Flex. Stay.

Somewhere between the Moots YBB and the 2016-ish Kona Hei Hei, I assumed that flex-stay bikes were dead for good. To put a more specific timestamp on that potentially 30-year wide window of time, I’d have put the dead-in-the-ground-shoveling-dirt-on-it time of the funeral at somewhere around 2011.

I had speculated about the demise of the venerable design before, though. I had watched with interest as DeKerf and Ibis made their steel and ti soft-tail variations of the YBB, and then gaped in horror as the odd aluminum version began cresting the horizon, wondering if anyone at all was wondering about the suitability of aluminum as a spring. I laid betting lines with fellow mechanics on what the warranty departments looked like at some of these places, but still they would trickle into the shop, year on year.

Nothing is new, it is said. We look to nature for inspiration, we shuffle through the old parchments of centuries-dead inventors, and we apply the new technology to the known physics and we try again. The flex-stay mountain bike is no different. There’s a Bianchi folding military bike from sometime back at the dawn of the 20th century (1912, according to the googles) that might as well be the direct inspiration for Moots’ legendary YBB. They were by no means the first. Before that, a patent filed in 1899 by Charlie Travis shows a flex plate at the bottom bracket and an air shock keeping things sprung. Just a few years before that, in 1896, there was the Hygenic. Hell, even though it was introduced ninety years later, the YBB has been trucking along since 1986. In mountain bike parlance, that is close to prehistoric.

Admittedly, bicycle suspension technology wasn’t really anything to write home about until the 21st century came knocking, so a soft-tail derived from a 90-year old Bianchi wasn’t necessarily a bad idea. The design added an element of comfort and tractability without a ton of additional weight or complexity, travel numbers were sub-inch so the actual amount of flexing was negligible, something steel and titanium can do pretty well for hundreds of thousands of cycles so long as the bending ask isn’t too high.

Aluminum, well, that was a different matter. Didn’t stop Ibis from tilting at that windmill, with the original Ripley in 2001; an aluminum soft-tail with a whopping 1.25” of rear travel. The argument from Ibis and designer John Castellano was “well, airplane wings are made out of aluminum, and they flex. You just gotta design it right.” To be fair, I have not seen a broken OG Ripley, but there weren’t many around. And Ibis Mk1 went out of business right around then anyway. Still, we are still talking soft-tails here, which, in comparison to hardtails, do feel like suspension, but are not really what we think of these days when we talk about full suspension. Nowadays you can get about that same amount of comfort by switching from 29x2.0 to 29x2.6 tires.

Over in the world of full-on full suspension, however, travel was inching upward along with a concurrent evolution in kinematics and linkage proliferation. To say that much of this early full suspension evolution took some liberties with both design principles and materials application is a polite understatement. And, as these bikes blew their dampers, exploded their bearings, sheared their pivot axles, snapped their links, cracked their frame tubes, all while lumbering around exhibiting almost as much yaw as they did vertical compliance, there arose some elements of dissatisfaction with all this unreliable complexity.

One way of confronting this was by reducing the number of parts that could break. In this somewhat volatile Wild West time of mountain biking’s adolescence, simplicity had its merits. Single pivot designs, exemplified by the Santa Cruz Tazmon, then Heckler, then Superlight, were a good way to get the benefits of full suspension, compromise some small amount on the absolute performance angle, and still have something good handling and reliable. But, skimp on material and build wimpy rear triangles, and as many brands and their hapless customers discovered, you end up with bikes that handled less than predictably. Schizophrenically, maybe.

Enter the flex-stay. Again. But this time, disguised as a single pivot mountain bike with a solid rear triangle, and a linkage driven shock. Basically, a multi-link design like either a Horst Link or walking beam, but instead of featuring an extra pivot in front of or above the rear axle, a section of chain stay or seat stay would discreetly flex a tiny amount, allowing the pivot at the linkage end to follow the correct arc to compress the shock. Fewer parts, like a single pivot, but a solid rear triangle that affords a higher degree of lateral chassis rigidity, AND a linkage allowing some degree of shock behavior tweaking, AND the ability to roll more than an inch of travel. Win-win, right?

