Dialing in Tire Pressure: How Low is Too Low?
It’s pretty much every fatbiker’s dream to roll effortlessly on groomed hardpack trail, leaving our friends amazed and blasted by our snowdust. Brrappppp!! So before throwing our rig on the rack and heading out to the nearest fatbiking trail, we pump up our tires. They’re hard as rocks. In road biking, a higher PSI means a faster rolling tire, so why wouldn’t you pump your fatties (your fat tires, of course) up, too?
This is a huge mistake that I see even experienced fatbikers make. They will pump a few extra pounds into their tires, knowing better, yet still anticipating a faster ride. After a few short miles they are on the side of the trail, letting out air.
The reason why fat tires are so, well, fat, is because in order to pedal around in the snow they need traction. A large contact patch. Surface area. Float. When you pump them up to a stout 15+ PSI you’ll notice something strange: all of the sudden you’re pedaling hard, but you’re not going anywhere. Awh! *Frustrated sigh*
Dialing in your pressure just right is a little more tricky on a fatbike than a dirt bike. When you’re thinking about tire pressure on your full suspension, you might consider the material the trail is built out of (rock vs. dirt) or the amount of rain you’ve had lately (am I going to hit a mud puddle?). Alternatively, you might just go shred instead of fussing with your tire pressure, because time is precious and who the hell is that neurotic?
Here it is: I’m asking you to be a little neurotic when it comes to your fatbike. Unlike dirt riding, when you’re riding snow, conditions can change by the hour. One minute you’re flying up a slightly frozen hardpack from last night’s freeze where you can run higher PSI, the next you’re slipping through warm, sticky mush where a low PSI is necessary to stay upright. And while you might not notice the difference of a few pounds + or - when you’re riding dirt, those few pounds will be extremely noticeable on your fatbike.
Surprisingly, the biggest reason to pay attention to your tire pressure when riding on groomed trail actually has nothing to do with you.
Here’s an unfortunately common scenario: A rider, we’ll call him Brett, sets his tire pressure too high. He rips out of the parking lot on his fatbike, stoked to show his buddies how fast he is. He hits the trail gunning it, full-throttle, spinning out in place and sweating like lemonade on a hot day. He unknowingly tears huge ruts into the beautiful, smooth hardpack that was carefully groomed by a kind soul the night before. Frustrated that he doesn’t have any traction, he hops off his bike and walks back to the parking lot. “It’s not rideable” he tells his buddies and they all post-hole their way out of the forest. Those gouges and footprints freeze solid overnight. The next morning you wake up early, having dreamt fat dreams about snowy berms and smooth, corduroy trail. You charge down the track, blissful, wind in your hair, grin on your face, until...BAM BOOM! You slip into Brett’s tire tracks, slam into his post-hole, and fly over your bars and crumple into a snowbank. Your pals behind you stack up like pancakes. Frozen, bruised, salty pancakes. You all curse Brett.
Fattie PSA: If you’re leaving gouges, it doesn’t necessarily mean it’s time to go home. Pull over to the side of the trail, step off your bike, let out a few PSI and try again. Hopefully you’ll be in business. If the trail is still so soft you’re leaving ruts, it’s time to go home and try again in the morning or evening, when the snow is colder. If you ever step off your bike and your foot falls through into the track (what you have created is now called a post-hole), then push some snow into the hole and walk back to your car on the side of the groom. Your local groomers will absolutely love you for it!
Here are two examples of leaving a rut. When these ruts freeze overnight they will become super hard by morning. While the rut on the left might not ruin someone’s day, the one on the right definitely will! Below, to the right, is an example of what happens when riders and hikers walk on the groomed trail. It’s still ridable, of course, but it’s not going to be very much fun.
Post-holing happens when a rider gets off their bike and stands on a soft trail. WHOMPH! Their feet fall through the semi-packed powder, and the track now has a massive hole in it. Last year, when a few of my friends started grooming our local trail system, I tagged along a few times and was floored by how much work it is, especially to get the first track of the year in. After four hours of fighting with the machine in the cold (usually at night—it’s the best time to groom) these good Samaritans were eager to go rip the new trail the next day. They were devastated to to find that two bikers had already post-holed through their beautiful track.
When anticipating dialing in fat tire pressure, ask yourself how hard the snow will be. If it’s basically an ice-skating rink, go ham! Filler up! Putter at 10! Get wild! These conditions happen often in sub zero conditions or early morning and evenings during the freeze and thaw cycle.
If it’s been consistently cold and the trail is pretty hard (think snowmobile trail) a PSI between 4-7 is likely a good place to start. These conditions are common in normal winter temps. Remember, you can always let more air out!
If it’s a soft, powdery day and you still need to get some time on a bike, let out air until you see the walls of your tire crumple ever so slightly. I’ve ridden at 2-3 PSI plenty of times (although, if you are a heavier/taller rider throw in an extra pound or so). These conditions are common after a big snow or when riding off groomed trail.
You’ll also want to consider the kind of snow you’ll be riding in. Wet and dry snow are both determined by surface temperature. When the surface temp is above freezing, the snow can become melty, heavy and wet. This ultimately creates more dense snow (sugar snow) due to a greater ratio of moisture to surface area. Alternatively, dry snow has a lower water content and, on average, 5 inches of dry snow will melt to only 0.5 of an inch of water. Source.
In thick wet snow (3 to 6 inches), riders will want to run really low pressure (2 to 5 PSI) in order to get the traction to be able to move. When the snow is sticky, it’s easier to do damage to the trail and therefore, the lower the PSI, the better. In dry powdery snow, conditions vary even more. If the snow is packed and groomed, you may be able to ride between 6 and 8 PSI.
Not all riders are the same, so I asked Molly what her preferred PSI is for winter riding. Here’s what she said:
2-4 PSI if it’s super soft
5-6 PSI for average conditions
7-12 PSI on hardpack
Something to keep in mind is that the only time you need to worry about going too low is if 1.) you have tubes (you really should go tubeless) or 2.) if you’ll be riding your fattie in mixed conditions (rocks, mud, ice). In mixed conditions, a rock or log could push your tire to your rim and dent it.
After reading this, you’re likely thinking, “That’s nice, but my tire pressure gauge isn’t sensitive enough to register 3 or 4 PSI.” During my first winter of riding I realized that I needed a new gauge.
All of these gauges are easy to use and light.
The Meiser Accu-Gage is the heaviest gauge listed here, but is also the most durable. I have two—one that hangs out in my garage and another that chills in the car. I like this gauge because it’s cheap and it doesn’t require batteries.
The Topeak SmartGauge is a winner because it’s accurate. No guessing involved. Another plus is that you can use this gauge for all your bikes, not just your fattie.
Quarq's TyreWiz monitors air pressure in real-time and relays the data to a cycling computer or a smartphone, so you always know what’s going on with your tires. They thread into your presta valve stem and take the place of your valve core, making them switchable between bikes. They allow you to set a high and low range, so when the tire looses too much pressure by your own standards, the LED will flash red rather than green. This is particularly nice for rides in varying elevation, since that can directly effect tire pressure.
I know this is annoying to hear, but that’s because it’s annoyingly true: the best way to get really, really good at nailing your PSI every time is to get out and ride. Take the time to make adjustments, and think about how your bike is handling. Check behind you every so often to look for those rivets in the trail. Most importantly, get your shred on.