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

[leorest]

A full four wheel geo setup and then buy an infra-red thermometer gun.

[spend]

leorest said:

.. buy an infra-red thermometer gun.

Absolutely worth their weight in gold IMHO.

Forget the accuracy of calibration blox, even cheapies are plenty good enough for most purposes.

[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

[leorest]

Mr Whippy said:

...am I looking for even temps across the tyre...

Not 100% sure but I think even temp will give even wear and even wear doesn't mean best handling. One setup will give best results for one track/car/driving style combination. Alter any of the previous and all bets are off.
Confused? Join the club. There are loads of good books on the subject. Staniforth/Puhn/Milliken.
Fred Puhn gets my vote for a well rounded "How to" starter book.

Mr Whippy said:

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

Most likely a bit of both. Obviously air can easily be done yourself whereas alignment takes a bit more equipment and/or effort.
There's a whole industry devoted to such tweakings. Some of the big car manufacturers (Toyota) spend millions developing new cars then employ consultant engineering companies (Lotus) to polish the design.

[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

[Sam_68]

Mr Whippy said:

No simple answer then

Nope, 'fraid not.

Just about everything about car handling and set-up is usually a compromise between several factors.

For example (WRT your original question) modern tyres generate maximum cornering grip when they're set at a fairly hefty negative camber setting, but this will wear the inside shoulders quite heavily (and give uneven temperature readings) if you spens a lot of time going in a straight line, and will give less grip when braking or accelerating.

[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

[Sam_68]

Mr Whippy said:

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?

All other things being equal, it is likely that:

• less negative camber at the rear = less cornering grip at the rear
• more negative camber at the front = more cornering grip at the front

...net result = move toward oversteer, and rear end breakway more likely.

Whether you think that's a good idea depends on whether the car understeers too much for your tastes at present.

... and you may well be able to compensate for the increased oversteer without effecting the improved tyre wear too much by playing around with the tyre pressures a little.

[anonymous-user]

I was at the track last year and the tyre manufacturer I use was present, one of their engineers spent a lot of time helping me set the car up for tyre pressure, and all he did was try to get even temperature across the tyre and to get all 4 pressure the same when the tyre was hot. Ended up running the left front lower than the others when cold, then when I came in with the tyres hot all 4 wheels ran the same PSI and the temperature measured across each tyre was even, I run almost 50/50 weight across the front and back, used a thermo thermometer to check.

[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

[Sam_68]

Berw said:

I was at the track last year and the tyre manufacturer I use was present, one of their engineers spent a lot of time helping me set the car up for tyre pressure, and all he did was try to get even temperature across the tyre and to get all 4 pressure the same when the tyre was hot.

That's absolutely fine for track use, when you're working the tyres consistently hard and you aren't too worried about wear rates, but for road use (where the car spends more time being driven gently and/or in a straight line)you may find that setting the geometry and pressures to give consistent temparature on a track will result in fairly extreme wear patterns in general road use.

[flemke]

I know less than nothing about BMWs (and everything else, to be truthful), but you're dealing with a moving target.
The "right" camber for the road will be different from that for the circuit. One will try to allow for this by adjusting pressures, but that will only get you so far.
As you reduce pressure, you'll be making the contact patch not only wider but also longer, so the inside corner ("hinge" or junction between the belt/tread and the sidewall) will spend more time in contact with the road. Also the more deformation (which you would get at the hinge by lowering the pressure), the more hysteresis, which in turn means more rapid wear.
It sounds as though now you have more camber F&R than you would want for public road driving. To compensate for the fact that too much camber is distorting the CP for low lateral-g driving you've lowered the pressures, but because of that low lateral-g the tyre rarely gets tipped into a more vertical position. If you were racing with that amount of camber, a high % of the total tyre wear (which obviously would happen more rapidly than on road) would be in centre of CP, but that's not your usage. So instead of getting 500 mi of circuit wear distributed across CP, you're getting 2000 mi of road wear, mostly on the hinge.

[flemke]

Another thing - have you checked your toe (without taking off your shoes)?
Excessive toe can rapidly accelerate uneven tyre wear (although it's possible that you would desire it in order to affect handling).

[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

[jonamacg83]

All sorts of relevant info here:

http://www.macgracing.co.uk/trackdays_setup1.html