Why you still need Snow Tires with ABS Systems
I often read on forums somebody saying "Well, I don't need snow tires because I have ABS." Or worse, "I don't need snow tires because I have AWD." The problem with the latter statement is obvious (AWD does not help you stop or turn) but the problem with the former is less obvious. ABS actually increases the need for good rubber between the car and the road.
Having good tires is the single biggest factor in winter driving. This doesn't just mean having snow tires. Having bald or worn tires is worse than having all seasons. Even if you do not live in an area where it snows enough to consider purchasing snow tires, it is very important to check your tire tread depth using the penny test. If you have insufficient treadlife remaining, replace your tires before it starts snowing.
But why does ABS increase the need for good tires? ABS works using a technique called "cadence braking" to prevent the wheels from skidding during braking. When the ABS controller detects a wheel has stopped spinning (locked) it releases the brake pressure on that wheel until it spins again. It then increases the pressure until it stops, and so on. More sophisticated ABS systems can do this quite fast, and can usually shorten braking distance on dry pavement compared to a human executing the cadence braking maneuver, and somtimes even a skilled driver executing a threshold braking maneuver. In any case, the tire's grip still matters because a stickier tire will grip longer before the wheel locks requiring less ABS action.
But we aren't talking about dry pavement, we're talking about winter driving on snow and ice. On snow and ice ABS drastically increases stopping distances. In snow, when the wheels lock a wedge of snow builds up in front of the tire causing a significant retarding effect. But ABS destroys this effect in its attempt to keep the wheels rolling. As a result, the ABS controller spends much of the time with the brakes released and it takes longer to stop. On ice, the wheels lock up so readily that the ABS hardly applies the brakes at all and stopping distances are nearly doubled compared to a locked wheels slide. The issues seen with snow are also seen in loose gravel and sand.
Modern ABS systems are significantly better at handling these situations, but are still subject to drastically increased stopping distances. The solution to this is twofold. The first factor is to make sure you are using good tires. As stated above, good tires mean there will be more braking ability before the wheels lock and the ABS system intervenes. The second factor is to be aware that the purpose of ABS is to allow the drive to retain some steering ability during braking and not to improve stopping distances.
There is a sophisticated solution to the problem of having to make an emergency stop where the ABS effect is detrimental. You can deliberately skid the car 90 degrees perpendicular to the original direction of motion. The car will continue to skid forward in the same direction, but all four wheels will be sliding accumulating the "wedge" and will enable a quicker stop. However, this is a difficult maneuver to get right and requires enough parking lot practice to make it automatic before using it in an emergency. In the meantime, just get some good tires and remember that ABS doesn't help you stop.
