Run-flat & non-run-flat tyre pressures

Alloy wheels and tyre discussion
Busterboo
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Run-flat & non-run-flat tyre pressures

Post by Busterboo » Thu Jan 12, 2017 12:53 am

:) 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.

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Run-flat & non-run-flat tyre pressures

Post by mr wilks » Thu Jan 12, 2017 9:54 am

Busterboo wrote::) 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
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Run-flat & non-run-flat tyre pressures

Post by Busterboo » Thu Jan 12, 2017 10:09 am

mr wilks wrote:
Busterboo wrote::) 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.

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Run-flat & non-run-flat tyre pressures

Post by Ducklakeview » Thu Jan 12, 2017 10:35 am

Busterboo wrote:
mr wilks wrote:
Busterboo wrote::) 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.

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Run-flat & non-run-flat tyre pressures

Post by Buckz » Thu Jan 12, 2017 10:49 am

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.
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Run-flat & non-run-flat tyre pressures

Post by Busterboo » Thu Jan 12, 2017 11:24 am

Just phoned Sytner BMW Leicester. Spoke to the Chief Technician, said that I'd switched from Bridgestone to Pirelli, from run-flat to non-run flat, and asked which pressures I should use.

His answer was (the same as Bridgestone's): "If it's the same tyre size, it's the same pressure."

:)

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Run-flat & non-run-flat tyre pressures

Post by Ducklakeview » Thu Jan 12, 2017 11:29 am

Busterboo wrote:Just phoned Sytner BMW Leicester. Spoke to the Chief Technician, said that I'd switched from Bridgestone to Pirelli, from run-flat to non-run flat, and asked which pressures I should use.

His answer was (the same as Bridgestone's): "If it's the same tyre size, it's the same pressure."

:)
Somewhere to avoid taking your car then...

Seriously, just do what you want, but make sure you post some pics after you have wrapped the rear end of your 35is around a tree, so we can all say "told you so" but I'm sure your knowledgeable BMW dealer will apologise and pay for the damage... Not!

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Run-flat & non-run-flat tyre pressures

Post by bmwaddict » Thu Jan 12, 2017 12:35 pm

Mate, don't do it, everyone is clearly advising you against it.
I work in the tyre industry and I don't care what bmw says, putting 45psi in a normal tyre of the type we use on our zeds (low profile, wide sporty tyres) will surely compromise the grip and therefore safety of your car if you ever push it and drive spiritedly.
I do mine 33 front 36 rear, and although it's not an e89 you must see from what everyone is saying that it's not a good idea to put loaded ford transit type psi in a tyre of a 1400kg sports car.
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Run-flat & non-run-flat tyre pressures

Post by Steve84N » Thu Jan 12, 2017 2:28 pm

One thing I have noticed is that higher profile tyres tend to run lower pressures. This can be seen on something like a 3 series where the door sticker has pressures listed for different wheel/tyre options. Therefore, the M might be a little low as they run a higher profile. This is consistent with what most people settled with as being a bit above M but below standard RFT.
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Run-flat & non-run-flat tyre pressures

Post by Busterboo » Fri Jan 13, 2017 1:31 pm

Have just phoned 4 tyre manufacturers and asked each one the same question:
"Are run-flat and non-run-flat tyre pressures the same?"
Their answers were:

Michelin: "The same."
Goodyear/Dunlop: "The same."
Bridgestone: "Tyre pressures depend on the vehicle."
Pirelli: "We do not give tyre pressures. Follow your car maker's advice."

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Run-flat & non-run-flat tyre pressures

Post by Busterboo » Mon Jan 16, 2017 1:37 am

Busterboo wrote:Have just phoned 4 tyre manufacturers and asked each one the same question:
"Are run-flat and non-run-flat tyre pressures the same?"
Their answers were:

Michelin: "The same."
Goodyear/Dunlop: "The same."
Bridgestone: "Tyre pressures depend on the vehicle."
Pirelli: "We do not give tyre pressures. Follow your car maker's advice."
The BMW figures for the E89 35is are 2.6 bar/37.7 psi front and 3.1 bar/44.96 psi rear.

These are, however, for (a) 2 passengers & luggage and make no mention of (b) speed.

I can find no figures for 1 passenger & no luggage or for high speed.

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Run-flat & non-run-flat tyre pressures

Post by Nictrix » Mon Jan 16, 2017 11:35 am

Just to throw something else into the mix, our X6 that has runflat tyres and the rears are 315 x 20 and front 275 x 20 are 2.0 bar front and back.
Our 23i Z is 2.5 bar front and 3.0 bar rear on 18" with the rears being 235s.
Why is the pressures lower for a much heavier car?
And we have been running the Z4 with standard pressures with non runflats for almost 3 years with no problems and no loss of grip for the driving that we do.
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Run-flat & non-run-flat tyre pressures

Post by Busterboo » Mon Jan 16, 2017 1:03 pm

Nictrix wrote:Just to throw something else into the mix, our X6 that has runflat tyres and the rears are 315 x 20 and front 275 x 20 are 2.0 bar front and back.
Our 23i Z is 2.5 bar front and 3.0 bar rear on 18" with the rears being 235s.
Why is the pressures lower for a much heavier car?
And we have been running the Z4 with standard pressures with non runflats for almost 3 years with no problems and no loss of grip for the driving that we do.
Your E89 23i's pressures, recommended by BMW (and confirmed by the major tyre manufacturers to be for both run-flats and non-run-flats) are, as you say, 2.5 bar front and 3.0 bar rear. In old money, that's 36.25 psi and 43.5 psi.

You say, however, "for the driving that we do", you've been running your non-run-flats at "standard pressures" - presumably in the low 30s - "with no problems".

Good, I'm glad. The 'driving that I do' can make my 35is's 335 bhp & 370 ft/lbs of torque a bit of a handful sometimes and I envy you your 23i.

For my part, however, I want to drive my car at the recommended pressures because, when I push it hard, I need all the help the tyres can give me. :)

(As for your X6, like any car, don't the tyre size, profile, laden weight & speed of use dictate the pressures?)
Last edited by Busterboo on Mon Jan 16, 2017 2:36 pm, edited 1 time in total.

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Run-flat & non-run-flat tyre pressures

Post by Nictrix » Mon Jan 16, 2017 1:14 pm

Busterboo wrote:


You say, however, "for the driving that we do", you've been running your non-run-flats at "standard pressures" - presumably in the low 30s - "with no problems".
You have grabbed the wrong end of the stick, the Z4 is run at standard pressures ie. 2.5 bar front 3.0 bar rear with no problems.
You seem to have pulled low 30s from nowhere as I mentioned nothing about this.
I was on your side with this discussion.
I do believe you have a point in wanting to know what pressures you should put in whether or not lower than the manufacturer states.
Just because you are being told these pressures are too high by people on the internet does not mean that they are correct.
This same argument goes on with bike forums as well as nowadays it is fairly normal for the rear on a bike to be 42psi which a lot of people say this is 6 psi too high.
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Run-flat & non-run-flat tyre pressures

Post by Busterboo » Mon Jan 16, 2017 2:39 pm

You're right. I misinterpreted "standard pressures". :oops:

But now I'm very glad you're 'on my side'. :)

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