Run-flat & non-run-flat tyre pressures

[Busterboo]

In another thread, in the context of changing from run-flat to non-run flat/normal tyres and wondering which pressures to use for the latter, I wrote,

"My understanding is that tyre pressures are determined by the car and not by the tyre. (So, the same tyre on different cars will be inflated to different pressures, but different tyres on the same car will be inflated to the same pressures.) In the case of the 35is, that's 37psi front and 45psi rear." [Emphasis added.]

These numbers still seemed high to me, so I checked direct with Pirelli and looked at a Bridgestone website.

Pirelli said, "Ask BMW."

Bridgestone write, "Contrary to popular belief, tyre pressure is not determined by the type of tyre or its size, but upon your vehicle's load and driving application, i.e. speed."

 

[mr wilks]

I think the true answer is there isn't one all i do know is 45psi in a 255 30 19 profile makes it easier to understand why so many of the alloys experience cracking

 

[Ducklakeview]

Personally, I'd just ue the recommended pressures for the ///M if moving from run flats, which I will also be doing shortly, going for a set of Goodyear Eagle F1 Asymmetric 3's. just under £400 delivered from Oponeo with the £25 Amazon voucher..

My car on std RFT's;



///M's on non RFT's;



Mike

 

[TomK]

My understanding is that run flats are inflated higher so that the car doesn't ride so much on the sidewalls (which are extremely stiff in a run flat).
You would never see a non run flat manufacturer recommended pressure for any car or tyre anywhere near that of runflat recommended pressures.
32 f 32 or perhaps 34 rear would be a good starting point for non run flats imo.

 

[Bing]

For some reason RFT tyre pressures always seem to be higher than non-RFT. No idea why. Driven wheels are also higher than non-driven.

The manual for the M says 30psi front, 32 rear :

http://www.z4-forum.com/forum/viewtopic.php?t=24559

Pretty sure the bar numbers on my door will translate to 32/33 though... will need to check.

On my 3.0Si when I moved to non-RFTs I played with the pressures a little and settled on 34/36 I think.

 

[Ducklakeview]

For some reason RFT tyre pressures always seem to be higher than non-RFT. No idea why. Driven wheels are also higher than non-driven.

I "think" that's because the pressures have to be higher to take the load from the reinforced sidewalls.

Mike

 

[Deddi]

Ducklakeview, that's a good price! I just checked them out but the 255 rears only available as Asymmetric 2 not 3. Did you manage to find the 3s? Cheers

 

[Ducklakeview]

Ducklakeview, that's a good price! I just checked them out but the 255 rears only available as Asymmetric 2 not 3. Did you manage to find the 3s? Cheers

Yup - £422 delivered, with a free £25 Amazon voucher



Mike

 

[Deddi]

Thanks, I'll try again!

 

[Busterboo]

Let me repeat what Bridgestone say: "Contrary to popular belief, tyre pressure is not determined by the type of tyre or its size, but upon your vehicle's load and driving application, i.e. speed."

They add: "To find out what your car's tyre pressures should be, consult the manufacturer's tyre placard, usually found inside the driver's door sill ...".

 

[mr wilks]

Let me repeat what Bridgestone say: "Contrary to popular belief, tyre pressure is not determined by the type of tyre or its size, but upon your vehicle's load and driving application, i.e. speed."

They add: "To find out what your car's tyre pressures should be, consult the manufacturer's tyre placard, usually found inside the driver's door sill ...".

Confusing me now a little as to what the exact purpose of this thread is for ?

Is it to determine that any given tyre size , brand , runflat or not can be at different pressures dependent on car ?
or
Is it to clarify that the manufacturers recommended pressures found on the car are always correct ?

 

[paulgs1000]

I ran 32 front and 34 rear on both my 2 litre and 3.0si Zeds - the pressure for my M is lower still - all non runflat tyres.

My belief is that runflat tyres have stiffer sidewalls and therefore need higher pressures to create the necessary shape (footprint), non runflats have a softer sidewall and therefore do not need the higher pressure to create and maintain the same shape (footprint).

The tyre manufacturer quotes you have, make sense if they are talking about the original fitment on your Zed - in other words the psi stated apply to any runflat tyre fitted. However we are now talking about non original equipment and therefore does the quote apply - I would say not.

You'll sharp find out whether what you are being advised is right or not but do take care. I found my Zeds were unstable, span up and slid about when I used runflat pressures with non runflat tyres.

Let us know how you get on.

 

[Busterboo]

Let me repeat what Bridgestone say: "Contrary to popular belief, tyre pressure is not determined by the type of tyre or its size, but upon your vehicle's load and driving application, i.e. speed."

They add: "To find out what your car's tyre pressures should be, consult the manufacturer's tyre placard, usually found inside the driver's door sill ...".

Confusing me now a little as to what the exact purpose of this thread is for ?

Is it to determine that any given tyre size , brand , runflat or not can be at different pressures dependent on car ?
or
Is it to clarify that the manufacturers recommended pressures found on the car are always correct ?

