Radial tires for motorcycles were introduced 30 years ago, and it created more than a bit of consternation among riders. Radials are for cars, motorcycle tires have to lean. Who wants bendy sidewalls on their tires? A rider needs grip, feedback and longevity, and the radial represented a real improvement in all three areas. The first thing I noticed about the radial was that it gave me much better grip and stability when cornering rapidly.
I could re-accelerate sooner, which I always tried to do when racing. The advent of increasing horsepower in production bikes coincided with the introduction of a Pirelli radial rear tire on the European model Honda VF R.
The VFR was one of the original mph superbikes and it quietly ushered in the era of the radial tire to the world of motorcycle racing. Spencer in particular, liked the handling characteristics of a radial rear and bias-ply front. By , when Spencer won both the and cc titles, Michelin radials were the tires of choice. The shift to radials on street machines was thus underway, confirming the old adage that racing improves the breed.
The overlapped plies form a thick layer that is less flexible and more sensitive to overheating. The advantages of radial tires over bias ply tires include: Flexible sidewalls. Reduced fuel consumption due to less rolling resistance. It may be tentative, but tires do have an expiration date. There is a general consensus that most tires should be inspected, if not replaced, at about six years and should be absolutely be swapped out after 10 years, regardless of how much tread they have left.
Radial tires have much thinner, more flexible sidewalls, and if the tire is inflated properly, will bulge at the bottom for exactly this reason.
Determine if your tires are unidirectional or multidirectional Many of the radial tires sold today are unidirectional, meaning they should only roll in one direction. When building a Bias Ply Tire, reinforcing cords are laid diagonally across the tire. Each additional ply has the cords laid at an opposing angle.
This creates a criss-cross pattern. Finally, more rubber is applied over the plies to create the tread. Bias ply tires are constructed with two or more layers of nylon cords wrapped at degree angles from bead to bead to produce the support and carcass the network of chords that gives the tire is strength. Steel belted bias uses two or more layers of nylon cord PLUS two steel belt layers beneath the tread. Atop those first thin layers, alternating layers of body cords are stacked in the tread portion of the tire only, so the sidewalls aren't as thick as they are on a bias-ply tire.
These drawings show the difference in construction between bias-ply and radial tires. Illustration by Michael Ennis. Radial tires were an attempt to correct some unwanted characteristics of bias-ply tires.
Bias-ply tires do not shed heat quickly, for starters. As motorcycles became faster and faster, the tires were limiting how quickly the riders could ride them. The second problem is that bias-ply tires have a very stiff sidewall due to the fact that all the plies run from sidewall to sidewall. Because radial tires have fewer layers of body cords on the sidewall, the sidewalls allow the tire to flex more.
This means the contact patch of the tire stays put on the ground a bit better, and the sidewall provides lots more feedback to the rider. This design also allows the sidewall and tread to work more independently of each other than a bias-ply permits.
Sportbike folks are usually interested in performance, which means speed. They need tires that can hold together under extreme stresses.
Touring riders typically are interested in performance, which means load-carrying capacity and longevity. They need tires that can hold together under heavy burdens. Happily, there's a method of tire construction that shines for each! When motorcycles come out of the factory with bias-plies and radials mixed, the bias-ply is always in the front.
Bias-ply tires are also the only tube-type tire available. Many riders like the classic look of spoked wheels, and in off-road situations, spoked wheels are usually a little more sturdy and wheel damage is not always catastrophic, as it usually is with cast wheels.
Different manufacturers have different positions on adding tubes to radials, but such a setup is usually not ideal. Radial tires, as we discussed, dissipate heat better and allow better sidewall flex.
Another issue related to their construction is the ability to create a tire that has substantially larger tread than sidewall. On bias-ply tires, the width of the tread and the height of the sidewall are typically similar.
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