Willow Koerber (Subaru-Trek) is now racing exclusively on a 29er and getting great results on it, too, despite her diminutive size (James Huang)

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That Willow Koerber is the best American cross-country mountain bike racer right now is no surprise – she won bronze at the 2010 World Championships despite crashing twice, and came second overall in this year's World Cup. What is surprising is that she's done all of this on a hardtail 29er – despite standing just 5ft 2in (1.57m) tall.

Koerber's not the only one, either, as Subaru-Trek team-mate Heather Irmiger is also a 29in-wheel aficionado, with both riders citing the larger format's faster roll on rough terrain, improved cornering prowess and enhanced stability relative to industry-standard 26in hoops.

"My bike feels like a natural fit to me and I'm always amazed at the flow I feel when I am riding the 29er," Koerber told us. "I had zero problems getting my fit dialled." Sizing may have been problem-free but is still not exactly standard, especially with the updated version of Koerber's Superfly Elite. Trek say the new chassis is 150g lighter and 15 percent more rigid thanks to its direct-fit bottom bracket bearings, larger tube profiles and tapered front end. Those aren't the only changes though.

The intended fork travel has gone up to 100mm, adding 20mm to the stack from last year's Superfly. Compounding things further is the carbon fibre steerer tube on RockShox's early-production Reba XX 29in, which uses a gradual taper that's great for distributing impact load but doesn't play well with Koerber's extra-short 103mm head tube.

Luckily for Koerber, team mechanic Matt Opperman and the rest of the Trek technical support crew have managed to get around the issues, adding a small spacer beneath the crown race to move the taper further down in the head tube, dropping the fork travel (and thus, crown-to-axle height) by 10mm, swapping to a -12° stem, and trading in last year's riser bar for a flat one.

A -12° stem, flat bar and reduced fork travel are required to lower the front end

Opperman says RockShox are revising the fork's steerer shape, too, so eventually he'll be able to add the travel back in or drop the bar height even lower depending on Koerber's preferences.

The build kit is relatively off-the-shelf, including a nearly complete SRAM XX group. Opperman has to use standard all-steel G3 rotors on the US domestic race bikes since he doesn't have enough XX rotors to go around, and a custom-fit SRAM Force front derailleur stands in while the direct-mount XX unit is finalised.

Bontrager Race X Lite wheels, tyres and cockpit components, CrankBrothers Eggbeater 11 pedals, a Fizik Vitesse k:ium saddle, Nokon cables and housing, and ESI Racer's Edge silicone foam rubber grips round out the build.

Koerber runs her Fizik Vitesse saddle far forward on the carbon wrapped Bontrager Race X Lite ACC seatpost

Total weight is 9.62kg (21.21lb) – undoubtedly race worthy but there are plenty of 26in race bikes on the circuit which undercut it by more than a kilogram. And in terms of rotating mass, Koerber's 1,692g wheel weight doesn't help, either.

Opperman says that Koerber's new World Cup bike, debuted at the final round in upstate New York, now closes that gap at just 8.9kg (19.7lb) thanks to a 1,390g wheelset built with Stan's NoTubes 29in rims, a made-by-DT Swiss rear hub and thinner Revolution spokes, and Trek's new FCC front hub. In addition, Opperman has included a more comprehensive complement of go-fast goodies such as more titanium hardware, ceramic bearings and "a few other bits to lighten it". 

Complete bike specifications:

• Frame: Trek Superfly Elite, size 15.5in
• Fork: RockShox Reba XX 29in, 90mm travel with custom crown race spacer
• Headset: Cane Creek
• Stem: Bontrager Race X Lite, 80mm x -12°
• Handlebars: Bontrager Race X Lite, 630mm
• Tape/grips: ESI Racer's Edge
• Front brake: Avid XX w/ 160mm G3 rotor
• Rear brake: Avid XX w/ 140mm G3 rotor
• Brake levers: Avid XX BlackBox
• Front derailleur: SRAM Force w/ custom adapter and cable pull hardware
• Rear derailleur: SRAM XX
• Shift levers: SRAM XX
• Cassette: SRAM XX, 11-36T
• Chain: SRAM PC-1090R
• Crankset: Truvativ XX, 39/36T, 170mm length, 156mm Q
• Bottom bracket: Enduro integrated
• Pedals: CrankBrothers Eggbeater 11
• Wheelset: Bontrager Race X Lite TLR Disc CL 29
• Front tyre: Bontrager 29-3 TLR Team Issue, 29x2.0in
• Rear tyre: Bontrager 29-3 TLR Team Issue, 29x2.0in
• Saddle: Fizik Vitesse k:ium
• Seatpost: Bontrager Race X Lite ACC, 5mm offset
• Bottle cages: Bontrager Race Lite
• Other accessories: Nokon derailleur cables and housing

Critical measurements:

• Rider's height: 1.57m (5ft 2in)
• Rider's weight: 48.5kg (107lb)
• Saddle height, from BB (c-t): 631mm
• Saddle setback: 12mm
• Seat tube length, c-t: 394mm
• Seat tube length, c-c: 323mm
• Tip of saddle nose to centre of bars (next to stem): 490mm
• Saddle-to-bar drop (vertical): +54mm
• Head tube length: 103mm
• Top tube length: 570mm (horizontal)
• Total bicycle weight: 9.62kg (21.21lb)

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