Yep, it seems the RYCA conversions and all those who follow their examples are installing "too wide' tires on the rims. I did get a 130/70-18 tire on my 2.50x18 rim just like all the other lemmings.....and after 2 years of riding on the bike I realize that is not the best way to go.
Here are links to several tires charts, and all of them confirm that a 110 tire should me mounted on a 2.50 rim, a 130 tire should be mounted on a 3.50 rim, and a 140 tire should be mounted on a 4.00 rim. (There is some variation that goes with the aspect ratio and the 140 tire can be mounted on a 3.50 - 4.50 wide rim). The tire carcass is molded to fit on the rims that match the tire width, and if you measure the distance between the tire beads on the unmounted tire....it is a good indication of what width rim it should fit.
http://www.mtsac.edu/~cliff/storage/gs/Motorcycle_Rim_Width_Tire_Size_Chart.pdfhttp://www.bridgestone.com/products/motorcycle_tires/products/battlax/bt45.html If you managed to squeeze the 140 tire onto the 2.50 rim - you have pulled the sidewalls in and it will do 2 things. 1) Modern tires are built with a profile that has a small amount of tread touching the road in the center of the tread - and the tread pattern is formed in an oval pattern that places a lot of rubber in contact with the road when the bike is leaned over. Squeezing the tire distorts this profile and makes the tread too round in cross section and the tire has less rubber touching the road. 2) Squeezing the tire together results in tread being rolled over on the sidewall where is never can be used, and it makes the tire too narrow. My 130 tire on a 2.50 rim was only 116mm wide.
I wore the center of my Pirelli Sport Demon tire out in 4,000 miles, and the edges look fine. The center of the tread was too arched and I was running on a tire that was not allowed to flex and move as it as designed - the narrow rim restrained the tire too much and did not allow it to work properly.
I now have a 110/80 tire mounted on my 2.50 rim. It looks a bit narrow if you are used to seeing sport bikes - but is fine if you are used to looking at Cafe'bikes and vintage motorcycles. The 110 tire mounted o the 2.50 rim is only 6mm narrower than the 130 tire was mounted on the same 2.50 rim - because the 110 tire is made to fit that rim while the 130 tires was made to fit a 3.50 rim (so that pulls the beads in a full 25.4 mm farther than they are supposed to be). Last night I saw a 1960's Moto Guzzi 750.....and the rear tires was a 110/90-18. A 110 tire is plenty wide to handle the weight and horsepower of our bike.
My new approach to this is:
The front 100/90 tire is fine on a 2.15 rim - even though it was really made to fit a 2.50 rim. It works OK.
The rear tire should either be a 110/80-18 on a 2.50x18 rim - or you should install a 3.50x18 rim if you want to run a 130 or 140 tire.
If you squeeze a wide tire on a too narrow rim - you are not going to benefit from all the high tech rubber and profile built into the tire. The folks design these tires really do know what they are talking about, and we should follow their recommendations. We are also not the only folks having this discussion. I was looking for tire recommendations for my Ninja 250 that I just bought - and all the young fellows want to throw a wide tire on the back of their little bikes....and the recommendations are that the best performance will come from the tires made to fit correctly on the rim.
Dave
Some reading I have been doing about this common issue:
http://faq.ninja250.org/wiki/Do_taller/wider_tires_work_better_than_the_stock...http://blog.jpcycles.com/2010/10/what-is-the-skinny-on-fat-tires/