Trek’s unique IsoSpeed road bike and cyclocross comfort tech has been translated to full-on mountain bike use with the Procaliber cross-country machine. We've previously spent a little time on the top-line 9.9 and mid-range 9.8 models, but recently got in the humbler 9.7 version to give it a thorough interrogation as to whether IsoSpeed is a winner on the trail.
The IsoSpeed coupling replaces the normal fixed joint between top tube and seat tube with a pivot. Add a flattened lower seat tube section, and the top of the seat tube is able to move independently of the rest of the frame if the bike hits a bump.
‘Boost’ rear hub spacing (148x12mm) for extra tracking stiffness, neat cable/hose management and two bottle cages complete a slick and contemporary frame. The smallest size (15.5in) runs on 650b wheels while the other four use 29in wheels to keep size and shape proportional.
The IsoSpeed coupling allows the seat tube to move independently of the rest of the frame
The Boost rear end is matched by a 110x15mm Reba RL Boost fork and RockShox’s new OneLoc remote lockout lever. While the asymmetric frame is ready for a twin-ring crankset and front shifter, the 9.7 SL gets a 1x11 SRAM GX setup. The Shimano Deore brakes are an awkward fit with the shifters but work reliably.
The rest of the kit is from Trek’s Bontrager sub- brand, including a narrow bar/ long stem XC cockpit and a surprisingly chunky post and saddle for a racer. Bonty also supplies the narrow Mustang Elite wheels, wrapped in minimal-tread XR1 Expert rubber.
The question everyone asked when we were riding the Trek was “does it [IsoSpeed] work?”. The answer is a definite yes, though it only has an effect when seated. Whether you’re riding the bike or watching someone else ride it, you can see the seat tube, post and saddle flexing back and forth in relation to the top tube. You can feel this flex taking the edge off smaller bumps and rocky trail chatter too.
While IsoSpeed adds some tangible softness, the Procaliber is still a rapid beast
You can also notice the seat bouncing underneath you in time with your pedal stroke as soon as revs drop and torque increases up steep climbs or through heavy mud. In some circumstances this pulsing feel seems to help you gain traction and sustain speed, but in others the bounce made us wish we had a rear lockout.
Despite the chunky chainstays channelling your wattage to the rear wheel and a rock-solid front end adding undiluted shoulder power into the mix, there’s a definite softness at full gas compared with a conventional hardtail. The smooth ride also means its inability to deal with mid-size hits can catch you out at first, leaving you stumbling over steps, rocks and roots.
Although they’re marked as 2.2in, the tyres are only 1.9in in width, with a harsh ride that undermines the smoothness potential of the frame. Swap to fatter tyres and the added composure combines with the precise ‘Boosted’ handling and a complete bike weight of just over 11kg to create a surprisingly confident and capable ride that’s still rapid to accelerate and flies up climbs with minimum effort.