Belt drives and 140 mm cranks: a look at Hattie Harnden's Gamux DH bike

World Cup downhill pits are a showcase for bleeding-edge mountain bike technology. But even among the prototypes and custom layups, Hattie Harnden's Gamux race bike stands out.

It’s not just one component, but a whole philosophy that challenges convention, from the Gates belt drive replacing a traditional chain to some of the shortest cranks you’ll ever see on a gravity bike.

Thanks to a detailed walkthrough with Gamux founder and mechanic Pascal Tinner, we can dig into the 'why' behind this unique build. It all comes back to a single idea: creating a totally integrated system to free up the rider's mind.

A Belt, Not a Chain

The most obvious departure from the norm is the Gates Carbon Drive. Belt drives are common on commuters, prized for their clean, low-maintenance operation. On a DH bike subjected to massive impacts, flying rocks, and brutal power spikes, it seems like a massive gamble.

According to Tinner, however, it’s a calculated one. "It's pretty consistent," he explained, adding that after extensive collaboration with Gates on custom sprockets and belts, "now it's super reliable."

The goal is predictability. A belt doesn’t stretch like a chain, its engagement is consistent, and it’s impervious to the mud and grime that can wreak havoc on a derailleur system. For a racer, that consistency translates directly to confidence when putting the power down out of a corner.

The Holistic Concept

This pursuit of consistency is the thread that ties the whole bike together. Tinner repeatedly refers to the build as a "holistic system that needs to work." The team's goal, he says, is to "make them forget about kind of the physical abuse... so have more capacity in the brain to focus on riding fast at the end of the day."

It’s a refreshing take. So often, the focus is purely on the hardware – saving grams here, adding stiffness there. Gamux's approach is to tune the machine to the point where the rider's mental energy is spent entirely on reading the track, not managing the bike. Every component choice flows from this principle.

Short Cranks and Dynamic Geometry

Nowhere is that philosophy more apparent than in the rider cockpit and fit. Harnden is running custom 140 mm cranks from North Shore Billet (NSB). Most enduro riders are on 165 mm or 170 mm cranks; seeing 140s on a DH bike is genuinely radical. As the presenter noted, "they're some of the shortest cranks I've ever seen."

Tinner confirms that "crank length is a big topic that fits into that with frame sizing together." The shorter length provides massive ground clearance, reducing the risk of pedal strikes in rocky, rutted terrain. It also influences body position and pedalling dynamics, all part of that holistic system.

The frame itself is a high-pivot design with a reach of 477 mm and a static chainstay length of 445 mm. But static numbers don't tell the whole story. Tinner points out the bike has 10 to 12 mm of rearward axle path as the suspension compresses, meaning its effective chainstay length feels more like 450-452 mm on a bike without a high-pivot – a crucial detail for on-track behaviour.

The Finer Details

The thoughtful spec continues at the front end. The Ergotec handlebar has a high 50 mm rise and a moderate 775 mm width, but the standout figure is its 12 degrees of backsweep. That's significantly more than the typical 7-9 degrees and represents a very specific ergonomic choice.

The wheels are Reynolds Blacklabel 307 carbon, and their durability is a key advantage. After a season of use from March onwards, a team representative noted the only maintenance required was replacing "just a couple of bearings." In a world of disposable race gear, that’s a ringing endorsement.

For this particular "super wet race," the wheels were shod with Schwalbe Dirty Dan Gravity Pro tyres, running at 24 PSI in the front and 27 PSI in the rear.

This Gamux isn't just a collection of expensive parts; it’s a cohesive, purpose-built racing machine. The belt drive, the ultra-short cranks, and the specific geometry are all evidence of a team that isn't afraid to deviate from the norm in pursuit of a faster, more confident rider.

Hattie Harnden's Gamux DH Build

• Frame: Gamux, mixed aluminium and carbon with interchangeable stiffness inserts
• Fork: Custom Manitou Dorado
• Shock: Manitou
• Drivetrain: Gates Carbon Drive belt system
• Cranks: NSB, 140 mm
• Pedals: Crankbrothers
• Wheels: Reynolds Blacklabel 307 carbon
• Tyres: Schwalbe Dirty Dan Gravity Pro
• Handlebar: Ergotec (50 mm rise, ~775 mm width, 12° backsweep)