This summer, we got an early sneak peek at the all-new Zleen Racerunner Radical, dubbed “the world’s first true radial bicycle tire“. Not only does it take what we saw revolutionize Schwalbe’s MTB tires last year to the next level, with just 2° off from a completely radial construction and a shockingly low sidewall. The new Zleen radial XC tire also claims to lower rolling resistance, reduce weight, AND let you run higher pressures with an increased contact patch. Plus, now that the new Racerunner Radical tire is getting closer to market, and we know how much it’s gonna cost you.

Zleen Racerunner Radical innovative radial XC MTB tire

(All photos/Cory Benson)

Zleen is an interesting young Czech tire company, which grew out of the long-running tire expertise in the eastern part of the country a few years ago. Their generally affordable high-performance bicycle tires are developed entirely in the Czech Republic, and manufactured in India by strategic partner Ralson. We first spotted them a few years back at the XC World Cup in Nové Město. And more recently on the bikes of 2012 Olympic & 2011/214 World Champ Jaroslav Kulhavý.

Their latest tire, the Racerunner Radical, is one of the biggest departures from conventional XC mountain bike tires that we’ve seen in a long time. And the radial construction inside is just the beginning…

What does radial construction matter?

(Sketches & renderings/Zleen)

The reason radial bicycle tires are being developed is to experiment with changing the tire’s contact patch, support & grip. By switching from conventional 45° bias-ply casings to running the main fibers of the tire casing at 90° relative to the path of travel, the tire will deform more locally, more closely following the irregular surface of the trail. This also results in a larger contact patch at the same tire pressures. That means you can run higher tire pressures to get the same contact patch, now shorter and wider for better grip overall.

There are a couple of downsides or tricky bits, however. One is that all else being equal, a larger contact patch with more rubber grip on the ground logically also means increased rolling resistance. Although, a more supple tire with more local flex also means reduced hysteresis – less energy lost in the process of the tire flexing, so possibly an offset of decreased rolling-resistance there, too. Rolling resistance is especially tricky when talking MTB tires over irregular surfaces.

Another issue, the stiffer 45° bias-ply construction actually gives needed support to the tire, important while leaning and cornering. Going fully radial would require some alternative strategies to maintain the necessary casing stiffness, which could add extra weight or other complications.

And how is this different than Schwalbe?

It’s hard not to immediately compare the Zleen Racerunner Radical radial XC tire to the new Schwalbe radial MTB tires launched last year. The dramatically improved performance of the new Schwalbe construction catapulted their enduro and gravity tires to the top of my own must-have list last year. But Schwalbe’s new construction was exclusively applied to all-mountain to downhill tires, because while increasing grip it also increased rolling resistance enough that they even created a new fast-rolling Albert tread pattern to offset the extra drag from more contact with the ground. Schwalbe didn’t change the tire shapes at all, just swapped the angle of the internal fabric casing construction to ~75° (15° off from completely radial), instead of traditional 45°. The Schwalbe tires are really ‘more radial’ than they were before, but not quite 90°.

Now, the Zleen Racerunner Radical is much more radial. It’s still not 100% at 90°, but it is really close. Zleen makes the Racerunner Radical with an 88° casing structure, just 2° off from being completely radial. It is also wildly lower profile – not just the tread, but the entire tire cross-section.

Squared off and low-profile construction

29×2.25″ Zleen Racerunner Radical (black, front) vs. Leopard Pro (gray, rear)

The radial construction is, of course unique in itself. And it results in a very squared-off profile for the uninflated tire, compared to a standard bias-ply tire.

But it also allows Zleen to make the Racerunner Radical tire about 1/3 shorter than a conventional 2.25″ XC tire. The rationale here seems to be that since the radial tire casing is more supple and deflects more locally, you will run higher pressures for the same amount of grip. That means you worry less about pinch flats, and sidewall cuts. But also that the radial tire performance seems to benefit from reduced overall air volume, too.

The end result though, is that while a regular 57mm wide XC tire might be about 50mm tall above the rim, this one will be only about 35mm tall. That’s a pretty big difference that we’re curious to try out on the trail.

A third less sidewall height also means less material in the tire, which unsurprisingly also makes this tire quite light, at a claimed 550g. The new tire is in fact more than 15% lighter than their comparable Leopard Pro XC race tire in the same 29×2.25″ size. Another unintended consequence is also that a standard 29×2.25″ tire would have a circumference of 2.3m, but this one would be 2.2m. That means the distance travelled with each tire revolution would be 10 centimeters or 4% less, effectively like slightly reducing the gearing of your XC race bike by a chainring tooth or two.

Radical Radial performance claims vs. a bias-ply XC tire

• 30% larger contact patch area
• run at 70% higher tire pressure, with the same grip
• 40% lower rolling resistance
• 25% weight reduction, from the lower profile

Tech details

• lightweight 120tpi radial XC mountain bike tire
• Radial Belt 88° radial casing
• tubeless-ready with a folding Aramid bead
• bead-to-bead fabric puncture and cut protection
• soft 47 ShA XC race rubber compound
• 29″ x 2.25″ (57-622) size only
• 550g claimed weight

Zleen Racerunner Radical radial – Pricing, options & availability

The new 29er-only Zleen Racerunner Radical cross-country tire comes in just one 2.25″ width for now. The all-new radial XC mountain bike tire retails for approximately 91€ (depending on the market), which makes it about 75% more expensive than the rest of their Pro- & Race-level bi cat-named MTB tires. But it also looks like a huge step up in performance if all (or even just some) of their dramatic performance claims are to be believed.

As for availability, that’s still a bit up in the air. Officially, Zleen has said that the tire would be available for sale this month, both via their international distributors and direct from their e-shop in Europe. But so far, they haven’t popped up in stock yet. We’ve reached out to Zleen for an update, but haven’t heard back.

In any case, we look forward to testing out the new tire as early as the start of next year to see how those radical radial claims translate to riding real-world cross-country mountain bike trails.

Zleen.online

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