35i tyres runflat or normal

[techathy]

If you set the suspension up harder at the front compared to the rear you'll induce under-steer. So if you're suggesting putting non-RFT tyres on the rear then yeah, I don't see much of an issues there. However, just like having a massively miss-matched spring & damper setup between the axles, which both work quite well in isolation, you may also find weird things happening with regard to chassis resonances.

Personally I'd be concerned with the situation of non-RFT tyres up front & RFT at the rear.

[David-H]

I'm not arguing per se, just asking for a valid reason of WHY, because asking why is how you learn. The difference between you and I is that I'm interested in facts whilst your after opinions.

and HIS mind, that's soooo insulting

Sorry, should have know that you're a self opinionated female.

BTW It is a fact that RFT have different stiffness and handling characteristics to non-RFT.

Also, watch your grammar - should have been "whilst you're after opinions" - or the grammar police will be after you.

[techathy]

I'm not arguing per se, just asking for a valid reason of WHY, because asking why is how you learn. The difference between you and I is that I'm interested in facts whilst your after opinions.

and HIS mind, that's soooo insulting

Sorry, should have know that you're a self opinionated female.

BTW It is a fact that RFT have different stiffness and handling characteristics to non-RFT.

Also, watch your grammar - should have been "whilst you're after opinions" - or the grammar police will be after you.

There was a thread on here, I think, not so long ago where someone had different brands of non-RFT tyres front & rear on their car. They were complaining about the handling doing weird things. I think they had HP on one axel and UHP tyres on another, so a much smaller difference compared to RFT & non-RFT.

[David-H]

I'm not arguing per se, just asking for a valid reason of WHY, because asking why is how you learn. The difference between you and I is that I'm interested in facts whilst your after opinions.

and HIS mind, that's soooo insulting

Sorry, should have know that you're a self opinionated female.

BTW It is a fact that RFT have different stiffness and handling characteristics to non-RFT.

Also, watch your grammar - should have been "whilst you're after opinions" - or the grammar police will be after you.

There was a thread on here, I think, not so long ago where someone had different brands of non-RFT tyres front & rear on their car. They were complaining about the handling doing weird things. I think they had HP on one axel and UHP tyres on another, so a much smaller difference compared to RFT & non-RFT.

That's what I was trying to advise the OP about but she - Sars - didn't seem to want to understand that the behaviour of different types of tyres can lead to unpredictable and dangerous response. But, then again, when ever do females listen?

[sars]

Sorry, should have know that you're a self opinionated female.

Not just an opinionated female (we all have opinions after all), but a Professional Mechanical Engineer with 30 years design experience, who just happens to be a woman, who sometimes likes to poke trolls

BTW It is a fact that RFT have different stiffness to non-RFT.

True, but relative sidewall stiffness is also a function of tyre pressure, increase the pressure by a few psi in a non-rft and you can obtain a similar stiffness as a rft, basic physics honey, certainly you can take steps to minimise the differences. As I have said previously the tyres for a staggered set up have different characteristics between each size, width alone makes a big difference. You also run different pressures front to rear, even on a square set up and again this will effect how a tyre and thus effects handling. There are so many tyre/wheel size combinations that are available at point of sale, for which engineers have to allow for when developing the car, this ultimately gives a safe window in which to play with. Just see Mr Wilks, he plays with non RTFM recommended sizes and he's still with us

That's what I was trying to advise the OP about but she - Sars - didn't seem to want to understand that the behaviour of different types of tyres can lead to unpredictable and dangerous response. But, then again, when ever do females listen?

Yet you still fail to produce any scientific evidence or test reports in support of your first statement, I mean if it's so dangerous, there must be litterally 1000's of accidents out there because of it. Alas no you just want to insult and bully like any school yard thug

[David-H]

Sorry, should have know that you're a self opinionated female.

Not just an opinionated female (we all have opinions after all), but a Professional Mechanical Engineer with 30 years design experience, who just happens to be a woman, who sometimes likes to poke trolls

If indeed you are a Chartered Engineer, with a decent degree, it's surprising that you have such poor grammar.

My first statement was that mixing RFTs & non-RFTs is not recommended, not that it is dangerous or anything else. I have no idea why you have chosen to start an argument over this or to ask for scientific proof etc. other than trying to sow discord.

Being as a troll (/ˈtroʊl/, /ˈtrɒl/) is a person who sows discord on the Internet by starting arguments or upsetting people, it would appear that you sometimes like to poke yourself?

Calm down dear, calm down.

