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Tires are one of the most important yet most overlooked components on a bike. Acceleration, braking, comfort, control-tires have a huge impact on all these things. Yet picking the “right” set of tires can be a challenge-particularly when it comes to mountain bike models, which vary wildly in size and shape.

What should you be looking for in your net set of tires? Here’s the skinny on fats.

DON’T SWEAT THE THREAD COUNT
A tire’s carcass, or casing, is made of layers of rubber-impregnated fibers, onto which the tread rubber is heat bonded. The number of threads in a given inch of tire casing (measured as TPI or threads per inc”) is sometimes tossed around as if it were a big deal. And when it comes to road-bike tires, it is-tires with a high thread count tend to boast a nice, supple ride quality.

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But when it comes to mountain bike tires, a high thread count isn’t necessarily something to wax poetic about. High TPI tires are more prone to punctures than lower TPI tires (which use fatter, less-penetrable fibers). Today, most all-purpose mountain bike tires have somewhere around 50 to 70 threads per inch-a good compromise of durability and feel. Stouter downhill tires might have as few as 33 very fat threads per inch.

Long story short, don’t sweat the thread count.

FAT VS. REALLY FAT
Mountain bike tires used to be fat. Then they got skinny. Then they got fat again. And now, with the advent of plus-size tires, they are getting grossly obese. What’s up with that? Fat, high-volume tires have a few things going for them. The larger-volume tires have a degree of innate give or compliance, which is why they were popular with back-in-the-day riders who lacked any other kind of suspension. High-volume tires are also less prone to pinch flats and offer better traction than narrow tires (all things being equal) because they possess a bigger footprint.

So, let’s get down to brass tacks here on tire sizes:

XC Racers: 1.9 to 2.2 inches
If you’re racing on relatively smooth, hardpack trails, you can still get away with 1.9 to 2.2-inch tires. This is the tire for people who obsess over grams and can live with less traction.

Trail and All-Mountain Riders: 2.25 to 2.4 inches
This is the meat of the mountain bike market right here. 2.2 to 2.4-inch tires still get the job done for most riders. There’s enough volume here to provide excellent traction over rocks and roots and a slight degree of suspension.

There’s also a lot of variety in this niche-with very lightweight models on one end and heavy-weight bruisers on the other. Heavier casings are a plus for riders who want to run lower pressures on technical trails or who simply live in a locale that routinely shreds tires to bits. In short, while this tire size is the mountain biking coin of the realm, you still need to match tire weight and tread design to your conditions.

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Gravity Riders: 2.4 to 2.5 inches
Rotational weight is something that few people are losing sleep over when shopping for a big, burly tire with gobs of traction. Heavy casings, reinforced sidewalls, a wide swath of soft rubber-the ideal tire for a freeride rig or park bike is going to prioritize traction and durability over all other considerations.

Plus Size/27+/29+/Mid-Fat: 2.8 to 3.2 inches
The bike industry is still working out the kinks on the hip name for this category of bike, but the basic idea here is a 3-inch wide tire that strikes a middle ground between a trail bike tire and a true fat-bike tire. Some people are shoehorning 650b rims with 27+ tires onto their 29ers, because the outer diameter winds up being about the same to a 29×2.3-inch rim and tire combo. Benefits? More flotation over minor trail chop and insane levels of traction. Downsides? Extra rotational weight and a bit of bobbing (it’s a big squishy tire, after all) at higher pedaling cadences. This market is just getting onto its feet, so everyone is still trying to nail the right tread patterns. You’ll see more of these things in the future, though. The bike industry loves nothing more than a new trend.

Fat Bike Riders: 4.0 to 5.0 inches
If you’re reading this sentence with any interest, you already know who you are and what you need; namely, a tire that’ll provide max floatation over snow and sand. This is still the tire size to go with for fat bikes. The plus-size tires (see above) aren’t going to cut it for you (the footprint is too narrow for snow riding) but I understand why you’d consider it: True fat-bike tires are heavy.

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GO KEVLAR
All clincher tires are equipped with beads that help lock the tire into place on your rim. Tire beads are either made of steel or aramid fiber (think Kevlar). Aramid fiber is much lighter that steel and comes stock on most performance, aftermarket tires, which are often listed as having “folding” beads. On average, you cut a quarter pound off your bike by going with a set of Kevlar-beaded tires. That may not sound like a big deal, but we’re talking about rotational weight here-you’ll definitely feel it out on the trail. If you can afford it, spring for the Kevlar. Get the folding tires.

NOT ALL RUBBER IS THE SAME
Most tires these days are made of synthetic rubber, but not all rubber is the same. Each tire brand has a very distinct formula. Pure rubber is incredibly soft and prone to dry rot, so tire manufacturers use proprietary blends of additives, such as carbon black, silica, elastomers, and hardening chemicals, to increase their tires’ stiffness and durability, as well as to fine tune how their tires roll and shed mud.

