35i tyres runflat or normal

[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. :roll:

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 8)

 

[sammyz]

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

 

[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.

 

[David-H]

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.

Nice that someone has addressed the OP's post with some useful advice / first hand experience.

 

[Nanu]

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.

Am I right in assuming that 4 good non run flats are cheaper generally than 4 good run flats which if true may also come into the equation?

 

[Nictrix]

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.

Am I right in assuming that 4 good non run flats are cheaper generally than 4 good run flats which if true may also come into the equation?

When we were looking for new tyres I looked at prices of both RFT and non RFT as well as gathering info on both and found that non RFTs were around £400 for 4 whereas RFTs were upwards of £600 for 4.
A puncture in a RFT tyre might not be repairable just the same as a non RFT tyre but could cost £70 more expensive to replace the tyre if it couldnt be fixed.
A slightly better ride, lower cost and wider choice makes non RFTs the better choice for me.

 

[ronk]

For two 19 rear RFT I've just paid £400
I don't think the fronts will be much less expensive . So a set will be close to £800

 

[techathy]

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.

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

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

What you’ve failed to comprehend that a tyre is a fully functional suspension system in which has both damping and spring rates. In terms of handling the tyre has to be considered with regard to its properties as an independent suspension system, not just a spring (which you have done).

Why does damping rate effect suspension stiffness? Say the tyre rolls over a small bump, let’s say an expansion joint siting 10mm proud of the road surface, and the car come to a complete stop on top of it. At this point the tyre deflection may be 8mm leaving the wheel to move 2mm upwards. Hit the same bump at 30mph the tyre may only have time to deflect 3mm meaning the wheel has to move 7mm. To complicate matters further, as the force being placed on the tyre is much higher at speed if you hit the same surface imperfection at 70mph and the tyre may deflect 5mm leaving the wheel to move up 5mm.

So because of the damping rates the deflection change depending on how quickly the force is applied & removed from the tyre. Thus tyre stiffness also has a time element to it.

The thing is in a tyre the damping rates and spring rates are both changed by tyre pressure so can not be independently controlled. This all means that while you’ll be able to match the spring rates of an RFT tyre by upping the tyre pressures the damping rates will be way out. The reverse is true. If you try to match damping rates, which can only be matched in a very narrow force range at one time if at all, the spring rates will wrong.

 

[ronk]

As a personal preference, I don't like mixing tyres in any way shape or form.
Tyre manufacturers do in depth testing and evaluation of their design and construction. They use sophisticated equipment to measure the levels of grip at a range of pressures and on various surfaces.

 

[sars]

What you’ve failed to comprehend that a tyre is a fully functional suspension system in which has both damping and spring rates. In terms of handling the tyre has to be considered with regard to its properties as an independent suspension system, not just a spring (which you have done)

Why does damping rate effect suspension stiffness? Say the tyre rolls over a small bump, let’s say an expansion joint siting 10mm proud of the road surface, and the car come to a complete stop on top of it. At this point the tyre deflection may be 8mm leaving the wheel to move 2mm upwards. Hit the same bump at 30mph the tyre may only have time to deflect 3mm meaning the wheel has to move 7mm. To complicate matters further, as the force being placed on the tyre is much higher at speed if you hit the same surface imperfection at 70mph and the tyre may deflect 5mm leaving the wheel to move up 5mm.

So because of the damping rates the deflection change depending on how quickly the force is applied & removed from the tyre. Thus tyre stiffness also has a time element to it.

The thing is in a tyre the damping rates and spring rates are both changed by tyre pressure so can not be independently controlled. This all means that while you’ll be able to match the spring rates of an RFT tyre by upping the tyre pressures the damping rates will be way out. The reverse is true. If you try to match damping rates, which can only be matched in a very narrow force range at one time if at all, the spring rates will wrong.

No, the tyre is PART of a fully functional suspension system, the tyre alone cannot damp oscillations in the same way as a spring cannot, simplistically a football has exactly the same properties as a tyre, less air gives less stiffness which gives a lower amplitude of first oscillation and thus less oscillations before stability returns , BUT it still oscillates and is technically un-damped . A damper reduces oscillation by dissipating kinetic energy through the viscous drag of a fluid, a tyre does not have this ability and you should not confuse the two. What you describe is a reduction in amplitude NOT a damping effect.

So the same tyre with different pressures will effect the amplitude of initial oscillation as you describe because of the altered tyre stiffness, however you can match the overall stiffness from non-RFT's to RFT's by the increase in air pressure AND it is mathematically possible to prove.

So because of the damping rates the deflection change depending on how quickly the force is applied & removed from the tyre. Thus tyre stiffness also has a time element to it.

This statement makes me believe you have little understanding of mechanics, Force and thus Stiffness is not time dependent, what you describe is the effect of velocity and mass, this is known as Momentum