Steering 'jump' on full lock

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Anonymous

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I noticed recently that when reversing with full turn on the steering wheel it sometimes feels like the steering jumps (for want of a better word)...it feels like the wheels have gone over a small rock.

Is this problem, or just a feature I have not noticed before ?
 
I'm sure there are many similar topics on here.

On full lock on a cold and wet morning the front wheels can "skip" at low speeds because the wheels are leaning over so much they hop sideways rather than roll round.

Mine has always done this on cold wet mornings on full lock. I've done 35k miles and have perfect tyre wear etc so believe it just to be geometry of the suspension.
 
I’ve noticed this when in ‘Drive’ and on full lock at very low speed. It does indeed feel like the tyre has gone over something like a small rock.

Glad to hear it’s a common occurrence which doesn’t appear to be a fault.....unusual though!
 
Had it on mine since purchase around 5 years ago. Friend runs a Hunter centre so know the alignment is spot on so I’m happy it’s just the way the geometry is set
I hear and feel it most at a Costa that has Acco drains. It jumps and feels like it’s lifting them when I reverse out and anyone sat outside looks up to see what happened.
 
I believe its the ackermann effect plus when on full lock the camber/wheel angle is tilted over.
 
I have Michelin Pilot Sport 4 (not PS4S) tyres on the front and the 'small rock' syndrome is sometimes felt on full lock at very low speed.

I see that Jaguar put out a Special Service Message (SSM73888) on this entitled.....”Low Speed Manoeuvring Front Tyre Skip / Jump phenomenon."

Taken from the attached SSM.....

Issue: Low Speed Manoeuvring Front Tyre Skipping Phenomenon.

Cause: Acute Ackerman angles employed to enhance driving experience.

Action: This SSM is a guide to explain the phenomenon of front tyre skipping while low speed manoeuvring with full lock applied to the steering causing a feeling of a slight jump, vibration or noise from the front tyres.

This is NOT an error state. It is a result of geometry focused for higher dynamic higher cornering speeds, manoeuvre, environment and tyre type.

Geometry: 100% Ackermann Geometry principle means that front lock angles allow for the radius of travel of both wheels to be defined by a common point through the rear axle. However in reality this principle cannot be achieved due to inner wheel lock angle constraints. Typically, vehicles have Full Lock Ackermann levels between 55 and 65%. More sports orientated vehicles have lower levels of Ackermann specifically to improve tyre utilisation.

Manoeuvre: A benefit of low Ackermann can be found at higher cornering speeds when the tyres are generating slip angles. At very low speeds the tyres do not generate a slip angle which results in an aligning torque being applied to the tyres that is absorbed in the tyre. Tyre slip is exaggerated at high lock angles.

Environment: Tyre slip is more perceptible when the friction level between road and tyre is lower, i.e. when there is ice and snow, or when the road surface is wet. Or brick, car park painted surfaces or concrete containing pea shingle may exacerbate the concern.

Tyre Selection: Changing the tyres from summer to all season or winter tyres may result in an improvement but there is a trade off in economy, road noise and tyre life. A customer experiencing this phenomenon can choose to fit an alternative tyre at their own expense.

A benefit of low Ackermann can be found at higher cornering speeds when the tyres are generating slip angles (represented in picture 1 by the green arrows).

More sports orientated vehicles have these lower levels of Ackermann specifically to improve tyre utilisation.
 

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