Diet by design

DN Staff

January 17, 2000

8 Min Read
Diet by design

Bethel, CT-Anyone who pedals a mountain bike up a steep trail knows that lighter is better, just as anyone who rides back down again at breakneck speed knows that stiffer is better too. When the engineers at bicycle manufacturer Cannondale set out to balance these oft-conflicting design goals two years ago, they came up with the Raven, a full-suspension mountain bike whose frame combined a lightweight aluminum spine with carbon-reinforced epoxy skins (see Design News, 3/02/98). Now, the Raven appears in a new version that takes weight reduction further without sacrificing the stiffness and strength that bikers need when the going gets tough. "We're always striving to make things stiffer with less material," says Tod Patterson, a design engineer who worked on the new Raven.

In a redesign so sweeping that the old and new Ravens have only a set of rivets in common, Cannondale's engineers removed more than a few grams of weight by overhauling the components that attach to mounting points on the frame's metal spine. A new lightweight sealed cartridge bearing saved 30g, new seat post clamps with extra cut-outs removed 28g, and smaller-diameter derailleur cable housings shaved another 17g. Still bigger savings, a half pound's worth, came from a new 1.9-lb swingarm, the component connecting the frame and rear hub. Made from thinner tubing than earlier models, it features an asymmetrical design in which the left side runs straight back to the hub while the right side curves to provide clearance for the chain.

But the new Raven didn't really begin to lighten up until Cannondale's engineers revamped two of the bike's most fundamental structural components: the front suspension fork and the frame itself.

Not really a fork. Actually, calling the new Raven's front suspension system a fork is no longer accurate. Instead, the bike features a radical new front suspension system that replaces the familiar two-sided fork with a single telescoping leg that weighs only 3.7 lbs, or two pounds less than what it replaces. Known as the "HeadShok Lefty," this air-sprung, oil-dampened shock provides the same 100 mm of travel found on the company's earlier top-of-the-line front suspension forks.

The Lefty also offers all the same torsional and side-load rigidity as Cannondale's premium two-sided forks, according to R&D engineer Todd Sorzano. "We didn't lose anything by going single sided--except weight," Sorzano says. In fact, the one-sided design appears to have gained strength. Sorzano points to independent tests showing that the Lefty has up to three-times the resistance to torsional deflection of other leading forks and up to 62% better resistance to side loads (see charts). Tests performed by Cannondale itself show that the Lefty's oversized single-leg design "consistently exhibits three-times the fatigue life of other top forks," Sorzano reports.

Despite its radical appearance, the Lefty borrows some technology from earlier HeadShok forks. For one thing, the Lefty and its two-sided predecessor both employ a patented "square peg in a square hole" construction rather than round tubes and bush- ings. Like the earlier HeadShok, the inside of the Lefty's upper tube (or stanchion) has four flat surfaces that correspond to four flat surfaces on the outside of the inner tube (or slider). These flats transmit the steering loads with the aid of 88 horizontal needle bearings that roll between the flats, 22 per side. According to Sorzano, the flat surfaces keep the slider from twisting within the stanchion. "A round tube within a round tube does nothing for the torsional rigidity," he notes. And the needle bearings reduce "stiction," or static friction, for a shock that compresses smoothly, Sorzano adds.

Unlike earlier models, the telescoping mechanism, with all its needle bearings, now resides entirely within the suspension arm itself. Earlier two-sided forks kept the suspension in the "head tube" that's centered under the handlebars. "But there was really no reason to center it," Sorzano says, adding that the offset telescoping arm may permit a future increase in the Lefty's travel distance without an undesirable increase in head-tube dimensions.

Going to a single-sided suspension system did require some other changes to the suspension system: The Lefty works only with disc brakes. And to support the wheel without the benefit of a second fork leg, Cannondale replaced a traditional axle with a new tapered spindle made from a 4130 chrome alloy. Sorzano explains that a standard axle "just wasn't good enough" when it came to providing enough torsional rigidity for single-sided support. The new spindle--whose strength Sorzano puts at ten times greater than a traditional axle--tapers from 25 mm on the side closest to the telescoping arm down to 17 mm near the wheel hub. The hub itself features sealed cartridge bearings that Cannondale also redesigned to accommodate the spindle. For easy wheel removal, the hub employs a self-extracting bolt that draws the wheel off of the axle as the bolt loosens.

