charlietravis
Beggars Would Ride

Flex. Stay.

Reading time

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.

hygenic

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.

silktistay

All conjecture aside, just look at that. Swoooon. Ibis Silk Ti.

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).

sugar

Ahhhh, the early 2000s, as the bike industry convulsed around itself in economic contraction, a big company with a wad of cash and a marketing agency could still get all weird and aspirational and completely confuse the bejabbers out of its own customer base...

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.

epicevo

This bike may have been the beginning of the turnaround for me on Flextails. It made me feel like I was about 100 watts stronger everywhere.

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!

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Comments

Jotegir
+9 BarryW Konrad Mammal Andy Eunson TristanC cheapondirt vunugu Todd Hellinga Hardlylikely

All stays are flex stays if you ride hard enough.

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xy9ine
+7 Mike Ferrentino Vincent Edwards Pete Roggeman Andy Eunson Skooks Velocipedestrian Todd Hellinga

as far as i know, this was the longest travel flex pivot design (9" travel). given carbon use in dh applications was in it's infancy at the time, there was some skepticism as to durability, but these proved to be bomber. the chainstay is flexed down at 0 travel (effectively a negative spring), which in conjunction with a high starting leverage rate, makes (for better or worse) silly plush first few inches of travel. 

the spot i reviewed (with carbon link flex plate) had a similar negative load, which (seemed) to reduce the air shock breakaway / increase sensitivity a bit.

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mikeferrentino
+1 Perry Schebel

The only way that bike could be made more awesome would be if they incorporated a pull shock. And a linkage fork. Amazeballs.

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velocipedestrian
+1 Mike Ferrentino

A Trust fork would suit the aesthetic of a Lahar so well.

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shoreboy
+6 Mike Ferrentino BarryW Pete Roggeman slimchances57 vunugu Todd Hellinga

Still ride my 94' YBB to work everyday. I still get the odd person on the bike route tying to tell me the frame is broken when they see the slider moving up and down. Moots still supplies replacement parts for it too when the slider and spring need a refresh.

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rigidjunkie
+6 Mike Ferrentino Pete Roggeman Geof Harries Dr.Flow bishopsmike DadStillRides

OMG that Sugar gives me flashback.  I had a friend who rode the largest one they made, he was around 6'5" and a solid 250 pounds.  If memory serves me correctly, he broke a dozen of them and GF kept sending him replacements.  The crazy thing was they broke in different places.  I personally witnessed one on its second ride crack at the bottom bracket.  We looked at it laughed and he once again walked out of a ride.  Eventually GF asked him if he would accept a handmade hardtail frame instead of another Sugar.  He agreed and rode that thing for a few years while waiting for full suspension design and manufacture to create something that could take his size and blunt ridig style.

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lacykemp
+6 Mike Ferrentino Pete Roggeman BarryW slimchances57 vunugu Hardlylikely

I'm not able to fully qualify why a flex stay bike has the get-up-and-go that it does. In my monkey brain, it's because the "flex" is wound so effing tight that of course it wants to propel you forward faster. But there's no science behind that. Just many bananas. The Hei Hei Trail, which I believe is the ~2017 Kona you were referring to, was an odd bike that really only seemed to tide Kona over until the release of the Process 134 which was not a flex stay bike. The Hei Hei Trail was dissolved from the lineup shortly after or maybe right when the V2 of that 134 came out. I have a current generation Hei Hei and tell myself that maybe I am super fit when I ride because, holy shit that thing is FAST. I guess I came here to say that I don't want flex stays to go away, because there's something weird and wonderful about how they work, so I'm glad to see more bikes continuing to use that tech.

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cooperquinn
+5 Vincent Edwards Timer Spencer Nelson Todd Hellinga trumpstinyhands

This is the only bike I'll call a "twanger"

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jt
+1 Mike Ferrentino

All the lateral compliance one could want and more!

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Roxtar
+2 Timer DadStillRides

lateral, vertical, horizontal,...

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slimchances57
0

All at the same time! ; D

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xy9ine
0

i rode a slingshot once - in a parking lot. had a neat springy feel under sprint that didn't feel entirely terrible. nice syncros spec; i had all the bits (including fork) with the exception of the cranks, which i lusted after.

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mikesee
+3 Mike Ferrentino nothingfuture bishopsmike

^^^ Most comfy hardtail I've ever ridden.  But it ain't no flex stay.

I raced one for a full season of 12hr, 24hr, and 100 milers in the 90's.

And then built myself a new, different one a few years ago.  Probably 20-30% of my annual mileage is on the Slinger.

P.S. You *can* make it twang.

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Roxtar
0

SO

F'n

COOL

I'd LOVE to show up for a group ride on that.

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Roxtar
+2 mikesee Timer

Could you just slacken it out by adding cable?

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mikesee
+3 Mike Ferrentino Mammal trumpstinyhands

You slacken it out by gaining weight.  Really comes into its own around the holidays...

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slimchances57
0

Had one....worst handling bike ever made.... transitions from right to left were opposing committee meetings with the front and rear wheels making different proposals on which way to go. Dreadful.

