FEATURED IN VEHICLE OPS
If you think about it, of all the equipment on a patrol vehicle, tires are probably the most ignored, but they’re responsible for more duties than just about anything else. Your tires must perform safely in temperatures ranging from the deep freezes of Minnesota to the hottest surface temperatures of Death Valley. They must carry the load of thousands of pounds of vehicle, occupants and equipment. They must provide stability while maximizing acceleration, stopping and turning. They must provide extreme resistance to impact and puncture. And they must do all of the above consistently over tens of thousands of miles. So when you think about the engineering behind those four rubber donuts, it becomes apparent that tire selection can make all the difference in how your patrol vehicle performs.
Contact Patch & Tread Design
Tires are the only thing connecting your vehicle to the road. In the simplest terms, the weight of a vehicle presses the tire surface at each corner of the vehicle into the road surface. The amount of contact between a tire and the surface is called the contact patch . The effectiveness of each contact patch determines how well your vehicle accelerates, stops and turns. The greatest influence on contact patch comes from weight transfer caused by driver input. For example, when a driver applies the brake, weight shifts toward the front on the vehicle, causing the front suspension to compress. This compression places more weight on the front tires, causing their contact patches to enlarge. The larger patch increases the friction between the road surface and the tire, thereby improving the vehicle’s ability to stop. The contact patches on the rear tires become correspondingly smaller, which reduces traction in the rear. You can affect the contact patch at each corner of your vehicle by the inputs you make, but how that contact patch is designed depends on the purpose of the tire’s tread pattern.
According to Toyo Tire Co., tread has four main functions: it provides grip in wet and dry conditions; it improves tire stability; it prevents hydroplaning; and it’s aesthetically pleasing.
A number of design factors affect the tire’s wet weather, dry weather, snow and durability parameters. The first is the void, or spacing, in the tread. The larger the void, the better the tire works in the rain. But this means less rubber actually touching the surface, so dry performance and stability suffer. Consider the performance of a tire designed for off-road use vs. a summer performance tire. The large gaps between the tread lugs on the off-road tire, which are necessary to grip rocks and channel deep mud, mean less tire surface for dry braking and handling. By contrast, a summer performance tire has few voids to maximize dry grip, but would be treacherous in the rain and useless off road.
Another factor is groove depth. Simply put, the deeper the groove, the better the tire can deal with snow and water. Deeper grooves also allow more air to circulate through the tread blocks, which keeps the tire cooler, but makes it noisier. In fact, much of the noise you hear from a tire is from air flow making its way through the tire. Groove depth also hurts dry-weather stability because it contributes to more lateral movement for the tread blocks.
The width of the slits along the tire face also contribute to its ability to channel water and snow. The wider the slits, the better it removes the liquid to preserve the tire contact patch, but this comes at the expense of being noisier, and it reduces the tread surface along the edge of the tire.
Siping is also important. Sipes are small slits in the tire surface that cause the tire to basically “squeegee” the road surface as it comes in contact with water or ice, which is very important in low-traction conditions. If you can imagine running a squeegee down a wet window, the pliable rubber blade is able to channel away the moisture. Now, think of doing the same thing with a rubber hockey puck. It will just slide on the surface because it’s too rigid, not gaining any traction or displacing any fluid. One of the biggest mistakes I’ve seen departments make in choosing tires is in assuming that an aggressively designed off-road tire is the best choice for snow and ice. In deep snow and mud, perhaps, but in conditions of heavy rain, slush or ice, surface contact is what’s needed, and sipes are critical. Yes, the tire must channel the water away, so grooves are necessary, but there still has to be rubber contacting the road surface to ensure grip. The many mini-slits required to create sipes compromise dry weather tire stability, so they are generally found on tires designed for winter and rainy conditions.
Tread block arrangement, pitch and other factors affect noise, but we should be primarily concerned with choosing the best tire for the environment in which we regularly work.
A Word on Inflation
The biggest enemy of a tire is excessive heat, and most of that begins with inadequate tire inflation. Just as you check your shotgun, trunk, back seat, radio, light bar and siren when you come on duty, checking your vehicle’s tire pressure is critical. Air in the tire is what allows it to perform properly, and millions of dollars have been spent in tire design and performance that begins with proper inflation.