Wheeled vehicles get their grip or traction or grip from that point where the tires touch the road. Whether there are two, four or 18 wheels, the available grip is determined by those points, contact patches or footprints, as they are known.
There are four contact patches for a passenger vehicle, each one roughly the size of the palm of your hand or sole of your foot. They alone determine how well the vehicle stops, turns or accelerates. The engine delivers power to the wheels, the steering wheel turns them and the brakes stop them – but it is those four patches of rubber tires that turn, stop or provide acceleration.
One of the most important things for any driver – novice or experienced – to be aware of is the role those contact patch(s) plays in determining a vehicle’s traction. This awareness, combined with a rudimentary knowledge of handling will help a driver retain control. Straying beyond the limits of a contact patch’s grip will result in a slide, spin or other loss of control. How a driver manages the distribution of weight among the contact patches determines the difference between staying on the road or sliding off it or into another object.
Just as a skater or skier carefully balances the weight shift from one foot to the other to maintain a grip on slippery surfaces, the driver must be aware of a similar weight shift. Maintaining the maximum amount of available grip is a direct function of how a driver directs the weight shift of the vehicle during cornering, acceleration or when stopping.
In an effort to graphically display the dynamics of a contact patch, I worked with the folks at Goodyear’s engineering lab in Akron. The accompanying graphics are actual photographs of the four tires of a vehicle – as seen by the road, or more accurately as seen looking up through a glass plate built into the surface of a special section of Goodyear’s test track. The different series of photos show the four contact patches of a vehicle in different situations and help us realize the importance of maintaining an awareness of what is happening beneath our vehicle in these situations.
The size of the individual contact patches determines how well a tire grips the surface. Obviously a large front contact patch provides more grip than a small one. Similarly if moisture on top of the road surface were to come between the tire and road, the amount of grip and ability to turn, stop or accelerate would be diminished.
STATIONARY – Here we see the four tires, as they appear when the vehicle is sitting still or stationary. The two front or steering contact patches are slightly larger because the engine and transmission are in the front of a vehicle making it slightly nose-heavy. We can also see from this view that the front of the compact patch is rounded, the tire it presents a leading edge to the road, especially important when trying to get a grip in wet or other slippery situations.
ACCELERATION When we accelerate, we shift some of the vehicle’s weight from front to rear. As the nose of the vehicle rises, weight is lifted from the front tires and those contact patches shrink. With more weight on the rear tires, those contact patches grow.
STOPPING The front contact patches grow as vehicle weight shifts forward under braking, presenting a much greater surface area and more grip. As much as 85% of a vehicle’s braking has to be managed by the two front tires. We can see how important it is to brake before starting to turn. The small rear contact patches have a tenuous hold at best.
TURNING: When we turn, weight shifts toward the outside of the vehicle, lifting weight from the inside tires. The outside patches grow and the inside ones shrink. This is a graphic display of why a vehicle can lose control or grip in a turn.
The photos also help us realize the significance of buying quality tires and maintaining them properly with respect to pressure and tread depth. As is apparent, if the tread was worn out or the tire was improperly inflated these contact patches would be even more distorted and unable to provide the grip needed to allow us to turn, stop or go.
TECHNICAL STUFF The vehicle used in this test was a Jeep Grand Cherokee with all-wheel drive – typical of SUV’s on the market today. The tires were brand-new and inflated to recommended pressures. The tests were performed at 0.5G – representative of fairly aggressive motion – somewhere between a panic or emergency evasive maneuver and a slow, mild change in direction or motion. Normal production passenger vehicles with good quality tires will generate up to 0.8 G or so on a reasonably grippy surface such as new pavement. On snow, ice or in heavy rain, that limit is much lower.