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mikesee
0

That’s the beauty of them. Put the wheels wherever you need to, whenever needed. Brilliant tech climbers because with enough body English the wheels come out of plane to avoid obstacles or gain traction.

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FlipFantasia
0

high school me still wants one of these

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MTBrent
+4 Vincent Edwards Mammal JohnC Todd Hellinga

Mike, don't call it a podcast, but have you ever considered an audio version of your columns/articles?  Like, literally reading them aloud?

I'll always read them, but I'd certainly enjoy listening to a few minutes of MF bestowing his knowledge and experience in a mic.

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lacykemp
0

ME TOO!

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mikeferrentino
+8 MTBrent UMichael Vincent Edwards Mammal vunugu Todd Hellinga Hardlylikely Grant Blankenship

Funny you should ask... There are a couple things afoot. One, we have been quietly tweaking an nsmb.com podcast (but kinda like a high school band we've been jamming a lot in the spare bedroom but aren't ready to gig just yet), and one of these days it'll exist. Two, I am working on a sidegig "thing". It is still embryonic, but there will be an audio companion/podcast component to it. Gotta cross some more eyes and dot some more tees first, though.

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pete@nsmb.com
+3 Velocipedestrian vunugu Hardlylikely

Keep asking and we'll make him do Beggars Would Ride, the Bedtime Stories version.

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dave_f
+2 bushtrucker mnihiser

For some MTB stuff I can't help asking myself how long the service life of a part might be. Aluminum flex stays, or wheels with aluminum spokes. I guess they just have to last long enough for most of them to be sold on to the next owner with no warranty coverage.

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nothingfuture
+4 Mike Ferrentino BarryW Pete Roggeman Timer

I dunno.

I've had some aluminum-spoked wheels for... 20 years? And they're still in great shape (despite my oversized self riding them). I routinely see 40 year old aluminum road bikes kicking around- and those are almost entirely "flex stay" in their construction.

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Roxtar
+2 Mike Ferrentino Pete Roggeman

Actually 7075 aluminum is sort of alu-magic. It can't be welded but it has surprisingly good springback properties, is extremely tough, and has a high fatigue life. That's why it's used on I9's aluminum spoked system wheels and many other high end products that don't require welding (Those airplane wings Castellano was looking at, for instance).

As a mech engineer and former machine shop owner, I was very suspect about the spokes on the I9 wheels, especially regarding impacts, but I've now been using them for about 8 years on all of mine, my wife's, and many customer's bikes without an issue. And I'm an enduro basher, riding the northern New Mexico jank.

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Timer
0

Many aluminium parts on bikes flex all the time. Forks, rims, brake adapters, brake calipers, brake clamps, crank arms, crank spindles, stems, handlebars. As long as the part is properly designed, its active life expectancy could rival that of its owner.

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vincentaedwards
+2 Mike Ferrentino Pete Roggeman

I never got to try that 145mm travel Hei Hei, but I remember your [favorable] review from back then. I did test ride the aluminum 2016 XC model, and really enjoyed it. 

I have an Epic Evo from the current (outgoing) generation. It’s probably the fastest bike I’ve ever owned, and very nearly accounts for the amount I’ve naturally slowed down since 2016. Late last year I got curious… was the comparatively unforgiving suspension down to the flex-stay… or the shock / tune? I swapped to a Cane Creek DB inline 190x42.5 giving the bike about 115mm of rear travel, and then put a 130mm Fox34 [fit4] on the front end. For where I ride, I don’t think the bike is any slower in this new configuration. It’s definitely more comfortable though! 

I still agree with you that there’s something inherently stiff and energetic in the flex-stay system. 

I’d love to hear from some Stumpjumper owners here in regard to a 130mm trail bike that uses flex stays. 

(I’ve heard some whispers from the rumor mill that we may see a new 130mm bike from Specialized this year… and it may not continue the flex-stay tradition… time will tell) Looking forward to your upcoming review in a few weeks. I hope you’re enjoying those epic XC rides in the meantime.

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mikeferrentino
+5 Joseph Crabtree taprider BarryW Pete Roggeman Vincent Edwards

When we roundtabled the twanger Stumpy back at the beginning of Beta/end of bikemag, we were all mighty impressed with how the bike accelerated, but were also all a bit flummoxed by the suspension. I am going to completely talk out my ass now, since I have ZERO real evidence to back any of this up, but I suspect what is going on with longer travel twangers is not so much that the suspension doesn't work all that well, but that it is more difficult to strike a good front/rear ride characteristic/balance. Meaning, the inherent springiness of the twanger causes a different ride sensation than might otherwise be achieved, and it is "differenter" than what is going on up front.

At shorter travels, this disparity between front and rear behavior is less noticeable, but it becomes more apparent the more travel you have to play with. Now, factor in that there is less ability to play with leverage rates or instant center positioning because there are fewer links, AND there is an additional spring for the damper to try and manage, and the longer travel twangers become more challenging to control in more dynamic environments. Maybe. Like I said, this is just me scratching my not very well educated chin.

