Last week's discussion of how tire size affects aerodynamics set off quite a little bit of discussion. We've provoked some big responses before, but nothing quite like that. The one thing that we hope people started to think about as a result of it, other than the direct component of narrower tires doing better than wider ones in the wind tunnel, is the importance of measured width. It's a big factor.
Now, measured width is a bit of shorthand, what we are really referring to is the actual volume of a tire, which includes the height as a variable. Height and width aren't in lockstep, as some rims actually hold the tire lower down within the rim, while some let the tire sit a bit higher. To investigate this more fully, we measured inflated width and height of 23 and 25mm Continental 4000s II tires on every rim we took to the tunnel, as well as estimated what they would be on a representative rim of the old standby 14mm between the brake tracks.
There is debate over what "counts" as tire volume - does only the inflated portion outside the rim's circumference count, or does the volume in the cavity count as well? Fortunately, the variances there weren't so extreme that they threw things out of whack. Our calculation was fairly rough and simple - average the width and the height, take the surface area of that circle, and call that overall tire volume. To eliminate the debated "dead zone," we took 5/8 of the overall tire volume and called that "effective tire volume." 5/8 simply because we are measuring the "outside half" of the tube, as it were, and that's bigger. As I said, a bit quick and dirty, but when you reference it against a bunch of other calcs, the way you peel that carrot doesn't amount to much in the wash.
Using a law of chemistry called Boyle's Law, which simply states that for a given mass of a gas, if you decrease the volume then the pressure must rise, we normalized how much pressure a given volume of air would yield in each tire/rim setup. The results are shown in the graphic below.
So what does this have to do with anything? It shows that as you increase tire volume, in order to keep the same "buoyancy," you need to decrease pressure. There are a lot of different ways to express buoyancy, probably the best of which is illustrated here - the wheel drop method. Ask 10 people what the ideal pressure is for any given tire and you are likely to get 20 responses. The point we're making here is that tire volume is probably the biggest determinant of how much pressure you should use in your tires, and it varies by a ton. Put 30 psi in a road tire and you are going to be riding around on the rim, put 30 psi in a 2.2" mountain bike tire and you are going to be bounced all over creation, put 30 psi in a cx tire and you are going to be pretty close to ideal (I know, I know, tubulars, lower psi, etc - I'm making a point). How big your tires inflate on any given rim will have a big effect on how that tire feels. The 100 PSI default for road tires was established when rims were much narrower than today's. The chart shows that to achieve the same tire volume as 100 PSI on a traditional skinny rim, you should only run only 79 PSI on a set of Rails with a 23mm tire, and only 66 PSI if you've mounted 25mm tires on your Rails.