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It’s almost like the concept of a balance board. If the load is directly in line with the center of the flexure, the only thing that will cause the flexure to deflect is articulation of the suspension that requires displacement at the shock. Where you run into problems is if someone has mistakenly put the flexure outside of the load path.
Well, it might chatter given the right (wrong?) construction, weights and frequencies. Arguably it’s kinda like attaching the wheel to the middle of a coil spring while the damper is attached only at the ends.
Conceptually I don’t love flex stays. But I do like simplifying things by using fewer parts, so I can see why these exist and how we get there.
That’s all theory though, in the real world I’ve ridden flex stay bikes that I’ve enjoyed and flex stay bikes that I’ve hated so the hate isn’t inherent to the suspension design.
@ :28 in this video, you can watch the flex stay (edit: meaning seat stay) chatter: https://youtu.be/XyAbIpOO0Zs?si=1Tnjq7HF_BQPhymX
I think you are right in a theoretical realm but in reality these things are chartering and undamped rebounding.
Shit I realized I am talking about two different scenarios and that I’m at least half wrong. Will update after work
The strange behavior at the shock you can see there is due to the tire.
In my opinion most, if not all, the suspension performance gripes that people have with flex stay bikes is mostly due to the fact that they are typically less gravity oriented so suspension performance for descending is not prioritized. The Stumpjumper 15 and the Stumpjumper 15 Evo are the same bike. In the previous Stumpjumper generation, the flex stay Stumpjumper was very different from the rest of the Stumpjumper models.
@:31 seconds the seat stay is oscillating/chattering/dunno what you prefer to call it. I don’t know how you are seeing a connection with the tire. Right behind his calf.
It’s gotten more complicated as I have researched it further. The concept of the “twanger” flex stay bike has been around for a minute, but what’s confusing our conversations on the matter is that there are a lot of different ways to design a flex stay bike. When I look at bikes that have a flex pivot on the chainstay I can see the way that the damper would control their return. so I get that on some level there is or can be damper control of the flex stay rebound.
However. If we are making one of the members of linkage flexible, especially a large section of the seat stay, I am still seeing way for loads to be transferred into the flex stay without even compressing the shock. It also could be option a and b simultaneously, that although the axle path is controlled there could still be forces exerted on the flex stay that do not fit into the path. I’m imagining a bottom out event where the flex stay flexes even more (you don’t really have to imagine this one, just watch the video I posted,) or weird off axis loads that don’t transfer neatly to the shock.
I just can’t get away from the fact that the axle is pushing into a spring that has no damper attached. I know that the flex stay is virtually connected to a damper but I’m replaying it over and over again in my head and it’s the same as on a rigid bike: the seat stays flex and the spring back undamped to their original form. I wonder if this is why people love the way a flex stay bike pedals: there is an element of “planing” happening (another hotly debated concept…)
I’m open to watching an animation or something but I’m wondering if I’m just beyond help.
Helpful contribution to the thread: my spur rips. That is all.
Ah! Ok! There is an assumption that the flex stay can only be flexed by articulation of suspension.My assumption was that the flex stay flexes before articulation even occurs, which I can see is not correct all of the time (especially under ideal circumstances or with a good design.)
I’m still stuck though on the video of the blur, because assumptions aside, the “load path” is kind of only a suggested route for loads to be redirected. When members of the suspension are stiff and the pivot locations are bearings/free to rotate, the load will be redirected to the shock. But I think we change things a bit when we have a flexible springy member AND ESPECIALLY when the force comes in perpendicular to the load path. If we have a forward axle path and we are getting forces that are perpendicular to the load path, I am seeing the flex stay being flexed.
Do you see any validity in that theory?
