Bontrager jackalope hodag tire pressure cold weather

You just got your new Bontrager Jackalope and Hodag combo set up and ready to go, now it’s time to hit the trails. But what about tire pressure? If you’re like most riders willing to brave the cold, you probably inflate your tires inside your garage, basement, shop, etc. After all, it can be much easier to work small parts like presta valves when you can actually feel your fingers.

Most people know that air pressure drops when it’s cold, but how much? If you’re checking your tire pressure with a gauge at 70ºF, your psi may drop by as much as 4 pounds by the time you get outside. While 4 psi might not sound like much, if you’re running 8 psi in a fat bike tire, losing half of your air pressure is a pretty big deal.

Check out Bontrager’s conversion chart next…

Bontrager Fat bike pressure chartStarting with the Ideal Gas Law, Bontrager put together the Conversion chart above based on the air volume of a Hodag Tire (3.8″) on a Jackalope rim (80mm). Other 3.8″ fat bike tires mounted to 80mm rims should have similar numbers, but wider tires will require some additional math and the use of one of the many PV=nRT calculators found online. That PV=nRT is the Ideal  Gas Law where P is pressure, V is volume of gas, n is the amount of gas in moles, R is the ideal gas constant, and T is the all important temperature in this case.

Using the chart above, it should be a bit easier to arrive to your destination with the desired pressure right off the bat. Obviously temperatures on the trail may change, and you need to know what the temperature is when you’re inflating your tires. This is definitely not for the riders who simply go by feel, but for those looking to run specific psi it could be pretty useful.

bontrager.com

29 COMMENTS

  1. Thanks, this is useful. I go mostly by feel, but nice to have a ballpark idea of how much the temps matter. It’s even got a proper water-based temperature scale! (in addition to the freaky body-temp-and-brine scale a few misguided souls still insist on using…)

    -23 is low enough for me, but since I hear that the lowest doable temps for bike tubes and tires is in the -40 to -45 range, it would be interesting to see that too.

  2. Shouldn’t be to hard to extrapolate -40, looking at the chart the decrease in pressure seems to be pretty linear with the decrease in temperature.

  3. The ideal gas law doesn’t account for moisture in the air that can condense out when the temperature drops. Yes, it’s a bit nit-picky, but still. Best bet is just to start your ride with tires firmer than you need and to adjust pressure to suit conditions as you go.

  4. Really? 3.8?
    I thought we were talking about fatbike. Oh, right- the Farley can only take a 3.8 or else the bike won’t ride. Genius design.

  5. booyah – +1
    Releasing excess air is easy. Adding it on the trail at -10 deg F, not so much.

    Also, don’t forget your suspension.

    And this applied to all bikes, regardless of tire size. The second paragraph makes it seem like its worse for fat bikes. Not true (although I admit they will tend to be ridden in colder weather than most bikes)

  6. how timely…
    did a -28 ride last night. Put 8psi in the fatbike before leaving home.
    the bike sat outside in the truck box all day for our afterwork ride.
    just by feel, I could tell at least half my air was out.

  7. Or you could just buy a pressure gauge and adjust pressure outside just before you roll off. Anyone who rides a fatback should own a gauge that can accurately read low pressures.

  8. I wonder if they made the mistake I did once upon a test…? The temperature needs to be absolute, as in Kelvin, instead of degrees Centigrade… Someone with time on their hands [not me, I’m lazy] please check those numbers? The change in pressure per temp seems high. I always used about 1psi per 10 degrees F in my mind for auto tires.
    Somehow all this makes Avogadro’s number, 6.023 times 10 to the big-mutha 23!, come out of my semi-functioning brain…

  9. This begs for an app where the parameters (tire diameter, air pump zone’s ambient temp in F or C, target pressure in desired units) can be entered rather than relying on the exact specs of Treks new fat combo in a warm space. I have a Beargrease, and after I buy those Trek hoops to fit the calculation table I won’t be able to affort to heat my house to 70F! 😉
    Note that the temperature T in the law is in Kelvin, not Celsius or Fahrenheit, though I reckon most people won’t bother with the algebra to figure out a target pre-ride-environment pressure anyway (I doubt I will without that dandy smartphone app).

  10. JBikes: It’s worse for fatbikes in the sense that they are often ridden close to the minimum possible pressure, especially when it’s cold. If my commuter loses pressure from say 5 to 4 bar, that’s hardly a concern. 5 to 4 psi on the fatbike might be.

  11. You don’t actually need the ideal gas law and you don’t need some other chart for differently sized tires. You just need Gay-Lussac’s’ law (http://en.wikipedia.org/wiki/Gay-Lussac%27s_law), one of several components making up the ideal gas law. Gay-Lussac’s’ law says P1/T1 = P2/T2. In other words, for constant volume (close enough even though fat tires change size a bit with pressure) and constant number of particles (I’d also say close enough despite booyah’s objections), that chart will work, regardless of tire size.

  12. @MBR I agree there seems to be a problem with the chart.

    Take the left most column and extrapolate. What pressure will your tyres be when the temperature hits -20F? Zero PSI? Plausible I guess…. But then at -30F? Will your tyres have a negative PSI value? Doubtful. Maybe Bontrager should stick to making whatever it is they make that is good.

  13. @Nathan and @MBR, the values in the table were calculated using absolute temperature and pressure. Relative pressure can very well go negative with a decrease in temperature… put some tupperware in the freezer if you don’t believe it. Also, volume is irrelevant as long as it remains constant (or approximately so), just as @feldy said.

  14. @Bill_Nye have you run the numbers? Unless I’m missing something, I don’t think they’re correct. Copied from my Facebook post:

    The ideal gas law PV=mRT reduces to P1/T1 = P2/T2 because R doesn’t change, and mass and volume are basically constant. So, P2 = P1(T2/T1). Using their example of 10 psi at 21.1 deg C, and converting to Kelvin, the pressure at -12.2 deg C should be 8.9 psi, not 7 psi. The ideal gas law isn’t exact, but the assumptions made in using it can’t possibly account for more than a 20% deviation.

  15. i didnt do the math but you need to use absolute pressure. 10 psi on your pump is really 24.7 psi from vacuum. The 10 psi is above atmospheric pressure. So redo the math with 24.7 psi instead.

  16. I don’t mind if my tires get softer by the time I’m riding in snow. I usually let out pressure when I get to the trail anyhow (don’t have a car)

  17. @ Georg: +1000

    “pound per square inch” what a laughable unit…

    I’m tired of this totally outdated Imperial system why are we still using this already?

  18. There’s another factor not taken into account as well. Tire rubber gets firmer/less pliable as the temperature drops which make a huge difference within these pressure ranges…….

  19. The table numbers are correct, I checked them at several points. Using P2=(P1T2)/T1, not only does temperature need to be in Kelvin but pressure needs to be in atmospheres (atm) not in PSI. The final values are rounded a bit to come up with whole number values. This rounding error will be amplified with the pressure unit in PSI as 1 atm = 14.7 psi.

  20. While you guys sit there and futz with your tables, diagrams, debating measuring units, and gas laws I will be out riding my fatbike. Like I always do. PSI and temp doesn’t mean crap when you hit the snow. You can’t make a table that tells you what psi you need for the given conditions.

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