All people know about run-flat tires that are able to work after a complete loss of pressure. We have conducted our own test to find out how it feels!
Run-flat tires appeared not very long ago but their popularity is steadily rising. BMW, for example, uses such tires for the original equipment of some of its vehicles thus allowing them not to carry spare wheel.
Today practically all tire manufacturers has run-flat tires in their product range. The names, however, differ: Bridgestone – RFT (Run-flat tire), Continental – SSR (Self Supporting Run-Flat), Goodyear – EMT (Extended Mobility Technology) or RunOnFlat, Michelin – Zero Pressure, Pirelli – EUFORI@. The concept is the same – after the loss of pressure the vehicle is supported by reinforced sidewalls. There are some restrictions – after the loss of pressure the speed should not exceed 50mph, the mileage – 50 miles. But after the “airless” riding all manufacturers recommend to replace the tire with a new one. Only Goodyear advices to show the tire to a specialist who will take decision about its further fate – maybe it will work. But such specialist can be found only at big service stations and the risk of taking a wrong decision is high…
We decided to check what would be the real mileage of a “nail-proof” tire. And how will it affect the performance of the vehicle? We took summer tires from three tire manufacturers (size – 205/55R16): Continental PremiumContact SSR, Goodyear Eagle NCT 5 EMT and Michelin Primacy HP ZP. We choose Volkswagen Passat without pressure monitoring system.
The peculiarities of the tires can be noticed already at tire service centre when mounting. The manufacturers insist that the thick and rigid sidewall requires special equipment (without an additional lever it would be impossible to mount such tire). Some experienced specialists say that they can do that using conventional equipment. Such tires can be mounted on any machine – it will be much harder not to leave scratches on a wheel even with the use of advanced equipment…
Two flat tires at one side – such situation is hardly possible but still may happen. That’s why we imitate such puncture and reduce pressure in two tires at once.
4x2 Mode
The first exercise – running-down from a 50mph speed. At first – when all four tires are OK, then – with two “damaged” tires. In the first case Goodyear was the winner, but Michelin won with “damaged” tires. “Continental” showed worse results in both cases. In general, running-down on “empty” tires is approximately two times shorter. It will considerably affect fuel consumption level what means that Michelin is the most fuel-efficient tire. Simultaneously we evaluate directional stability, noise level and comfort. The best directional stability is achieved by Michelin, the worst – by Continental (as it has the lowest rolling resistance level).
Goodyear practically doesn’t “vocalize” empty state. The only hope is a vehicle with a standard pressure monitoring system. You will not be able to detect pressure loss by noise level or vehicle behavior changes. The empty Michelin, however, rattles as a regular empty tire.
Michelin is the softest tire both with and without pressure. Continental and Goodyear perform like rigid sport tires.
Maximal lateral loads are experienced on dry asphalted surface. As soon as front wheel rim touches asphalt the vehicle will be turned upside down. That’s why this moment is watched by a colleague who stands outside and warns the driver by walkie-talkie. The ride is stopped when 0,98 – 1,18 inches is left between the rim and the road. The task is very complicated as we deal with swift changes. At the same time we feel responsible as we don’t want to miss the critical moment.
In both conditions the first place is taken by Continental, the second – by Michelin, the third – by Goodyear. Speed decrease when maneuvering on empty tires is practically equal for all tires – approximately 13%. Yet, Michelin shows a little bit better performance.
It is interesting that the vehicle doesn’t “show” empty tire when riding by 3mph slower than the limit speed – up to 35mph there are no problems with the steering control. But if You increase speed just a little bit maneuvering will lead to skidding.
At the same time we draw Your attention to the fact that none of the tires even tried to come off the rib.
The next stage is extreme braking from 50mph to 3mph (in order to exclude ABS interference). At first – on four “full” wheels, then – on two “empty” wheels. We make marks on the rib and the tire to trace their possible crank.
The leader is Continental, then goes Michelin. Goodyear shows the smallest difference in braking distance change, the biggest difference is shown by Michelin.
The behavior of the vehicle when braking also differs with different tires. Continental causes slight skidding, Michelin causes almost no skidding, Goodyear makes Volkswagen “bend” its front requiring from the driver increased attention. Note that none of the tires cranked when braking.
Durability
Conventional empty tire usually overheats when riding. What about run-flat? We used infrared thermometer for temperature control as it allows to conduct measurement without vehicle stoppage.
After the test Michelin turned out to be the coldest tire. It seems that this tire is subject to deformation to a less extent. It means that its durability must be the highest. Let’s check!
We calculate the mileage of all tires in empty condition after all tests. The result is within the range of 25 – 30 miles. What will be the ultimate mileage?
Turn by turn we drive with empty tires. Continental was the first to give up – cord threads appeared after 100 miles. Nevertheless it is two times as much as the promised 50 miles. The result is not at all bad if to consider adverse conditions of test conduct! Goodyear “showed” cord at 262 miles – 5 times as much as 50 miles. Michelin turned out to be the winner – 350 miles (7 times as much as 50 miles).
Well, If You anyway have throw the tire away after a 50-mile ride in empty condition – why not to continue driving up to the full destruction of the tire? And You shouldn’t buy used run-flat as it is impossible to say by appearance whether the tire has been used in empty condition.
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