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BMW 3-Series (E90 E92) Forum > BMW E90/E92/E93 3-series General Forums > Regional Forums > UK > UK Technical Forum > do i need to use run flats???



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      12-07-2011, 04:13 PM   #1
pez
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do i need to use run flats???

Hey guys

i am looking to buy the MV3 18inch alloy wheels and have noticed many people selling them with non run flat tyres on them.

it has made me think do i need to use run flat tyres or do the rims sup[port normal tyres?

this would be a lot cheaper!

cheers

Pez
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      12-07-2011, 04:22 PM   #2
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plenty to search on here.....

.....but basically ANY wheel can take run flats or non runflats. There is no difference between any wheel for either type of tyre these days.

Any tyre place that tells you otherwise is talking BS.
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      12-07-2011, 04:23 PM   #3
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Thats like asking if you should wear a condom.

Always safer, but NOT NECESSARY.
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      12-07-2011, 05:24 PM   #4
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      12-07-2011, 05:35 PM   #5
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Quote:
Originally Posted by scott135i View Post
Thats like asking if you should wear a condom.

Always safer, but NOT NECESSARY.
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      12-07-2011, 08:55 PM   #6
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Quote:
Originally Posted by scott135i View Post
Thats like asking if you should wear a condom.

Always safer, but NOT NECESSARY.
They take most of the fun out of it too...
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      12-08-2011, 04:08 AM   #7
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Wheels for Run Flats

2 points to know when considering different types of tyres

BMW rims designed for runflat tyres are all designated EH2. This stands for E-xtended H-ump 2-two humps. This is essentially a design that locks the Run Flat tyre bead onto the rim and ensures that the tyre remains properly mounted in the event of a sudden high speed deflation.

Secondly, standard chassis suspension and damping rates for run flat tyres are quite different. This is due to the lower compliance of the sidewalls, which requires greater suspension compliance. Changing from run flat to normal tyres will change the handling characteristics of the car (some may claim for the better. Personally I don't like to 2nd guess a group of BMW engineers and thousands of hours of testing)

Whether a tyre can be mounted on a rim and successfully inflated is a very different question vs. how a tyre/rim combination behaves in a sudden high speed deflation. 120mph on a German Autobahn is not the place to find out that the combination sucks.

In a typical high speed deflation, a normal tyre on a normal rim will be unstable and may actually come off the rim after the deflation, causing the driver to lose control

A run flat on a purpose made rim is designed to stay on the rim after sudden deflation, allowing the driver to maintain control.
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      12-08-2011, 04:51 AM   #8
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Quote:
Originally Posted by doughboy View Post
plenty to search on here.....

.....but basically ANY wheel can take run flats or non runflats. There is no difference between any wheel for either type of tyre these days.

Any tyre place that tells you otherwise is talking BS.
Not true - ANY wheel can take non runflats, but not the other way around, having said that all modern BMW wheels are designed to accept runflats. If you are buying older used or non BMW wheels you would need to check to be sure.
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      12-08-2011, 05:25 AM   #9
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Quote:
Originally Posted by SteveC View Post

Secondly, standard chassis suspension and damping rates for run flat tyres are quite different. This is due to the lower compliance of the sidewalls, which requires greater suspension compliance. Changing from run flat to normal tyres will change the handling characteristics of the car (some may claim for the better. Personally I don't like to 2nd guess a group of BMW engineers and thousands of hours of testing)
But on E90 (and other BMW) models that don't come with RFTs (ES models) there is no difference to any of the suspension part numbers AT ALL (bushings / arms / dampers / springs). This proves the range-wide alterations (see 'compromises') made to vehicle supension are nothing to do with safety but purely to try and accomodate the awful characteristics of RFTs and mitigiate customer complaints. If they were safety critical, then RFT and non RFT models would have to be different of course.

I'm sure the BMW engineers were gutted about having to destroy great handling in order to accomodate a marketing led push to RFTs.... Thus why M division don't won't use RFTs.

Quote:
Originally Posted by Falmouthboy View Post
Not true - ANY wheel can take non runflats, but not the other way around, having said that all modern BMW wheels are designed to accept runflats. If you are buying older used or non BMW wheels you would need to check to be sure.
Apologies if I got that wrong, but I thought the extended bead hump was only required for older RFTs and that modern RFTs can go on any rim?

The only legal requirement being that the vehicle must have some kind of low or differential pressure warning system (TPMS)


A quick google finds the following statements many times:

Quote:
Self-supporting tires (SSTs) are the original and most common run-flat type. Heavily reinforced sidewalls support the vehicle after air departs the scene. This sort of run-flat is designed to fit on normal wheels with no modifications.

Michelin's PAX, a patented auxiliary support run-flat system, is a relative newcomer. PAX sidewalls, while still stiffer than normal tires, are not as rigid as SSTs. Instead Michelin designed a unique wheel that positions a semi-rigid "support ring" inside the tire to hold the car up when the air goes bye-bye. A non-standard bead design is necessary where wheel and tire meet.

Enter TPMS

A side effect of the stiff sidewalls found on run-flats is that they never look flat. As a result, the danger of driving on underinflated tires is even greater, as many people don't check their tire pressures until they "look" low.

To counter this problem, the use of tire-pressure monitoring systems (TPMS) has become mandatory in run-flat applications. Since run-flats only provide a limited zero-pressure driving range, TPMS is critical to help the driver know when the mileage clock starts ticking, and more importantly, when time is up.

Last edited by doughboy; 12-08-2011 at 05:34 AM.
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      12-08-2011, 09:48 AM   #10
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Quote:
Originally Posted by scott135i View Post
Thats like asking if you should wear a condom.

