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Show don’t tell! I definitely hear you telling me that I’m wrong, an telling me how you think it works, but you still have zero data to back up your claim. I’m not saying you’re wrong I’m just saying that expecting people to believe you “because you said so” is weak.
Hey @seanfisseli there are already enough negative misconceptions surrounding flex stay suspension systems and (as a guy making them) I’d love to help put those misconceptions to bed. But it’s pretty difficult to show you something that isn’t happening. Can you help me out?
The vid you posted shows a lightweight XC bike hucked to flat, I think I see what you do but that isn’t going to result in any meaningful axle travel that would be perceptible to the rider and it only happens when the shock is at full travel under a fairly heavy impact for such a bike. Shit happens at bottom out, if not in the seat stays then elsewhere. Have a look at the G-Out Project right here on Vital for many ‘undamped springs’, unlike the huck to flat vid, you won’t need a magnifying glass. But both show stuff pushed to the limits.
I’m not worried about perceivable axle travel I’m talking about twangy, chattery ride feel. Some in the threads have said that a flex stay will deliver a lot of force to the seat stay under braking, which plays a role for sure! “Negative misconceptions” is a funny way of saying that people don’t like this suspension layout, which I think is fair since everyone has a preference.
There is a huge amount emphasis placed on making bikes that “work,” that are “fast,” that feel “efficient,” but myself and others are chasing the qualitative experience of riding a bike that feels good throughout the ride, a bike that feels pleasurable, not just “fast.” The old way of thinking is being challenged (see also: long cs debate, pedal kickback/chain feedback debate.) and folks get very defensive when their precious bike designs are being “attacked,” but we are just looking to progress our understanding of what makes riding mountain bikes intrinsically enjoyable, rather than what makes a bike “faster” and therefore more fun because it’s “faster” than another bike.
It’s valid to say that I am being too picky. It’s valid to say that most riders haven’t noticed or don’t care about this phenomenon, but I am still very interested in the issue because I love shorter travel bikes and want one that feels really good to ride!
A pivot will allow the designer to make the ss stiffer because it does not need to flex. I don’t know how you are missing the fact that the ss on a flex stay bike is designed to be more flexible (at some location if not the whole stay) than other designs.
I am totally ok with some riders enjoying the very thing that I don’t like. What some call lively I might consider chattery/twangy. It doesn’t mean that either of us are wrong!
Edit to add: I’m 200 lbs so maybe it’s a Clydesdale problem 🤠
You raised some really good points and I have had to really think on this since you first posted it. Here is what I’m thinking:
Damping isn’t only done with a “damper” like on a shock. Tires are actually tuned to be more or less damped through chemical makeup and construction. Bars and seat posts are damped by our grip or body weight.
When the wheels were on the ground my flex stay bikes were great. They weren’t bad in big compressions either. They didn’t feel bad when pedaling, they felt prettt good. In those cases the system seemed to stay loaded enough that the SS were controlled and effectively “damped” (in the broader sense of the word) by all of the mass/forces at both ends of the system.
One final thought from this session: “its pretty difficult to show you something that isn’t happening” but I am asking for you, the group of people denying that I have any valid claims here, to prove that what I am saying can’t happen. No one has done that. Y’all just keep saying “it can’t happen.” “That’s not how it works.” Which is especially frustrating when some of the most vocal/aggressive of the group refused to acknowledge or account for the chatter in the blur video (finally people are starting to address it, but it’s still the same ol “that will have no effect on rider quality.”) show me why that won’t have an effect on ride quality. Just telling someone they are wrong is a silly way to prove you’re right.
If we’re now moving the discussion on to braking then that’s a completely separate thing and just as we can’t lump all four bars, faux bars, flex stays, etc together; nor can we brake designs.
I don’t feel attacked, that’s okay. Trolled maybe.
And it’s not a funny way to say anything, it’s first hand experience. I get many queries from riders about the flex stay arrangement of my frames. Folks are rightly or wrongly concerned about flexing elements of their frame. I’m always pleased to explain that the degree of flex is so small that it’s insignificant to the frame structure and also to the suspension design.
