Staggered wheels, tyres, pressures, alignment, questions!

[Mr Whippy]

When I bought my Z4 it had RFT RE050 on, and as usual the inner front tyre wear was very bad, and the rear inner wear was bad. 225/40r18, 255/35r18.

The OEM pressures are 33f, 36r

I moved to FK452 tyres, and now, after almost 10,000 miles, my tyres have worn nicely.

There is a problem though.

The rears have worn the insides a fair bit, but also the inside has worn flat (too much pressure)... they ran 33psi.

The fronts have worn the inside quite a lot (more than rear), but the centres have not worn as much... they ran 31.5 > 32psi.

So, I'm starting to think that I might need to have run about 33-34psi front to wear the main patch flat, and the rears 31-32psi...


This seems a wide contrast to the OEM pressures for RFT. Running higher front than rear.



Is it ideal to run pressures to wear flat, is this the ideal way to choose the appropriate pressure?
Does the odd alignment of -1deg front camber, and almost -3deg rear camber, mean that tyre wear will never be ideal, and so using the apparent wear as a guide for pressures is not ideal?


I'm confused basically. On any other car I'd tweak the pressure to get flat wear, but the Z4 seems to wear tyres oddly full stop, so am I tweaking pressures while other variables are making that a faulty process?


Since running lower pressures at the rear (running more on the sides than the centre of the tyre), the rear end feels a bit less planted on bumps under power or on neutral turn-in. Is now the excessive (-3deg is lots in my view) camber and softer sidewalls meaning some odd tyre behaviour under lateral loading?


I'm very tempted to take the car to someone clever and have it set up with a decent alignment config.

Anyone done much to the OEM alignment with BMW's on staggered wheels?


Thanks

Dave

[christer]

Mr Whippy said:

a lot of stuff

Hi Dave

I wouldn't play with tyre pressures to get more even wear. Uneven wear is almost always a suspension setup symptom but not necessarily a negative thing. A car with negative camber will *usually* wear the insides more than the outside - for obvious reasons - it shouldn't be too extreme but it will be noticeable. If you insist I am fairly sure it would be possible to set the car up as neutral to try to squeeze some more life out of the tyres, but you would likely lose some lateral grip and the car may feel less responsive when cornering - but thats your choice.

Just to reiterate though, I wouldn't set tyre pressures in order to try to get even wear. It could be dangerous depending on how you drive.

In fact, I have *never* heard of anyone doing this before....?????

[Mr Whippy]

I think the big problem is that standard the car runs RFT, and so the suggested tyre pressures just seem excessively high, 33psi front and 36psi rear... they feel very high on non-rft tyres.

There is always a sweet spot, and I assumed wear was a good way to aim.


Currently fronts are about 6mm in centre, 2-3mm at sides, and worn to near 1mm on very inside edge (literally on that bit as it curves around onto sidewall as they all do)
Rears are about 5mm at sides, 2-3mm in centre, and about 1.5mm on very inside edge.


It's not so much the very insides that are bothering me, but the general patch wear. It *seems* like I need to run less pressure at the rear as I have worn the sides off, and more at the front to wear the centre!?

I've had a good 10k out of the rears, but that doesn't seem great. The fronts will likely go to 15k. Either way, I'm not happy that they will perform well with such a disparity of wear across their width?! Just feels odd and not 'normal'


Hmmm

Dave

[christer]

Hi Dave

On my car I run 2.3bar and 2.6 bar which is what it specifies in my door on my 550 and it does not specify anything different for RFT's. These pressures in PSI are approx 33-34 in front and 37-38 in rear. These are of course not goin to be the same fro our cars, but somehwere in the range 31-34 in front and 35-40 in back seems to be the case in most cars I have owned.

From the additional info given, it does seem like something strange is going on - I can't remember if you said that you have had a full 4 wheel laser alignment done? If not, I think this is where you should start. In the meanwhile, I recommend that you stick with the correct recommended pressures and run for a while longer after the alignment to see how you get on. I think it is very weird that your tyres are worn more on *both* edges - but to mee this may be a sign that you are running too low pressures, because if you are runnign too high then the middle would wear more in most cases.

In addition to the 4-wheel alignment which I would highly recommend (and you don't have to pay more than £50-90 for that - though I am sure BMW would like far more!) are you sure you are running the correct size tyres on the appropriate sized wheel (are they standard BMW and fit for example?)

I am just trying to explore the simple things first

p.s. 10k miles on the rear of my car would be good in my eyes, but different cars and all that!

[Mr Whippy]

The centre wear on the rears at 33psi suggests they are still too high though, but going lower means way below the reccomended 36psi rear!
The fronts are the other way around, I need to run them higher to wear the centres down, so 33psi (that would be around reccomended pressure actually)

Car is now on 70k, so I'll probably get some powerflex bushes on and then go for a full alignment to get it checked

Also due some new tyres, so will just experiment with what is on now for a few more weeks

Thanks

Dave