Help - have I made an expensive mistake?

[DevonZ4]

Hi,

Had my 3.0 2004 Z4 for about a year now and love (or rather loved) the way it handled but wasn't that fond of the harsh ride over bumps and potholes etc and the occasional tramlining.
So earlier this week I noticed that the rears were low on tread so wanted to replace them.
Having read a lot of forums about replacing the run flats with conventional tyres to improve the ride and handling and also the fact that they're cheaper I decided to take the plunge.

As I enjoy a spirited drive - but don't exactly drive on the ragged edge I was after some mid-range tyres and having read a lot of good reports went for a set of KUMHO 255/35R18 KU39 XL 94Y.

All I can say is that the car now feels dangerous. When I first took it out on the dual carriageway taking a slight bend at about 70mph - the traction control light came on and the back end felt really loose and jittery. Coming off the dual carriageway onto a bend doing about 50mph the exact same thing happened - the ABS was kicking in with the traction control - and basically it just felt like there was something massively wrong wit the back end.

I checked the tyre pressure which was fine, I checked that they were on the right way round - they are.
So I then took the car to get a 4 wheel tracking and alignment done at a Hunter specialist - it was out (including the camber on the rear) - so they adjusted it all so it's all in spec. However the problem is still there.

They suggested that perhaps changing the fronts to non run flats might help - but said that they didn't really think this would help (as why should the fronts affect the grip on the rear). So now my dilemma is - change the rears back to run flats - which I hope should solve the problem but will cost another £450. Try some Kumhos on the front (cost about £220) - but this probably won't work. Or try better (Bridgestone, Pirellis etc) non run flats on the rear - i.e. premium brand and see if this helps.

Any suggestions would help massively as I'd like to move away from run flats if I could - but not at the expense of making the car dangerous to drive (god knows what it will be like in the wet). Also I'd like to do this without losing as much money as possible.

Thanks in advance for your help.

[mr wilks]

TBH the difference from RF to Non Rf is more noticeable when switching the fronts so i doubt its the fact the rear tyres are standard that is making the car suddenly feel "twitchy"
Maybe give your location & a friendly member may lend you a set of rears (even if 17 inch) to try out & see if the problem disappears ?

I switched from RF,s to standard on my ZC but even though the front end twitchiness went & was a less bone crunching it could never be described as "planted" when pressing on
Maybe the front end needs more weight in like a V6 or V8

Oh & welcome to the forum , maybe put a quick intro & some pics in the newbie section so you can enjoy special attention

[-Tom-]

How many miles have you done on the new tyres? They have a coating/layer which can last a few hundred miles whilst you have little grip.

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[DevonZ4]

Thanks Mr Wilks - I'm in South Devon if anyone is close.
Tom - I've done about 400 miles on them so far (went to Bristol and back - SLOWLY!!).

Does anyone have any experience of only swapping out the rear run flats and this affecting performance?

Cheers

[Adamski]

My Avons took 500 miles to scrub in. They were fine after that. Remember you've also gone from having very little tread to nearly 8mm of tread.

Give it time and the tyres will start to grip again


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[paulgs1000]

I changed to non runflats on the back of my first Zed and immediately felt an improvement in ride (changed to mid range Falken 452s).

Your new tyres will take time to 'bed in' and for the coating used to form the tyres to wear off - (normally 1-200 miles).

Also - have you had your suspension checked at the rear - may be an associated problem?

Due to a mistake, for the time being, I am running Falken Z1Bs on the back of my 3.0si and though I have the traction light blinking from time to time (which I expect) I am surprised you have had so much insecurity with the Kuhmos.

I contacted Falken directly when I had the issue with my Z1B rears - their technical department were very helpful - maybe contact with Kuhmo direct might help?

I hope you get sorted with your Kuhmos soon.

[StevenH72]

probably a "scrubbing in" period of the new tyres. Give them a few more runs and I'm sure that they'll feel much better.

[Spikey]

Yeh as said before give them time to bed in sure they will be fine

If you can't wait find a big car park and have some fun scrubbing them in lol

[El Taco]

Find a place four wheel alignment jig and get the thing checked out to establish a straight edge

Then follow whatever advice you feel you need to.

[Finisterre]

what pressure?

[paulgs1000]

what pressure?

Very good point. . . (front and back)

[DevonZ4]

Thanks for your replies everyone.
The thing I find strange about the behaviour is that the tyres feel fine at lower speeds - so taking sharp bends and putting my foot down at say 30mph I don't feel any problem - grips well - no traction control light. However get up to 50mph and especially if I lift off before a corner - the back end becomes jittery and the traction control light is on and the ABS is kicking in.
If I take the traction control off and floor it from a standing start - I don't have any less noticeable grip than my Bridgestone run flat previous tyres - i.e. it's not wheel spinning and fishtailing around - the tyres appear to be gripping. It's just at higher speeds when they feel really loose and dangerous.

As I said - I had all four wheels aligned and camber done on a Hunter system today - and it didn't fix the problem - I'll try and figure out how to upload a picture of the print out.
Tyre pressure is at 2.5 bar or 36 PSI which is what is written on the plate on the car. I'm wondering if this should be less because they're not run flats though? Can anyone tell me what pressure they run on their non run flats please?

The rear springs were changed about 24 months ago by the previous owner (I have receipts etc to prove that) - but I'll check them tomorrow - i.e. to make sure they're not broken - but I expect I'd feel that.

Thanks for all your help though - maybe a call to Kumho might be the order of the day on Monday.

[DevonZ4]

Hopefully this will work

[beanie]

Personally I'd stick with the tyres for a while and see how it goes. New tyres can be greasy for quite a while and just changing one axle set will make this even more noticeable.

Geo looks fine, as per the rule book though I'm surprised at the factory camber settings from to rear. That said, I can't see those settings being the cause here.

36psi seems high to me though perhaps some others with 3.0l that have gone this way with the tyres will chime in with some experience. Personally I'd try dropping the pressure a bit and seeing how it goes, nothing to lose.

only other thing worth checking is the production date of the tyres-this should be written on the sidewall. Have a look on google to give you some idea of what and where to look. If they are really old then you may have a case for getting them changed- the tyre industry generally recommends changing every four years due to deterioration of the rubber but it's a fairly grey area as some tyres live outside whilst others remain on the shelves for a couple of years never seeing the sun. Still, it will at least rule something out either way.

For what it's worth, my eagle f1's were shocking for quite some time, less precise, less grip and generally not that nice. Given a good few heat cycles and a lot of miles they are easily on par with my old ones and my scientific, full throttle tyre test corner backs this up

[Claireysmurf]

I went over to non RFTs on the back whilst I got some more wear from the front RFTs and a couple of times thought I was close to spinning the car at 60-70. At 90 I didn't notice it.
I have non RFTs all round and wish I had done it sooner.
I am using Hankook K117 S1 Evo2 all round on mine.
I think I have felt a bit more front end bounce on mine since swapping but that may be my imagination.