I have several frames hanging in the workshop and my own bike sitting right here. If there’s nothing I can show you on video that will change your mind I’d suggest trying some other flex stay frames. There are differences and nuances in all designs and that’s good cus we’re not all the same and it’d be super boring if everything was.
It’s a part of the convo for sure. Not the whole convo. Again, I am speaking of the majority of mainstream carbon flex stay bikes wherein seat stays have gotten super pinner (and possibly floppy.) I would bet your bike feels a lot different than say the blur or epic Evo and I wish I could give it a good ride to see for myself!
Also, I agree with you that people afraid of fatigue or whatever on a flexing stay are silly. That’s never been my issue with a flex stay. I’m not even concerned with the durability of the thinner stays. I have made one point and one point only: I don’t like the weird chattery/twangy sensation on the flex stay bikes I have tried (that get a shit load of praise btw) but I am open to find bikes that don’t have that!
It doesn’t have to be bottom out, it just has to be a point where the force needed to compress the shock is greater than the force needed to flex the stay. Just because the shock doesn’t want to move doesn’t mean that the axle can’t continue upwards, flexing the stay off axis. We are trying to constrain the axle to the path defined by the linkage but the force just want that thing to move in the direction they are hitting it. If the members of the linkage are more flexible than the shock, won’t the forces at the axle with just flex that member?
Edit: I get that this might be a simple physics issue that I’m not understanding. But I would need to see it drawn out for me to understand how it really works if not this way…
This feels like a discussion with a Flat Earther.
I was thinking you guys felt that way about me lol
The burden of proof is on the claimant, you are claiming flex stays are creating increased axle movement due to the extra flex, therefore you must be the one to present the data.
Your exact claim is, because i said so, my ass can feel it. There are experienced engineers here telling you they haven't seen it in making real bikes. So the burden of proof rests on you to prove YOUR claim.
Thank god experienced engineers have never been wrong about bike design! Cmon dude, if they’re so good at engineering they could have explained why I am wrong, especially considering how much typing they have done to tell me I am wrong.
Also, WTF at this lawyer language, bro we are on a mountain bike forum talking about drawbacks of a specific suspension design. You guys are acting like I’m telling you that God isn’t real. Relax!
Get a broom handle with no broom on it. Hold it vertically with your palm over the top of it, and thrust it downwards into a concrete floor as hard as you can. The broom handle hits the floor, and the high degree of compressive stiffness of the broom handle transmits the impact efficiently into your hand, and now it hurts. Your hand didn't get any closer to the ground than the length of the broom handle because it did not compress, or bend, it just transmitted the force along the length of the handle.
Now with the bottom of the handle still on the ground, and your hand on the top, grab the middle of it with your other hand and give it a wiggle. Wow, isn't it flexy?
This is how the forces resolve in the seatstay of a flex-stay bike. The stay is wiggling in an extremely minor way that is inconsequential to its compressive stiffness, which is what matters when driving the shock. To get any real change in length of the stay, which is what you need in order to get any extra wheel travel, you need to bend the fuck out of it, which doesn't happen. If you had a broom handle that had a 2 degree bend right near one end of it you could still slam it into the ground and hurt your hand in the exact same fashion.
The wheel path is constrained by the arc that the chainstays define, just like a normal bike. The seatstays still act in near-pure compression to drive the shock, just like a normal bike. You can make the rear triangle nice and stiff in every other way you want to, just like a normal bike. Making one small part of the seatstay flex while retaining stiffness of the overall triangle is not a difficult problem to solve at all.
You’re right, the axle can only travel on the arc defined by the chainstay, but looking at how the seat stays chatter/wobble on the blur tells me that there is a window where the axle wants to rotate while the seat stays chatter off axis, allowing the whole rear trying to compress into the seat stay. Flex Stay Apologists keep making the mistake of assuming that all loads going into the axle will be diverted nice and tidy on axis into the seat stay, but that’s in an ideal, abstract way. In reality the system looks a lot messier and there are ample opportunities for the seat stay to flex off axis.
Also, you’re all making a mistake of assuming a large scale compression going through the seat stay, when we really only need that leaf spring effect on a very small scale to create very unpleasant sensations, especially with sustained descents!
