The truth about run flat tyres
Often these are the only option when you’re buying a particular car - and generally they’re packaged up with no spare tyre at all. So: Good idea or bad idea?
In the spirit of fairness, the two advantages to run-flat tyres are: First: you get a puncture - you can drive on them, flat. So if you’re on the way to that big job interview, or in that part of town you really shouldn’t be in because of the free trade in drugs, firearms and sundry other illicit services, you need not get out and loiter, wearing your ‘victim’ T-shirt over your G-string and fishnets.
Second advantage: if you have a rapid deflation - a big hit that lets all the air out, fast - run flats are more stable than a conventional tyre.
Unfortunately durability in this deflated state takes a real hit - there’s a maximum serviceable distance, usually 80 kilometres - that’s 50 miles - and a speed limitation as well, when internal pressure is lost.
There is simply more bad news than good. Run flats - commonly sold by Bridgestone and Pirelli - use one of two systems to get the job done with no air.
The self-supporting kind uses reinforced - meaning stiffer - sidewalls to carry the load for a short distance in the absence of internal pressure.
The other kind is based on an internal support ring, which the flat external carcass sags down and rolls on when the air escapes.
Couple of problems here: First up, run flats are heavier than conventional tyres. And it’s exactly the wrong kind of additional lard-arsedness - they’re heavier in unsprung mass. And that hurts everything dynamic that you care about as an owner: like ride and handling.
It’s a big challenge for premium carmakers like BMW, which has embraced run-flats. It’s hard to overcome the inherent ride and handling deficiencies when you bolt additional unsprung mass to a car.
According to the JD Power 2015 Original Equipment Tire Satisfaction Study in Retardistan (bit of a mouthful) in the performance segment, run flats scored 612 out of 1000 possible points on average. Conventional tyres scored 685.
See it at J.D. Power >>
That’s about 11 per cent worse for the run-flats. This is a survey of almost 30,000 owners (although they didn’t all have run-flats on performance cars, clearly).
In the luxury segment: 712 points for conventional tyres; 688 for run-flats. That’s a little over three per cent.
So you get a flat and you drive out of Sin City unmolested, without stopping. You, the boss’s secretary and her prized collection of latex evening wear are safe. And that’s the main thing. All you got was a warning on the dash.
But the run-flat is a throwaway at this point, and these things are expensive to replace. They’re significantly more costly than a conventional tyre.
Tyre repair places generally won’t repair them after you’ve run on them, flat. Which is a bit of a bastard.
And even if you never get a flat, they’re also generally noisier, they don’t ride as well and the tread doesn’t last as long.
Run-flats come packaged up with a tyre pressure monitoring system - otherwise you might not know the air’s been dumped out. So when the dashboard lights up you’ll need to reduce speed to the maximum allowed - usually 80 kilometres per hour (50 miles an hour).
That’s not a big deal around town, but it is a bastard on the freeway, in the wet, at night, with a couple of hundred kilometres left to drive - and your run-flats will not take you that far in that deflated state.
So, what do you do? Drive 80 km and then stop and call for a tow? Not likely. Most people continue to drive - and that’s unsafe.
As much as I would enjoy donning my white ceremonial hood, to burn a cross and string up every run flat in the developed world as a penalty for extreme design deficiency, I guess run-flats are acceptable if, as a car-buying customer, you know all this on the showroom floor.
My position on run-flats is that this is one area where the old ways really were best. In my view, nothing beats a full-sized alloy spare wheel and tyre. Especially here in ‘Straya, where (from time to time) we do drive long distances between tyre dealers.
Secret life of run flat tyres >> (on Wikipedia).