Unsettled behind-tyres/lsd/bushes?

[iain]

First post lurker here.....
I run an 07 4mc <13K miles, I've recently swapped the rear worn out Contis for a pair of 255 michelin PS's and still running the oe conti's at the front.....pressures as per spec.....

I've now got a very unsettled car ...... long high speed sweepers, even fast straight lining are very nervous, with constant minor steering adjustments and a decidedly unplanted feel to the car its almost as though I've got a slight rear streering effect.....I've always loved the predictability that Michelin has brought to my cars but I'm pretty sure that the tyres are the cause of this problem...

Prior to spending to resolve the issue can anyone cast light upon other areas that might contribute to the effect's I've described. Are their any known issues around the geometry at the back? what about suspension bushes and roll bars, can a failing lsd generate the issues described? Any thoughts and experiences would be welcomed....

Assuming its the tyres....do I swop the front for Michelin or the back for OE conti's.....to be honest I'd pref the Michelin route, I accept that BMW-US may well have selected the conti as the best performer.... but call me cynical ....I sense that the conti's might better contribute to their profitability than the cars performance.

many thanks Iain

 

[JayD]

Iain

Mixing the tyres like this is likely to be the problem. You need the same boots on each corner, not a mix of two.

Ditch the conti's and put some Michelen's on the front as well ,then I am sure the car will be just fine.

 

[iain]

JayD thanks.....any experiences with the other points noted?
all the bets IAin

 

[Gids]

Have a word with Blacklines on here, he changes tyres more often than most people change their underwear. I'm sure if anyones mixed tyres front and back he will have.

 

[Rama]

JayD thanks.....any experiences with the other points noted?
all the bets IAin

100% sure tires will fix this.

 

[Top pup]

Did you go for the michelin PS2's or just the PS's? I know the PS2's are pretty slippery for the first hundred miles or so until they scrub in a little.
The RTA bushes could also make the rear of the car unselted but I doubt you would have worn them out in 13000 miles.

 

[enfield]

Differnt tires means differnt grip means unsettled car.... Just my $0,02

 

[Blacklines]

Very interesting post - a similar problem has been playing on my mind lately. I have experienced exactly what you are describing.

I am sure tyres are the root cause however do you mind if I ask a few questions?

1. Does it happen on both L and Right corners?
2. Does it happen when you have a full tank of fuel?
3. Does it happen when the traction is off?
4. OE wheels and tyre sizes (225/45 ZR 18/255/40 ZR 18)?

If the answer is YES to all 4 then I would start with tyre pressures. Assuming the fronts are still too good to throw away and the pressures are set accurately(!) then try dropping the rear pressures by 3 or 4 psi. That should help - but let us know what happens.

My Z4 tyre experience (as Gids said - far too much!) consists of full matching sets of Michelin, Contis and Vredesteins. I have a suspicion the Z4 is susceptible/sensitive to front tyre wear but may seek the assistance of Simpsons. Re:

... suspension bushes ... roll bars ... a failing lsd

I am sure you would not anyway but might I also suggest being particularly careful about braking in a corner. I have found the instability is quite dramatic under brakes.

You have spurned me on to get to the bottom of this one. I will throw on my brand new OE wheels and tyres this weekend (provided they will fit over the AP brakes) and see if I am still suffering.

And welcome!

 

[Curtis]

I think it's safe to say that if the problem started when you fitted mismatched tyres then it's probably down to mismatched tyres. The Z's geometry, bushes etc leave a lot of room for improvement but I assume the handling characteristics didn't bother you before the tyre swap? Different tyres perform better at different pressures. The factory pressures for the Conti's may not suit the Michelins so have a play around with the pressures until you find your preferance but not until you match the front and rear tyres. If your still having problems then it's worth improving the geometry and replacing the crappy OEM bushes but start with the obvious.

 

[iain]

Very interesting post - a similar problem has been playing on my mind lately. I have experienced exactly what you are describing.

I am sure tyres are the root cause however do you mind if I ask a few questions?

1. Does it happen on both L and Right corners?
2. Does it happen when you have a full tank of fuel?
3. Does it happen when the traction is off?
4. OE wheels and tyre sizes (225/45 ZR 18/255/40 ZR 18)?

If the answer is YES to all 4 then I would start with tyre pressures. Assuming the fronts are still too good to throw away and the pressures are set accurately(!) then try dropping the rear pressures by 3 or 4 psi. That should help - but let us know what happens.

My Z4 tyre experience (as Gids said - far too much!) consists of full matching sets of Michelin, Contis and Vredesteins. I have a suspicion the Z4 is susceptible/sensitive to front tyre wear but may seek the assistance of Simpsons. Re:

... suspension bushes ... roll bars ... a failing lsd

I am sure you would not anyway but might I also suggest being particularly careful about braking in a corner. I have found the instability is quite dramatic under brakes.

You have spurned me on to get to the bottom of this one. I will throw on my brand new OE wheels and tyres this weekend (provided they will fit over the AP brakes) and see if I am still suffering.

And welcome!

Thanks to all for your thoughts, responding specifically to Blacklines questions;
1 It happens left/right and staight ahead
2 Seems to be irrespective of fuel weight
3 Not checked if traction control affects...good thought though
4 Yep all OE 'cepting rear PS's

 

[iain]

Just changed the fronts to michelin PS's and suprise suprise the car seems to have lost the twitchyness......One note of caution .... when the guys were stripping the contis of the fronts they pointed out a significant number of cracks in the rubber 10-15mm up from the bead and running inline with the bead. When mounted these cracks would be hidden by the wheel rim.

As they were part worn I'd intended to store them for later use.. the guys at the tyre shop described the cracks as 'stress cracks' and recommended throwing them. Anyone seen this before

 

[Blacklines]

Just changed the fronts to michelin PS's and suprise suprise the car seems to have lost the twitchyness

Glad to hear that.

I have a suspicion the Z4 is susceptible/sensitive to front tyre wear