Full of air: tire inflation

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 methodAsk 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. 

Back to blog


the problem with all of the charts above is that it's "lab data". I have a set of 38/50 clincher carbon wheels. I am running 25mm continental GP4000S tires and continental 28 tubes. the braking surface is 25mm wide and the rim ID is 18mm. they weigh 1440G (no skewers or rim strips), sapim cxray spokes, ceramic endura bearings. they are similar to zipp's most current models.I tried running 88lb. in the rear and 83lb in the front using the enve pressure chart. I weigh 130-132lb depending on the day. I had 2 pinch flats in the first couple of rides and moved my pressure up to 95lb. on the 2nd flat, I could feel the tire almost bottom out. previously on 23mm tires/22mm braking surface carbon clinchers I ran 105 rear/100 front with only the usual problems. unfortunately, in the real world of crappy chicago suburb roads we have pot holes that are unavoidable sometime with 40 crazzies riding a 30 mph. low pressure might be OK for a TT or triathalon, but not for my use. as an FYI, I raced as a cat 2 for years and am 64 and can still stay with the 20-30 year old racer types.


Mario – Different riders, different roads, different flats. It happensEric – I think aerodynamics is within the decision tree of 34 customers, but from what I know it's not at the top. A manageable, responsive, fast-across-all-that-that-might-mean wheel set is what they're after. Probably anyone who's spent time on 34s will tell you their #1 impression is absolute invisibility to crosswinds. Our launch of the 52 was dependent on its success in the tunnel. Our even taking the 34 to the tunnel was incidental. The fact that it posted a quite creditable comparison to the 3.4, an unabashed aero-speed-at-all-costs wheel, is a great showing for it. Dave

Dave K

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…


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?


Dave, What size tires? I'm guessing 25s?Dave


Leave a comment

Please note, comments need to be approved before they are published.