THE GOODS : Shocking News About Seats, Tires

Question: Almost every time I get out of my new Ford Taurus, I get walloped with an electrical shock. It doesn’t happen every day, but on most days as soon as I touch the outside of the door I get zapped. I asked my mechanic what caused this and he didn’t know. He checked out the electrical system and it was fine. He told me the tires could be at fault. What causes these shocks?

--B.R.

Answer: The shock comes from static electricity, which is created in a number of ways. The problem occurs when your body has an opposite charge from the car or the ground.

The static charges are known to occur in at least two ways, from the car tires or the seat fabric.

The polyester and other synthetic fabrics on seats, when rubbed by your clothing, can create a static charge. The charge increases depending on the fabric types and builds up as you slide across the seat when you are entering and exiting. (Cars with leather seats rarely cause these types of shocks.)

Often, this charge exists between you and the car’s frame. So, you get shocked any time you touch metal that is ground to the frame--which would include most of the metal in the car--either inside or outside the car.

Tires have long been blamed for static shocks, though tire manufacturers denied their tires contributed to or caused the problem. In the past, the tire makers were right, but recent technical changes to tires may renew concern that tires can cause static buildup.

As a car moves through the air, it will build up a charge if climatic conditions are right, particularly if the relative humidity is low. If static builds up faster than the tires can dissipate it, then an electrical charge exists between the entire car and the ground.

Tires have traditionally contained carbon black--in addition to a mix of natural and synthetic rubbers--a form of pure carbon invented in 1912. Carbon black, produced by burning natural gas or oil, is used to reinforce and strengthen the rubber. The reason tires are black is because of carbon black.

Rubber is a poor conductor of electricity, but carbon black is a very good conductor. So as static electric charges build up, they are dissipated through the tires, thanks to the carbon black.

But recently, some foreign tire manufacturers have begun substituting silica (the main ingredient in sand) for carbon black. The silica is intended to reduce the rolling resistance of the tires. But it also reduces their conductivity.

The shocks caused by this type of static charge usually occur only after a driver has put his foot down on the ground and is touching the car. Then, the body becomes a conduit for the charge to travel from the car to the ground.

Experts say there isn’t much you can do about the problem, short of changing tires, if yours are what are causing your shocks.

Honda has acknowledged that its 1994 Accord models caused electrical shocks last fall, as the result of Michelin tires. Michelin changed its tire compounds earlier this year, which solved the problem, a Honda spokesman said. Honda owners may get credit on a case-by-case basis, he said.

Meanwhile, Ford uses tires on its Taurus made by every major tire producer, including Michelin, though it did not use the same Michelin tire that caused the Honda problem. So, it is not clear whether the tire or the seat fabric or some other cause is at fault in the Ford’s case.

You ought to contact your Ford factory representative. If the car is under warranty, you could say it’s not acceptable for the car to cause shocks. You might also want to locate and contact the manufacturer of your tires.