The 27.5″ x 3.8″ (27.5 Fat) and the 26″ Fat are both available wheel and tire sizes for Fat Bikes. Let's dig into these two options and see what we can learn. If you're looking for more info on Fat Biking check out this blog and this one. Want to understand tires better? View our Bike Tire Buying Guide here.
Let's compare the 27.5″ and 26″ Fat Tires in their capacity to Roll and Float as well as their individual Weight.
Rolling
Pros
- 27.5″, with its larger diameter, has a greater attack angle. This means that the wheel should roll over obstacles easier. It's the same idea as a 29″ wheel vs. 26″.
Cons
- Because there is more tire to achieve the larger size, there is more weight pushed to the outside of the rotational mass of the tire, which will have a greater effect on the handling and overall feel of the bike. With a traditional 29er, that extra weight is offset by the improvement in grip and rollover. When we're talking about a 4″ tire, that's a LOT of extra weight compared to smaller gains, especially in the snow.
- When you add weight to a tire and a wheel, especially with fat bikes, they can be more susceptible to auto-steering. If you've ridden a fat tire at low pressure you probably understand what this feels like. Without getting into a long discussion of the physics, auto-steer happens when you're turning and the tire begins to pull you into a corner by itself. You can feel this in the handlebars as the inside grip pushes towards you and the outside grip pulls away from you. This happens with all tires but is even more pronounced with lower pressures and wider tires.
Float
Pros
- 27.5 Fat gives the rider a LONGER contact patch with the ground. This should mean better float in loose conditions like snow, sand, and mud.
Cons
- Due to that contact patch being along the LENGTH of the tire, the 27.5″ tire can be susceptible to “biting” or “knifing” in corners. That is, once the tire leaves its forward path and is asked to turn, it's “sharper” than a 26″ Fat tire with a wider width and it's going to want to dig into the loose material.
- With the wider overall tires possible with the 26″ Fat, you're able to run lower air pressure. This allows the tire to “squat” more and provides a bigger contact patch, allowing the tire to float better and grip stronger in turns.
Weights
Now, let's look beyond the tires (and wheels for that matter – those are bigger and thus heavier with more rotational mass) and take a look at the weight of a few of the different frames in these two tire sizes.
| Model (size medium | Wheel size | Frame/Fork Weight |
|---|---|---|
| Specialized Fatboy Carbon | 26 Fat | 1770 grams |
| Salsa Beargrease Carbon | 27.5 Fat | 2110 grams |
| Salsa Mukluk Carbon | 26 Fat | 2313 grams |
| Specialized Fatboy M5 Alloy | 26 Fat | 2381 grams |
| Trek Farley Carbon | 27.5 Fat | 2554 grams |
| Trek Farley Alloy | 27.5 Fat | 3310 grams |
As you can see, the Specialized Fatboy carbon, which is a 26″ Fat model, has a significantly lower weight than other carbon models and the Specialized Fatboy M5 Alloy is lighter or very comparable to even some carbon offerings.
Fatboy with 27.5″ x 3.8″!
Now, with all that said, some people just prefer the 27.5″ tire. Because of this preference and our desire to provide customers with options, we did some internal R&D and determined that the 27.5″ x 3.8″ wheels and tires are fully compatible with the Specialized Fatboy with room for even wider tires. And compared to other options on the market, the Fatboy handles much more like a traditional XC hardtail.
So, there you go, a Specialized Fatboy with custom-built 27.5″ Fat wheels! This particular build uses DT Swiss 350 hubs and Sun Ringle Mulefut 80 SL rims. Stop by your local ERIK'S to check out the latest Fatboys or to gear up yours with 27.5″ Fat wheels.