Driving Tips
Jim Roberts

Tires and Driving Safety
Beyond round and black
 
While many people take tires for granted – except for the occasional blowout – and feel they are all about the same, this opinion grossly underestimates their importance.
Pneumatic tires today are quite remarkable: These rubber structures filled with compressed air are the only contacts between your two-ton-plus vehicle and the surface of the earth. Each of the four contact patches is about the size of an adult’s footprint. This small area of contact with the pavement allows steering, acceleration, and braking, and is the only thing that separates you from maintaining or losing control of your vehicle.
Let’s talk about how your choice of tires will influence the effectiveness of that contact patch, and then why tire pressure is critical to make sure the tires can do their job properly.

Pick the right tires for the job

Three factors control the effectiveness of those patches of tire contact: Rubber compound, tread pattern and tread depth.
Modern science has made great strides in improving the effectiveness of tires. Interestingly, the Shrader tire valve that allows air into the tires is the only aspect of the tires that has remained unchanged since Carl Benz first started putting rubber tires on his motorwagens.
Because tire grip (or adhesion, to use the proper term) to road surface is dependent on the compound of the rubber as well as the surface area, different compounds are recommended for different temperature ranges.
You should also be aware that tire compounds deteriorate with age and storage conditions. For that reason, federal law requires that all tires have the year and month of manufacture molded into the sidewalls; for safety, experts recommend that tires be replaced within five to eight years after they are manufactured.
Tread pattern and tread depth are important, too. Contrary to what many people believe, treadless tires actually adhere better to dry pavement than treaded tires.  This is the reason race tires are slick.
Of course, most of us have to drive in the wet, so grooves or “sipes” are cut or molded into the tread surface to pump the water out as the tire rotates. Obviously, the deeper and wider the grooves, the better the tire pumps away the water between it and the pavement. However, unlike race drivers, we can’t change our tires whenever it rains, so most tires on the road are a compromise between dry- and wet-weather traction. Some tires are promoted for their wet traction while others are superior in dry conditions. 
The tread depth also determines how effective the tire will be in inclement conditions, and federal law also mandates that tire manufacturers incorporate wear indicators into the tires that will become visible as the tire wears.
Federally mandated wear indicators are at 1/16th inch. An often-recommended  measure is to insert a U.S. penny into the tread, and if the top of Lincoln’s head is visible, that is 1/16th of an inch tread depth. However, for added safety, tread should be deep enough to cut through one-eighth inch of standing water. To check, use the top of Washington’s head on a quarter, which is one-eighth inch.
Tires are rated and advertised as summer, high-performance, winter, or all-season (or M&S, for mud and snow), which takes into account different tread compounds and tread designs. Which should you use? The answer depends on your climate and driving habits.
Summer tires are made from a tire compound that will produce maximum grip at higher temperatures. However, they will not have as much grip at very low (e.g. below-zero) temperatures. High-performance tires are effective in similar temperature ranges, but have less-aggressive wet weather tread patterns. They will put more rubber on the pavement but are more likely to hydroplane or skid in wet conditions.
Winter tires usually have a very deep, aggressive tread that is designed to get traction on snow, but they are not as effective in rain. Their compounds are blended to provide better adhesion at a very low temperature and some will actually adhere to ice. However, they may overheat in warm (more than 85F) temperatures and should only be driven in the winter.
The important thing to know about snow tires is that their effectiveness decreases dramatically when the edges of the grooves have worn down due to dry weather driving. Winter tires are at their best the first season. It’s also important that winter tires be fitted on all four wheels for control in snow conditions to maintain balance in handling whether you have rear-wheel or front-wheel drive. Remember, you have to turn, and stop, as well as accelerate.
All-season (or M&S) tires have tread designs designed to channel standing water off to the sides of the tires. They are a compromise: not the best in summer and not good for deep snow either: They aren’t as bad as summer tires in snow or winter tires in hot and dry conditions.
Keep all this in mind, taking into account average temperatures and conditions where you’ll be driving, when choosing tires.

