Feature Article: Siping Strides
Pat Keating, a senior technical engineer at Yokohama Tire, sees siping’s most critical function in “steering traction. The direction of the vehicle is provided by the driver’s input via the steer tires. Drive tires normally have blocks and wide grooves as their major features, so sipes are less critical there, though we do provide a single sipe for wet traction.”
Harden says, “The width, depth and other dimensional characteristics of the siping which are used within a new tire’s tread pattern are customized across the entire pattern to obtain the desired performance balance for the product. Each specific tread pattern, compound and tire size combination may require unique siping characteristics.”
A sipe’s biting edge cuts through water on the road, Walenga says. “As far as reducing treadwear goes, consider that every time a section of the tire enters the footprint, it has to roll flat,” he says. “It is compressed, and then when it leaves the footprint, it kicks out. This is a lot of what produces tire wear.” In action, a sipe “reduces flexing or scrub, and expands and produces a biting edge. It does a lot for traction, and it stretches to make treadwear more uniform.”
Curtis Decker, an engineer with Continental, says a sipe works almost like a squeegee. “A sipe is there to remove the last layer of water to get the best grip.” Decker explains that three-dimensional sipes lock together in the footprint and then spring apart, allowing the blocks to jump out of the footprint when it leaves the road. “With long tread blocks, you get a snapping effect, which generates a dynamic force on the road,” he says. “It creates what we call ‘heel and toe wear.’ It’s like rubbing an eraser across rough paper. With our sipes, we break up the tread block, and that means less snap effect as it comes out of the footprint.”
William Estupinan, director of technical services at GITI Tires, says even molded sipes are likely to close up when they arrive in the footprint. “Sipes work by increasing the traction bite surfaces and thus increasing the area of contact with the road,” he says. “Sipes create micro-blocks and micro-lugs that provide a more flexible and ‘adaptive’ interface tread.” He also believes sipes reduce treadwear mainly by keeping the tire from losing traction and sliding.
The risks and rewards of aftermarket siping
While major tire makers have elevated siping to a refined, computer-modeled science, some truckers take a relatively low-tech approach and do their own slicing. This aftermarket surgery is particularly common among those who operate in a rainy, snowy climate, where the additional siping can provide better traction. It can be effective, but there are risks, say the big tire companies.
Even Weir Schankel of Van Alstine Manufacturing, creator of a tire siping tool for aftermarket sipers, urges caution. “We recommend a depth of just 1/8-3/16-in.,” he says. “If the tread wears down and starts to eliminate these sipes, you can always go back and re-sipe.”
He says an operator holds a siping tool square in front of his chest, similar to grabbing the dual handles of a rowing machine, then pulls it across the tire. The hand-held tool allows adjustments and different blades to determine the angle, depth and width of cut, and the spacing between sipes.
Some makers of automotive wheel balancing and alignment equipment also make machines for aftermarket siping. This is often done with the wheel still on the truck. Unlike manufacturers’ processes, these machines typically can cut sipes in only one direction, says Bridgestone/Firestone’s Guy Walenga.
If aftermarket siping ruins the tire, the warranty won’t apply. If that’s a concern, check with the manufacturer before proceeding.
“We don’t recommend or endorse it,” says Walenga, although he sees few problems “unless it’s not done right. The biggest problem is the depth of the cut. If too deep, a sipe can catch extra debris.” Excess debris can create stress near the bottom of the sipe, leading to “chunking,” or breaking out of pieces of tread. “You could always cut down to 2/32 and then do more later if you need to. You may still have a useful casing. Just don’t cut into the belts.”
Other industry spokesmen are a little less tolerant.
“If we really believed more sipes would help, we would put them in,” says Continental Tire’s Curtis Decker. He adds there can be “terrible consequences.”