And just a reminder, this applies to the carbon flex stay bikes with really slimmed down seat stays that are engineered to be strong enough on axis. This convo keeps getting turned around and around but there is a very specific phenomenon that doesn’t apply to all flex stay bikes (sorry Thom.) but that I believe is very real.
Can someone help me understand - I think picture 1 is an accurate depiction of our didactic conversation on flex stays, but picture 2 feels appropriate as well. Curious to hear opinions, please provide supporting data, thanks in advance.
1)
2)
The bike industry is sandra bullock and Im the the guy in picture two
But seriously I feel your pain. This broomstick thing is a big development though. It really demonstrates nicely what I have been trying to say for a while.
See: this point (I think i know what this means…)
The tyre nor your backside has the bandwidth to transmit/sense the vibrations of the amplitude and freqency caused by resonance in the chain stay. I'm like 95% sure at this stage you're confusing suspension design charecteristics for some fact about the flex stay.
None of what the people with expertise in this forum are discussing is opinion or conjecture. An engineering degree and industry experience actually teaches you real things about dynamics, vibration, kinamatics and materials.
So many of you are hating on me for having an issue with flex stays, telling me that “engineers told me how they work.” But engineers made the Stumpjumper referenced in this reply. Engineers measured zertz working. They couldn’t measure the difference between bb30 and pf30.
I don’t know why I’m still trying to have an honest, good faith discussion with you guys about this, but it’s so wild to me how much faith you’re putting in engineers being perfect, and how many assumptions there are that this crop of carbon flex stay bikes are without design flaws!
I know I am not as smart as a lot of you but I really wish that some of you were kinder and more open minded. This industry has been seriously held back by bullies and by closed minded folks who ignore feedback. I’m sure you want to apply that to me but whatever. I’m just saying that working together, even with someone who has a different viewpoint than ours, can be a lot more effective long term than dogpiling the person who thinks differently.
When did I say anything about chainstay resonance?
6/21/2025 at 10:11am and 12:34pm
In one of those instances I mistakenly typed chainstay instead of seat stay. But I was referencing the blur video, where I brought up the wobbling seat stays
If that typo effed up the whole convo I’m sorry
There has been considerable effort by several in this thread to explain the rationale in approachable terms. IIRC the physicist Neil De Grasse Tyson made a comment claiming that it's not up to the universe to explain itself to you - it's up to you to understand the universe. If a person does not understand the universe, it does not imply that the universe is wrong - it implies that the person does not understand the universe. In order to make meaningful challenge of existing methodologies, the parties debating must speak the same language.
It's possible that the current understanding of flex stays can be improved - but pressing those who speak a more complex language to explain it in simple terms is almost guaranteed to not push the boundaries of our current understanding, but instead push the boundaries of those who possess the ability to reason about flex stays in a non-trivial way.
Let's cut the act. Your claims of 'good-faith' discussion are laughable considering you've repeatedly insulted, dismissed, and openly mocked anyone who challenges your gut feelings. Your entire argument hinges on the logical fallacy that because you personally can't imagine flex-stays not affecting ride quality, everyone else must be wrong. Sorry, but engineering isn’t dictated by your personal disbelief or anecdotal impressions.
And spare us the lecture about kindness and open-mindedness. You set the tone here yourself by calling others 'apologists' and sarcastically ridiculing anyone who provided factual explanations. You don't get to aggressively dish out insults, then cry foul when people push back. If you genuinely cared about progress or openness, you'd drop the self-righteous attitude, provide actual data, and acknowledge your own hostile approach before accusing everyone else of bullying.
Nobody here is 'hating' on you. You're getting called out because your arguments are weak, your reasoning flawed, and your attitude aggressive and disrespectful. Accusing everyone else of 'hate' just because they're not validating your personal beliefs is a cheap deflection from the fact that you've offered zero evidence and leaned entirely on incredulity and sarcasm. If you constantly mock and dismiss people, don’t act shocked when they push back. You’re not being 'hated on,' you’re being challenged to actually back up your claims, or drop the victim routine.
OK, I'm gonna lock this thread, feel free to start another one if you feel the need...