Diving deeper into tire construction

holistic understanding of tire construction reveals subtle details that can help you choose the best rubber for you.

Chances are most of us shop for mountain bike tires the same way—lots of tread for more grip, less tread for fast rolling, all for the right price. Or, we just ask our buddies what they ride and go with that. As it turns out, there’s a lot more to a mountain bike tire than just the chunkiness of the tread. And by learning a bit about how tires are designed, you may end up with a better tire for your style of riding. To start, it’s time to dispel the myth that grip only comes from big, knobby lugs, and it’s equally important to stop thinking of less tread as faster. To get a better sense of which tire is right for you, pay attention to the details and expand your tire vocabulary.

Siping is basically putting grooves in the tire, usually in the tread blocks. It allows for directional flex, which means it’s possible to tailor traction in individual tread knobs.


Get tread-smart

To a tire novice, it seems fairly simple: the tread knobs stick out of the casing and provide grip. The further they stick off the casing, the grippier they will be, right? Not exactly. It’s important to think about tire design holistically; all elements work together to create grip, low rolling resistance and durability. Big lugs are a good place to start when it comes to grip, but that’s certainly not the whole story. Take siping, for example—the small cuts you see on the face of a knob. It’s a small detail that pays big dividends, yet it rarely enters into tire-choice conversations. Siping is basically putting grooves in the tire, usually in the tread blocks. It allows for directional flex, which means it’s possible to tailor traction in individual tread knobs.

“If you were to put a sipe across the knob, that sipe will allow a little bit of air inside the sipe to collapse,” says Ken Avery, Senior Vice President of Product Development at Vittoria Tires. “You can actually keep the base of that tread block stable, which is critical to making sure that the tread pattern does what it’s designed to do, yet you can actually engineer a bit of surface flex in that tread block where it hits the ground. That’s really important going over rocks and roots and different types of terrain.”

Vittoria has taken advantage of siping in a few cool ways. One way is progressive siping: putting three sipes in a row at varying widths on a tread. On a center tread, progressive siping works like this: the leading edge of a tread knob has a small, narrow sipe. Behind that is a slightly wider one, and then at the trailing edge of that knob, a wide sipe. When you’re braking, this design allows the tread to grip by letting the tire roll forward slightly under braking forces. The smaller, leading-edge sipe means the tread can bite in when you’re climbing. So, you’re actually getting a unique combination: exceptional grip under braking, grip while climbing, and low rolling resistance in between. It’s possible to apply this same principle to side knobs to create cornering grip.

Tires like this Vittoria Mazza have ramped and stepped leading edges on the center knobs for less rolling resistance and sharp, full-height trailing edges for braking traction.


The Dead Zone

When you’re riding straight, your tires run perpendicular to the ground. The center tread gives you traction, and if it’s designed well, it should also reduce rolling resistance. When you’re cornering, your tires lean over. The side knobs bite in, giving you the traction you need. Simple enough, right? But, in between those two positions is a whole lot of gray area. That empty space can end up being a dead zone in your tread, which leaves you with a vague, grip-less feel. But, it can also provide the space necessary to allow your side knobs to flex and do their jobs. With certain tread patterns, this dead zone needs to be filled to give you grip as you roll the tire over. That’s where transition knobs come into play.

“In the center tread, you’re going to be digging to climb, and you’re going to be braking,” says Avery. “So, you’re going to want to make sure the leading edge and the trailing edge are properly designed, and then you look at that side tread and, honestly, it’s arguably even more important because there’s nothing worse than having your bike drift in a corner unexpectedly or losing that traction when cornering; but, often riders don’t think about what happens in between. There’s this dead zone when you’re in your mid lean.”

To counter that dead zone, Avery says it’s helpful to think of your tire as a soccer cleat. The cleat has spikes spaced throughout, but in between those spikes is a lot of empty space. That allows the spikes to dig into the terrain, providing grip. Your tire works similarly. But, depending on the position of the tire in your lean, you may find yourself in a dead zone with no grip at all. It’s most noticeable in hard-packed terrain. Transition knobs fill that void and provide grip in between the center tread activation and the side knob activation. It leads to a stair-stepping process: center tread, transition tread, side tread.

“It’s a tricky thing to design, because they can often fight each other,” Avery says. “Sometimes, if it’s not done correctly, the transition knobs can prevent that side tread from doing the work it’s designed to do. So, it needs to be done a certain way, and it needs to still have enough space around it.” Avery says a good way to accomplish this is to offset the tread patterns. This essentially fills those voids between the center tread and the side knobs without interfering with either.

