Fat tire bikes have had a surge in popularity on off-road trails, especially in the adventure biking and bikepacking communities. You may have even seen them in urban environments with road tires. With fat bikes in all those different appearances, it begs the question: What is a fat bike?
A fat bike (or fatbike) is a mountain bike with oversized tires between 3.8″ and 5″ wide. These tires provide increased surface area and require less air pressure, resulting in improved grip and comfort while riding. Due to this, they are typically used for adventure biking on loose terrains like gravel, snow, sand, and even ice.
The name would suggest otherwise, but it’s actually not about tire-shaming. Arguably branding like that wouldn’t fly in this day and age. But a couple of decades ago it did.
Fat Bike 101
Fat bikes are designed for adventure riding, and that’s exactly how they got invented. They had their start in 1986 with the goal of riding over the Sahara desert. A use case not that accessible, to be honest. That’s why they only really started gaining in popularity and exposure with an adventure race in Alaska that went a thousand miles over frozen lakes and snow.
While the temperatures couldn’t be more opposite, there are striking similarities where the fat bike just excels at: loose surfaces like snow, sand, and gravel. It’s in these extreme environments that fat bikes are in their natural habitat.
What is a fat bike good for?
Fat bikes are geared towards very specific purposes, in which they excel. The massive surface area of fat bike tires allows for incredible grip on soft and loose surfaces like gravel, snow, and sand. Additionally, the large volume of air inside acts as suspension smoothing out small bumps. This is why they are popular adventure travel bikes.
While fat bikes sure look heavy and cumbersome, the tire casings are not thicker, there is just more rubber material and more air in them, which by itself is not much heavier. What you notice is the rotating mass while riding. This gives the fat bike a stable, planted feel and incredible grip on the trail, especially on loose surfaces. On the flip side, the increased traction comes at the cost of higher rolling resistance. For more on why this is, you can read more on tread patterns here.
What’s the difference between a fat tire bike and a regular bike?
The biggest difference between fat bikes and regular bikes is the tires’ width of up to two times as wide. They’re designed to provide flotation to ride over surfaces like snow and sand, that a normal bike would be impossible to ride on. To fit tires this size, fat bikes frames are fundamentally designed differently.
At a width between 3.8 and 5 inches, fat tires are up to double the width of the commonly used 2.4″ MTB tires. With these kinds of tire dimensions, fat bikes are one of the few remaining bikes that still use 26-inch rims.
Compared to very common 27.5×2.4 wheels, the outside diameter for a complete 26″ fat bike wheel with a tire is pretty big and more like a regular 29“ wheel. It’s the size of the rubber that gives the tire its massive volume and provides these special riding characteristics.
The size also allows for fat bike tires to require lower air pressure compared to narrower tires. This allows them to conform to the ground better and get more contact area at all times. (More detail on the topic of tire pressure here.)
Fat Bike Tires
Not to be confused with the already big plus-sized mountain bike tires, fat bike tires overshadow any other bicycle tire in terms of sheer footprint and volume. At up to 5.0″ they can get absolutely massive. They look ridiculous for an MTB, more like dirt bike tires than mountain bike ones.
With unusual dimensions come unusual tire characteristics. This starts at the basics with tire pressure. Normal tire pressures for fat bike tires are between 5 and 15 psi, which is really low. At the minimum of 5 psi, the tire will really squirm around and be really good over rocks and roots as it can conform to the ground like no other kind of tire.
You’ll get simply impressive grip, which you’d never think was possible on a mountain bike, due to the huge surface area of a soft tire.
So, the main selling point of fat bikes are these massive tires. With dimensions this unusual, the „rest of the bike“ has to be designed around them to make them even fit.
Fat Bike Frames
There are full suspension fat bikes, but hardtail fat bikes are far more common even for mountain biking on rougher terrain. The reason is that the dampening of the soft tires is enough suspension in most circumstances you would ride a fat bike in. In fact, a suspension may even complicate things as it’s likely to work simultaneously with the tire dampening – in the worst case even against it.
I personally feel that a hardtail is the best of both worlds. It has a predictable back end while still being able to go quickly without your hands getting rattled off the handlebar. The tire dampening is still not enough to comfortably ride downhill sections at pace. If rougher terrain isn’t on the menu, a fully rigid fat bike for adventuring is the better, maintenance-friendly option.
