Despite all the different sizes and purposes and appearances, tires only really come in two varieties: You’ve got your bias-ply, or cross-belt tires as they’re sometimes called, and your radials. Up until the ’70s all bikes rolled on bias-ply tires, but as motorcycles got faster and more powerful and as riders began carrying knee-dragging lean angle, it was clear that a more performance-oriented technology was needed, which is how radials came into existence.
Before we explain how bias-ply and radials differ let’s talk about the general anatomy of tires, which is similar regardless of the design.
Looking at the cross section above, you’ve got the tread cap which is the part that rolls on the road. Beneath that are layers of fabric that form the tire’s carcass and give its structure. Those fabric layers extend down to form the tires sidewalls and wrap around the bead, which is a thick steel cable. All of it is splashed in rubber and represents the insides of a tire, whether it’s bias-ply or radial.
The difference between the two is how those fabric layers, called carcass plies, are oriented. On bias-ply tires, the layers, which are often made of polyester, nylon, or aramid, are laid down diagonally—hence the term “bias”—across the tire in a criss-cross pattern from bead to bead. Occasionally an additional layer or two of fabric will be applied along the tread to strengthen that area, but it’s still applied at an angle and as a whole the tire’s construction is uniform, so the sidewall and the tread area have similar thicknesses and flex properties.
On radial tires, the fabric carcass layers are laid down perpendicular to the bead rather than at an angle, which makes for a more flexible sidewall. Then, on top of the carcass plies, there’s an added layer of belting, most often made of thin steel cable or aramid thread that’s located only beneath the tread and is specifically there to strengthen that area. So with a radial tire the sidewall and tread have independent constructions, giving those two sections of the tire different flex characteristics. And that’s a big deal if you’re trying to design a tire with very specific behaviors. With a radial, you’re not stuck compromising tread rigidity in favor of sidewall flexibility like you are on a bias-ply, which again has that uniform construction.
What all this boils down to is two families of tires with different behaviors. Bias-ply tires overall are going to be thicker and more robust, with taller sidewalls and rounder profiles. That may have been a good fit for bikes back in the day and it still works well for cruisers and touring bikes, but on the racetrack in ’70s bias-ply tires were overheating because they couldn’t cope with the power of the bikes and they weren’t providing the handling that riders needed.
Enter the radial tire, first used in the automotive world shortly after WWII, but first tested on racing motorcycles in the 1980s on the 500 GP bike of one baby-faced Freddie Spencer.
Rather than relying on layer after layer of fabric to form its structure like a bias-ply, a radial’s carcass might have just two fabric plies and a steel belt, so you end up with a thinner, lighter tire that manages heat better. And because you can tune the stiffness of the sidewall and tread independently, you can design a tire with optimized flexibility, handling, and grip, among other benefits. With radial architecture you can also build a tire with a broader, flatter crown and shorter sidewalls as is common on modern sportbikes.
So radials are better for high-speed applications, where handling and outright traction are a priority. The technology was developed for and evolved alongside sportbikes. Meanwhile, bias-ply tires, even though they’re old tech, are still a good fit for bigger, heavier bikes since the tires’ more robust design offers higher load capacity and durability. Also, bias-ply tires are a must if you’re rolling on spoked wheels since modern tubeless radials aren’t designed to be used with tubes, the concern being that the soft, butyl-rubber layer that seals the inside of a radial tire will create too much friction and heat.
In the end, radials are the newer, more sophisticated technology and claim the lion’s share of the market, but bias-ply tires still have a place on a certain kind of motorcycle. So which tire ends up getting used isn’t so much about what’s newer or more advanced, but what works better for that kind of bike and the way it’ll be used.