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Front tyres scrubbing / jumping when on full lock.

Ackerman geometry, if I remember my engineering education correctly, is about getting the wheels to describe approximately correct radii when cornering as, obviously, the inner wheel follows a smaller radius than the outer wheel. Again, if I recall correctly, when looking from above the car if lines are drawn between the swivel axis of each wheel and the centre of the track rod ends, the lines should converge just ahead of the rear axle centreline. Obviously, this is set up for forward motion as we don't tend to reverse at any sort of speed or for great distances.

I think the phenomenon being discussed is a product of castor angle being totally wrong when reversing, especially as the wheels are turned out on full lock. When turning in this way the wheels try to tuck under but are resisted by the anti-roll bar and so they skip instead.

All of this is exacerbated by the lovely wide wheels we all have. You should detect it if you do the same in a small hatchback on gravel.

 

Quite so Stuart.

As I mentioned earlier in this post, I likened the "reverse castor" effect to wheeling a supermarket trolley backwards when the wheels are still trailing - very unstable..! And when you factor in suspension compliance effects, wide (summer?), cold tyres on icy ground you get the effects we're all encountering.

Jeff
 
[:)] This is interesting, although I have not noticed this scrubbing from my Carrera 2, it's very pronounced on my MK1 MX5, it's almost painful to the ears, I don't want to think what it's doing to the front tyres!
 

Good to hear that the effect's not confined to Porsches Geoff.

My brother owns an early bog standard 1.6 Mk1 MX5 with skinny tyres - I'll have to check with him to see if he gets the same as you.

Jeff
 
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