[sars]

Calm down dear, calm down.

sorry should have added sexist and patronising

[David-H]

Calm down dear, calm down.

sorry should have added sexist and patronising

Just taking a leaf out of our former Prime Minister's book!

[David-H]

Not just an opinionated female (we all have opinions after all), but a Professional Mechanical Engineer with 30 years design experience,

An Engineer eh? Me too, once upon a time.

my profile

[sammyz]

Sad to see sexism and derision of others opinions are alive and well on this forum. Should be better than that

[techathy]

True, but relative sidewall stiffness is also a function of tyre pressure, increase the pressure by a few psi in a non-rft and you can obtain a similar stiffness as a rft, basic physics honey, certainly you can take steps to minimise the differences. As I have said previously the tyres for a staggered set up have different characteristics between each size, width alone makes a big difference. You also run different pressures front to rear, even on a square set up and again this will effect how a tyre and thus effects handling. There are so many tyre/wheel size combinations that are available at point of sale, for which engineers have to allow for when developing the car, this ultimately gives a safe window in which to play with. Just see Mr Wilks, he plays with non RTFM recommended sizes and he's still with us

The problem is 'stiff' as we perceive it isn't actually is a load of things, like spring rates, lobbed together and turned into a sensation in the human brain.

Anyway an RFT tyre the stiffness is due to the lack of suppleness in the side wall , compared to a non-RFT tyre, meaning it takes a long time to move & it also has a shorter maximum travel. Compared to and RFT running the non-RFT tyre at a higher pressure tyre it will likely deflect further, also as you increase the tyre pressure the return spring rate of a non-RFT tyre is higher meaning that it'll maintain its contact patch better. It's a bit like running the RFT axle of your car with short throw suspension with a low spring rate and over damping it, then running other axle under damped with a higher spring rate. Dynamically either is okay(ish) if you run both axles like that but it'll produce a horrible result if you mix n' match.

With regard to running outside the manufactures specs. You can change things a lot more if they're working in a balanced way than in a completely unbalanced manner. I run non-RFT 245/35R18 front & 265/35R18 rear & absolutely love it.

[sars]

Anyway an RFT tyre the stiffness is due to the lack of suppleness in the side wall, compared to a non-RFT tyre such that it can support meaning it takes a long time to move & it also has a shorter maximum travel. Not only doesn't the tyre deflect as much it takes longer to deflect &return.

The SI unit of stiffness is N/m and it is defined as the ability to resist deflection by a force. It is a function of material strength and it is NOT time dependent (that is a force of 1N will deflect a material 1mm regardless if it is 1 minute or 60 days)

Both a tyre and a spring have stiffness (for springs this is known as rate), I can duplicate the rate of any spring exactly by using the compressive nature of a gas in a pneumatic cylinder (increasing or decreasing volume). A tyre filled with compressed air induces an increase in stiffness of the tyre in exactly the same way as the extra sidewall reinforcement of a RFT.

However compared to running the non-RFT tyre at a higher pressure tyre it will likely deflect further.

No, because if we match overall stiffness of the tyre by over pressurising the non-RFT slightly, deflection will be identical given the same force

Also as you increase the tyre pressure the return spring rate is higher meaning that it'll maintain its contact patch better than the RFT tyre at the lower RFT pressure, and even at the same pressure.

I'm sorry that doesn't make any sense, yes you can reduce contact patch of a tyre by over inflating obviously, and to be honest I am unaware of the exact over pressure required to align the stiffness between the two tyres, when I changed to non RTF's in the zed it was about 0.2 bar to get the feel right, with pretty even wear across the tread

[Nanu]

surely the answer is to fit the non run flats and see what happens, but before you do accept that you may have to change all four.

[sars]

surely the answer is to fit the non run flats and see what happens, but before you do accept that you may have to change all four.

That's a useful and positive post

[35isDreamer]

My 35i has Bridgestone Potenza runflat tyres and the rear ones are ready to be replaced. They have not lasted long not much more than 10K miles. Are there options to replace, anyone fitted standard 255/25/R18 tyres like Falken or others.
I ask as the runflat rubber is a hard compound and the rear end is twitchy which I think is due to hard rubber compound.
There appear limited options with that size

I swapped the 19 RFs on my 35is recently. Went for Continental Sport Contact 5, as these were the only nonRFs that my dealer had that day. No idea how these compare with other tyres.

Difference is noticeable. Much more grip and better ride + quieter. Not revolutionary, but better. Very pleased I did it.

Dealer said it might invalidate insurance if you don't let them know. The also confirmed it doesn't impact my BMW warranty.