I just recently posted this on another site, I ran a radial 160/18 Metzler and a Bias 130/18 on the frt of my SEEG for at least 20,000 miles of its life, never any problem. I ride two up almost all the time, always exceed the speed limit and never had any problem with the combo. Per the Metzler guy at Daytona in frt of Millers during Bikeweek this is a hoax. At that time, three years ago, all new Big Dogs came that way (radial/bias). The only problem I had at all was the rear 160/18 Radial would only last about 9 - 10K vs the 150/16 bias at 10 - 11K. But no one makes a 160 bias.
Maybe the Metzeler guy at Daytona had not read the caution statement in their own catalog:
IMPORTANT NOTE REGARDING TIRE UPGRADES
Metzeler allows builders and owners to mix radial rear tires and bias ply front
tires when the following conditions are met:
> All Metzeler tires must be used on the proper permissible rim size, see chart
enclosed to verify correct rim size and tire fitment.
> Mixing radial rear tires and bias ply front tires is allowed only when fitting a
stock Harley-Davidson® with the proper custom width wheels for the tire
application. (see application chart) OR when fitted by a custom manufacturer
(i.e. American Ironhorse). OR if listed in this fitment chart!
Don't get me wrong - I am glad you did not have a problem - I just heard it might be a problem in some situations.