Full of air: tire inflation

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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. 


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  • Dave K on

    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.

  • Dustin @ Southern Wheelworks on

    I totally agree that you can use less air pressure with larger tires and more tire volume, anyone who's ridden a while can ridden different wheels/tires should have figured that out. My MTBs are both rigid, so I use big front tires so I can get away with lower air pressure to help smooth things out. I’m 160lbs and use a 29×2.35” front tire with about 15-18psi, and a 29×2.2” rear tire with around 22-25psi. It rolls crazy fast and is smooth and hooks up well in the corners.I'm not quite following the math here. You took the average width and height of the tire to come up with an approximate diameter to represent the cross section of the tire, I get that. Did you then find the cross section area and multiply by the circumference of the centerline of the tire? Volume = area x length. Larger tires not only have more cross sectional area, but they also have a larger centerline circumference by a small amount. Or did you ignore the actual volume of the tire and simply look at the cross section?Also, doesn't Boyle's Law assume the mass of gas is the same? As tire volume increases with larger tires, how can you assume the air mass needed inside the tire is the same? I don’t understand why you look at “effective volumes” since once you reach a certain pressure the volume doesn’t really change, the tire casing will only stretch so much. A 25mm tire at 95psi will have the same volume (and “buoyancy”) as a 25mm tire at 80psi. Buoyancy isn't really the best way to describe the physics here, it's more about the force of the air pressure supporting the load on it. You use less air pressure in larger tires not because of the buoyancy of the tire (if thinking of buoyancy in the traditional sense, where it is the difference in weight of an object and the weight of the fluid/gas displaced by the object), but because the air pressure is acting on a larger surface. PSI = pounds per square inch. If you have more square inches, you need fewer pounds to get the same resultant force.Then there's also the tire itself to consider, as the rim gets wider and the sidewalls become more vertical, they provide more support and influence what the ideal pressure is. And some tires are just flat out stiffer than others and require different air pressure to feel best even if they’re the same size as another tire. Magazine articles/reviews/etc about The Great Tire Size vs Rolling Resistance Debate always bugged me: they often compare rolling resistance at the same pressure. But as we all know, you don’t use the same air pressure with different size tires. Magazines have taken the same approach when comparing MTB wheelsizes, riding them all at the same air pressure, which isn’t ideal. I use less pressure on my 29er than I did on my 26” bike.I don't think there's really a good way to model this with simple math, there's just too many variables. Just gotta ride and fiddle with it!

  • Mario on

    Bravo guys! Simply superb! You just prevented me from doing all those computations I felt the difference between a 24( attack) and a 23(4000s) when I inflated them to the same pressure…had a hard time figuring out what pressure I need for the 24mm…this is using 15.5mm inside the brake track width…. Can't wait to get a 17 or 18 between the brake tracks…By the way… It seems from your other post… The kinlin works better than the pacenti…more cost efficient too

  • Dave Kirkpatrick on

    Well, Andy, in that case I'd definitely recommend more pressure. But you'd need "more more" if you were on skinny rims. I pinch flatted a road tubeless tire this spring, pretty sure I'd have flatted a tank tread on what I hit though. Generally the tri/tt guys pump pretty hard to optimize rolling resistance. Lowered pressure primarily helps with comfort, cornering, and traction, which those guys don't seem to car about.

  • andy on

    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.



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