MB we have a problem... Run-Flat Tyres......RFT

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wheels-inmotion

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Joined
May 18, 2008
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Location
Hemel Hempstead
Car
Daily hog is a Vectra
The evolution
The RFT made a entrance way back in the 80's with the Dunlop Denovo. This was a split-rim wheel filled with gel canisters that would burst and cool the deflated tyre. Needless to say the evolution failed due to cost and I think the public wasn't interested or understood the safety value.

Nevertheless, Dunlop stayed with the evolution and eventually added the Demlock. Again this was a gel filled tyre but this time it was a past rather than canisters. In addition, the wheel was conventional but the tyre had an extra rail in the bead area that locked into the wheel to avoid departure once deflated.

Again this failed and again it was due to cost and demand so only the high-end manufactures added these, classing them as an extra.

For about 20yrs things went quite until Bridgestone developed the RFT without the cooling gel but with an armoured sidewall. BMW initially added the RFT to their build, I think part cost-cutting ( no spare tyre ) and part selling safety.

The Pro's
Safety is without question. No more fitting a spare tyre on the side of the road and there's no such thing as a blow-out on a RFT. These points can't be denied and since they save lives I welcome that.

The con's
The RFT's are way more expensive than a conventional tyre.

The RFT cannot be repaired if punctured. Reason being the manufacture says the sidewall cannot be examined correctly for damage unless it's X-rayed. I've argued this with them because an experienced tech would know if there is damage and what if he/ she did repair the tyre and it failed. The worst thing that could happen is the tyre would have to do exactly as promised and "run-flat".

The RFT has extremely loud harmonics.

The RFT's traction is less than remarkable due to its composition silica content, simply because they struggle to get hot.

The RFT is heavy?... Due to this the wheel is made from a lightweight alloy composite. This is done to reduce unsprung weight. The problem is the first line defence buffer offered by a conventional tyres sidewall isn't there so concussion is transmitted directly to the wheel, and they crack.

So what's the problem?
BMW, who have committed the RFT to their entire models and also apply a chassis geometry to perform perfectly at the cars suggested top-speed. The problem I found was a train of BMW's with tyres worn down to the wire in as little as 4k. At £400 a tyre owners are not happy.
The reason for this is the chassis positions are applied to a tyre that cannot deform and adopt a sports camber position. Solution is simple, dial out some camber but stay within BMW's datum. Will the car handle as well at 160mph? Probably not but what do you want, preservation or performance.

Point to note BMW M series do not fit RFT's and AMG didn't until recently.

Point of this post is because I'm now seeing MB's on RFT's and the whole episode is starting again. MB has adjustability issues unlike the BMW and although things can be made better I wouldn't call it a cure at the rear of the car, but I'm working on it.

I hope this helps.......
 
Whatever happened to Tyron bands?!
 
Tyron bands are designed to stop the tyre migrating to the wheels recess in reply to a deflated tyre coming off the rim. All rims have a recess to allow the tyre fitting since it needs to be done at about a 45 degree angle. Logically you cannot fit a 19" tyre to a 19" wheel without an off-set.

Historically the Tyron band is fitted to trailers and caravan wheels because the driver is normally unaware a tyre has deflated, so the band isn't a run-flat but it is a safety addition which can only be a good thing.
 
I don't mind my runflats, maybe the Airmatic suspension helps, but recently replaced my two fronts as the centre of each tyre was down to 1.6mm but each edge still had about 5mm. Mercedes state 36psi all round, obviously thats too high. The Goodyear F1 Assymetric 3 replacements were £30 cheaper per tyre if you opted for "OE" BMW fitment instead of "MOE" Mercedes, not sure why?
 
Point to note is within reason the tyre pressure suggested is simply that "a suggestion". Put it this way we both have the same car, I always drive on the motorway, you don't. You have three kids, I don't. I always run near empty on fuel, your's is always full. Given those factors there's no way our tyre pressures could be the same.

So again i advise within reason add or subtract 1 or 2 psi from the "suggested" platform, but please no more than that otherwise the tyres contact patch could be compromised.
 
Point to note is within reason the tyre pressure suggested is simply that "a suggestion". Put it this way we both have the same car, I always drive on the motorway, you don't. You have three kids, I don't. I always run near empty on fuel, your's is always full. Given those factors there's no way our tyre pressures could be the same.

So again i advise within reason add or subtract 1 or 2 psi from the "suggested" platform, but please no more than that otherwise the tyres contact patch could be compromised.
You know me so well!
 
Also I think it's only fair to add, I bought my wife a BMW purely because of the RFT's?..... She is disabled so the RFT does have it's place on the planet despite how much I dislike them.
 
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I once bought a set of old BMW alloys to use as winter wheels. They came with knackered RFTs fitted which were going straight in the bin to be replaced by my non-RFT winter tyres. The bald RFTs seemed to weigh nearly twice the brand new winter tyres. I couldn't get over the difference! Whilst the safety benefits of the RFTs are great, their effect on ride and handling is not so good...
 
@wheels-inmotion I am not sure where you get the £400 per tyre cost. Maybe if you were talking X5/6 rear that is 305 wide, then yes. My GT runs on 245/45/19 and 275/40/19 and I only go for RFT Michelin which just so happen to be BMW homologated at £220-230 a corner.
 
The £400 price wasn't literal. A rear RFT on a GTR costs £600. Point is the cost -V- mileage just isn't right and the type of wear is down-right dangerous.
 

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