Ask a Mechanic: The reality of having run-flat tires

Dear Ask a Mechanic,

I have a three-year-old Mini Cooper. Recently two tires developed a bump on their side walls. I do hit the occasional pothole, but never at any high speeds. Because the wheels on the Mini do not have a conventional inner tube, I was told I would have to replace both tires (they have less than 18,000 kilometres on them). There is no warranty or insurance coverage, and it has cost me $750. What is going on here? – Tire Dinged

Unfortunately, you’re the victim of a couple of automotive industry trends. One is the market’s preference for larger diameter wheels and correspondingly shorter sidewalls. With less sidewall height to flex and compress, the likelihood that the tire will get pinched by something, like the edge of a pothole or a driveway lip, becomes even greater.

Mini’s use of tire sizes on the Cooper that are a smaller in overall diameter than what is common in Canada and the U.S. results in even shorter sidewalls.

Your cost sounds quite high, but you probably tried to match the remaining tires, which would be a major factor. Even if you didn’t, the Cooper’s less popular tires sizes mean there are fewer alternatives on the market. One of the Mini’s tire sizes, the 205/45R17, is a common factory fitment on Mazda’s MX-5, but the Japanese roadster doesn’t use run-flat tire

Run-flat tires are another factor in your replacement costs. BMW, Mini’s parent company, began transitioning to primarily using run-flat tires in the early 2000s, eliminating spare tires and saving on vehicle weight, space and production costs. The German automaker isn’t alone in using run-flat tires, Chevrolet’s Corvette has used them for three generations, but BMW is among the higher users of them in the auto industry.

The key benefit of these extended mobility tires is the ability for the heavily reinforced sidewall to safely permit continued driving without air pressure (albeit at reduced speed) far enough to reach a service station or garage. Most tire manufacturers have a policy of replacing and not repairing the tire once that happens. Some do not permit repairs at all.

Due to their heavier and specialized construction, run-flat tires are typically anywhere from 10 to 30 per cent more expensive than comparable tires. They also tend to increase ride harshness, a side-effect of the super-stiff sidewalls necessary for low pressure support. Many owners opt to replace run-flats with other tires (and rely on roadside assistance in a flat tire situation) when they buy a vehicle with them.

While it won’t help you with your car’s original tires, pro-rated Road Hazard warranties are available from select tire retailers and brands (often at additional cost) for future tire purchases, offering you a limited form of insurance protection.

Ask a Mechanic is written by Brian Early, a Red Seal-certified automotive technician. You can send your questions to wheels@thestar.ca. These answers are for informational purposes only. Please consult a certified mechanic before having any work done to your vehicle.