Wednesday, January 13, 2016

Fat tire pressures for snow.

A good friend of mine recently got the fatbike bug, largely so that he could ride on snow, and wanted to better understand appropriate tire pressures for that surface.

He's ridden bikes for decades and skied for even longer, but the correct range of pressures for 5" tires on soft snow is not as auto-intuitive as you might think.

I spent a few minutes writing a detailed response, and after sending it to him it occurred that many here might benefit from it as well, if only as a resource for their new-to-fat friends.

Without further ado...

It takes some time to wrap your head around appropriate PSI for snow--it's probably gonna be a lot less than you think.  The standard credo for tire pressure when snow riding is 'when in doubt, let air out'.  

Best way to be sure is to take a little low-pressure (0-15psi) gauge with you for the first month or so, and check pressure frequently with both the gauge and your hand to get your hand calibrated.  The idea being to learn what works by feel, so that you can ditch the gauge sooner than later.

This is a rough guideline.  The absolute number is irrelevant, finding a pressure that works, and then being able to both recognize the conditions and duplicate the appropriate pressure is what matters.

10psi and up = pavement pressure.
6-8psi = *very* hardpacked snow.
4-5psi = softer or less consistently packed snow.
2-3psi = deeper snow, when more flotation is needed.  If you need this kind of
pressure, you'd probably be having more fun with skis on!  But
sometimes you start a ride on hardpack and have an ambitious
objective, then it snows or blows and you have to dump air to keep
0-2psi = what I most often ride at, due to lots of light, dry snow and very little traffic.

As temperatures and conditions change the appropriate pressure for the surface can fluctuate pretty dramatically.  1psi makes a big difference.  My way of staying safe (avoiding flats or rim damage) is to lean all my body weight on the saddle, while looking down at the rear tire.  Any wrinkles in the sidewall?  Add psi until the wrinkles go away.  That's your baseline for hardpack.  The flipside of that process is that for the softest, least-packed snow (the kind where you should have chosen to ride lifts with skis on that day!) you can go as low as four or five wrinkles in the sidewall as long as you're being delicate.  More than five wrinkles and you're generally just adding resistance without increasing float or traction.  That said, conditions in my neck of the woods often require 5+ wrinkles just to keep pedaling, and since pedaling beats walking...

One last bit of editorial: Not many people understand how far you can go in a short time on one of these steeds when conditions are good, but how absolutely hosed you can be if it's nuking or blowing or both on the return.  Like 7-8mph when it's good, and hours per mile when it's bad.  I don't take a sleeping bag with me on every ride, but I *always* have a puffy, firestarter and lighter, and some food on winter rides.  Seems like about every other year I get antsy to do something epic, and conditions change halfway through the ride, leaving me out for the night and into the next day (or til a sledneck comes along and packs the track back in).

Don't hesitate with questions!


  1. Hey Mike. What has been your experience on dry beach sand? I'm riding a single speed 32-22 with 4" tires here in Mexico. I don't have a gauge but using the wrinkle test it seems like I'm able to ride on the flats with more pressure than on snow; maybe the equivalent of hardpack snow? Hills are pretty much out at any pressure unless the sand is wet. Thanks for the article!

  2. Super post. Love the wrinkle test (like I'll ever use it).

  3. Thanks for the post, first of all. The pictures helped me visualize your words better.

    By the way, you don't cover front tire pressure here, but I've seen elsewhere that you will sometimes use a lower front pressure, (lower than the rear tire?) to get a pseudo steering damper effect. Did I get that right? Seems to be something that works for me, since I caught on to that.

    Again, many thanks.

  4. Hi Mike, You are right on with your write up! I have found the easiest way to explain FatBike tire setup is a simple motto...

    "Leave a Flat Track"

    As long as the track you leave behind has a flat bottom with no arc in it, you're riding on top of the snow instead of wasting energy cutting the surface. Once the interlocked snow surface has been "cut" by a tire with too much pressure, the next guy through the trail will have to deal with the loose surface. The closer you get your air pressure to that "arc" vs "flat" threshold the more efficient you'll be. This principle is the key to being a good Nordic citizen. The skiers will like the flat groomed track you leave and it eliminates their "Rutted" trails argument against Nordic Biking.

    Another factor to consider... is the Combined Gas Law we learned in high school. When the temperature of the air in the tire drops 10 degrees, the pressure changes significantly enough to require the rider's attention. Often, we set up our bikes in a place where its warm and then go out and ride. Pretty soon, you're tire feels totally flat and you assume you've got a leak. That assumption usually leads the rider to over-inflate in an attempt to compensate. It is best to let your bike sit in the environment that you plan to ride on for a bit before you try and set it up for your trail conditions. Just like when the temperature drops, the pressure drops... if you leave your bike tires sitting out in the sun (at least out here in the west), suddenly the tires feel hard. Just be aware that temperature changes will affect pressure and may necessitate an adjustment by the rider.

  5. I weigh 265 pounds. How should I compensate for that? I assume my pressure will go up because of my weight? I like the wrinkle test and will use it tomorrow eventing. Thank you for the great write up.

  6. Dan
    I'm 6'7" and 230. On my Clownshoe/Bud/Lou wheels, I run 15-16 PSI for 90% of my winter riding and commuting. Its fast, and I hate deflection. Thats including hard packed singletrack and snowmobile trails. If its softer and I'm starting to dig in, I just start airing down in increments until it feels "right". I really, really avoid any wrinkles, because if you run wrinkles long enough, you will get the sidewall cords start to break down, which can either start rubbing holes in your tubes, or you can get enough wear to get leaks out of your sidewalls themselves if tubeless. For me, If its so soft that I have to see wrinkles to make progress, It has to be long enough distance wise that riding is the best option. Otherwise, your tire life will suffer. So one mans 10 PSI is another, bigger mans 15 PSI. Your wheel set up will make a huge difference as well. I hope you are on 100mm/4.8s. So much better. Mikes stats are great starting points, but as they say, your results may vary. I've been able to run 15 PSI on my set up in fresh, windblown where other, smaller guys on like Darryls and Nates needed to air waaaaayyyyy down.

  7. I think you are spot on with the PSI and conditions.

  8. These are some awesome pics! I really enjoy reading your blog, keep up the posts!

  9. Great post and awesome pics, what would be the starting point for a 210 pound rider on 80mm rims with 4" tires? I'm sturggling finding the sweet spot for non packed snow riding.

    1. It all depends on your local conditions, which makes it hard for me to say. Best bet is to go really, really low to start -- like 2psi -- and then (if necessary) slowly increase in .5psi increments until you find what feels right.