that came as standard.

It drives faultlessly other than this, although the tracking may be just slightly out, could this be the issue?

Many thanks

dave

 

[BrianJ]

Don't worry about it - they all do it. It's the way the steering geometry is designed. You need only get concerned if you manage to notice it on full opposite lock, but in that case you'll have more to worry about than that!

 

[poobarr]

I did wonder that, thanks very much Brian

 

[Nick_USA]

.... will add that expect it to be worse when the tyres are cold during winter.

 

[Mark_Bennett]

It's those great big fat tyres trying to describe rather different circles on hard-lock. Somethings got to give and it's not likely to be the tarmac.

 

[poobarr]

I'm just pleased its not a fault

thanks again

 

[Timbo2]

Ah!!!! thank you, you've answered somthing I was concerned about.

 

[grahamwhite]

mine does this too, there is a specific name for when this happens, Its called the Ackerman effect.
The Ackerman effect wiki article

 

[poobarr]

How did you find out it was called the 'Ackerman effect'?!

 

[tscaptain]

I thought the Ackerman steering principle was supposed to stop it happening? Good explanation here - it's R/C website but the info is sound!

http://www.rctek.com/technical/handling/ackerman_steering_principle.html

Like Mark said earlier I think it's the width of the tyre and the different distance the inner and outer edges have to travel when there's lots of lock on. Same thing is happening on the rears too?

 

[garyw]

ORIGINAL: poobarr

How did you find out it was called the 'Ackerman effect'?!

Apprenticeship []

Alan, its to keep the radius similar not the tyres, I believe its more of the KPI that causes the juddering... []
I'll ask Chris next time I see him.. []

 

[tscaptain]

To save others looking it up......
KPI
-Provides steering pivot near to the tyres centre line,
-reduces steering effort,
-aids directional control,
-aids self centring of the steering,
-helps to distribute vehicle weight evenly across the tyre []
I quite agree Gary - so that both wheels describe concentric circles the inner wheel puts on more lock.
This from our own 996 mod - on another forum!

"I think the 'juddering' effect occurs when a wide tyre is turning a tight radius - the outside edge is having to move a greater distance than the inside edge. I believe that it is the slippage that has to occur which causes the effect. On mine, it is only noticeable when reversing, and I have no idea why that is........ "

Will be interesting to hear what Chris reckons![]