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Editorial: On Tires and Compromise

Featured Posts, Interesting | January 2nd, 2012 by 15
bs_pot_re960as_pprft_1000

The perfect tire would stick to any surface, never go flat, run in any weather, provide plentiful steering feedback, and wear forever. Fat chance we’ll …

The perfect tire would stick to any surface, never go flat, run in any weather, provide plentiful steering feedback, and wear forever. Fat chance we’ll ever get a perfect tire because a big set of those features are mutually exclusive. So tires are about compromise, compromises
between traction and wear, traction and wet performance, traction in hot weather and traction in cold. Oh, and they should generate no noise, no vibrations, nor add any harshness to the ride. Impossible.

Tires range from wide and sticky for racing slicks and narrow and hard for low rolling resistance e-mobility specials and then everything in between. Putting the right tire on the car for its intended use, and finding a suitable tire that will minimize NVH, increase fuel economy, and yet deliver appropriate traction in dry and wet/hot and cold is the stuff of nightmares.

Editorial: On Tires and Compromise

Once the appropriate set of compromises are reached, the tires reach the public (on suspensions tuned to that specific tire – in the case of high performance and luxury cars). And up until recently, that was five tires per car, then four tires and a donut spare, and even more recently, four run flat tires.

The run flat tire allowed manufacturers to save weight by eliminating the spare tire, jack and accessories. The weight savings would help with fuel economy, every pound shed adds up to fractions of MPG – the more weight lost, the more MPG improved – and car makers will take MPG gains wherever they can find them.

The manufacturers could accurately claim that the use of run flat tires as improves safety. After all, it is dangerous to change a tire on the side of the road. But they could not claim that they improve feel, that ever elusive notion of road information being transmitted from the contact patch up through suspension bits, and finally emerging in the driver’s fingertips. In addition, they couldn’t deliver the ride comfort expected on a luxury car (and of course the suspensions of cars had to be tuned to deal with the harshness of a run flat). And from an enthusiast’s point of view, they just didn’t feel good.

The reason they didn’t feel good was the compromise required in the sidewalls of the tire that provides the run flat capabilities. They are extra stiff/thick to provide support for the tire when air is lost. The additional sidewall stiffening changes the spring rate of the tire (yes, tires act as springs and their spring rate has to be accounted for when tuning the suspension – and then there’s the fun with increasing air pressure which increases the spring rate and also changes how the tire acts dynamically).

Tires, which for all the world appear to be round black lumps attached as an afterthought to a car, are in actuality one of the most complex and hard to get right components of the driving experience. And one that can cause the most grief if not properly maintained. And the run flat was designed to provide some piece of mind for one major potential driving danger, a flat tire on a busy highway.

Now a third generation run flat tire has emerged. It promises to be a bit less inflexible than its predecessors. Bridgestone has released its third gen run flat and Tire Rack had an opportunity to test the tire in both run flat and non run flat configurations (essentially the same tread pattern, belt construction configuration).

Editorial: On Tires and Compromise

Available at Tire Rack

The third gen run flat was the Bridgestone Potenza RE960AS Pole Position. It was developed by Bridgestone with BMW for use on BMW automobiles. Surprisingly, the run flat felt more responsive on the track than the non run flat version of the tire. Tire Rack chalks that up to the slightly stiffer sidewall on the third gen run flat. However, the run flat was still harsher than the non run flat version, though much better than the previous generation run flat sampled in the same test.

Left unsaid up to this point is what happens to a run flat tire that’s been driven without air pressure. Quite simply if a run flat tire is driven for any significant time without air pressure it must be replaced. The stiffer sidewall breaks down supporting the wight of the car and loses its effectiveness subsequently. However, running a regular tire without air for any significant distance results in the total destruction of the tire.

Regardless, if a tire on a high performance car has been compromised by a puncture, it should be replaced. The tire, even when properly repaired, loses its speed rating from a puncture, and if you routinely drive your car at extra legal speeds, you need to have the best rubber
possible on the road. No compromises.

In the end, yes run flats are getting better, but there are those of us who swear at them, not by them. I personally ditched the run flats when they wore out and no run Continental ExtremeContact DWs and carry a ContiComfortKit (air compressor & can of ‘slime’) in the trunk.

So what have you done (or plan to do) when it was time to replace the run flats and, if you’d like, tell us why you decided to do what you did in the comments.

For more tire selections, visit TireRack.com

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  • D Foutty

    I live in Alaska where it can be 100-200 miles to civilization,or be -30 F in the winter so safety out ranks performance unfortunately,the compromises of life (:

  • LaMa

    Run-flats are for people who are incapable or unwilling to change a tire. Unfortunately its about 50% of the drivers in the US.

    by the way the “…less inflexible…” could be said as “…more flexible…” and would be more appropriate imho.

