When I began riding (22 months ago) I had to buy a new front tire for my 400F. A man who should have known better sold me a tire too big for the rim. It took me a few months to figure out why the tire was too big. It wasn't too big to fit on the rim at all. It appeared, to my novice eyes, to fit very well, in fact. It also looked pretty good, I thought.
After about 3 months I met a fellow who builds and races vintage motorcycles. The first thing he said when he saw my bike was "your front tire's too big". He explained to me how the bigger tire actually put less rubber on the road in a turn than the smaller, closer to stock size of tire. The apparent paradox got my attention.
I didn't really understand it as clearly as I would have liked too, however. It still seemed to me that bigger was somehow better.
That notion was pretty much dispelled when I attended some modern and vintage motorcycle track races. Basically what I saw was this - regardless of the size of the tire or of the rim, all the racing bikes seemed to have one thing in common - the angle of the sidewall measured from vertical on the wheel was always pretty flat. (I mean the angle from the actual rim edge to the outside margin of the tread was minimal.)
On my front tire, because it was meant for use on a bigger rim, the same angle was 25 to 30 degrees or more. I'd never really noticed that before, and if I did notice it, I didn't think it was important. When I saw racing bikes however, I quickly saw where the angle of the sidewall between the rim and the tread perimeter is integral to the performance of the tire under load or stress (that is, under operating conditions).
So that's when I got interested in getting more appropriate size tires. I moved down a size with my current front tire, and I feel better with the finer tread and straighter sidewalls.
I can't really talk knowledgeably about all the technical specifics and such, but I have a lot of faith in my own intuition, and after a certain point of taking in information, with the stock rim size it just makes sense to me to use the size of tire that the designer(s) of the bike determined provided the most safety, performance, and durability.
You might be right however, when claiming that advances in technology allows drastic deviation from the factory stock recommendations. (I think I have good tire sizes on my 400f, and they're still a bit bigger than the stock recommendations.) But I can't get over the jangle in my brain whenever I see a sidewall jutting out at a pronounced angle from a rim. The most modern technology regardless - it just doesn't seem to make sense considering what I've learned about motorcycle tires over the last couple of years.
I also have a lot of faith in the tire companies to be quick to advertise and promote the latest features of their respective lines of motorcycle tires. That faith leads me to believe that if a certain size tire would be safe to use on a certain size rim, they'd let me know it. Their guidelines and tire fitments charts are probably already stretching fitting ranges to the maximum limits (legal/liability wise) as it is.
..... anyway - I run on ........
