About Those Skinny Rear Tires…

[Dirk_S]

For my V9 cafe, I built an 18” rear wheel to go with an 18” front. Both wheels have a 2.5” wide rim. I’m running a 100/90-18 in the front, 110/90-18 in the rear, same tire sizes as on the Le Mans (which uses 2.15” rims for the front and rear). When lacing and trueing the rear wheel, I noticed that it had an offset that favored the left side of the bike’s frame when mounted, so I took it all apart and attempted to center it up. It’s still offset, maybe by 0.5” (I had Buchanan’s prep my hub and rim for the custom 18” size, so I’ll have to let them know about the offset if and when I decide to have another 18” rear built for my V7 III dual-sport).

Wheelbases:
My bike = ~1455 mm;
LM III = 1505 mm;
LM I & II = ~1470 mm
V50 Monza: 1400 mm.

I’ve taken the bike on a few 1-2 hour rides now, including some interstate slab, and I really dig the nimble feel of the V9 motor with these lighter 18” wheels. However, I’ve also noticed one or two days where the wind picked up on the highway and significantly influenced the path of the bike. I had some trouble keeping it in the center of the lane. This bike sways more easily than when it had the fat bobber wheels, and more easily than my V7’s 130/18 rear tire (3.5” rim).

Is this highway-speed inebriation simply due to the skinny tires (Monza and Le Mans owners, speak up!), the shorter wheelbase with the skinnier rear (in contrast to a Le Mans’s wheelbase), the 1/2” offset, or…all of the above? Note that I’m not suffering from any shake or steering wobble, and I have prior experience with extreme wind conditions (100 kph crosswinds in Argentina while riding at a speed of 110 kph with big trucks flying past me in the opposite direction also at 110 kph).

[faffi]

I have never heard anyone comment on narrower tires being more sensitive to winds. On the contrary, narrow tires tend to be less influenced by rain grooves and other road undulations, so I have always believed that to also be true for strong winds. But that does not mean that my "intuition" is correct. Some tires, however, can be very unstable. When I fitted a Heidenau K36 to the rear wheel of my KZ650, the bike became dangerously unstable. In fact, if I shook the handlebars at 60 mph or more, the whole bike went into a wobbly state strong enough to throw my feet off the pegs. Fitting a different tire returned the bike to unflappable stability. I have had similar experiences with other bikes and tires, like the Metzeler ME33 on the front of my CB1100F that worked very poorly regarding stability.

[nc43bsa]

It's been my experience that motorcycles do not necessarily need to have the wheels aligned in the same plane.  Parallel planes, yes, but not the same plane.

For example, several decades ago in the age when all mass-produced motorcycles had wire spoke wheels, there was a very common brand that had two different models that had similar front wheel assemblies.  Each assembly had the same part numbers for its individual components, but each complete wheel had a different number.

The assemblies were interchangeable, but if the wheel from bike model A was installed on bike model B the bike was unstable;  if the wheel from model B was installed on model A, it was not unstable.  Made from identical parts, but with different offsets.

Neither bike, with the proper front wheel, had both wheels in the same plane, and both were stable and balanced.  It was related to the side-to-side weight distribution.

Guzzi Content:  I noticed during my racing days that most of the modified Guzzis had the rear wheel offset to the left to accommodate a wider rim and tire.  They seemed to handle just fine.

But I digress.  As to your question about skinny tires, as a rule, smaller tires are more responsive than larger ones.

[Dirk_S]

…Some tires, however, can be very unstable…

Thanks, faffi—Forgot to mention my tires! I’m riding with a pair of Kenda K657 Challenger bias tires, a budget-friendly set marketed by Kenda toward lighter weight sport-touring. First time trying them.

Another factor I hadn't thought of are the clip-ons. I’m still fairly new to riding on the highway with clip-ons, so perhaps they’re contributing.

[faffi]

As nc43bsa mentioned, offset wheels are fairly common. BMW airheads had that, the Virago gen2, the original CB750 and the ZRX1200, for instance.

[chuck peterson]

V1000 had the same size f/r on 2.15 rims

Go 110/90 on the front to slow it down?

[Groover]

I agree with bumping the size up a bit on your next tire change. I have 120/90r18 and 110/90r18 Avons on my G5 seems very resilient to wind on highway. What also helped (I believe) was having Ride-On sealant in my tires - I don't in the current wheel set, but I felt it helped with the factor you mention when I had that in my last set (which were Pirelli Sport Demons in the same size I have now)