It's to find out which pressures should be used in non-run-flat tyres on an E89.

In my case, a 35is.

 

[Busterboo]

I think the true answer is there isn't one all i do know is 45psi in a 255 30 19 profile makes it easier to understand why so many of the alloys experience cracking

Since at least the late 1990s, Porsche 911s have run on 36psi front & 44psi rear alloys. I had one for 5 years, used the local Official Porsche Centre and my local 'indie', and read the main UK Porsche website daily. During that time and from those sources, I never heard of an alloy cracking - except in an accident, of course.

Accordingly, it seems even "easier to understand" that 19" BMW alloys don't crack because of high tyre pressures or even the hard Bridgestone run-flats, but because they're inherently poor wheels.

 

[Wrs]

It seems you are really dead set on going with run-flat recommended pressures for your non run-flat tyres :| so I suggest you crack on then let us know how you get on... Be cautious with your rears at 45 psi though. The ride will be pretty harsh too.

Personal choice I guess and test various pressure combos to see what works/feels best to you .

 

[Busterboo]

I'm not "dead set" on anything. I'm just trying to confirm what's right.

So, as I've said above, if 36psi & 44psi are right on a 911, why aren't similar figures right on an E89?

After all, they're the ones that BMW give.

And Bridgestone state clearly that pressures are not dependent on tyre type - they're the same for run-flats and non-run-flats.

 

[mr wilks]

I'm not "dead set" on anything. I'm just trying to confirm what's right.

So, as I've said above, if 36psi & 44psi are right on a 911, why aren't similar figures right on an E89?

After all, they're the ones that BMW give.

And Bridgestone state clearly that pressures are not dependent on tyre type - they're the same for run-flats and non-run-flats.

Has it crossed your mind the rear of a 911 carries all the weight & cornering at any speed will have significant effect on the rear tyres :roll:
From my time with 2 x 911s i recall tyre pressures were also a talking point on the forums & as with this topic subjective to personal choice & not everybody found the manufacturers numbers to be the best for them

 

[Busterboo]

I'm not "dead set" on anything. I'm just trying to confirm what's right.

So, as I've said above, if 36psi & 44psi are right on a 911, why aren't similar figures right on an E89?

After all, they're the ones that BMW give.

And Bridgestone state clearly that pressures are not dependent on tyre type - they're the same for run-flats and non-run-flats.

Has it crossed your mind the rear of a 911 carries all the weight & cornering at any speed will have significant effect on the rear tyres :roll:

Yes, the first time I drove my 911 and thought, 'Kuh! These tyres are hard.'

The 'front lower pressure, back higher pressure' is true of the Z4, too, however, whether it's on run-flats or not.

It doesn't relate to where the engine is, but to where most forces occur on the wheels.

 

[Ducklakeview]

I'm not "dead set" on anything. I'm just trying to confirm what's right.

So, as I've said above, if 36psi & 44psi are right on a 911, why aren't similar figures right on an E89?

After all, they're the ones that BMW give.

And Bridgestone state clearly that pressures are not dependent on tyre type - they're the same for run-flats and non-run-flats.

Has it crossed your mind the rear of a 911 carries all the weight & cornering at any speed will have significant effect on the rear tyres :roll:

Yes, the first time I drove my 911 and thought, 'Kuh! These tyres are hard.'

The 'front lower pressure, back higher pressure' is true of the Z4, too, however, whether it's on run-flats or not.

It doesn't relate to where the engine is, but to where most forces occur on the wheels.

OF COURSE engine position will have an effect, as it means that the rears on a 911 will be carrying more weight from the off. IMO, if you put those run flat pressures into std tyres on the rear of your 35is, then you are asking for trouble.

You've asked for advice, been given it by numerous people, based on their experiences, and still choose to ignore/question it.

Can we ask WHERE Bridgestone state that the pressures are the same for both RFT's and non? I was under the impression that all E89's came with RFT's from the factory, so how would they be in a position to give this advice? And yes, BMW may give those figures, and they may be correct for the car, as supplied, ie with RFT's, but again how can they say the same car on non RFT's should run at the same pressures, IF they have never supplied such a car?

I'll say it again, RFT's need EXTRA air due to the fact that they are inherently more rigid than normal tyres, and as such, if operated at lower pressures, the sidewalls will start to take more of the weight of the vehicle, leading to rapid wear and overheating. Think of it this way, would you be happy dropping say 12 psi from the recommended pressures on "normal" tyres? I wouldn't, for the same reason, ie the sidewalls will begin to carry the weight, and as they are not as strong in a traditional tyre, overheating and wear will take place at those sorts of pressures below what they were designed for. It's not rocket science...

Therefore, my analogy regarding my own, in that I will be running pressures close to those recommended for an ///M, ie pretty much the same car in terms of weight/tyre size etc.

I would have said, in response to Mr Wilks quaestio, that this really is a thread without purpose.

Mike

 

[Buckz]

I'd probably stick M pressures on on non RFT's tbh! this is as close as you can get to manufacturers spec being a very similar car.

imo over 40psi on non RFT is too much.