Join Date: Jan 2005
Location: Portsmouth, Hampshire, UK
I have had trouble finding a radial BT020 front tyre for my 03 LT which can use bias ply or radial, read all the stuff on the forum about Metzellers and Bridgestone and pressures and eventually wrote to Bridgestone to ask whether the radial was being phased out as the latest catalogue shows the bias ply as being required for all years now. The reply I got was very interesting, I have included it below, and I can identify with some of the problems mentioned like the "shimmy" and the slow turning when I'm teaching people to ride figure of eights.
Here is the response:
The V rated radial version has not been deleted from the range and is still an approved fitment for this bike. However, they are far less popular than they once were. The K1200LT is very sensitive to tyre fitment and has a natural shimmy in the handling, and in many cases this is highlighted even more when using radials.
Using cross ply's is not a retrograde step and the main differences between using these types of constructions, are.
Radials have a much stiffer carcass than bias ply tyres and their self damping properties are not as good as bias ply's. This means the forces the radial does not damp transfer to the suspension and chassis of the bike and basically make the components work much harder, this increases shimmy and creates a less comfortable ride.
The bias ply has better self damping properties and therefore takes a lot of the stress away from the bike, therefore creating a more comfortable ride and reduces shimmy.
Radials are heavier to turn at slower speeds and can sometimes (on certain bikes) self steer at what we call "car park speeds" and fall into the turn.
You may or may have noticed that even bikes of 10 to 15 years ago still have bias ply approved and the reasons are the same. Their chassis' can not cope with radials and can make the bike very unstable, certainly at higher speeds.
The above is not always a given but it just highlights some of the issues one may encounter and are the main differences between the two.
With regards to pressures, the approved pressures are 42 psi & 42 psi. You also have to take into consideration the governing bodies' regulations, too. The tyre pressure works in conjunction with the load index and the speed rating and the maximum permissible cold pressure in Europe is 42 psi. It may be different in the USA.