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tim-lane
+6 Mike Ferrentino Vincent Edwards Mammal BarryW slimchances57 vunugu

I agree with these theories, Mike. I'll add that I suspect that rear through axles enabled the resurgence of flex-stays.

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mikeferrentino
0

Ohhhh, good point!

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velocipedestrian
0

Ah, this feels familiar. My last 26" FS had a coil fork, and no matter what I did with the adjustments (there were many - CCDBA) it never felt balanced. 

Got a cheap old DHX5 for the rear, and boom! Balanced suspension feel. 

I think this disparity is OK in reverse - a coil rear/air front certainly feels fine on the current steed.

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snowsnake
0

I'm not a current Stumpy owner, but I am a REEB SST owner, and I think there really is something to the flex stay system that helps make the most out of short travel, air sprung bikes. The way the SST's stay's preload the shock make it very supple off the top, and it takes big hits quite well, but without the "mush" of an overactive and soft suspension design. When the trail is smooth, the suspension just...fades into the background, so much so that folks have had good luck single speeding it!

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tim-lane
+1 Mike Ferrentino

IIRC, Yeti's bonded in flex-joints fell foul of a GT/Busby patent, that might've driven the move away from their use. It was about the same time that GT were putting the hammer down on the Maverick Mono-Link patent. They also came unbonded from the frame tubes - I repaired a few of these.

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gdharries
+1 Mike Ferrentino

In 2016 I bought, rode and loved a XL Kona Hei Hei Trail. The bike was so fast, sharp, lively and fun.

Enough rear travel (100mm) to take the edge off, but also get me into trouble in all sorts of ways. It was awesome. Here it is on Day 1 of its life with me.

I also broke the Hei Hei's flexy chainstay twice. Kona replaced it both times.

Sold the bike in 2018 and instead got a Process.

I miss the Hei Hei. One of my all-time favourite bikes.

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mikesee
+1 Vincent Edwards

"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."

If you're willing to think outside the relatively narrow bun that the industry has distilled itself down to, you can run 3", 3.25", 3.5", or even 4" tires on a flex stay bike.  

Then you have the ability to tune the suspension toward stand-and-deliver efficiency and damping out the bigger hits, while the wheels and tires can be tuned toward that magic carpet you alluded to.

Is this gonna fly for everyone?  Ha.  What one bike suits everyone now?

My idea of the quiver killer, for where I live and ride:  https://lacemine29.blogspot.com/2023/09/fresh-metal.html

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mikeferrentino
+1 mikesee

Ha! I was just reading Whit's musings on twangers in the lead up to this post. Looks like a mighty cool bike!

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vincentaedwards
0

Good read- thanks for sharing!  The bike turned out great too thumbs

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MTB_THETOWN
+1 Mike Ferrentino

In around 2005 I got a stumpjumper and my dad got a 575. A few years later he broke his collarbone and decided to stick to road since he's a klutz and he passed me the 575 and i gave the stumpjumperto a friend (although last fall he and a friend rode into each other on road bikes and he broke the other collarbone). I should have kept the stumpjumper, it ride much better, but I was sucked in by the yeti allure. 

In around 2017 i got a felt decree with a flex stay. It was such a huge upgrade from the 12 year old 575 I loved it. But I quickly moved on to a Transition Patrol with a horst link. It rode better in every way, up or down, even though it was much heavier.

Recently I rode a few laps on a friend's spur and came away unimpressed. It just felt harsh. I got myself a rocky mountain element and love how much more plush it rides.

I still strongly prefer a horst link.

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mikeferrentino
0

I hear ya. And I loooove the Element. But by the same token, having now gone back and forth between twangers and similar travel multi-links, and some of those multi-links have been insanely light and efficient, when it comes to something I'd slap a number plate onto I would willingly compromise on ride quality for that springy je ne sais quoi.

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vincentaedwards
0

also- there’s still at least one longer travel flex pivot frame out there in 2024… 

https://www.last-bikes.com/Tarvo?Language=en&ClientID=a30f076f-d7c3-4db1-8649-8599fa71aee6#:~:text=The%20new%20TARVO%20is%20still,the%20focus%20of%20our%20development.

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Jotegir
+4 Mike Ferrentino bigbrett Vincent Edwards vunugu

But will the stay Last?

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jt
+2 Mike Ferrentino cedrico

Don't forget Merida (not applicable to us NA folks, but still). Or have your cake and eat it too with Spot and their 'living link'.

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BarryW
0

Good stuff as always Mike.

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Roxtar
0

"gaped in horror as the odd aluminum version began cresting the horizon"

I'll never forget reading about an aluminum flex stay bike (forget the brand) and thinking, "This company really needs to hire an engineer before they kill someone"

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slimchances57
0

My first 29er was an 07 Scandium flex stay Salsa Dos Niner ...loved it while it lasted, frame cracked @ the brake side rear drop out after @ two years. Salsa was great about replacing it.

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