I received a fresh set of tires last week for Project 24, a 1995 RC36 model which wears radial tires, and out of curiosity set them next to a set of bias-ply tires from the RC24/26 Interceptor ('86-87). The increased width of the newer generation radials was immediately evident, especially on the rear — the fronts are spec'd at 20mm wider and the rears 30mm. In the U.S. the change to radials came with the 1990 model. Why radials? Are they inherently better than bias-ply?
(REARS) Left = 4th gen., Right = 2d gen
Yes...in general. Both designs have their place, even today. Like many street bike advances, the use of radial tires on high-performance motorcycles was born of racing demands. As power and speeds increased back in the 80's, the old bias-ply designs couldn't take the heat of competition and so became a limiting factor in winning races. Radial design allows more sidewall flex but run cooler, increasing tire life. Other desirable effects include the ability to conform a bit better to road surface irregularities and provide a slightly larger contact patch (more grip). From a race tire engineer's perspective, an inherent limiting factor with bias-ply design is that the sidewall height must increase as the width increases, so to get a nice fat tire, the diameter grew — not good. Radial design has no such correlation.
But wait...it's not all bad news for the old bias-ply, and for the sport bike crowd it centers around the concept of "handling." That all-encompassing word has a whole boatload of variables, but tires are a big one. When you ride an RC24/26 ("skinny" tired) beside a later model ("fat" tired) what becomes noticeable is something called "turn-in." The older bike seems a little lighter on its feet...but the bike's not physically lighter. This is due to two forces of physics. One is that, other factors being similar, a narrower tire has less rotating mass (think gyroscope) and so can change direction more easily (this was also the thinking behind the popular use of a 16-inch front wheel back in the day).
The other is a combination of the contact patch's offset from the bike's centerline (when leaned) and its resulting effect on the bike's center of gravity. The net effect is that a narrower tire can take the same turn at the same speed, but at less lean angle compared to a wider tire. For us mortal street riders, it provides a much appreciated benefit — we don't have to lean as far (for a given speed) on our skinny tired machines, giving an extra margin of safety.
An interesting historical fact: The hallowed RC30 spec'd a bias-ply/radial combination during its production — the front a Bridgestone Excedra 120/70 17, the rear a Bridgstone Cyrox 170/60 R18. While such a mix is generally considered sinful, the engineers must have had their reasons.
I like radial tires as much as anyone, but don't feel sorry for us RC24/26 riders. Our bikes' handling mirror their look; light and athletic.
The illustrations below help to illustrate the forces at work here.
(source: Total Control, by Lee Parks)
But wait...it's not all bad news for the old bias-ply, and for the sport bike crowd it centers around the concept of "handling." That all-encompassing word has a whole boatload of variables, but tires are a big one. When you ride an RC24/26 ("skinny" tired) beside a later model ("fat" tired) what becomes noticeable is something called "turn-in." The older bike seems a little lighter on its feet...but the bike's not physically lighter. This is due to two forces of physics. One is that, other factors being similar, a narrower tire has less rotating mass (think gyroscope) and so can change direction more easily (this was also the thinking behind the popular use of a 16-inch front wheel back in the day).
The other is a combination of the contact patch's offset from the bike's centerline (when leaned) and its resulting effect on the bike's center of gravity. The net effect is that a narrower tire can take the same turn at the same speed, but at less lean angle compared to a wider tire. For us mortal street riders, it provides a much appreciated benefit — we don't have to lean as far (for a given speed) on our skinny tired machines, giving an extra margin of safety.
An interesting historical fact: The hallowed RC30 spec'd a bias-ply/radial combination during its production — the front a Bridgestone Excedra 120/70 17, the rear a Bridgstone Cyrox 170/60 R18. While such a mix is generally considered sinful, the engineers must have had their reasons.
I like radial tires as much as anyone, but don't feel sorry for us RC24/26 riders. Our bikes' handling mirror their look; light and athletic.
The illustrations below help to illustrate the forces at work here.
(source: Total Control, by Lee Parks)
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