Ride a high-pivot swingarm DH or AM bike like the Orange and you'll discover that the rearward axle path and strike angle that the configuration produces dominates over the rocks and roots. Unfortunately, chain-torque issues caused by the conventional chain-line of a derailleur bike defeat any advantages of the high-pivot suspension design. Chain growth is a huge negative factor with conventional high-pivot design - to the point that most bike makers have long abandoned the concept. Zerode, however, has taken a different, more successful tack on the matter. By placing a gearbox high in the frame, aligned with the swingarm's pivot, the Zerode G-1 circumvents the chain-growth problem entirely. In fact, the Zerode gearbox design only requires a minimal chain take-up device on the rear and a single guide mounted near the upper side of the gearbox drive sprocket to manage a whopping, 9.25 inches of travel.
Zerode G-1 Frame Details:
-Travel: 9.25 inches
-Gearbox:8-speed modified Shimano Alfine hub
-Frame: Aluminum, 1.5-inch head tube, high-pivot swingarm with a primary and secondary chain drive
-Near-Concentric chain line eliminates suspension bob and produces active suspension
-Frame supplied with Fox DHX RC4 shock, shifter, gearbox, chain tensioner, rear sprocket and spacer kit, rear axle
-Price: $3495 USD
While the Zerode G-1 is known by many riders. I was intrigued by the well-executed design and thought I'd give the New Zealand DH design a closer look. The frame is beefed up where it needs to be, with a pair of CNC-machined spars holding up the gearbox and drive bits. The shock is tucked low in the frame and driven by a large aluminum rocker that doubles as a stabilizer for the chainstay suspension link. In effect, the G-1 is a single pivot suspension turned up-side down. The gearbox is then conveniently located in-line with the high-pivot swingarm so that a lot of suspension movement creates only a tiny bit of chain slack - which is easily taken up by a derailleur-style jockey pulley arrangement near the rear sprocket.
Zerode takes an eight-speed Shimano Alfine planetary gear hub and converts it to a gearbox with the addition of two torque plates that incorporate chain adjusters and a sweet looking drive-sprocket that bolts onto the hub's spoke flange. It looks pretty sexy.
Zerode says that the weight of the modified Shimano Alfine hub and its trigger shifter is nearly the same as the parts that it replaces - roller chain guide, rear derailleur, and cassette, but that probably refers to a heavy low-end drivetrain setup. I'd bet that the Zerode gearbox arrangement gains some weight where it supports the Alfine hub and extra drive gears that isn't calculated into the equation. Still, a look around the Zerode G-1 indicates that its designers have been around the block. It's well executed. The Zerode G-1 comes in small and large sizes and its geometry and purchase options can be found on the Zerode website.
A look under the Zerode G-1 shows that chainstay link of the swingarm pulls on a beefy rocker link as the suspension compresses, which then drives the shock. Two machined-aluminum plates surround the moving bits of the G-1 - a sturdy configuration that keeps much of the frame's weight centrally located.
The Zerode website claims that the rearward axle path that the G-1 swingarm scribes is the natural angle that the rear wheel is deflected when hitting a square-edged or substantial-sized bump. This plays out in the real world, as similar suspension designs have simply owned the rocks on many of the world's toughest courses. Science also says that the in-line chain path of the Zerode will negate the negative chain torque issues that plague the high-pivot genre. In addition, by transferring the weight of the cassette and rear derailleur to the center of the bike and off of the wheel, the Zerode reduces unsprung weight in the suspension, making it quicker to respond to terrain. So far, so good.
A 34-tooth crankset drives the hub-gearbox and then a separate chain runs from the gearbox to a single rear drive sprocket. The transmission has the spread of a conventional 11 x 34 cassette. Photo on right courtesy of Zerode
Two items, however, remain questionable. No high-pivot suspension that I have ridden has been neutral in braking. In fact braking has a tendency to lock up the suspension with minimal pressure on the rear brake lever. Also, 9.25 inches of rear wheel travel is almost unheard of on the Pro DH racing circuit these days, mainly because riders want low bottom bracket heights for cornering and thus must be willing to sacrifice suspension travel to achieve this goal. It would be interesting to ride a Zerode to see if the rough-ground benefits of the suspension outweigh the slightly higher bottom bracket necessary for its extra wheel travel. When that happens, expect a full report- RC