By Paul Abelson
senior technical editor
Here’s a multiple-choice quiz. Your tires are:
- An important contributor to comfort and ride.
- A factor in achieving fuel economy.
- The largest single component of maintenance costs.
- A structural part of your suspension system.
- The same as they were 10 years ago.
If you answered “A” and “B” and “C” and “D” - you’re well informed. If you included “E,” you need to read Land Line more often.
Tires have undergone significant changes in the past decade. Structures have been improved and inner liners now allow less air to permeate the casing. The most dramatic and visible change, however, has been the introduction of wide-base designs that allow one tire to replace two standard duel tires. Are they for you? (See the accompanying article on Page 88.)
Tires perform many functions. Tires have their own spring rate, the amount of deflection a tire or a spring experiences for a given load. By load, engineers don’t just mean cargo weight. The term includes impact loads from rough roads and potholes, and lateral weight transfer during cornering.
Designed to operate within a given range of air pressures, tires influence suspension design. Although it’s not actually part of the tire itself, the air inside your tires also plays a key role in your suspension. Michael Burroughs, product manager for Michelin, told Land Line that only 5 to 6 percent of the load is borne by the tire structure. The air actually carries 94 to 95 percent of the load.
Air pressure is mentioned in every discussion about tires for a very good reason. It affects so much: ride quality, fuel economy and tire durability and maintenance.
The rubber meets the road
As tires roll, they flatten to form the contact patch with the road. When any point on the tread is in contact with the road it can transmit fore-and-aft traction, braking and lateral cornering forces. When there is insufficient air, these forces are less precise, as forces are transmitted through softer, less-supported sidewalls.
As the point on the tire rotates, the patch compresses the tire. After the point goes past the contact patch the force is released and, like a spring, the point cycles beyond the circumference before resuming its normal position. Engineers call this a standing wave.
The depth of the standing wave is calculated using speed and air pressure. The standing wave flexes tire cords, generating internal friction that heats the tire. If there isn’t enough air to resist excess flexing, the tire gets so hot that rubber can melt. Just as repeated flexing will break a paper clip, prolonged excessive flexing weakens tire cords. Combine damaged cords with melted rubber and we get road gators.
Each tread strip represents two ruined tires; the one that lost its tread and the other dual that becomes overstressed from carrying double the load. Even if it doesn’t blow during a limp home run, the remaining tire will be damaged internally and won’t handle retreading. It should be replaced along with the other tire. Or, you should pull over immediately and wait for road service.
Low air pressure lets the tire flex more for a given load. It gives a more comfortable ride, but makes handling imprecise and lets more weight transfer during cornering.
Fuel economy - so critical as diesel surpasses $3 per gallon in some regions - is greatly affected by both tire design and air pressure. Rib designs are more fuel-efficient than lug types. Worn tread is more fuel-efficient than new deep tread, because rolling resistance is affected by design and depth. The lower the rolling resistance, the less energy is wasted for flexing.
Tread with care
Now that LEDs and sealed harnesses have improved safety lighting, it is no longer the number one cost item in vehicle maintenance, according to TMC studies in the 1990s. Tires have become the greatest contributor to maintenance costs.
Gauging tires often, aligning and rotating them, and checking for irregular wear can control maintenance costs. When wear is detected, take corrective action. Eliminate the cause and change the tire’s position to normalize wear.
With fuel and operating costs skyrocketing, tire care is an important way to keep costs in check. It doesn’t matter if you spend less for fuel or maintenance. Every penny saved goes right to your bottom line.
Paul Abelson may be reached at email@example.com.