I would say that the load rating is not that important as all of the tires are capable. The Avon's, however, are built as a rear/sidecar tire and are not built for the torsional forces they undergo as steering tires. I believe (could be wrong! not first time) that the 4x19 tires are straight copies of what the Triumph automobiles used after WWI so they are bias ply car tires.
The 4x19 tires were probably chosen for the aesthetic appeal on the design end. The steering assembly was cobbled together by designers who hadn't read the manuals of which there are many, hence the bump steer debacle.
Don't get me wrong. I love my M3W. It is a terrible kludge of bits of this and that thrown together that somehow works most of the time. The rest of the time we spend tearing our hair out and shouting obscenities to the sky when the fit hits the shan...
