With all the fatbikes hitting the road, I thought I'd share some information about tire pressure. I've been planning on this 'test' for a while and finally got the dusting of snow I needed.

I ran my Pugsley three times down the fresh snow, plus once with the Marin Nail Trail and once with my Kona Dew Drop - this was just for a little comparison with tires and pressures most people are familiar with.

Top to bottom: fatbike at 15 psi; fatbike at 8 psi; fatbike at 4 psi; Marin at 35 psi; and Kona at 50 psi.



Here are the three fatbike pressures in one image for comparison (it's not perfect, I know, but it gets the point across)



Bikes:
Runs 1, 2 & 3 were with a Surly Pugsley with Nate 3.8 inch tires
Run 4 was a Marin Nail Trail on Bontrager Connection Trail 26 x2.0
Run 5 was a Kona Dew Drop on Clement X’PLOR USH 700x35

Rider is 170 lbs and the air temp was 32F.

Hah, a picture is worth ....

Five sweet wheelies!

On a fat tired bike, I think that tire pressure is more about ride quality than tire contact patch but a rider would certainly want a good combination of both. When the traction is poor, low tire PSI will maximize grip but cost rolling efficiency.
Of course, most fat tire riders aren't too worried about pedaling efficiency but would be looking for maximum tire "give" for a suspension effect.
Good results from the posted pictures, they clearly explain what a difference a few pounds of pressure can make.
Bottom line, ride and have fun, lol

The extra work of low pressure tires, I looked up a couple rides to illustrate the effects of tire pressure on a fatbike:

In the summer with 12 psi, I rode 24 miles with an average speed of 15.7 mph (the only summer ride listed)
Today with 9 psi, rode 12.2 miles with an average speed of 10.7 mph
Yesterday with 6 psi, I rode 25 miles with an average speed of 9.4 mph (it was -10F... so there's that to factor in too)
Coming home from Theo (tight technical single-track trails with 3-4 inches of fresh snow) I left the tires at 4.5 psi and rode 12 miles home with an average speed below 9 mph. (I had a pump, but was "lazy" - not going to happen again!)

The fatbike forums on other message boards are full of posts extolling low tire pressures -- people saying they are running 5 to 7 lbs. My pump may not be very accurate (I've ordered a 1-30 lb presta pressure gauge to be sure) but at an indicated 12 lbs, I find the pedaling effort to be enormous. Pumped to an indicated 20 lbs (the max that the tire recommends) I find that the bike rides great. I'm mostly on paths and packed single track without much in the way of deep snow or steep off-road hills (so max traction or float is not a goal). When I get the new gauge later this week (hopefully) I'll be doing some similar tests to find the optimal tire pressure for the kind of riding I do.

The LBS recommended a 15 psi gauge, I thought "NO WAY" but took their word. Couldn't be happier! This is the one I got - Meiser Accu-Gage Dial Pressure Gauge > Accessories > Pumps & Inflation > Pressure Gauges | Jenson USA . I never run more than 15 psi (summer or winter)

I've ordered the exact same gauge in a 1-30 configuration. I have a 1-60 Accu-Gage schrader version for autos that I have found to be very accurate. I spoke with someone at the company last week to find out when they were shipping as they are all back-ordered. The massive boom in fatbikes surprised and delighted them.

I mostly use a digital gauge, as in I use my digits (thumb) to press on the tire and feel how hard it is. But the gauge on the floor pump shows less than 10psi. Rolling resistance doesn't feel bad (Dillinger 4, I've used them tubed and tubeless)

I think when I had Endomorph and Larry tires I probably ran them a bit higher but I'd guess not much more than 12psi.

While Hypno Toad's average speed/pressure numbers are interesting, it is hard to draw conclusions because conditions have a huge impact on rolling resistance. I can tell you that on his ride at -10F rolling resistance was way higher than it would be on a ride at 20F. It takes a lot more effort the colder it gets. Tires get stiffer, you have more clothes on, snow gets grippier... and it just takes a toll on you.

What pumps would you use (on the road, not at home/in the shop) to inflate a fat tire? Even a full-length frame-style pump would likely require a fair bit of pumping.

I use a standard floor pump for the fat tires and carry a Serfas compact pump. I have never tried pumping the tires with the hand pump. However, I will tell you with the floor pump, ~10 strokes gets you 1 to 2 psi. Since the tires are high volume and low pressure, it's honestly not too different from a high pressure, low volume tire.

I totally agree and that's why I added temps, it make an amazing difference riding in the cold... I don't have a great way to compare pressure versus speed at the same temp. But clearly, lower pressure equals more work and slower speeds. Shocking, I know

Take this for example, two rides over the same route:

-10F, 25 miles, average pace 9.4 mph with Strava Suffer Score of 134
45F, 28 miles, average pace 14.6 mph with Strava Suffer Score of 56

I know that my tires were higher pressure on the 45F ride (no snow or ice) but I don't know what the pressure was at, likely 12 psi since that's my typical 'summer' pressure. Regardless, the level of work (Suffer Score) and the average pace, people get a great idea how much more work it is to ride in the cold.

