Run-flats Vs. Non run-flats - Can I mix?

Compudaro · 02-12-2015 08:34PM #1

Evening all!

So here's my pickle...car needs 2 new front tyres (2 back tyres have plenty of thread left). 4 run-flats fitted at the mo but I want to change to conventional tyres. Can I (with some degree of safety) replace the 2 worn run-flats with 2 non run-flats. Then when the rear run-flats wear down my plan is to replace those with non run-flats too. Any advice appreciated!

xabi · 02-12-2015 08:38PM

I have heard it's a no no to mix them, car won't handle properly at speed.

TBi · 02-12-2015 08:45PM

I mixed tyres on my mini and BMW. No issues. You aren't supposed to but I saw no problems.

Main thing is you don't mix on a single axle.

sunnysoutheast · 02-12-2015 08:50PM

Generally the advice seems to be that this shouldn't be done due to the differing sidewall stiffnesses but many people say they have done so.

In the end when I had this dilemma I bought four non-RFTs and sold the two RFTs at about 6mm tread on DoneDeal.

Remember the change may or may not be declarable to your insurance and your breakdown service may not cover punctures if there is no spare in the car.

bazz26 · 02-12-2015 10:52PM

Run flat tyres and non run flat tyres have different characteristics which effects the way the car handles. Both car and tyre manufacturers tend to not recommended mixing them and I wouldn't do it myself, I'd either go with all of one type or the other.

PaulKK · 02-12-2015 11:19PM

I did the opposite to you.

I had non runflats front and rear and when replacing the tyres I replaced the rears with run flats approx 6 months before the front since the front were not worn too badly.

I did not experience any issues while they were 'mixed'.

Compudaro · 03-12-2015 12:10AM

PaulKK wrote: »

I did the opposite to you.

I had non runflats front and rear and when replacing the tyres I replaced the rears with run flats approx 6 months before the front since the front were not worn too badly.

I did not experience any issues while they were 'mixed'.

Thanks for all the replies on this. I spotted these, which will fit mine...tempted to just stick another 2 run flats on the front... http://www.adverts.ie/alloys-wheels/bmw-1-2-3-5x120-16-run-flats/8479982

What you think?

5W30 · 03-12-2015 12:12AM

PaulKK wrote: »

I did the opposite to you.

I had non runflats front and rear and when replacing the tyres I replaced the rears with run flats approx 6 months before the front since the front were not worn too badly.

I did not experience any issues while they were 'mixed'.

This.

I don't see how run flats mixed with non-run flats can make a difference if you're running the same tyre per axle e.g. run flats on the rear and no run-flats on the rear or the opposite.

There is no imbalance so it shouldn't offset the car. It's the same as having different brand of tyres on the front and rear of the car. One axle will have more rolling resistance than the other, but that doesn't mean that your car will be fishtailing as you drive along.

The only thing I'm not sure about is whether you can use non-run flat tyres on a run flat alloy.

George Dalton · 03-12-2015 12:17AM

5W30 wrote: »

This.

I don't see how run flats mixed with non-run flats can make a difference if you're running the same tyre per axle e.g. run flats on the rear and no run-flats on the rear or the opposite.

There is no imbalance so it shouldn't offset the car. It's the same as having different brand of tyres on the front and rear of the car. One axle will have more rolling resistance than the other, but that doesn't mean that your car will be fishtailing as you drive along.

Have you ever seen/felt the difference in the sidewall of a standard tyre and a runflat? The runflat sidewall is pretty much rigid (as it has to be to be able to do what it is meant to do, run flat) The standard tyre sidewall is very flexible.

This means that both tyres will behave differently when cornering. In normal driving conditions this won't be noticeable but it could cause very strange things to happen under more extreme situations.

I most definitely wouldn't mix them if it was my car.

PaulKK · 03-12-2015 12:25AM

George Dalton wrote: »

Have you ever seen/felt the difference in the sidewall of a standard tyre and a runflat? The runflat sidewall is pretty much rigid (as it has to be to be able to do what it is meant to do, run flat) The standard tyre sidewall is very flexible.

This means that both tyres will behave differently when cornering. In normal driving conditions this won't be noticeable but it could cause very strange things to happen under more extreme situations.

I most definitely wouldn't mix them if it was my car.

I have to be honest George I noticed no difference whatsoever when I had a mix on mine and I can assure you I don't hang around. Mine are 16" though so they may handle it better than a lower profile.