    English isn’t my 1st or even 2nd language but that hurt my eyes :-)

    • http://www.bmwblog.com Horatiu B.

      You’ll live with it :)

      I think the 3rd generation RFTs will change the perception and will become accepted. I don’t see BMW changing their direction back to conventional tires.

    • Xeniczone

      I completely disagree.  Run flats are amazing technology as far as tires go.  They come in handy in many situations.  If you’re a VIP and someone start shooting at you are you going to hit pause while you change the tire?  Driving down the interstate, your TPMS says you have a flat and you want to change a tire while cars fly by you at 80-100 mph?  You save 50-70 lbs of weight not carrying a spare+tools+storage.

  • Phantom of the Opera

    Although in my country (Argentina) RFT are useless if you intend to travel, as there is no guaranteed supply of these type of tyres across the geography.. I still prefer them, because of the safety they provide, specially in “red zones”… where your life is at risk, if you stop to change a tyre…
    Here many BMW users have two sets.. conventional for long trips, and RFTs for urban usage.

  • http://twitter.com/EndrasBMW Endras BMW

    Very exciting news! We can’t wait to feel out the 3rd gen run flats. 

  • http://www.facebook.com/rtcalo Ricky Calo

    I have a 2011 135i and I really have no beef with the run-flats aside from the fact that they dilute steering feel I think they are great for street driving. The roads here(Austin, TX) are pretty good therefore rough pavement is hardly an issue. However I would love to ditch the run-flats but i have read that it will throw off the suspension and make the car behave different. Big price to pay for more steering feel IMHO.

  • BeEmWe

    I live in Dubai, so performance tires are used all year round as it never gets colder than 10 degrees Celsius (50 degrees Fahrenheit). On the other hand, it can reach 55 degrees Celsius (130 degrees Fahrenheit) in the desert areas in the summer. 
    The speed limit is officially 120 km/h (75 mph), but there’s a generous tolerance of 20 km/h in Dubai and 40 km/h in the neighboring city of Abu Dhabi. As a result, you can actually cruise at 160 km/h (about 100 mph) legally! 
    With this combination of heat and speed, one would better play as safe as possible. I experienced a flat tire at 100 mph (160 km/h) in my 2008 530iA and only noticed it through the iDrive warning. On the other hand I had a flat tire in a Toyota Camry at 80 mph (130 km/h) and it was terrible. In my BMW i drove a further 80 km (50 miles) at lower speed until I got to a Gas Station. You simply can’t change a flat tire in such heat on the way to work as you ‘ll be soaked in sweat! I used the donut spare from the Station as I was still half-way to work (I drive 205 miles ( 327 km) to work!).
    The combination of speed, heat and distance I deal with is probably extreme, but is very real in here. I had damaged rims to be replaced under insurance, but due to the unavailability of my OEM 18 inch 246 rims, I paid the difference to upgrad to the 19 inch 272 M wheels which came wrapped in Continental rubbers. I didn’t notice at first that they were not runflats! I only found out after a couple of thousand miles. They’re softer than the Runflat Dunlops I used to have despite one size bigger rims. Still, I’ll go back to runflats once these wear out as it suits my situation much better (I change tires almost once a year). 
    I must admit that roads are very good in here, and potholes are a rarity! I drove the same car with the last set of runflats on roads in a neighboring country where potholes are common, and I’m sure if I lived there I’d switch to the smallest possible rims as every road unevenness got transmitted right to my weak neck and shoulders.
    Hope I’ve given a clear picture of my situation and haven’t bored you with this long response.
    Cheers!

  • ANDU

    REASONS NOT TO BUY RFT TYRES AGAIN (EVEN THOUGH I DID IT ONCE):
    -PRICE
    -PRICE
    -PRICE…LET ALONE THE LACK OF PERFORMANCE PLUS HARSHER RIDE AS
    OPPOSED TO CONV.TYRES.
    I BOUGHT LAST SET OF RFTS IN 2008 AND AFTER 3 YEARS THE PRICES WENT UP FOR THE SAME EXACT TYRE,SO I JUST BOUGHT REGULARS AND 2 CANS OF TYRE REPAIR SPRAY.
    PS   I LIVE IN ROMANIA.

  • Billy K

    These new Potenza RFTs are only available in a few sizes and not available for my BMW 550i.  My current run flats ride horribly. 

  • IslandMan

    Just ditched the RFTs for Conti DWS.on my Z4…smoother, quieter and no tram-linig. With the M sport suspension package the softer sidewall on the DWS makes for a much more pleasant ride.

  • 88E30M3

    I am ditching the run craps on my E90 for Michelin PSS & a new BMW Mobility kit.

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