I,m too old for this ****!
Join Date: Apr 2005
Location: San Juan, Puerto Rico
r6gr8t1: What's this 16 stone bit? Some medieval form of weight measuring? Come on..! You criticize our tire preessures but give us your weight in stones???
Just kidding you! You have some valid questions, even as you say you're in neverland with tires. I'll attempt to give you some answer, but don't take it as gospel, as I'm not an evangelist. This is my opinion and NOT advice. Actually, DON'T do as I do. I'm just trying to explain why I do it.
I BELIEVE motorcycle (and tire) manufacturers are VERY scared of liability issues.
I believe they make their recommendations for tire pressures based on a worst case scenario. Namely, a 275 lb. rider, carrying a 250 lb. passenger (pillion to you Brits). Then you definitely need the 36F 42 R recommended by Yamaha, and even then you're probably overloaded and should not go very high speed ever or fairly high for very long. Tire pressures determines tire flex. More flex= more heat. Too much heat=no good. Why speed requires higher pressures? For a given mile a tire will rotate only so many times, but as you go faster, it will rotate this many times in less time. So it heats up more. Again, heat past a certain point =no good.
Now, I don't weigh but 165 lbs. Don't know what that is in "stones". I never take a passenger. I do very little highway riding or sustained high speeds (over 100 MPH) and most of my riding is in the twisties at really much more like 45-70 MPH. I've arrived to 30R, 32F by experimentation. I've tried everything higher or lower and these pressures seem to give me and my R-1 the best handling combination for the riding I do. This willl vary according to bike, riding style, make and model of tires (see my post on the Bridgestone's 002's) and so on. I'm not alone, all my buddies using the same Michelin Power Race tires have ended up with the same pressures after experimentation. Again, this will vary, and only YOU can determine what works for you.
I hope this hasn't confused you more. Maybe others in the forum can pipe in and explain better.
As for the profile change in contact patch, there's still a difference between the 180/55 and the 190/50 if I put them both at the same pressure. And normally I'd have the bigger tire at slightly lower pressure anyhow.
P.S. I'm so short my knees only touch the ground if I fall off, which I try not to do.
"Speak softly, but ride like hell!"