Gary Fisher Sugar, take a bow. Introduced in 2000, the Sugar bucked the single pivot paradigm of the prior half decade, and, in spite of boasting a whopping 2.5” of rear travel with a rear triangle being made entirely out of aluminum, didn’t explode apart nearly as often as I anticipated it would. Cannondale’s Scalpel appeared in 2002, followed in-turn by the Yeti ASR SL in 2003. The Scalpel was somewhere between soft-tail and flex-stay. mimicking a sorta Horst Link by way of a carbon fiber chainstay with a flex zone built into it, and travel was similarly modest – about the same as the Sugar. The Yeti, meanwhile, featured tiny titanium flex pivots bonded into the seatstays of the rear triangle and produced 100mm of travel. At the time, this was a lot of travel for an XC race bike, and an unheard of amount of travel for anything flexy (Ibis BowTi, we see you raising your hand. You don’t count. Sorry).

Yeti then upped the ante again with the 575 in 2004. 5.75” of rear travel, using the same bonded in flex-joints first found on the ASR. They would go on to then render the entire bike in carbon fiber and keep the name and the design rolling until 2011. That’s the year I planted the headstone the last time I thought flex-stay bikes were dead. Yeti had just come out with their Switch suspension design, followed by the Switch Infinity suspension that we know today. Yeti was doing the same as everyone else; employing new multi-link suspension designs that allowed for solid rear triangles, bringing a lot of carbon fiber to the party, and reaping the benefits of the arriving maturity of suspension kinematics that pedaled well, ate up bumps beautifully, offered exemplary chassis stiffness and managed not to eat all their pivots and bearings every other week.

In this new long-travel trail bike world, flex-stays began to display some limitations. Effectively, these are still single pivot designs. Light, simple, yes, but hey cannot offer the same highly tailored anti-rise/squat/instant center behavior, and there is less ability to mechanically manipulate the shock behavior during travel. The more travel you are dealing with, the more this matters. And the flex-stay, regardless of where it is located, is a spring. It has an effect on the overall spring rate of a given suspension. This can be taken into account during design, but there’s still a certain twang-factor to every single flex-stay bike that I have ever ridden that is undeniable.

So, in the face of all that, I thought back in 2011, the ol’ flex-stay has gotta be a goner.

Then I rode a 145mm travel Hei Hei a few years later and fell in love with the damn thing. No, the suspension absolutely did not work as well as any of the other 145mm travel bikes I tested that year, but I loved it anyway. Then Yeti reintroduced yet another ASR in 2015, as an XC race bike. It was a flex-stay, and it was a rocket. A few years later I adopted a stray Specialized Epic Evo, then Kona dropped a new, shorter travel, XC race oriented Hei Hei, and Transition unveiled the Spur. Nowadays, the XC race market is dominated by flex-stay bikes.

And I think they might be sticking around for good. My Epic Evo was an awesome bike, and I spent enough time on it over the course of a couple years to really come to terms with the compromises involved in flex-stay designs. And now, in the middle of a test block on another flex-stay XC bike, I think I am beginning to understand the bigger picture. I may have to eat these words in a year or two, but so be it. Here’s the bigger picture that I think I understand:

Flex-stay bikes are fast.

There’s a twanginess to the ride, a sense of energy being returned. In longer travel packages, it becomes more difficult to corral that twang and make it go where it is pointed, from a design standpoint. But in what we call shorter travel applications these days, sub 120mm, for example? Damn, they get after it.

They feel lively and responsive when it comes to mashing pedals and turning calories into forward motion. This may be placebo effect, a knock-on from having a loaded spring as part of the suspension instead of neutral and more linear pivots and bearings, but I’ll take it. When it comes to the ethos of XC, whether racing or spirited trail riding, getting after it counts for a whole lot. AAAAND, this is just my opinion here, I think that when you’re dealing with 120mm travel or less, the quality of suspension performance from a “give me a magic carpet” standpoint isn’t as critical. People who want plush, highly effective suspension are going to want more travel anyway. People who want to go fast are (maybe) willing to deal with a little compromise, a little less perfection to their cushion, if they feel like they are getting a fistful of giddyup in return.

Flex-stay bikes, I’m glad you’re sticking around. I apologize for spreading rumors about your demise, and hereby nominate that we rename you, respectfully, “Twangers”. Long may you twang!