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.
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The experiment I wanted to do was to put an equivalent mass of gas into several different tires and see what happened. Not a practical experiment. The effective volume was taken by the cross sectional area of the tire (avg of width x height) and multiplied by circumference. I used the same circumference for each, the .1mm it would have changed by would have changed the resultant pressure implication by .01% of nothing. Whether this is a textbook application of Boyle's Law or not, the point is that as tire size increases, you have to decrease pressure. The significant sub-point is that a "23" on rim A might be significantly different than a "23" on rim B. The results that our application of Boyle's Law gave matches our iterative/anecdotal experience with almost frightening exactitude. "..it's more about the force of the air pressure supporting the load on it." If that doesn't describe buoyancy, well…Our point is categorically not to tell people to blindly follow a simple instruction about pressure. It's to help them think about it and to elucidate the factors involved. Most people don't tell them anything, they pat them on the ass and say "these are the bees knees, they'll make you 11% faster and when you come home your partner will find you more attractive." A 2.2" Cross Mark LUST rides terribly at any psi, but becomes acceptably unterrible at about 23. A 2.2" regular Cross Mark rides wonderfully at 27. Nearly a 20% difference in pressure, same volume, different construction. But we have to make the big points first, then make the more subtle ones. It took about 9 posts on one forum last week just to convince a few people that outside rim width meant jack to tire inflated size.
I weigh 200 pounds and use GP4000S on Rails or A23s. Three times I have tried lowering my F/R tire pressure to 90/95 and three times got front pinch flats in shallow potholes/cracks. My min is therefore 95/100, and I usually go with a little more, ~98/103 on the Rails and 105/110 on A23s. The tires definitely feel softer to thumb pressure on the A23s than the Rails at the same pressure.
Dave, What size tires? I'm guessing 25s?Dave
Pinch flats at 90/95 – weighing 200lbs? I used to weigh at 210 using the same pressure on 23mm 4000s conti. Never had a pinch flat in 4500 km.( tire life to date) . I had several flats due to debris ( staple wire, nail, and tube failure… But no snake bite pinch flats….. Using shimano alum rims at 21mm outer and around 15.5inner width Tire – tubemounting maybe? After installing tube and tire, before inflating. Do you pinch check the tire to see if there is some tube sticking out on bead interface?
I ride a set of Pacenti SL23's with WI hubs and GP 4000S 25mm tires on my bike. I weigh ~180-185 lbs and I haven't had a single issue with mine at 85/100 psi. When I first built the wheels and was experimenting with pressures, I tried 95 in the rear, but when I would look down I could see the tire flexing too much while I pedaled. I find the ride to be amazingly smooth and have not had any issues with over 1000 miles since I built them at the beginning of the season.I find your wind tunnel results to be interesting for sure. Have you guys had any time to ride sets of the Rails back to back on the road switching from 23 to 25mm tires (assuming comfortable pressures for yourself)? I'm curious as to your thoughts of the ride qualities between the two. I think that someone who's looking at your Rail 34's shouldn't be sweating that last % of aero efficiency, but I know how people get when they see numbers and want to have the "best" if nothing more than for something to brag about at the club ride…