Tread Pattern and Tire Construction

I have a 77 Moto Guzzi that I'm seriously considering fixing up and putting back on the road. There are a couple of mechanical issues, one of which is tires. The old rubber is a set of 15 year old bias tires, and I remember riding back from a rally and having major weave problems on milled roads. There was also an issue whenever I hit sections of 'erased' lane divider lines - the tires wanted to follow these irregularities in the pavement. Of course, my ST with radials exhibits NONE of these traits and rides over road defects with aplomb. (I've always been scrupulous about tire pressures and maintenance so I attribute the bike's instability to the bias tires.)

Tires for this bike (in Guzzi's recommended size) seem to be mostly bias type, and tend to have an 'aggressive' tread pattern - crosshatching would be how I would describe it compared to say a PR4's or Angel GT's mild wiggles or V's tread pattern. These latter patterns are what I call 'road tires'. Pardon my own nomenclature - I don't know if the industry describes tires this way but I hope it conveys to you what I mean.

My first question is this: Can I put a modern 'road' tread radial on an old Guzzi T3? What is the difference in handling that I can expect due to the tread pattern? What would be the difference that is due to the construction - radial vs. bias? Would the bike be unmanageable with these tires compared to the older bias types with that cross hatched tread pattern? I'm assuming that the stability of the tire has a lot more to do with the plies angle (radial vs. bias) than the tread pattern - am I right?

Second Question: Not all the mfrs offer the tires in Guzzi's recommended sizes. What difference in handling can I expect if I increase the rear tire size from 110/90 to a 120/90? This would be my only tire size change based on what is available today.

I've had good luck with Bridgestone S11 Spitfires on older bikes.

I ran Michelin Macadams (40's ?) on a 77 Honda 750f2 SOHC. They transformed the bike and were available in tubed or tubeless.
Not sure they're still around but there Pilot Activs are supposed to be a good classic tyre.
Hope it helps.
Upt'North.

Many of us that have owned ST1100s over the years have run both bias tires (stock) and radial tires. While there are subtle differences between the two, they're not radically different in handling feel, so I would answer no to your question about radials being unmanageable on a bias designed bike.

The tread pattern will make a difference in terms of the tire following grooves in the pavement. Going from memory in the old days when tires were skinny and bias, I seem to remember the tread patterns that were more diagonal to the direction of travel tended to follow grooved pavement less when riding in a straight line. Seems to me that over the years as tires got wider, and tread patterns more diagonal, the tires have less tendency to follow grooves in the pavement, but I have no hard evidence to support that claim. The ones that had a single deep groove in the center of the tire, or multiple grooves in the direction of tire travel, seemed to follow grooves more.

for your Guzzi, the narrow rims are going to limit your available choices, but I wouldn't worry too much about which tire you finally choose. I've always noticed differences between tires, but nothing significant enough that I would consider any of them unacceptable or unusable.

I'm not sure if they have anything applicable to your Guzzi, but I found some great tires from Heidenau when I was restoring my '73 CL 350. In addition to their "modern" tires, they have line of vintage rubber. Possibly worth checking at
http://heidenautires.com/
I found mine in stock at Revzilla.

I noticed the same thing, as far as poor handling of the "vintage" tires. I switched to Michelin Macadams on both of my vintage BMW's, and noticed a HUGE difference. Prices were about the same, IIRC, and handling was FAR improved.