Trek mountain bikes have improved dramatically over the past few years. Bikes like the Remedy, Fuel and Slash have been introduced to compete well in their categories. The Bontrager tires have lagged in this revamp a bit and some might say that they have held back the bikes from their full abilities.

Trek knew of this and they have installed members of their A-Team to work on the Bontrager tires and components. The goal was not only to bring the bikes to their full potential but also to make some of the best tires available for the aftermarket. Michael Browne, formerly on the mountain bike group, called to say that he was genuinely excited about this year's tire line. He said the XR1 and the XR4 tires have been revamped and invited us to push them on some dirt corners.



The revamp started by recruiting one of the premier experts in the field, Frank Stacy. Stacy was a pro motocross racer for over 20 years and he moved on to tire development after that. He has worked for companies like Dunlop, Pirelli, Specialized and Kenda developing tires in the moto, utility and bicycle areas. A good article on his career can be found here from Motorcross Action. Stacy is a student of the tire development field and he focuses on lab testing, field testing and really identifying and quantifying the elements like traction and rolling resistance. His own site here called Stacy Testing gives you a peek at his methodology.

Ok, enough science. The UPS man arrived with a bunch of XR4 tires and we were rightfully excited. The tread pattern is much different from the previous generation as it is more modern with a simple, blocky and more open style. The casing is supple and tires mounted true. The Team Issue tires are light and are dual-compound. The center knobs felt normal but the side knobs were gummy and pinching them with your nail resulted in a scar that last many seconds as the tire slowly morphed back into its original shape. All the knobs had some siping or cuts to give them extra traction.

The 26er tires are called XR-4 and the 29er tires are 29-4. The '4' refers to their trail/traction abilities on a 0-4 line with the XR0 being an ultra lightweight xc race tire and the XR4 being an aggressive all mountain tire. There is also an XR-Mud for wet and muddy excursions.

This graphic from Bontrager explains the tire line-up very well.



The XR4 tire size line-up is as follows.
Bontrager XR4TPIMSRP*BeadTPIWeight
29 x 2.30 (Team Issue)12069.99Aramid120794g
29 x 2.30 (Expert)6049.99Aramid60
26 x 2.20 (Team Issue)12064.99Aramid120
26 x 2.20 (Team Issue TLR)12064.99Aramid120
26 x 2.35 (Team Issue)12064.99Aramid120730g
26 x 2.35 (Team Issue TLR)12064.99Aramid120760g
26 x 2.20 (Expert)6049.99Aramid60
26 x 2.35 (Expert)6049.99Aramid60770g
26 x 2.206034.99Wire60
26 x 2.356034.99Wire60
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We mounted the tires to a few bikes, a Santa Cruz Blur TRc, an Ellsworth Evolution 29er and a BMC FS01 and have been riding them for the last month. The tires had good size and shape to them as they lived up to their 2.2 and 2.30 inch claims. We rode them in the loamy singletrack of Santa Cruz, loose fire roads of Los Gatos and wet trails of Waterdog and Skeggs in Woodside.



Cornering

This where it's won and lost as cornering is often the defining line between having fun and hitting the ground. The Bontrager XR4 corners like a champ. It is consistent, predictable and it grabs a hold of the trail with authority. We ride a lot of loamy, bermed singletrack so we actually had to make the effort to ride loose terrain for the sake of tire testing. For hero dirt singletrack, these tires grab with more to spare. In fact, we actually preferred putting the more XC XR1 tire in the rear just so we could slide the rear a bit and throw the bike around more.

On loose fire road, these tires hooked up. We used the Team Issue as the gummy side knobs were really able to find side traction where it was scarce. We believe that the supple casing also helped this tire hook up in the rocky, loose trails of the Kennedy trail in Los Gatos, CA. The siping or cuts on the knobs of the tire seemed to help too as this tire was hooked up as we pushed it a little on some off camber turns.

Wet and Mud abilities

Again, this tire did very well. The open block pattern of the tire allowed it shed mud pretty well. Wet rocks and roots are pretty daunting but this tire with the soft casing and soft side knobs kept us upright.

Rolling Resistance.

Again, it seemed to roll pretty well. It's not a race tire for sure but it never felt like a slow tire like the old Nevegal. For fast Strava runs, we put the Bontrager XR1 in the rear for speed and the XR4 in the front for traction. Braking and climbing traction was excellent in all conditions for us.

Durability and Tubeless.

The tires came through with flying colors for us but we don't have a ton of sharp rocks here in Norcal. For tubeless, Michael Browne sent us some TLR (tubeless ready) versions and we pumped those up tubeless with no issues whatsover. We put some Stans fluid in and the setup never leaked.

Conclusion

We're pretty darn impressed so far. The tires will definitely bring the best out of any Trek bike and we believe this can be a great component choice for any bike brand. If you have an All Mountain ultimate bike build, definitely take a look at these. If you want a balance of speed and grip, put these in the front and put the XR1s in the rear. 29er or 26er, these tires are a good choice.

We'll keep riding them throughout the season and update this review.