TBi · 03-12-2015 10:55AM

My E60 has different sized wheels front to back, by design. A lot of cars can have this setup and also cope with matched wheels front to back. So on my car tyres have to have different characteristics. What's the difference between this and running run flat and non run flat?

5W30 · 03-12-2015 11:11AM

George Dalton wrote: »

Have you ever seen/felt the difference in the sidewall of a standard tyre and a runflat? The runflat sidewall is pretty much rigid (as it has to be to be able to do what it is meant to do, run flat) The standard tyre sidewall is very flexible.

This means that both tyres will behave differently when cornering. In normal driving conditions this won't be noticeable but it could cause very strange things to happen under more extreme situations.

I most definitely wouldn't mix them if it was my car.

So if I follow your theory then a car with 4 conventional tyres will be like driving on jelly.

I don't really see your point. Yes a run flat has a rigid sidewall, so what? That doesn't mean conventional tyres are bad now...

I would see it as a danger if you had a normal tyre on one side and a run flat on the other, but different axles won't make any difference at all.

Under-inflated tyres (e.g running 15 PSI instead of 30 PSI) also have weaker sidewalls. Doesn't make it an instant death trap though!

TBi · 03-12-2015 11:36AM

5W30 wrote: »

Under-inflated tyres (e.g running 15 PSI instead of 30 PSI) also have weaker sidewalls. Doesn't make it an instant death trap though!

Good point. My car says the front tyres should be 2.3 bar while the rear should be 2.7 bar. That would make the rear tyre wall stiffer. Again instant death trap!

George Dalton · 03-12-2015 11:40AM

5W30 wrote: »

So if I follow your theory then a car with 4 conventional tyres will be like driving on jelly.

I don't really see your point. Yes a run flat has a rigid sidewall, so what? That doesn't mean conventional tyres are bad now...

I would see it as a danger if you had a normal tyre on one side and a run flat on the other, but different axles won't make any difference at all.

Under-inflated tyres (e.g running 15 PSI instead of 30 PSI) also have weaker sidewalls. Doesn't make it an instant death trap though!

The problem is the difference in the behaviour between front and rear. If all 4 tyres are the same type then there is no imbalance.

Using your example running tyres on one axle at 30 PSI and on another axle at 15 PSI is just as dangerous.

I didn't say it was an instant deathtrap. I said that under extreme (emergency) conditions the car may not behave properly. It's a chance I wouldn't want to take.

VeVeX · 03-12-2015 03:47PM

George Dalton wrote: »

The problem is the difference in the behaviour between front and rear. If all 4 tyres are the same type then there is no imbalance.

Using your example running tyres on one axle at 30 PSI and on another axle at 15 PSI is just as dangerous.

I didn't say it was an instant deathtrap. I said that under extreme (emergency) conditions the car may not behave properly. It's a chance I wouldn't want to take.

Don't get sucked in George :P

5W30 · 03-12-2015 04:21PM

George Dalton wrote: »

The problem is the difference in the behaviour between front and rear. If all 4 tyres are the same type then there is no imbalance.

Using your example running tyres on one axle at 30 PSI and on another axle at 15 PSI is just as dangerous.

I didn't say it was an instant deathtrap. I said that under extreme (emergency) conditions the car may not behave properly. It's a chance I wouldn't want to take.

Well then a different brand of tyres on the front compared to the rear, or all seasons tyres on the front and summer tyres on the rear will cause the exact same thing!

If you walk into any garage and ask for a new set of tyres for the front or rear, they will do it without a bother.

If it was really unsafe the NCT wouldn't pass cars with mixed tyres.

Toyotafanboi · 03-12-2015 04:31PM

5W30 wrote: »

Well then a different brand of tyres on the front compared to the rear, or all seasons tyres on the front and summer tyres on the rear will cause the exact same thing!

If you walk into any garage and ask for a new set of tyres for the front or rear, they will do it without a bother.

If it was really unsafe the NCT wouldn't pass cars with mixed tyres.

yeah but these are the same garages that will sell LingLongs and HappySunshine tyres to customers telling them that they are of reasonable quality and as regards the NCT, well...

