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Which Tire is Best for Sand, Rock, Snow and Mud

Posted in How To: Wheels Tires on February 1, 2011 Comment (0)
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Photographers: Courtesy Continental Tire North America

Choosing the right tires for your 4x4 is a lot like picking out a rental car for a family vacation. You have many different models to choose from, and most will get the job done, but some work much better than others. Where a front-wheel-drive minivan might be well suited to handle a trip to Disneyland with several small children, you wouldn't necessarily want one for an exotic overland safari through the Serengeti. The same holds true for tires. Where a run-of-the-mill all-terrain might provide excellent traction in the paved confines of suburbia, you certainly would not want to show up at the local mud bogs on a set. Therefore, with every tire choice comes a set of compromises. With this story, we'll help you understand some of the key variables in picking the right tires, and we'll point out which type of tread design and compounds work best for a given environment. Keep in mind that we could not cover every tire made in this article-space is limited. So if we missed your favorite tread, drop us a line and let us know what tread type you prefer for a given terrain. Send responses to fourwheelereditor@sorc.com.

The Basics
The first consideration of any tire selection should be intended use. While most tires are designed with a wide array of environments in mind, some target specific usage and subsequently lack attributes that are necessary to excel in other specific terrain types. For example: The BFG Baja T/A, which is primarily designed for desert racing, does not work well in snow and ice. Conversely, other tires, such as General Tire's Altimax Arctic, are true thoroughbreds for winter conditions, but feature a soft rubber compound that degrades quickly in warmer climates.

In addition to compound, you must consider tread design or pattern. It is unfortunate, but many enthusiasts purchase tires based on appearance and price point. Tire companies sometimes use these fancy patterns to attract the consumer who makes buying decisions based on emotional impulse rather than rational real-world data. However, in other cases, what might seem unnecessary for a daily driver may actually ensure great performance on the trail. For example, the split-personality Interco TSL Thornbird features a three-stage lug sidewall that protrudes noticeably from the carcass. These oversized lugs serve no real purpose on the street. However, when aired down, the protruding lugs become an impressive part of the traction equation. Don't let the good looks of a tire fool you-research the reasons for buying a particular design or tread element before making your purchase.

While the BFG Rugged Trail T/A does a great job on pavement, gravel roads and in the wet, without decreased air pressure, they-like all OE tires-are prone to simply sinking in sand at street pressure. However, once aired down, these meats were able to propel this Nissan Titan through the dunes with ease.

Here are some key things to keep in mind when shopping for tires.

When selecting a tread pattern, keep in mind that large uninterrupted tread blocks tend to decrease traction in the wet. Tires with copious amounts of siping stick like glue on rain-soaked roads.

High void-ratio mud-terrain tires are often noisy, while all-terrain designs' tight lug spacings are generally quiet.

Alphanumeric load-range values are expressed on the sidewall as load range "C," "D," and "E," but are not always reflective to the actual load the tire is capable of carrying. The more important numbers to consider are the maximum load numbers expressed as pounds per square inch at a specific psi cold.

Sometimes you don't know how well a tire will do in the sand until you almost get stuck. This occurrence was definitely one of those scenarios. With one dead leg dragging in the sand, the author was able to drive our project Mega Titan, broken lug studs and all, over one mile through the difficult terrain of Pismo State Beach. The 46-inch Mickey Thompson Baja Claw tires got the job done with relative ease-a testament to the high void-ratio/big-power arrangement.

Radial versus bias ply: Radial-construction tires are best for street and occasional trail use; bias-ply tires are best suited for dedicated trail rigs.

Size matters. Be sure that the tires you pick will work on your rig before you make your purchase. Refer to Four Wheeler's tire fitment chart before ordering. This chart can be found at www.fourwheeler.com

Void ratio relates to how much rubber is touching the ground versus open space between the lugs. Ideally, you want a specific amount of contact pressure for the weight of the vehicle. Tires with a higher void ratio tend to increase surface pressure-concentrating the weight of the vehicle to the individual lugs that are in contact with the road surface. A lower void ratio tends to spread out the pressure across a larger contact patch. Finding the right void ratio depends largely on the type of driving you do most.

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Construction
The way a tire carcass is constructed makes a huge difference in how it performs both on and off the pavement. There are many different configurations that manufacturers employ to produce a tire carcass. Essentially, sheets of rubber are applied to a rotating drum and then wrapped with a reinforcement material such as steel, fiberglass, nylon, polyester, or Kevlar. After each reinforcement layer, another layer of rubber is applied. When all the layers are in place, the unit is known as a "green tire." The "green tire" could have reinforcement cords that are oriented radially, as is common to a radial tire, or at opposing angles, as is found in a bias-ply tire. Either way, it's these belts, and the way they are assembled, that give the tire its strength and puncture resistance while allowing it to conform to terrain.

In Moab, rock crawlers may encounter everything from slickrock to sand dunes, so it goes without saying that a good rock tire should be a jack-of-all-trades.