Always safer, but NOT NECESSARY.
Depends upon application - just like tyres
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      12-08-2011, 09:54 AM   #11
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Quote:
Originally Posted by SteveC View Post
2 points to know when considering different types of tyres

BMW rims designed for runflat tyres are all designated EH2. This stands for E-xtended H-ump 2-two humps. This is essentially a design that locks the Run Flat tyre bead onto the rim and ensures that the tyre remains properly mounted in the event of a sudden high speed deflation.

Secondly, standard chassis suspension and damping rates for run flat tyres are quite different. This is due to the lower compliance of the sidewalls, which requires greater suspension compliance. Changing from run flat to normal tyres will change the handling characteristics of the car (some may claim for the better. Personally I don't like to 2nd guess a group of BMW engineers and thousands of hours of testing)

Whether a tyre can be mounted on a rim and successfully inflated is a very different question vs. how a tyre/rim combination behaves in a sudden high speed deflation. 120mph on a German Autobahn is not the place to find out that the combination sucks.

In a typical high speed deflation, a normal tyre on a normal rim will be unstable and may actually come off the rim after the deflation, causing the driver to lose control

A run flat on a purpose made rim is designed to stay on the rim after sudden deflation, allowing the driver to maintain control.
Which always begs the question -

Why do BMW not fit them to the high performance, bahnstorming M cars
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      12-08-2011, 09:57 AM   #12
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Quote:
Originally Posted by scott135i View Post
Thats like asking if you should wear a condom.

Always safer, but NOT NECESSARY.
Quote:
Originally Posted by JJD View Post
They take most of the fun out of it too...
Yes they seriously numb the sensitivity with very little feedback


Only a few cars have fixed support rings for rts. BMW not one of them.
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      12-08-2011, 02:51 PM   #13
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Quote:
Originally Posted by scott135i View Post
Thats like asking if you should wear a condom.

Always safer, but NOT NECESSARY.
Quote:
Originally Posted by JJD View Post
They take most of the fun out of it too...
Quote:
Originally Posted by briers View Post
Yes they seriously numb the sensitivity with very little feedback
So to recap, having RFT tyres is like having sex while wearing a condom
  • They are a little bit safer
  • They take most of the fun out of the experience
  • They seriuosly numb the sensitivity with very little feedback
  • Oh and the make the exercise cost a little bit more than neccesary


Any more?
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      12-10-2011, 03:12 PM   #14
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ive had both on my e90 and the normal tyres are a lot better handling in my opinion than the runflats, they are also smoother, quieter, cheaper and eisier to fit... but u are foooooked if u get a punture..... are u a gambler??? lol
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      12-10-2011, 03:57 PM   #15
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what about all the Fords, Vauxhalls, Toyotas, Hondas etc etc that DON'T have a spare and DON'T have runflats either?

You don't hear their owners fretting about having a puncture.
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      12-10-2011, 04:20 PM   #16
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Quote:
Originally Posted by doughboy View Post
what about all the Fords, Vauxhalls, Toyotas, Hondas etc etc that DON'T have a spare and DON'T have runflats either?

You don't hear their owners fretting about having a puncture.
No, but you do see them regularly parked on the hard shoulder with either the owner, perplexed, looking at the flat tyre or with an RAC van fixing it.
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      12-11-2011, 03:54 AM   #17
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Run flats aren't like condoms.

If you get a hole and a leak in a run flat it doesn't ruin your day!
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      12-11-2011, 06:38 AM   #18
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Quote:
Originally Posted by JJD View Post
So to recap, having RFT tyres is like having sex while wearing a condom
  • They are a little bit safer
  • They take most of the fun out of the experience
  • They seriuosly numb the sensitivity with very little feedback
  • Oh and the make the exercise cost a little bit more than neccesary


Any more?
Very accurate summary
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      12-11-2011, 08:29 AM   #19
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Originally Posted by E93Ant View Post
Run flats aren't like condoms.

If you get a hole and a leak in a run flat it doesn't ruin your day!
Still an expensive accident though...
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      12-11-2011, 12:27 PM   #20
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run flats are a waste of money

After recently needing a new set of tyres I spent hours upon end researching what tyres I would buy, I eventually decided that run flats were not the way to go and fitted my car with some Michelin pilot sports, the first thing I noticed was the quietness and smoothness of the ride, then came the handling which was much better in every way, also these tyres should last a lot longer, so getting rid of run flats turn a great car into an Amazing car, just throw a can of Holt's Tyre weld in the boot
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      12-11-2011, 01:28 PM   #21
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I'd driven enough BMWs over the years to clearly see the first cars I tried on RFTs, were compromised by the run-flats.

AutoCar ran a varied tyre test, on the track with a Z4, and quickly advised "remove the run-flats" and get the chassis that was masked by the RFTs.

Besides the Z4, it was clear the first E60/1 cars were compromised, the early E9* cars were also not sorted, far too many weird characteristics, besides the awful ride quality and unpredictable dynamics.

All sorted with a tyre change. As I've said for many years, BMW compromised the cars for RFTs, not designed them for RFTS.

Sure things have moved on a bit, but IMO, still not completely sorted.

I agree if the flagship M-Division used them and the purists were happy, then we'd have no cause for not using them.

If it is a safety product, then BMW's M-Division and Alpina should definitely be leading the cause.

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      12-11-2011, 01:49 PM   #22
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I don't know enough about the technical aspects of suspension and set ups but I changed from RFT to NON RFT about 6 months ago and on my 19" tyres the ride is smoother, less harsh, quieter, no tramlining. I have a can of tyreweld and AA membership.
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