DAY 13 - Run-Flat Tires

The BMW 335i comes with run-flat tires. If you’re not familiar with them, I thought I would write a post today that would give you a primer on them and the pros and cons. 

There is a long-running argument about them. Some drivers love them. Others hate them. But, beyond all of this disagreement, it’s safe to say that no one in their right mind looks forward to experiencing the inconvenience and danger of a blow out. 

Also, it’s never cool to be stranded on the side of a busy highway waiting for roadside assistance or, heaven forbid, having to figure out how to use that jack that’s been hiding in the trunk since the car was new.

But exactly what is a run-flat tire and what makes it different than the tires we grew up with. According to Edmunds.com:

Run-flat or zero-pressure tires are intended to support the weight of the car for a short time, providing the driver with 100 or so miles of range to get off the highway and find a repair shop. Two kinds of zero-pressure tires exist in the market today. Both types still require the usual amount of air to provide day-to-day performance. Self-supporting tires (SSTs) are the original and most common run-flat type. Heavily reinforced sidewalls support the vehicle after air departs the scene. This sort of run-flat is designed to fit on normal wheels with no modifications. Michelin's PAX, a patented auxiliary support run-flat system, is a relative newcomer. PAX sidewalls, while still stiffer than normal tires, are not as rigid as SSTs. Instead Michelin designed a unique wheel that positions a semi-rigid "support ring" inside the tire to hold the car up when the air goes bye-bye. A non-standard bead design is necessary where wheel and tire meet.

Since these tires have stiff sidewalls, they don’t appear flat, even when they’re out of air. So how does a driver know if their tires are low on pressure without continually checking them?

Edmunds.com says:

TPMS (tire pressure monitoring system, ed.) has become mandatory in run-flat applications. Since run-flats only provide a limited zero-pressure driving range, TPMS is critical to help the driver know when the mileage clock starts ticking, and more importantly, when time is up.

Do the advantages outweigh the negative aspects? Let’s see.

PROS:

You can drive on a flat tire. You’re typically limited to 50 mph and 100 miles maximum distance.

Better stability after a blowout. Since these tires can support the car without air, steering and handling remain pretty much normal after a blowout.

Repairability —Run-flat repair guidelines are similar to standard tires, provided the speed and distance limits are not exceeded after a flat.

CONS:

You have no spare. Of course, you really don’t need one with a run-flat tire. That’s the whole idea behind them.

You’re going to get a harder ride. The stiff sidewalls that are the prime design feature of the run-flat tire result in a harder ride.

They don’t last as long. The jury is still out on this, but most resources tell us that run-flats tend to wear more quickly.

Blowouts are still possible, as they are with standard tires.

Run-flat tires are heavier, due to the weight of the reinforcement.

Run-flat tires are more costly. Compared to standard tires, expect to pay double, or more.

And one other thing that Edmunds failed to mention is that run-flat tires are a little noisier than standard tires. The Goodyear Eagles I had on the Corvette were terribly noisy. When I switched them out for the MIchelin Pilot Sports, the sound level dropped dramatically.

So there you have it. And just so you know, the new 335i currently has a little over 400 miles on it. The break-in period limitations on speed are partly in place so that the new tires can get a little wear on them so they grip the road a bit better.