Front tyres scrubbing / jumping when on full lock.

When on full-lock (particularly when it is cold and I am slowly reversing) I notice the front tryes scrubbing / jumping. It is as though the geometry is wrong and they are not in alignment so load each other up until on or other of them lets go and slips sideways to relieve the pressure. It has got considerably worse since the cold snap and a colleague with a 997C2 is experiencing the same issue, he decribes the sound as though one tyre is flat. Anyone able to shed any light on what is going on please? Thanks, B

Don't they all do that? Mine always has.

Hi Boggie, Yep, Mine does it too. I guess it has to do with the fact that all the weight is in the back. May be wrong though (I'm sure someone will correct me if I'm wrong[8|])

Mine too but after reading the forums for a few years they all do. I believe it's called tyre skip? it only happens in the winter for my C4 and nothing has ever been picked up on servicing. I think this is a well known experience with the 996.

Thanks Chaps, I had assumed that excessive toe-in or something broken were to blame. Glad to hear this is normal behaviour! B

I get it too on my c2 but its only on full lock.

Yep. Mine too on full lock and more when cold. Thx

Although the steering system incorporates Ackerman (or modified Ackerman) geometry in which the outer and inner wheels trace out different circle radii aimed at avoiding tyre scrub, at low speeds the rear differential (2-wheel drive) may not be 100% effective and this - coupled with wide tyres, suspension compliance and the fact that the front wheels are loaded by torque reaction to the drive - leads to the front wheels being dragged across the ground to some degree. I suspect that the effect will be exacerbated where there is also a front differential coming into play (as on your Turbo?). This leads to the effects you're describing. Jeff

I'd been having a few people ask the same question, so wrote about it in the club magazine this month. Ackerman is to blame but the good news is it's normal.. garyw

As well as Ackermann, I think it is a simple function of the tyre width. When turning, the outside edge of the tyre has to travel a further distance than the inside of the tyre. At high steering angles the tyre cannot compensate for the difference in flexing, and the tread will slip on the surface, in a skipping motion. Obviously, this is more pronounced when the surface is cold or slippery. If you take it to the extreme, with a rubber roller on a grippy surface, it will always roll in a straight line. Like paper in a typewriter roller, for those of us old enough to remember them....

An excellent analogy Richard (and, unfortunately, I am old enough to remember typewriters..!!). Interestingly, my front wheel drive Golf GTi doesn't exhibit the same skipping effects as my Cayman when reverse turning out of the drive, despite the fact that they have similar front wheel tyre sections. No doubt due to the separation of drive and steering (dragging the rollers in your analogy?) and lighter front wheel loading on the Cayman. Jeff

Much lighter loading on the front end. I believe the tyres will flex a certain amount before skipping occurs, and the extra weight of the engine in the VW will help a lot. I noticed it more with the Cayman than my 996s, due to the 235x35x19 fronts (wider and stiffer sidewalls). When I put my 225x40x18 winter wheels and tyres on, it reduced dramatically.

Interesting that the skipping only seems to occur when reversing - as far as I'm aware. Using your roller analogy, you'd think that there'd be a similar effect when rolling forward with the same steering lock applied. Maybe a degree of instability is created due to "reverse" castor (wheeling a supermarket trolley backwards when the wheels are still trailing) coupled with front suspension compliance effects. Plus, as you say, using summer tyres in winter on low friction surfaces aggravates the problem. Jeff

How many times do you pull forward with your wheel on lock? I only experience it when reversing off my drive in a morning or out of a space.I think whoever mentioned temperature could be on the money.

I think Jeff is probably right about the effect of caster angle. I'm just theorising, by the way. It might be utter tosh!

Well Chris, I do have to go from full left lock (reversing) to full right lock to get out onto my rather narrow village high street from the drive. And Richard's right. This is all just conjecture but it's made an interesting thread - especially since I can't be bothered to clear my drive of snow to get the Cayman out to verify our theories...!! Jeff

Thanks chaps, much appreciated. I am glad to hear that there is not a large bill looming..... B

I reckon Herr Ackermann is being unfairly blamed for this concern - surely the Ackermann angle is designed to alleviate the very scrubbing being discussed? Without his intervention in the geometry of the steering system, tyres would be wearing out after hundreds rather than thousands of miles and before the common fitment of PAS, drivers would have needed Popeyes forearms just to turn a corner. I reckon the most likely answer is a combination of the width of modern tyres, meaning the inner and outer edges want to describe a different arc around a curve (but can't), coupled with the cold weather hardening the tread blocks and taking away the little compliance they had in the first place. But then I could be completely wrong...

To me it still doesn't explain why the effect only happens when reversing but I guess that it'll just have to remain a conundrum. Incidentally, manufacturers often incorporate a modified rather than a pure Ackerman geometry to induce either toe-in or toe-out of the front inside wheel on turn in. It's all part of the front suspension kinematics juggling act aimed at inducing understeer/oversteer/stabilty where tyre slip angles, roll-induced toe and suspension compliance also play their role. Jeff

I also noticed that my front wheels were jumbing and screaching more in the cold. The car had a full alignment done just a month ago so I know its not of alignment. I'm glad to hear that its not only mine and more cost..

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