Anyone an expert of tyres and their impact on steering feel?

[Reggie183]

My 335d had it's rears changed last week. Original tyres were Bridgestone Potenza 255/35/R19's and they have been replaced with Bridgestone Potenza 255/30/R19's. Identical tyre, just a slightly lower profile (I was told the 35's weren't available). The fronts are still on the original 225/35/R19's.

Since then at motorway speeds the front end feels light and twitchy?

Any explanations? or could this be to do with something other than tyres?

Any help gratefully received before I take it in on Friday.

[jon-]

You've changed the sidewall by 0.5" so not massive, couple that with old worn tread vs new tread and it gets less.

Initial thought is something else. Were they the same model of Potenzas?

[belleair302]

Tyre Pressure, new bushes required and how about a KDS geometry check. I would have myself gone for Michelin's.

[Reggie183]

Thanks guys. I checked my front tyre pressures and they were a little low, but correcting them seems to have made no difference.

I've got a man looking at it on Friday.

[cptsideways]

I'm sure these cars are well corner weighted from the factory, what you have done by effictively reducing the height at the rear is shift a bit of weight back. You'll also have changed your effective castor angle. Which might be even more noticeable.

Just my opinion but I think you'll find its the cause

[rassi]

Also depends on how many miles you put on the new rear tyres? Normally they need a bit of scrubbing in (say 2-300 miles) to get normal grip. But IMHO don't really understand why you had fitted undersize tyres, would have gone somewhere else to get the right size.

[Baddie]

cptsideways said:

I'm sure these cars are well corner weighted from the factory, what you have done by effictively reducing the height at the rear is shift a bit of weight back. You'll also have changed your effective castor angle. Which might be even more noticeable.

Just my opinion but I think you'll find its the cause

35% of 255mm is 89mm.
30% of 255mm is 76.5 mm - ie there's a 12mm difference. I'd be very surprised if the profile change is to blame alone. But as the tyres are the difference between a good and bad car it must be related to that. My usual place says these BM's can be very sensitive to changes of make. You haven't chnaged the make but in these days of tyres being tailored to cars perhaps you got one with a different carcass.

I would agree that your new tyres may need "scrubbing in" for a few hundred miles and until then may have bigger slip angles than before, which would manifest as an oversteering sensation. My guess is that it will settle in time. I'd be more upset at the ride quality you must be getting!

[anonymous-user]

You've lowered the rear ride height by 12 mm, which will not make any significant difference to the weight distribution or the front castor. But it will have a significant affect on the aerodynamics. Dropping the rear 12 mm reduces the rake in the chassis, which will increase the aerodynamic lift at high speed. This is what you are most likely to be feeling here (front goes light and twitchy). Fitting the wrong tyre size is generally a bad idea and often causes other effects.

Edited by anonymous-user on Wednesday 12th September 17:55

[Reggie183]

Thanks guys. I had my tyres done by the local BM garage, so they "should" know. Amazingly competitive on price as well.

I have been on the Bridgestone web site and they no longer do the '35' ratio - so could explain why BMW fitted the '30' ratio.

I've got a mate from an independent garage looking at it on Friday, before I go back to BM.

I have already put over 700 miles on them since fitting on 3/9.

Thanks for the info/advice - will update over the weekend

[anonymous-user]

Reggie183 said:

Thanks guys. I had my tyres done by the local BM garage, so they "should" know.

They should know, but probably dont.

[F355GTS]

I can;t beleive anybody, least a dealer would recommend fitting the wrong profile tyres, pretty certain if you call BMW technical they''d be less than happy about it

I'd be back ot them to have the correct items fitted

[computamedic]

www.mytyres.co.uk list the right size.

Potenza RE 050 A 255/35 R19 96Y XL M0, BSW, BZ, double marking 96ZR

£181.50 supply only.

Dave

[Reggie183]

Upon investigation the tyres fitted are the new standard size (checked 2 new models on the forecourt). The 255/35's quoted above are not run flat's - Bridgestone don't appear to make them anymore.

Took it to a friends garage and he reckoned nothing to do with my rears - it would appear on of my fronts is worn on both the inner and outer edges (low pressures) and this apparently can give you the feel of driving in the tram lines you get in lane 1 of a motorway. He reckons that's what BM will tell me when they take a look next week.

Looks like the new tyres, with but extra grip just exagerated the poor performance at the front.