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Tire Construction Technology: Bias and Radial

Posted in How To: Wheels Tires on February 1, 2011 Comment (0)
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Tire Construction Technology: Bias and Radial
Photographers: The Four Wheeler Staff

Once a hotly debated subject for wheelers everywhere, radial technology seems to have won out over the one-time stalwart, the bias-ply tire. While radials are what you will find under the vast majority of trucks, there is still a place for bias tires in the realm of wheeling. We'll take you through the differences between bias and radial tires and where each one fits into the broader picture. Radials have become so good these days that there are very few manufacturers even producing bias-ply tires, but they are out there if you want them.

Bias-Ply Construction
Bias-ply tires are typically made using nylon cords that create layers, or plies, that extend diagonally from one bead to the other, meaning that the sidewall and tread have the same construction. Each successive ply is laid at an opposing angle to the first, creating a crisscross pattern, and strength is built using additional layers. This layering system makes for a very durable carcass with a high degree of puncture resistance. Bias-ply tires that are reinforced with a belt are known as bias belted. You can typically spot a bias-ply tire because of the rounded tread profile.

Because of the way they are put together, a bias-ply tire is just as strong in the sidewall as it is in the tread. A bias-ply tire is very resistant and tolerant to carcass deformation and offers unmatched performance on the trail because the tire is designed to self-clean more easily and compounds are generally softer for better grip. Because the bias-ply sidewall carries some of the load, it allows the sidewall lugs to extend far past the shoulder to expand the available contact surface. Bias-ply tires conform much more readily in the tread, offer a more compliant ride off-highway, and are typically less expensive than an equally sized radial counterpart.

Pit Bull and Interco still offer bias-ply lineups for the extreme wheeler. Here you can see how readily the carcass of a Pit Bull Rocker conforms to uneven surfaces.

Despite all the good that bias-ply tires offer in the dirt, they have several inherent drawbacks. Because of how a bias-ply tire is constructed, they don't provide the high speed stability that a radial does, nor do they offer the precise steering feedback of a radial. Increased rolling resistance and heat build-up mean bias-ply tires will wear very quickly if used much for highway travel. Another tidbit for those who don't drive their rig very often: Bias-ply tires are susceptible to flat spotting, which can make the first several miles noisy and uncomfortable until the bias ply regains its shape.

Radial Construction
Unlike bias-ply construction that uses cords that are laid at relatively shallow angles to the direction of travel, radial construction tires use cords that are laid from bead to bead at 90 degrees to the direction of travel, preventing internal friction by eliminating plies rubbing against each other. Steel belts are then used in the tread for superior stability and tread squirm resistance. Radial tires can be identified by a flat tread and a slight sidewall bulge in profile, even when correctly inflated.

The advantages of radial construction are lower heat, lower friction, and rolling resistance. These tires are much more stable at speed than their bias-ply counterpart and radials last longer, too. Steel belts hold the tread flat, creating a consistent contact area, while the flexible body aids in absorbing shock. Bias-ply tires are no match for radials on the road. Radials offer superior directional stability, unequaled driver feedback, and vehicle control.

Here you can see the roundness of the tread of this bias-ply Pit Bull Growler.

When discussing radial tires in off-highway applications, it is important to note that while a radial doesn't offer the same level of conformability as a bias-ply tire with its shape-shifting tread, the radial's tread and sidewall can act independently of one another. This allows for a very stable tread for traction and big-time sidewall flex. The radial's more consistent contact area also makes for a larger contact patch that spreads the load out more evenly, allowing for better floatation on loose surfaces such as sand.

Disadvantages of the radial tire include a weaker sidewall than a bias-ply tire, decreased self-cleaning ability when compared to a bias ply, and lower grip at low speeds. Also, unlike bias-ply tires, the radial's plies don't reinforce each other, making them vulnerable to irreversible damage.

Our Take
With the advances in radial technology, we'd have to side with the majority when we choose radial tires for our own rigs. Not only are radials very good on the trail, AND exceptional on the pavement, their all-purpose nature fits the multi-purpose use of today's four wheelers that see both daily driver and weekend-warrior duty.

However, if you are a hardcore wheeler who has a trail machine that is used solely for trail work, a bias-ply tire is worth considering. In a trail environment, you can't go wrong with the durability and capability of a bias-ply tire.

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