Of course, while transition knobs fill much of the negative space between the center tread and cornering tread, sometimes it’s better to leave the transition knobs out altogether. Negative space on a tire, as it turns out, allows for more than just mud clearing. It’s vital to leave empty space that allows the various treads to flex effectively.

Vittoria’s aggressive trail/enduro tire, the Mazza, doesn’t have a transition tread; instead, it has a staggered center tread that does the job of a transition tread. “There are two shapes within the center tread, and they’re offset so they’re not just in a straight row as the tire turns,” says Avery. “On the outer edges of that center tread, you’ll see that there’s a bit of a flare. It gives you that ability to still have room for the side tread to work, but at the same time, you have some support there as you lean it over. So that, coupled with the fact that the side knobs are a bit larger footprint and with the progressive sipe, all those things work together on the Mazza to create a sticky cornering feel.”

From top to bottom are peeks inside Vittoria’s enduro, trail and cross-country tires. You can see the difference in casing and puncture-protection layers as well as knob shape, height and layouts.

A case for casings

The common casing equation goes like this: enduro casing equals heavier but more durable; trail or XC casing equals lighter but not as durable. As you might imagine, that math glosses over a whole lot of X factors. It is true that enduro casings tend to be heavier and more durable, while trail and XC casings do indeed tend to be lighter and therefore more susceptible to damage from big hits. But, the casing has more complexity than that. Much of a tire’s ride quality is derived from the casing.

That’s where hysteresis comes into play. Without getting too far into the technical weeds, hysteresis is the process of deformation and a return to shape and the lag time in between those two events. The rubber that comprises your tire deforms when it encounters an impact. Then, after the impact, the rubber will rebound back to its original shape. A tire with a thinner casing will rebound more quickly than a tire with a thicker casing, which is why enduro tires deliver more of a muted feeling when you encounter a chunky rock garden. A trail or XC tire may feel a bit bouncier because that rubber rebounds more quickly.

In a sense, tires are an air spring, just like your suspension fork, but there’s no internal mechanism to control that rebound, so tires must rely on the rubber compounds to do the job. “A slower rebound compound will give you just a little bit more time before it wants to spring back to shape and potentially sacrifice traction in doing that,” Avery says. “If your tire deforms and springs back, you’re going to lose traction. But, if it rebounds a little slower, the deformation of your tire will stick slightly longer and you’ll maintain that traction.”

Now we arrive at the reason so many companies are tight-lipped about what’s in their rubber compounds. It’s a difficult design challenge to create a grippy tire that’s fast-rolling, supple and durable, so manufacturers experiment with additives that can control those parameters. Tire rubber needs to be flexible yet supportive. Treads need to be grippy yet fast. So, all of the elements—the casing, the tread, the siping, the negative space between treads—need to work together to accommodate the rider’s style, the trail conditions, and the type of riding that the rider will do. Throw in the specific needs of a rear tire versus a front tire and you’ve got a lot of decisions to make when you walk into the bike shop to buy new rubber.

Theory into practice

That sure is a lot of information about just one component of your bicycle. How do you choose the tires that are right for you? The ancient Greeks would like a word. From those giants of philosophy came the saying, “Know thyself.” That is exactly where you should begin your search for the perfect mountain bike tire. Know what kind of riding you intend to do the most, what kind of terrain you’ll be on most, and how you tend to ride.

Aggressive riders who change direction quickly and frequently, and who are likely to take on big impacts, probably need an enduro tire that will provide a muted, supple feel over chunk and will withstand the abuses of such riding. There’s a weight penalty here, of course, but that shouldn’t matter much to riders who spend most of their time focused on the descents.

And, consider the cornering knobs. Big and blocky are great, sure, but if you’re looking for more bite, look for siping and plenty of negative space to let those big knobs do the work. And, don’t forget to consider transition knobs if you want that mid-lean support.

If you’re more of an all-around trail rider—the kind who enjoys the climbs as much as the descents—you’re going to prefer a trail tire more than the heavier and more muted enduro tires. A good center tread, particularly on the rear tire, becomes vital for such riders. Look for a tread that will provide grip while braking and climbing but offers low-rolling resistance.

If you fall somewhere in between those two styles, you’re in luck. The combination of a chunky, aggressive front tire and a lower-profile, fast-rolling rear tire is probably right up your alley. That combination will give you the cornering grip you need up front to cut through a top layer of loose dirt and grip what’s beneath, yet you’ll get low-rolling resistance in the rear with enough grip to keep you feeling planted while cornering.

Ultimately, the fastest tire is the one that gets you from point A to point B in the shortest amount of time. Analyze what that area between point A and point B looks like to figure out whether you need an aggressive tire or a fast-rolling one—or both. Know thyself. Know the elements of a tire, and know the trails. Know those three things, and the right tire choice will become obvious.

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