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.
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).
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.
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