Interpretation ID: 1984-3.46

TYPE: INTERPRETATION-NHTSA

DATE: 12/01/84 EST.

FROM: NATIONAL SCHOOL BUS REPORT

TITLE: SLASHING TIRES FOR SAFETY AND SAVINGS

ATTACHMT: DECEMBER 30, 1988 LETTER FROM JONES TO SPRUNK, OCTOBER 8, 1987 LETTER FROM SPRUNK TO JONES, BROCHURES ON TIRE SIPING, 1978 NSC WINTER TEST REPORT, AUGUST 19, 1986 LETTER FROM KEIL TO SPRUNK, ARTICLE FROM AUGUST 19, 1986 ISSUE OF "SCHOOL BUSINESS AFFAIRS," MARCH 20, 1985 LETTER FROM GIFFORD TO SPRUNK, OCTOBER 15, 1982 LETTER FROM PALMER TO MARCY MANUFACTURING, AND APRIL 1983 AND APRIL 1984 ARTICLES FROM "GW SAFETY TALK"

TEXT: Coping with western Montana winter roads used to be a problem for NSTA member Bob Beach, until he made the switch from studded tires to slashed tires. That's right: slashed tires -- thin cuts across the tire, from 5/32 to 11/32 inches deep, and eighth of an inch apart, all the way around. The process is called "siping."

The problem with studs wasn't safety, but cost. "Studs will wear tire casings out," says Beach, "and though they are legal in Montana, they will wear highways out. We used to have a studding machine which made it cost-effective at one end, but we needed another set of tires for summer months, so at the other end, studded tires were costly."

Beach Transportation operates in the city and county of Missoula, Montana. His fleet consists of more than 70 school buses plus seven MCI coaches travelling 793,000 miles per year.

Winters are surprisingly short in Beach's part of the country, from about November to the end of February, but in those months the usual hazardous conditions prevail -- snow pack, slush at times and black ice at others. Schools are rarely closed due to snow, so school buses are expected to perform daily and safely. With siped tires, they do.

According to Beach, the little slits created by siping make the footprint of a tire spread, giving it more flexibility. "This means the tire grips the surface better, and it makes braking and steering on snow or ice much more effective." He added, "you get much better traction spin for starts."

Before the invention of an easy-to-use and low-cost machine that slashed tires, Beach used to cut tires by hand. "Back then, it was called cross cut," he explained, "but it was a costly operation, and the cuts weren't uniform. We wanted something safer."

Siping was invented back in the 1930s by a packing plant worker named John Sipes, who made a series of small cuts in his rubber-soled shoes to prevent slipping. Sipes extended his idea to tires, but it was several decades before the idea became a commercial success.

Today, tire-slashing machines are manufactured by Saf-Tee Siping and Grooving, Inc., the only manufacturer of after-purchase siping equipment.

Purchasers claim that the machines are trouble-free and cost effective. Each blade used to make the sipes is good for 80 to 90 tires, and costs around $ 5 each. Also, the machines are easy to manage and can be operated after 10 to 15 minutes of instruction.

The cost to sipe a school bus tire is $ 10. It takes about five minutes once it is off the vehicle. Any type of tire can be siped -- new, re-cap, or used, as long as it has 5/32" of tread left. Beach Transportation runs new tires on the front of his school buses, and lug re-caps on the rear. His tire inventory is around 500 and every tire is siped, including personal cars and the company tow truck.

Tire engineers have been impressed with test results which show that on snow and ice, siped tires were impressively better performers in terms of traction, braking, and cornering. And, contrary to the engineers' expectations, tread life increased.

Now, molded sipes, a process done during manufacturing, are an almost standard part of tread designs, but are not as effective as siping, an after-purchase process, since molded sipes are spaced only 1/4 inch apart. Also, molded sipes can displace rubber and reduce tread volume, whereas siping does not.

For siping to be most effective, the cuts must go straight across the tire, be of the same depth as the tread, and be at regular intervals. The result of siping with the Saf-Tee Siper is that hundreds of tread elements are created, and each individually grips the road surface and flexes under the stress of acceleration and braking.

Ice traction tests, comparing siped and unsiped tires with identical tread patterns on all four positions, showed the stopping distance of the vehicle to be 155.6 feet with siped tires, versus 200 feet for unsiped tires -- an improvement in performance of 22 percent. And tests for break-away traction on ice show that a significant advantage in force is developed by siped tires compared to non-siped. Once break-away occurs and the tires begin spinning, there is an increase in spinning traction developed by the siped tread in excess of 25 percent.

There are a few eases(Illegible Words) a tire may not be the(Illegible Words) go. First, when the tread design(Illegible Word) already complex and siping would result in numerous unsupported tire elements, chunking might result. Second, using siped tires on gravel roads is not recommended since gravel can cause chunking. However, some companies prefer siped tires in all conditions, including gravel because of the increased safety.

Bob Beach used to run his siped tires only in the winter, but he began using them year-round when he noticed that tire life was increased after siping. He also discovered that siped tires are very effective in Montana's June and September rains. "The siped tread elements open up and the sharp edges penetrate the lubricating film of water in what might be called a squeegee action, and the openings between the tire elements created by the sipes channel water away, minimizing hydroplaning."

Various tests have shown that on slick, wetted surfaces, such as concrete, a new tread with cut sipes will provide up to a 30 percent increase in braking traction, at speeds between 20 to 60 mph. One Canadian transit manager testing low-tread siped tires reports, "I know some of you will say 16 percent (his test results) is only three feet at 20 mph, but three feet can be the difference between an accident and no accident, or a $ 2,000 accident and a $ 1,000 accident. The less accidents you have, the lower your insurance premiums are. The less accidents your fleet has the more confident your drivers are, and the company president is happy."

That certainly is the case with Bob Beach. Siped tires are not only safer on roads that are icy, snow-packed, or filmed with water, they are also cost-effective even on dry roads. They run cooler and dissipate heat. "Increased safety and reduced operating costs don't always go together, but with machine-siped tires, they do. There's no way I'd go back to running without them."

This article was written by Richard Chapman for Saf-Tee Siping & Grooving, Inc., 3467 E. Sunrise Dr., Minnetonka, MN 55345. For further information, contact Wes Sprunk at (612)935-4540.