[subxero]

Originally Posted by Alex

I don't know about divide by 2, but it seems that the discussion is missing the multiply by 2 that is necessary when calculating tire height. Yes - 140/70 means that the height of the tire is 70% of 140, or 98mm. But that's on both the top and bottom of the wheel, so the overall diameter of the tire is 98mm for the top of the tire, + 17" for the wheel diameter, + 98mm for the bottom of the tire. A difference of 7 mm in the calculated tire height of the tire itself, by tire size, would be a 14mm difference in actual tire diameter on the bike, right?

All of this gets fuzzy in the real world, as the tire sizes themselves aren't exactly accurate, and differ based on manufacturer and type of tire.

I guess the devil is in the details and it pays to be as clear as possible.

Originally I was only talking about how much the rear would raise the rear end of the bike. So I was only using the change in aspect ratio (x1) as the other side of the tire would be facing up and have no effect on ride height. But yes, you are correct the overall tire outer diameter would be different
(aspect ratio x2)

And I totally agree on different tires, I did make it appoint to say this is only accurate if it was the same tire just in different sizes. I know far less about street tires than off road tires but in the off road world off the top of my head Dunlop, Pirelli and Bridgestone I think, use the same sizing, Then Michelin uses a different sizing, and so does IRC. In off road tires a Dunlop 120/100 18 is comparable to a IRC 140/80 18, and the Michelins run "small" as well. it might be a 130 = a 120 Dunlop there, its confusing and annoying at best.