For every mechanic who has an enthusiasm for what he does, there are ten who are just stuck in a job who won't have any desire or interest to check, so what "garages" will and wont do isn't really a good gauge of anything.

i was thinking myself that runflats on one axle and not the other would be something akin to having two new premium tyres on the front and two ditchfinders on the back (for example), which i can tell you myself is fine 99% of the time, but one day it will catch you right out, i've been there.

5W30 · 03-12-2015 04:35PM

Toyotafanboi wrote: »

i was thinking myself that runflats on one axle and not the other would be something akin to having two new premium tyres on the front and two ditchfinders on the back (for example), which i can tell you myself is fine 99% of the time, but one day it will catch you right out, i've been there.

I see where you are coming from. Naturally any sort of imbalance could cause the car to oversteer or understeer but the amount of imbalance between both axles won't be big enough.

For example what happens if your run flat goes flat? The imbalance will be a hell of a lot worse. It doesn't necessarily mean it will be dangerous. I know you're not supposed to be flying when your run flats go flat, but still!

The same is with mixing any kind of tyre, there will always be an imbalance. Front wheel drive cars wear front tyres quicker than rear tyres - imbalance.

VeVeX · 03-12-2015 05:12PM

Can I mix Run-Flats and Non Run-Flats?

Yes you can if you choose.

Is it recommended by vehicle manufacturers or tyre manufacturers?

No, it absolutely isn't.

Is boards.ie the place to ask a question and based on the answer arrive at conclusion on an important safety system of my car?

No, it absolutely isn't.

5W30 · 03-12-2015 05:14PM

VeVeX wrote: »

Can I mix Run-Flats and Non Run-Flats?

Yes you can if you choose.

Is it recommended by vehicle manufacturers or tyre manufacturers?

No, it absolutely isn't.

Is boards.ie the place to ask a question and based on the answer arrive at conclusion on an important safety system of my car?

No, it absolutely isn't.

The NCT will pass a car with them so I don't see your point.

Keep telling George to not get "sucked in" though, you bring wonderful posts to this community

VeVeX · 03-12-2015 06:22PM

5W30 wrote: »

The NCT will pass a car with them so I don't see your point.

Keep telling George to not get "sucked in" though, you bring wonderful posts to this community

If you're basing motoring safety on what passes the NCT and what doesn't you're deluded especially where tyres are concerned.

5W30 · 03-12-2015 06:30PM

VeVeX wrote: »

If you're basing motoring safety on what passes the NCT and what doesn't you're deluded especially where tyres are concerned.

Well the NCT do quite a good check on tyres if you read the manual.

True, even the ditch finders can pass the tyre requirements but that's not the NCT's fault. EU Regulations set out the requirements for tyres to get an "E-mark" and it's up to the EU to make sure that crappy tyres don't get an E-mark.

mickdw · 04-12-2015 12:32AM

I think this argument comes down to the characteristic of a particular car.
There should be a huge difference in resistance to sidewall roll and feel from the two types of tyre.
My car for example cannot live with sidewalls that are soft on the rear axle. It genuinely handles dangerously if such tyres are fitted while soft sidewalls on the front suit my driving fine.
I can therefore imagine that a car designed for very rigid sidewalls could handle very very strange on non run flats.

TBi · 04-12-2015 10:22AM

mickdw wrote: »

I can therefore imagine that a car designed for very rigid sidewalls could handle very very strange on non run flats.

I've had a mini and BMW e60 which were both designed for run flats. Changing to non run flats was the best change I ever made. It's very strange alright having a comfortable car that handles and grips better....

sunnysoutheast · 04-12-2015 10:34AM

mickdw wrote: »

I think this argument comes down to the characteristic of a particular car.
There should be a huge difference in resistance to sidewall roll and feel from the two types of tyre.
My car for example cannot live with sidewalls that are soft on the rear axle. It genuinely handles dangerously if such tyres are fitted while soft sidewalls on the front suit my driving fine.
I can therefore imagine that a car designed for very rigid sidewalls could handle very very strange on non run flats.

I believe the main issue is that - where the two types are mixed - the axle on the RFTs would effectively be much harder sprung than the non-RFT axle which would have a consequent effect on the handling.

I changed the RFTs on my F30 to non-RFTs and there is definitely a difference but not as much as I expected - probably due to the profile (225/45R18) and the non-RFTs being XL-rated.

The tyre place in the UK I used to use told me that they wouldn't fit a mix of RFTs and non-RFTs on the same car.

Run-flats Vs. Non run-flats - Can I mix?

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