Stiffer and lighter. Cannondale saved even more weight by upgrading the bike's frame materials. Like the original Raven, the new bike's frame consists of a metal spine for absorbing vertical loads and composite skins, which mount in channels in the spine's lateral faces and absorb the torsional loads from pedaling. But the cast aluminum spine on the original bike has given way to a lighter magnesium spine, while the original's thermoset epoxy skins have been replaced by thinner carbon-reinforced nylon composite. Together, the more advanced materials contributed to a half pound weight reduction for a total weight of 2.5 lb for the skin and frame. Add all the frame-related hardware and the revamped frame weight comes to just 5.3 lbs1.3 lbs less than the original frame and roughly twice the weight savings that Cannondale saw with the introduction of the first Raven.

Though lighter, the new Raven frame didn't lose any stiffness. "We maintained the same stiffness for less weight," Patterson says. While he declines to reveal Cannondale's specific torsional strength targets, he does note that the nylon composite provides the same stiffness as the thermoset composite. It does so, however, with a new wall geometry that features internal concave recesses and averages 0.010 inch thinner. "That allowed us to remove a bit of material from the frame," he says. Impact strength--or what might be termed "rock resistance" in the bike business--improved by 10% with the switch to the thermoplastic composite, according to Patterson.

To simulate the worst-case loads the bikes might encounter in the field, Mountain Bike Germany magazine measured the amount of torsional force required to rotate the Lefty and competitive forks one degree. Similar tests assess the amount of side-load force required to bend the Lefty and the competitors one inch.

The new frame also enabled a cost reduction, which Patterson estimates at roughly 20%. It springs in part from the material saved by thinning the skin walls and in part from increased manufacturing throughput. "The thermoset frame had to cool for an hour," he explains. "The thermoplastic part only has to cool for a few minutes." So significant is the cost reduction that it offsets the extra manufacturing steps--a primer and a nylon-based powder coating--needed to protect the magnesium spine from corrosion.

The Raven's weight and cost reductions prove that good things do come to those who wait. According to Patterson, the molds for the nylon-based parts took far longer to make than those for the thermoset skins. In fact, Cannondale engineers originally considered the thermoset version a prototype. Yet it was such a successful prototype that the company went into production with it right away, rather than wait to tool up for thermoplastics.

Riding the Raven

What goes up tends to come down. At least, that's been my experience on mountain bikes. I get up and ride. I hit a rock, downed tree, or other natural obstacle. I fall down. My sad efforts to win the good fight against gravity would clearly be aided by better biking skills. A better-developed sense of self-preservation wouldn't hurt either. Or, as a recent test ride on the new Raven showed me, I could just get a better bike.

The new Raven has one hard-to-miss feature that should appeal to any rider wanting to smooth out a rough ride. Its one-sided front suspension system, called the Lefty, really delivers long travel without sacrificing steering responsiveness. In my case, while riding the Raven in the pouring rain in upstate New York, the Lefty's 100 mm of travel helped me roll and jump my way over rocks and ruts that would have surely resulted in my downfall otherwise. Sadly, the Lefty does nothing for wet leaves on slick rock, but that's my problem.

When it came to maneuvering the bike, the Raven's steering mechanism felt smooth and responsive. And contrary to my initial skepticism about the single-sided support of the front wheel, the Raven had a rigid feel to it even during speedy descents chock full of nature's speed bumps.

Now, it's true that my idea of a speedy descent might not be enough to really put the Raven through its paces. But what's overkill for me is good enough for the pros: Brian Lopes of the Volvo/Cannondale team has ridden a Lefty-equipped bike to the top spots in both the World Cup Slalom Series and the National Off-Road Bicycle Association (NORBA) Dual Slalom Series.

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