Bernard Kerr puts the Mach 5.5 to the test.
Bernard Kerr puts the Mach 5.5 to the test.

The Case for Size-Specific Mountain Bikes

Pivot's new Mach 5.5 isn't built for women—but it fits them. And that's the Holy Grail of women's-specific design.

Bernard Kerr puts the Mach 5.5 to the test.

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Every woman’s challenge is finding gear that fits. That’s the point of women-specific gear design: We don’t necessarily need gear that performs differently than the men’s stuff. We just need it to fit us—and gear that’s built for a size medium or large man often doesn’t.

Chris Cocalis gets that. He started building custom bikes in 1988 and, after looking at the body measurements of more than 5,000 clients, started to notice some real trends—not between men and women, but among sizes. Short people (from 4'11″ to 5'3″) tend to have shorter femur lengths, so they need a steeper seat tube angle to push them forward. That way, their knees track properly over the pedals and they feel more balanced when cornering. Meanwhile, the extra-large set needs their own size-specific parameters for their bikes to perform as well as the ones that Cocalis builds for his 5'11″ frame.

So, around 1990, when Cocalis founded Titus and started making bikes in stock sizes. He engineered each size with its own unique geometry and even materials. Cocalis continues the practice with Pivot, his current bike company. His new Mach 5.5 Carbon trail bike comes in extra-small to extra-large sizes, and each size in the range is optimized to that rider’s body type.

(Courtesy of Pivot Cycles)

The XS Mach 5.5 that Pivot sent me for testing uses the same head tube angle (a fairly slack 66.5 degrees) as the larger sizes. But it uses a completely different carbon layup in the rear triangle than the medium through extra-large versions. Its tubes also use a modified layup and have a smaller diameter.

So, no, the extra-small (a size that’s ridden almost exclusively by women) is not as stiff as it could be. But “it’s not a dumbing-down at all,” says Cocalis, explaining that a smaller triangle is naturally stiffer than a bigger one—and that the same stiffness that feels great to a six-foot, 180-pound rider feels overbuilt to someone who’s 5'2″ and 110 pounds. “The bike doesn’t corner as well, and there’s too much hack because the shock and the carbon frame can’t absorb all the small, high-frequency bumps,” he says. When you take a large bike and simply shrink it—keeping the geometry and materials the same—the bike “rides like shit,” says Cocalis.

Of course, the best riders can adapt to anything. But even pros enjoy an extra boost of confidence and capability when they’re on a bike that truly fits them. At 5'2″, cross-country racer Chloe Woodruff had trouble finding an optimal bike fit among her former sponsors, but once she joined the Pivot team and started riding an extra-small Mach 4, her descending ability significantly improved, and she’s since notched her best-ever World Cup finishes.

I’m no pro, just an avid mountain biker who rides about five days a week and plans vacations around great singletrack. But when I got on the Mach 5.5, I immediately felt at home. Most bikes (even really great ones) take some getting used to. I need a few hours or days to dial in the suspension and figure out where I have to put myself in the cockpit to feel balanced while climbing, cornering, and descending.

But the Mach 5.5 felt optimal from the get-go. There was nothing to compensate for, nothing to figure out. Yeah, I’ve fiddled a bit with the shock, switching between the open and the middle settings while climbing to explore what felt better. Everything else just felt right, like a great-fitting pair of jeans—if jeans made me want to pound my chest like an overjoyed gorilla.

The Mach 5.5 Carbon promises to be a do-everything trail bike, with 140 millimeters of rear suspension and 160 up front. Its 27.5-inch wheels come with tires that are 2.6 inches wide (an emerging size that’s not quite “plus” yet fatter than the traditional 2.1- to 2.35-inch standard). And it’s light: The top-notch Team build that I tested for a few weeks on my home turf around Steamboat Springs, Colorado, weighs less than 27 pounds (and costs $8,299).

Given the slack geometry and brawl-ready Fox 36 Factory fork, I wasn’t surprised that the 5.5 is a gas on the downhills. But this isn’t a big-bump obliterator. It doesn’t smooth away drops and ledges, Cadillac-style. Instead, the bike feels sprightly and agile and sticks my intended line. Wherever I point, I go, and that holds true for big bumps and small. On straightaways littered with brake divots and fist-sized rocks, the 5.5 tracks reliably, which gives me the confidence to ease off the brakes and up the speed. In fact, rolling over fast, little bumps is where the 5.5 feels the most buttery.

That precise handling made it a blast on one particularly rough and awkward user-built trail that’s peppered with silly-tight switchbacks and eroded drop-offs. Hitting such features in rapid succession, the 5.5 felt nimble and responsive, with a light rear end that I could easily flick around the hairpins. That maneuverability made it a lot easier, mentally, for me to tackle sketchy trail with significant exposure and high consequences for misdirection—because there was no misdirection. The bike seemed to obey my every wish.

It even has a few wishes of its own. The 5.5 loves to hop, and it urged me to take a more playful approach to familiar terrain. Why simply roll over that water bar when I can launch it?

So, yeah, it’s a hoot on the descents. But the big win is how well the 5.5 climbs. Midrange effort is all it takes to coax it up steep, loose trail. It cruises across rocky streambeds and snaps smartly up rock ledges. That’s partly the result of the extra traction you get from wider 2.6-inch tires (credit also goes to the Reynolds carbon wheelset with Industry Nine hubs). But it’s also due to the DW-link suspension, which seems to translate 100 percent of my effort into forward energy.

What impressed me most, though, is that the extra-small size feels tailored to my height and weight. Such modifications aren’t cheap. Each size mold costs $75,000 to $150,000 to develop, and changing the geometry of each size in the range adds significant manufacturing costs. “If you hold all seat tube angles constant, it would simplify everything and save a lot of money,” says Cocalis. But he wants his bikes to feel like magic to every rider, not just the 5'11″ guys.

“That’s what gets me so excited,” says Cocalis. “When you put someone on the bike that really fits them, it makes them a better rider, a more confident rider. It’s about having more fun.”

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