I don't disagree, but I wanted to point out that the increase in rolling resistance is (probably) less dramatic than the numbers suggested.

And yes, it is very difficult to compare winter rides. Temperature and snow conditions have a huge influence. A one hour ride can turn into a three plus hour slog in soft snow. Last year's Iditarod Trail Invitational (ITI) record to McGrath was less than two days. There have been years people took two days to the first checkpoint.

I want to add this information to this thread - exposing things I 'know' but don't 'understand'.

I took a 25 mile ride at -10F earlier this month, it was a crazy amount of work (talked about above). I did not give enough thought to the change in temp and it's effect on the tire pressure. The night before this ride, I worked on the bike in the basement at ~65F checking tire pressure, moved it to the garage overnight, then took it on a ride at -10F. So, looking at the chart below, I started at 9psi at 65F, but when the tires cooled to -10F the pressure was down to 5.4 psi. That's a big difference!




Source: Tire Pressure and the Cold: Bontrager's PSI Conversion Chart Will Keep You Inflated This Winter - Bikerumor

Oddly, I had just watched this episode of MythBusters that basically covering the same subject of changing pressure with temperature.

Ideal gas law, PV=nRT.
Where P is pressure, V is volume, n is the number of gas moles, T is the temperature and R is a constant. So if you keep the volume constant, the pressure will go down as temperature goes down. The relationship is linear, but you have to use absolute pressure (on a tire you are measuring psi above atmospheric pressure)and temperature in degrees Kelvin.

The same thing happens when you inflate in the cold and then temperature goes up. If you leave a balloon in the sun they can pop.

It does have a big impact when you are riding in sub-zero temperatures, as you already found out. The thing to do is either overinflate at home, or on the trail. (Disclaimer: I never remember to overinflate and then I'm too lazy to do it on the trail)

Is there significant difference from 12 to 15 psi and 15 to 18 psi? I took mine to 18 and noticed very little diff in rollability vs getting a much harsher ride.

This ^^

Per Surly, the fat tire operating pressure is 5-15 psi. I honestly never go over 15 psi since my pressure gauge is limited to 15 psi. My basic pressures are:

Summer, dry riding - 10-12 psi (including some road riding getting to and from the trails)
Winter rider, pack snow - 8-10 psi
Winter riding, fresh/soft snow - 5-6 psi (very soft snow, I've been down to ~4 psi)

I'm sure you can ride higher pressures than I have listed, but IMHO you are removing the fat tire advantage.

What Hypno Toad said. You can probably run the tires at 18psi, but they will beat you up like 23s on a road bike. And if you go on any trails, they will have about as much traction as 23s too.

I rode my fat bike 17+ miles yesterday on some single track trails. Nothing very technical but average single track riding with roots, logs, rock gardens, climbs, mud, frozen earth, ice and other similar obstacles. I ran with 8 pounds of pressure on a fully rigid bike and was surprised how confident inspiring the the fat bike handled. Note, we have no snow yet but maybe some coming Saturday
Lesson learned, when talking fat bikes, tire pressure is the key to an enjoyable or harsh ride. I recommend that everyone experiment and see what works for them. I like the idea of over inflating at home and letting air out along the way to find the sweet pressure for the ride.

It depends somewhat on your tire size and other factors. That said, in my limited experience with 4.5"-5" tires, it makes more difference at lower pressures than higher. 5 psi to 8 psi to 10psi resulted in very noticeable differences handling, rolling resistance, and ride cushion.

I'm currently running studded Snowshoe XLs (~4.6") at about 10 psi; For me that gives the most acceptable balance of float, grip, handling, and rolling efficiency. I'm going to be doing an electric conversion on the bike shortly, at which point I'll probably drop the pressure some since I'll care less about rolling resistance.

For dry pavement with slick or semi-slick tires, I am approaching a new conclusion. The ideal pressure is the lowest pressure I can ride safely without the risk of pinch flats. Before I hit middle age, I aimed for the highest pressure I could tolerate. Now I aim for the lowest.

I tried the 18 on pavement. The last time I did some serious fatbiking I was down around 5-7, off road. Framed factory tires, 120 tpi.

So true. I used to commute in Canada at -30 to -40C on my Pugs and had far too many mornings of leaving the warmth of my apartment, rolling the bike downstairs, and getting 3 miles into a 7 mile commute before realizing I was pumping it really hard with pressures that were too low.

The danger at those temps was you needed to over-inflate so much that the first few miles were a little slick with the lack of footprint. I wanted to make a note about this though. From experience, even when studs are added, fat bike tyres are not the best on pure ice. My wife's studded 700/32 outperformed me there any day. However, in snow or frozen over slush/rough ice and ill take the fat bike.