Check your tire pressure frequently

The most critical requirement for the tires to do their job properly is correct air pressure. This is what supports the sidewalls and prevents the metal rim from contacting the road surface. National surveys have reported 40-60 percent of tires are underinflated. Too little air pressure results in a tire that squirms or is unstable when braking or cornering, even though it has a larger area in contact with the pavement.
Notice that drag racers maintain very low air pressure to increase the contact patch for acceleration; but they don’t have to turn, and they brake by using a parachute to create aerodynamic drag, rather than relying on the tires to reduce the vehicle’s speed. This is not really applicable to street driving.
Soft sidewalls also will allow the wheel to shift sideways in relation to the tread and slow any turning maneuver. You may get away with this at very slow speeds, but in an emergency the lack of responsiveness can be the difference between avoiding an obstacle or hitting it.
The other downside to under-inflated tires is that the lower air pressure allows the sidewalls to flex more, reducing steering response. This extra flexing also produces excessive heat, one of the most common factors in tire failure. Flexing and heat will eventually destroy the sidewalls or separate the tread from the rest of the tire.
Proper vehicle control can be maintained by checking the air in your tires at least monthly. Look at the manufacturer-recommended pressures that are posted on the door jamb, the inside of the fuel filler door, and the trunk lid on most Mercedes-Benz vehicles and inflate each tire at least to these pressures.
In the United States, Mercedes-Benz recommends a pressure that is at the low end of the safe range in order to provide the softer ride that many Americans are accustomed to in luxury cars. However, the tires are safe up to the maximum inflation pressures molded into the sidewalls (usually 40-51 pounds per square inch – PSI). In fact, the maximum load capacity is calculated at this higher number, so if you are loading your vehicle to maximum capacity, you need to use this pressure.
For most street use, somewhere between the recommended and maximum pressures will work. As a general guide, adding 3-5 PSI more than the Mercedes-recommended pressure will produce better turning and braking response, though at the cost of a slightly firmer ride. This will also provide a little safety margin before the tire becomes underinflated and dangerous.
We strongly recommend that you carry a tire pressure gauge in your glove compartment and one in your garage and learn how to use them as well as learning how to add air to your tires.
In recent years Mercedes-Benz has added tire pressure monitoring systems (TPMS). If you see a little warning icon that looks like the cross section of an underinflated tire with an exclamation point, this means that one or more tires has dropped more than three pounds from where you set the monitor. This will also light up if you raise pressure more than three pounds. If you want to drive on tires with the higher pressure, you can reset the warning light. The owner’s manual explains this simple operation. You should note, however, that these systems are not completely reliable under all circumstances, so they don’t replace manually checking tire pressures at least once a month.
In the next article, you’ll learn how to use the throttle, brake, and steering wheel to keep those four all-important contact patches on the road at all times.

Jim Roberts, a member of the MBCA-Educational Foundation Safe Drivers – Safe Families task force, is a periodontist in Birmingham, Alabama, and a four-time Sports Car Club of America, and seven-time Historic Sportscar Racing National Champion race driver. He is national director for High-Performance Driver Education for Lotus, Ltd, North America’s Lotus Club, and teaches high-performance driving for several national marque clubs and track-driving associations.
 
Two of your most important tools for tire safety, an analog tire gauge for the garage for monthly checks, and a pen-style gauge for the glove compartment to check tire pressures whenever you’re in doubt or while at the gas station at fill-up time.

A good analog tire gauge, available from  high-performance auto parts suppliers, is easy to use and will give you accurate tire pressure readings. This example was ordered online from performanceproducts4benz.com, and costs around $20.

Try to make it a habit to check your tire pressures when you fill-up the car, or each morning when you’re on long road trips out of town. If the tires are warm, they will read about four pounds higher than recommended cold pressures.

Above: A sticker on the inside of the fuel filler cap lists recommended tire pressures, including increases for hot tires and high speeds. Below: A sticker on the driver’s door jamb lists general tire information, including maximum weight for passengers and cargo, maximum passenger capacity, recommended tire pressures, and standard and winter tire sizes.