What you should care about is the rubber’s hardness. Harder rubber compounds tend to roll faster and wear more slowly than tires with softer compounds. Softer tires, on the other hand, often boast better traction than harder compounds, especially in wet conditions.

A sticky downhill tire will have a rubber hardness rating, or durometer, of 50/A, while a low-wear commuter tire might have a rating of 70/A. Which is best for you depends on where you ride and how often you’re willing to replace tires. I ride in the Pacific Northwest and since two-thirds of my ride season is spent hurdling wet rocks and roots, I tend to go with the softer, stickier models. If I was racing cross-country on hardpack fireroads, I’d veer towards the harder compounds. It’s all about location.

MATCH THE TREAD TO YOUR TRAIL CONDITIONS
While tread shapes vary wildly from one tire model to another, there are some general rules governing the spacing of the tire tread. Tightly-packed knobs tend to roll well and offer excellent traction on dry, hardpack trails. Conversely, they usually suck in wet, muddy conditions as the tread often clogs up with mud. For wet conditions or in loose soils, wide-spaced knobs are the answer-they shed mud well and offer a more consistent grip throughout the ride. On dry, hardpack trails, however, the widely-spaced knobs pose plenty of rolling resistance-providing a sluggish feel.

WATCH THE PRESSURE
The best mountain bike tire in the world will still ride poorly if you over-inflate the sucker. This, I’m sure, sounds like a no-brainer, but you’d be surprised. Riders, particularly those who grew up riding narrower 1.9 and 2.1-inch models that required relatively high pressures, often ride around on overinflated tires. The result? Their expensive, full-suspension bikes are ping ponging around the trail and, in a word, sucking. Mightily.

Your tire may have a maximum air pressure rating of 45 PSI, but you sure don’t want to ride them that way. Thanks to my love of all things hoppy and fermented, I tip the scales at 175 pounds. Nevertheless, I rarely exceed 24 or 25 PSI on my 2.35 to 2.4-inch tires. Sure, I pinch flat a bit more often than some riders, but the other ninety-nine percent of the time I’m enjoying better traction and a more forgiving ride that doesn’t have me scrambling around the trail, looking for the fillings that rattled loose from my molars. Take the time and experiment with your tire pressures, particularly when you switch to a new tire model.

There’s no hard and fast rule about tire pressure–other than this: to avoid pinch flats, skinnier tires (1.9 to 2.1-inch models) mounted on narrow rims require more pressure than wider (2.3 to 2.5-inch) tires on wider rims. How low is too low when it comes to air pressure? If your tires feel unstable (as if they want to rolling sideways and peel off the rim) while you’re cornering, add air pressure–two or three PSI at a time–until you hit that happy point where the tire seems to mute small rocks and roots, but doesn’t squirm when you lean the bike in corners.

TUBES OR TUBELESS?
Here are the benefits of using tubeless tires: You can run tubeless models at lower, traction-boosting air pressures with no fear of pinch flatting and, with some sealant inside the tires, small punctures (such as thorns) are less likely to flat your tires and ruin your ride. Still, it took more than a decade for tubeless tires to really catch on because the first iterations (UST models) weighed more than conventional tires and required specially sealed rim beds.

Nowadays, there are countless “tubeless-ready” tires that incorporate the tubeless tire’s special beads, but sport lighter, conventional tire casings. Likewise, you don’t need special UST-approved wheelsets any longer: plenty of wheels come with rim strips and valves that seal the rim bed. Add a bit of liquid sealant and you’re good to go.

Downsides? Installing a tubeless tire correctly (so that maintains its air pressure) is a bit more work than plopping a tube into a tire. A small air compressor, for instance, can really help when it comes to seating the beads on the rim, but with a little practice you can seal the deal with a garden-variety floor pump or CO2 cartridge. I always bring a spare tube with me on rides, even when I’m running a tubeless set up–it’s just easier to fix a flat out in the wild with a regular tube. Fortunately, you can easily go back and forth between tubes and tubeless.

THERE’S NO SINGLE “BEST” TIRE
Which tire is right for you? It all comes down to your riding style and the type of terrain you ride. If a tire is good in mud, it’s probably not as good on hardpack. If it boasts a ton of traction and gobs of high-volume cushion, chances are it’s slower and harder to pedal up hills. In other words, it’s all about tradeoffs. Every type of tire has its them-you just have to know the best balance given your riding style and soil type. Sure, there are some stellar jack-of-all-trades tire models to choose from, but they are far less common than the slick ad campaigns would have you believe. There’s no free lunch when it comes to product design: Every tire design involves trade offs. Pick wisely.

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