Environment: Sand
To drive successfully in the sand, a tire must provide flotation. Obviously, this has a lot to do with the size of the tire-the wider, the better. Flotation is a product of the sidewall's ability to flex as air pressure is lowered. Too short or stiff of a sidewall, and flotation will be hampered. Flotation is most important for sand because without it, the vehicle would simply sink up to the chassis or axlehousing.

In virtually all cases, the amount of flotation a tire can provide is directly proportional to the aspect ratio or sidewall height. As the sidewalls begin to bulge outward, the overall footprint of the tire grows. On tires with shorter sidewalls, the growth is limited and can create a problem, as too little air pressure may actually allow the wheel to come into contact with the ground while traversing bumps. This leads to sidewall degradation and/or wheel damage.

The ideal tread pattern for sand driving depends on the vehicle's configuration. For example, vehicles with high horsepower can benefit from aggressive lug spacing and a high void ratio, i.e., paddle tires. For underpowered rigs, the opposite is true. The best method for driving an underpowered rig in the sand is to stay on top of the terrain and not let the vehicle get bogged down.

Environment: Rock
Tires for rockcrawling require both carcass flexibility and puncture resistance while also delivering numerous biting edges per square inch. Conventional wisdom says that bias-ply tires are best suited for the rocks. However, aside from competitive rock racers and purpose-built buggies, most rockcrawling rigs are also driven on the street, in mud, and through dirt on the trail, so we believe a good rockcrawling tire should excel in all categories of driving. Benefits exclusive to rock-specific tires are typically related to compound makeup, sidewall protection and carcass flexibility. Additives such as silica help the rubber resist the cutting effects of sharp rocks, and reinforcement materials such as Kevlar further fortify the carcass and sidewall for the same reasons. Some rockcrawling terrain, such as the trails of Moab, Utah, consists of sandstone, which provides virtually any tire with traction to spare. Other trails consist of granite, shale, and limestone-each with its own particular challenges to tire design. Items to look for in a good rock tires are durability and flexibility.

Environment: Snow
Due to the unique nature of snowfall, four wheelers may encounter a variety of different types of snow on paved roads or while negotiating the trail. Similar to variations in rock and mud, snowpack can provide ample traction, or stop any tire in its tracks. However, one commonality to all snow types is cold temperature. As such, the first element that all snow tire designers strive for is what is known as hysteresis. This is the rubber compound's ability to remain flexible at sub-freezing temperatures. By remaining flexible, winter tires are able to grip the terrain surface better and return abundant traction.

The second most important element of a good snow tire is siping, which consists of little cuts in the tread blocks that provide additional biting edges for grip. Together with hysteresis, a tire so designed can propel a vehicle through the harshest of wintry weather.

When a tire operates in snow and ice, a strange thing happens. The cold outside temperature air temperature causes the rubber to stiffen up. This hardening effect results in a loss of traction as the contact patch cannot conform to the various contours of the terrain surface. To combat this problem, winter tire engineers have employed new technologies in rubber composition. These improvements help winter tire compounds remain flexible at lower temperatures. Tire engineers further improve traction in snow and ice by adding sipes to the tread blocks. A sipe works by opening up for a fraction of a second as the tread touches the road surface. When this happens, snow is forced into the micro-thin voids between the sipes. Here, the snow remains trapped as the tire rotates. When a snow-filled void comes back around to the snow-covered road surface, the snow inside the void clings to the new snow on the roadway. This effectively improves traction because snow is naturally attracted to itself. Think about the "snowball effect": Snow has greater interrelative friction with itself over rubber, therefore the vehicle sticks to a snow-covered road better when it has snow embedded in its tread pattern.

In a perfect world, we would not need snow tires because track conversions like the ones shown here would be affordable to the masses. In Yellowstone National Park, tracked vehicles are the only ones allowed to negotiate snow-covered roadways during the winter months.

Environment: Mud
Good mud tires typically have a high void ratio and an aggressively tapered lug design that allows the tread to eject mud as the tire rotates. This cleaning effect ensures that the lugs do not become clogged, allowing the lugs to maintain a solid grip with the loose surface. The trick to finding the right mud tire for your particular rig is relative to how much power you have on tap. A high-horsepower engine is capable of spinning an aggressive mud tire such as an Interco Super Swamper Bogger. A power-challenged vehicle typically works better with a lug pattern that features smaller individual lugs with more aggressive lug tapering. A tire's sidewall can make or break a rig in the mud, too. A super-aggressive sidewall will help propel a vehicle forward even when the tread pattern becomes clogged. Smooth sidewalls do very little for traction in the mud. Tire pressure is less critical in soupy mud, while thick gooey mud requires a high level of flotation. Tractor tires employ a very different strategy. Instead of using flotation, tractor tires sink in and attempt to find bite on the underlying hard-pack surface under the mud. In most cases, this approach works great, but your vehicle is basically undrivable on legal roadways. Tractor tires are also very heavy and require gobs of power to keep moving.

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