How to Choose Tires

It is said that 90 percent of tires are purchased based on two things: price and looks. Realistically, the odds are very slim that those tires would work. It’s the best for the vehicle they’re on. Tires and tire design are far more complex issues than most users believe, and every tire is a series of compromises. So, typically, the best solution is to find the tire that is the least compromising for your particular needs. Getting good street performance dictates choosing a different tread design than one that works well on the trail, maybe even a different type of tire. What tire works best depends on a number of things, including (but not limited to) where it is used, what vehicle it’s on, how that vehicle is driven, and how much it weighs. Are we heading for a large can of worms here? You bet. Let’s also make it clear that there is no “perfect” tire—but it sure can’t hurt to get as close as possible.

Determining your needs, in combination with a better understanding of tire construction and design, can be a big help when deciding what tire is best suited to your vehicle and use pattern. Basic components in tires do certain things, and your goal should be to match up those features and benefits with your needs. First of all, the tire should be of the right type, as in radial ply, bias-ply, or bias-belted. While these three different casings may look much the same to the untrained eye, they sure don’t work the same.

RADIAL-PLY

Radial casings are generally made with polyester and are great for highway use. They have extremely good directional stability, they are smooth-riding, and they have very good flotation properties. Plus, radials tend to last longer (at least in miles, but not necessarily in time). Radials usually have steel belts, which hold the tread flat and are largely responsible for the above-mentioned good properties. That’s very desirable for highway driving as well as on snow and ice.

Drawbacks include weaker sidewalls that are more prone to impact breaks, and the steel belts hamper the radial’s ability to conform to trail obstacles. The radial plies act much like a tank track, meaning they only want to deflect in one plane, which also leads to a harsher ride over irregularities both on- and off-highway.

BIAS-PLY

Usually made with nylon, bias-ply tires have the best casings for conformability and strength. These tires are just as strong in the sidewalls as in the center of the tread (not counting the tread itself). While the sidewalls may not flex as readily as a radial’s, the flex in the tread more than makes up for it, and ride quality on the trail is superb. Also, for those who put more trails than miles on their tires, time doesn’t seem to affect them much. Bias-ply tires are the originals and still the best—for trail use.

On the highway it’s a different story, as bias-ply tires tend to have less than great road manners, wear faster, and don’t exactly help fuel mileage any.

BIAS-BELTED

Bias-belted tires are built as a bias-ply casing but with a circumferential fiberglass or steel belt, just like a radial. This helps the belted tires get a little of the on-highway qualities of a radial, and it makes them very cut-resistant. It generally takes a heavy vehicle to make a bias-belted tire work well on the trail since the tread area can be quite stiff, at least with a steel belt.

FINDING THE FOUNDATION

Since most four-wheelers use their vehicles mainly on the street, a radial-ply tire would logically be the best choice for the vast majority. For those who trailer their four-by to the trails and/or severely limit the street driving (consequently racking up only 60–70 percent, or less, of the mileage on pavement), a bias-ply is probably the better casing to use.

If you picked a radial or bias-belted casing, drive a relatively light vehicle, and trail performance is important, try to find the best one. While radials and bias-belted tires tend to work better on the trail when under a heavy vehicle, not all belted tires (radial or bias) are the same. Some are built stiffer, with an emphasis on weight control and mileage, while others are more suitable for trail use. How can you tell? Push on the center of the tread on an unmounted or completely deflated tire. The more flexible the tread area, the better the tire will work on the trail, since it conforms more readily. On the down side, the highly conformable tire will wear faster, especially on a heavy vehicle in around-town use. A stiff casing on a light vehicle will last a long time but perform poorly on the trail and ride harshly.

Whether you decide to favor the qualities of a radial, bias, or bias-belted casing, which is likely the easiest decision in the process, the tire also needs an appropriate tread to work well.

SIZE DOES MATTER

What tread type is the best for trail use on a given vehicle depends largely on tire size and vehicle weight. Here the goal is to get the correct contact pressure (weight per square inch), which is largely a function of tread void ratio. As the name implies, tread void is the relative amount of open space between the tread blocks, and it doesn’t matter if there are a few large tread blocks or many small knobs. A high void ratio will put more of the vehicle weight on a smaller amount of rubber, increasing the contact pressure. A larger tire with the same tread will decrease contact pressure, although the void ratio remains the same.

Likewise, a lighter vehicle will have less contact pressure than a heavier one with the same tire. The two extremes would be a heavy vehicle on small, narrow tires with a high void ratio and a light four-by on big fat tires with a low void ratio. In one case, there’s too much contact pressure, in the other there’s not enough for optimum traction.

So how do you know when there’s too much and when there’s not enough? Try going up a hill of moderately packed dirt. If the tires start spinning but do not dig down much, the contact pressure is likely too low. You’ll want to try a smaller tire or one with a higher void ratio, or else air up a bit. Should the tires try to trench more so than go forward, chances are the contact pressure is too high. In that case try a larger tire or one with less void ratio, or air down more.

CONFORMABILITY

Driving up a flat hill is different from driving over obstacles such as rock and other uneven surfaces, because now the tread also has to conform to the terrain, as discussed earlier. Since all street-legal tires are designed with highway driving in mind, the load transitional characteristics (how the weight of the vehicle moves from one tread block to the next) that help the tire last on pavement can work against us in the dirt. Large tread blocks can’t flex very well, for obvious reasons. While the full explanation is outside the scope of this story, this is why you see custom-tuned (cut) tires on the trails these days. The drivers have increased the tires’ conformability by cutting a large tread block into smaller sections.

OUT ON A LIMB

Most ’wheelers already know what kind of tire works the best in specialized areas such as sand and mud, but we’ll try to give you some general ideas, just in case. For sand, size is everything. The wider (not necessarily taller) the tire, the better the flotation, and the idea is to stay on top. Little or no tread works fine in sand, and the rounder the tires’ shoulders, the better. Vehicles with a lot of power can run tires with seriously aggressive tread, even paddles, since the sand gets propelled rearward accordingly. To some extent, mud running is similar in that tire bite and power availability should be matched. Generally, though, the most aggressive and self-cleaning treads work best in mud. Light vehicles can benefit from wide tires, allowing them to float over the worst, while heavy rigs may be better off with skinnies that can find traction on the bottom—if there is one. Either way, the taller the tire (if there’s power to turn them) the better, keeping the drivetrain out of the slop.

We’ve seen everything from passenger-car whitewalls to tractor treads on rock trails, and they’ve all worked reasonably well. We suspect it’s more of an issue of what’s on top of them and who’s driving than what tires they are.

WHERE RUBBER MEETS THE ROAD

Now you’re on your own. But at least you’ve gained some insight as to what tire designs work where, and why, so you now have a better opportunity to buy tires based on what should work the best on your vehicle. As for the other 90 percent, we hope they like the looks and price of their tires.