When you stop and think about it, tires are probably the most ignored item on a rescue rig, yet they have more responsibility than just about any other piece of equipment. Let's consider for a minute what a typical tire must do. It has to perform safely in temperatures ranging from the deep freezes of North Dakota to the hottest surface temperatures of Phoenix. It has to carry the load of thousands of pounds of vehicle, occupants and equipment. It has to provide stability while maximizing the vehicle's ability to accelerate, stop and turn. It has to provide extreme resistance to impacts from hazards, such as debris, rocks, glass, metal and many other hazards. And finally, it has to do all of the above and still be able to do it consistently over tens of thousands of miles. So, when you think of the engineering behind those four or six rubber doughnuts that are keeping the corners of your rescue rig from smacking the earth, it becomes very apparent that the proper tire choice can make all the difference in staying safe on the road.
Houston, We Have Contact
Tires are the only thing connecting your vehicle to the roadway. 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 will determine how well your vehicle accelerates, stops and turns. The contact patch is affected by many things, but its biggest influence comes from weight transfer caused by driver input. As an 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. This is favorable because this larger patch helps to increase the friction between the road surface and the tire, thereby increasing the ability of the vehicle to stop. Because this weight has moved forward, conversely, 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.
If we drove on perfect streets under clear skies all the time, street tires would resemble those on race cars. That is, the tires would look smooth despite having a lot of rubber on them. Unfortunately, the real world isn't as tidy, so grooves or treads are a reality.
According to the Toyo Tire Company, there are four main functions for tread design. The first is to provide grip in wet and dry conditions. The second is to improve tire stability. The third is to prevent hydroplaning, and the last is for aesthetic reasons.
According to Toyo, a number of design factors affect the tire's wet-weather, dry-weather, snow and durability parameters. The first is the void, or spacing, between the tire's tread blocks. The larger the void, the better the tire can work in the rain, but this means less rubber actually touching the road surface, so dry performance and stability suffers.
Take an off-road tire versus a summer performance tire. The large gaps between the tread lugs on the off-road tire and are necessary to grip rocks and channel deep mud off road. But this means there's much less tire surface for dry braking and handling. By contrast, a summer performance tire has few voids to maximize dry grip, but it would be treacherous in the rain and useless off-road.
Another factor is the groove depth. Simply put, the deeper the groove, the better the tire can deal with snow and deeper water on the roadway. These deeper grooves also allow more air to circulate through the tread blocks, which keeps the tire cooler but noisier. 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 contributes 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 makes it 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 essentially "squeegee" the road surface as it comes in contact with water or ice, which is very important in low-traction conditions. 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. In conditions of heavy rain, slush or ice, surface contact is necessary, and sipes are critical. However, the many mini-slits required for sipes compromise dry-weather tire stability, so they're generally found on tires designed for winter and rainy conditions.
Other factors, like tread block arrangement and tread pitch, affect tire noise. But in the EMS world, we should be concerned primarily with choosing the best tire for the environment we regularly work in.
All Rubber Not Created Equal
The science behind tire rubber could fill a book, but in the simplest of terms, the heat range of a tire is critical to its performance. As an example, a racing tire is designed to heat up very quickly, often to temperatures exceeding 200 Fahrenheit. The rubber compound in these tires is designed to become very sticky as they reach these extreme temperatures, which is exactly what the driver meeds to get maximum grip. Unfortunately, this performance comes at the expense of tire life, so it would be impractical to have such tires on the street.
Instead, tires are designed to operate within specific temperature ranges depending on their purpose. According to Toyo, there are three basic categories for street tires: standard, ultra high performance (UHP) and studless. Standard tires are designed with tread patterns and rubber compounds that provide "jack of all trades, master of none" performance. An example of this would be an all-season, mud and snow (M+S) tire that usually comes standard on most street vehicles. It doesn't do anything poorly but isn't stellar in any particular category. Standard tires typically have harder rubber compounds for longevity and lower fuel consumption.
UHP tires provide softer rubber compounds to allow for heat build up for dry weather traction, and unidirectional tread design for maximum water drainage without large voids as discussed earlier. This means more surface contact and better grip. They also feature stiffer sidewalls to enhance turn-in, tire stability and cornering ability.
In the case of studless tires, a softer compound is used because it's more pliable for gripping ice, and multi-directional siping is used instead of unidirectional designs for traction enhancement. Sidewalls on studless snow tires are softer than UHP tires to allow the tire to better conform to the surface demands. The end result is a tire that performs markedly better than a standard or UHP tire in the snow and ice but wears out faster and shouldn't be driven in warm temperatures.
Don't Be an Airhead
A tire's biggest enemy is excessive heat, and most of that begins with inadequate tire inflation. 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. A moderately under-inflated tire will heat up excessively, make the vehicle sloppy to drive and affect high speed handling. It will also make the tire wear out faster. A severely under-inflated tire will cause significant handling issues and provide the opportunity for catastrophic failure. Both conditions will reduce resistance to hydroplaning.
If you aren't sure what the tire should be inflated to, the place to find this is not on the side of the tire. The number listed there is for inflation under maximum load. What you should be checking is the manufacturer placard usually found on the inside of the driver's door sill. If it's not there, ask for an owner's manual.
One thing to keep in mind is that the recommendations by the manufacturer are based on the weight of the vehicle at the time of design. A typical rescue rig often weighs much more, so a few extra pounds in each tire might be necessary. Make it a habit to check yours. This will also let you identify any tire damage because, as we know, rescue rigs are driven a lot of places other vehicles aren't.
Choosing the type of tire, its tread design and temperature range is as critical as choosing the proper gurney or light bar. When considering which tire to put on your EMS rig, or personal vehicle for that matter, examine what environmental conditions you and your fellow officers encounter on a regular basis, then choose appropriately. In some cases, depending on where you work, this may mean having two sets of tires for the seasons. Ultimately, tires are the only thing connecting you and your EMS vehicle to the roadway, and making the right choice will go far in keeping you safe in your duties.