This starts at the very front with forks, that are custom to fat bikes. For suspension forks, manufacturers like RockShox make extra suspension forks like their Bluto. Just looking at the front of a fat bike, you can see how much wider the stanchions are apart to make room for the massive tire.
So, at the axles, the wheel hubs are wider. Further back the bottom brackets are wider to actually make enough room for the cranks to spin without touching the wider chainstay.
As you can see, changing one component to extreme levels manifests through the whole design.
The biggest difference on a fat bike frame can clearly be seen in the rear end. Here not only the rear axle is wider, but the entire rear triangle of seatstay and chainstay needs to be much wider as well that tire into the frame without rubbing.
The actual front hub width on a RockShox Bluto is 150 mm compared. Compared to the 110 mm on a regular mountain bike, that is almost 1.5 as wide. With that obviously comes increased stability and durability.
Similarly, the rim width is not even in the same league. While regular MTB rims are 30 mm wide (even for plus tires), fat bike rims can get up to 100 mm wide! That’s more than 3 times as wide. It’s the rim width that allows for the big tire volume of a fat bike tire.
The pros and cons of fat bikes
- Rideable in extreme conditions
- Traction for days
- Tire dampening
- Comfort & confidence
- Sluggish handling
- Slow acceleration
- More energy required
In other words: A fat bike leans heavily towards stability at the cost of agility.
How a fat bike rides
I was lucky to have had the opportunity to try out all variations of fat bikes on my home trails: a carbon full-suspension, an alloy hardtail, and a steel fully rigid fat bike all by Surly. I rode them my home trails which have a nice asphalt and gravel fire road uphill and nice downhills in the woods. The terrain is rooty, rocky in some parts, has wide and sharp turns, even the occasional berm, and jumps.
Either one obviously rides exceptionally well on terrain, they are designed for. They float very well on loose surfaces like snow, sand, and gravel. It’s quite the surreal feeling you can’t get on a regular MTB.
If you get the chance, take a fat bike for a spin in snowy conditions. This is where an extreme bike like this truly shines and leaves any other type of bike in the dust. I had the opportunity to do it on a fully rigid fat bike. And I tried to do the same on my downhill bike, which I can not recommend. While I had mud tires on, they simply could not dig into the snow or conform to the ground. The casing and rubber in those temperatures were just too hard and the suspension was basically useless. I’d compare that experience to riding on ice, slipping and sliding at all times. There was hardly a time to rest and simply roll as I was correcting constantly. None of those problems exist on a fat bike.
Once you’re up to speed you can really plow through the trails and over off-road terrain. The biggest difference to a regular MTB is that the huge tires give you so much grip on the ground. This feature makes a fat bike really confidence-inspiring.
But as soon as repeated small bumps over root carpets came, the bike started to bounce like a bouncy castle. I felt it to be unpredictable. Over those kinds of trail obstacles, I couldn’t go as fast as I wanted to or as I was used to in a controlled manner.
With much more rubber on the wheels, there’s also a lot of rotation mass that you notice immediately. So on normal trails, it does take a little bit of work to get them accelerating from the start and slowing down a little bit more difficult. So is initiating turns and direction changes. It just feels more sluggish in tight sections. That stability can also be a boost in confidence and comfort as these tires are really forgiving with small riding errors.
The increased tire friction is noticeable even on flat, hard ground compared to a normal MTB. This leads to slower acceleration, but when up to speed you are just trucking along over rocks and roots. It’s quite the experience to be on-board as the bike is not impressed by the loose marbles and leaves in the fall. It’s simply unreal traction and nothing you can ever get on normal MTB tires half the width.
Can you convert your mountain bike to a fat bike?
A common-sized mountain bike cannot be converted into a fat bike. The main reason is that the rear triangle simply doesn’t provide enough clearance for a fat tire, only a plus-sized tire at best. The fork could be changed to one compatible with fat bike wheels, but the bike frame is what dictates the type of bike you can build.
Fat tires can only be mounted in purpose-made fat bike frames. There is no swapping back and forth between normal-sized and fat tires. This is why you can’t convert any mountain bike to a fat bike.
Fat bike wheels are designed not only with wider rims, but also wider hubs. The part where the axle goes through. So a fat bike frame is designed to accommodate those two extraordinary dimensions at the seat stay and at the axle.
Additionally, those wheels have a large overall diameter, even though the rim is comparatively small. So the frame geometry has to be designed with not only wheels in that width, but also in that height.