Finding optimum tyre pressures/alignment issues!?

[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.

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 pressure front than rear.



Is it ideal to run pressures to wear flat, is this the ideal way to choose the appropriate pressure? I've aimed to do this but still seem to have missed the mark by a fair bit (centre wear bars vs outside ones differ 3-4mm in my case)
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. Is this a good move? I am starting to feel BMW had some odd motives when choosing the alignment setup on the Z4/E46, mainly around RFT... I've not seen settings like them on other cars.

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


I've posted similar on the BMW section, but thought it was a fairly general question too so posted here as well.


Thanks

Dave

[Mr Whippy]

So after some new bushings and a geo setup, am I looking for even temps across the tyre for my intended road use?

Am I best tweaking pressures to achieve that, or can geo be looked at as well?

I just can't help but think as standard that the sportier (but non-M) BMW's have really odd standard alignment settings. Some people suggest adding more neg camber front to reduce the understeer, but shirley when the rears are running nearly 3deg negative that you are best removing some camber at the back to get less understeer? Can you STILL be gaining grip from camber at -3deg?

I thought -2deg was quite alot for road cars on road tyres...?!

Dave

[Mr Whippy]

No simple answer then

Seems like experimentation is key...

My thoughts now are if the standard pressures and alignment are defined more for RFT.

I might try buy some firmer non-run flat tyres this time, since I know the Falken FK452 are known to be a bit softer.


First thing will be new tyres on new wheels, then bushings, then a geometry check. I am tempted to try going for the minimum range on the manufacturer spec for rear camber, and try go to the highest end on front camber... hmmmm

Dave

[Mr Whippy]

Yeah, I'm pretty familiar with the principles and that everything is a compromise

But often I'm not finding myself questioning the setup of a car, but the Z4 just seems to be a bit 'wrong' to what I am familiar with.

OK, it's not entirely relevant, but a simulator I play with (car sim, has it's faults, but still is quite interesting to play with), gives much better expected dynamic results with MZ4 alignment data (not sure if kinematics vary alot though), than with what my car comes with standard.
Ie, lift off oversteer, return to straight ahead with open diff and an induced power oversteer situation, are all better with less rear camber. I could see the benefit with 300bhp+ maybe, but for now the Z4 feels to suffer more for all road work for having that camber setup for grip at the back


I guess what I'm pondering is, and asking, is the Z4/E46 non M setup renowned for being a good handling setup?
I'm thinking a bit less rear camber, and a bit more front (ie, still within tolerances, but each one taken to the extreme of the range), might be worth a go?! Or is that bad logic?

Dave

[Mr Whippy]

Might have to get one of those little thermometers then.

I once saw someone who had mounted an array of those over their tyre contact patches (say 4 per tyre), and ran logging from them all for active tyre temps. Was pretty cool. I don't think I will do that though

Have new wheels/tyres in a few weeks so will start to run from OEM pressures and get all my tracking checked and go from there... (might get this setup erring towards the most front camber and least rear within the specs )

Cheers

Dave

[Mr Whippy]

I'll have to go for a full check on geometry.

I'm also going to try get some kinematic data, because early tests seem to show that the fronts go more negative in bump, and the rear does the same, but even more so.


So it seems in a bend at least, the rear cambers up even more than the front... the harder you roll, the less grip the front gets (a safe handling setup?!)

Since I don't drive like a tool on the road, and the grip levels are so high, I've not really explored the oversteer/understeer at the limit issue, but it doesn't seem to suffer from lift-off oversteer, and generally seems pointy but relatively numb all said and done (my old 306 seemed more aggressive)


The problem is, I'm not sure how much the M models are different kinematically, but the static cambers are less at the rear by about 1.5deg iirc, and a bit less at the front.


As said above, I think I'll err to the higher end for front camber, and the low end for rear camber when I get the alignment done. Still within spec, but more towards equal contact patches, and flatter ones for road use.

Dave