By Mac Demere, automedia.com
Driving on ultra-high-performance tires on a snow-covered road is like trying to negotiate Copper Mountain's expert-level "Drain Pipe" ski run on water skis. Both are skis, so what's the problem? Even on tires billed as "ultra-high-performance all-season" driving in the snow is like negotiating that black-diamond run on cross-country skis: It's possible, but it's not the best choice. Also known as "max performance summer" tires or, more accurately, "three-season" tires, they come standard on every Porsche (except the Cayenne), all Corvettes, the Viper, all Ferraris and more.
The reason: Summer tires offer more grip on both wet and dry roads than all-season tires, much less winter (aka "snow") tires. The only exception: when the thermometer drops below freezing or snow covers the pavement.
Tire performance differences
Winter tires, all-season-tires and summer tires differ in the pliability and durability of their rubber at different temperatures. Tire engineers call it "glass transition temperature": Get them cold enough and every tire will have the grip of a Formica kitchen counter. For a race tire, it might be 40 Fahrenheit. For a winter tire it could be 60 degrees below zero. Summer tire tread starts becoming Formica-like somewhere just below freezing. Most all-season tires are still pliable below zero, but some "ultra-high-performance all-season" tires start losing grip well before that.
Grip and sipes
Tire grip in the cold is much like oil viscosity, largely because tires contain many petroleum-based products. A 50-weight racing oil will protect an engine at temperatures approaching 260 degrees, but at 70 degrees it's almost as thick as gelatin.
Today, most racing oils are "multigrade": A 20W-50 racing oil remains liquid enough to adequately lubricate the engine while it warms up but still protects at prodigious temperatures. Tires haven't advanced that far.
There are many other things that give tires grip in the snow. "Sipes" small slices in the tread are among the top features. But you can't put enough sipes in a summer tire to give it useful mobility in the snow.
I have personally experienced summer tires in the snow many times. Once, I got stuck on a perfectly level-packed snowfield. Another time, an ultra-high-performance summer tire would not go forward if the snowy road had any incline, but in reverse it could maintain about 5 mph. A third was the safest snow tire in the world: It couldn't get out of the garage.
Most recently, I drove for a video that was designed to sing the praises of a new ultra-high-performance all-season tire. We were using a snow-covered track. Air temperature hovered in the low 20s. The client supplied a BMW 3 Series, but that wasn't flashy enough for the ad agency. So, they hitched a trailer to a bright yellow Dodge Charger SRT-8 Super Bee fitted with 20-inch tires from a different tire company.
I said, "Uh, those are (Brand X, Model Y)." When they looked at me as if I was wearing an aluminum foil hat to keep the CIA from reading my thoughts, I elaborated: "They're summer tires."
Attempting to placate this wacko driver they replied, "Oh, you won't be able to tell the brand in the video."
"Absolutely right," I said, "because the car won't move with them on it." I attempted to explain the differences between winter, summer and all-season tires, but their California eyes quickly glazed over.
The ad guys' next bright idea was to have me borrow some all-season tires from Dodge, which was testing at the same facility. I drove the Charger over to their garage and almost crashed about 25 times in the quarter mile trip. I never exceeded 15 mph: I didn't want to go that fast, but with the brake pedal buried to the floor it picked up speed on a slight downgrade. I've run 210 mph in an Indy Car at Texas Motor Speedway, but I scared myself more times on that short trip.
Dodge graciously offered to loan us some 20-inch high-performance all-season tires. But over the two-way radio came their test driver's fourth request that day to be pulled out of a snowbank. His car was fitted with the tires we would be borrowing. I said, "No thanks." The Charger went back on the trailer.
Here's the bottom line: If you have a high-performance car on summer tires, don't drive it in the snow or when it's much below freezing. Don't count on ultra-high-performance all-season tires to provide anything more than limited mobility in snow, but you can be assured of less wet and dry grip. And a BMW 335 on first-class snow tires is a blast on a snow-covered track.