When you apply a vertical load to the axle, the chain stay is largely in tension and the seat stay is largely in compression. In the case of the Blur, the seat stay is the bit that flexes. It happens to be one of the ones where the flex designed to occur over the length of the seat stay as opposed to one specific spot. To avoid increasing the potential of buckling, this relies on minimal displacement of the seat stay end relative to the rest of the rear triangle. As a result, the compressive force is always more or less in line with seat stay tube and the stiffness is good in that axis. I also drew in a goofy arc at the seat stay. If the seat stay were to be shaped like this, what you are describing would happen.
i swear i am not being obtuse on purpose, i really respect you (and run your link on my bike) and I am really trying to understand it the way you do, but I dont understand why the phenomenon that occurs on a rigid bike, wherein the seat stay flexes, does not apply here.
in my mind the seat stay would be flexing the opposite way. At first I thought there was an opposite force to the axle like in the first pic, but at the least i see something like the second pic being the actual case, and I would imagine this is approximately how it would function on a rigid bike, probably where I am deriving my understanding of flex stays from, tbh.
The arc I drew is just a random arc that would deflect under load without require movement of the shock.
The sum of the chain stay and seat stay force vectors is equal and opposite to the vertical force the axle. So the green arrow you added in the first image isn’t a thing. This is pretty much a statics truss problem at a simplistic level so I’d suggest looking into that.
What you’ve drawn in the second image will occur under very high load. This is buckling. Any thing loaded in compression will eventually buckle as force increases. A long slender seat stay is more prone to buckling. But you wouldn’t use buckling as the mode of deformation for a flex stay.
I understand that you wouldn’t use it for deformation but it doesn’t mean that deformation won’t occur (see: blur video that shows [edit: said chainstay, meant SEAT STAY] buckling/chattering.) I get that you are talking about intended design/ideal operation, I am talking about side effects/unintended consequences of having a seat stay member that is flexible. (I read that treks flex stay activates with less than 5lbs of force. That doesn’t seem like a big impact.)
Also, you still haven’t explained why this is so different than the rigid bike. We know that a flexible seat stay will allow for rear axle deflection in that application, so why doesn’t that happen in this system?
Rigid bike flexing vertically doesn't happen. It's a triangular structure, in order for the seat stay to flex you would need either the seat tube or the chainstays to also flex and force needed to bend those is huge.
Also to add for the seatstay in a flex design to buckle something else has to also give. The chainstays in tension would need to stretch/bend or something at the other end needs to move (rocker,top tube rocker mount, shock mount, top tube flex). No one thing can move in isolation without something else in the chain moving.
@seanfisseli I think your confusing activation force with the axial compressive stiffness. The 5lbs force will be refering to the force at the wheel required to move the the suspension system i.e. to overcome the spring force.
What you describing is the seat stay buckling stiffness and will be order of magnitudes greater that the shock stiffness .
Can you please watch the video of the blur chainstays wobbling and tell me what you’re seeing?
I love that you guys are saying “buckling” rather than “flexing” to make it sound more implausible.
Brother, road bikes are designed with vertical compliance. Some have a lot.
Also, you’re assuming that everything is in tension in the rear triangle but not all flex stay bikes are designed with the flex stay in tension.
Buckling is a well defined phenomena in engineering and is distinct from the tube compressing.
https://en.wikipedia.org/wiki/Buckling
Short of saying trust me bro I'm an engineer, there's not many more ways i can think to explain this 🤣
they are two different things and I’m sure as an engineer you know that. Cmon, don’t do me dirty!
This will be my last attempt at explaining why rebound damping at the shock will control the flex stay. If you take the blur rear triangle and attach the upper pivot to the lower pivot with a rigid member, you are forming a triangle and locking out the flex stay. If the shock were to be locked at any given position, it’s effectively doing the same thing. As such, for the flex stay to move the shock just move and therefore the damping at the shock is controlling rebound of the flex stay.
You still haven’t mentioned how a seat stay will flex on a rigid bike, and why that doesn’t apply to a flex stay bike. I’m not trying to argue I’m really asking for help understanding this!
Edit to add: I am not assuming that the only way the axle can move is by the suspension cycling. I am positing that the axle can move if the rear triangle flexes a.k.a. if the flex stay flexes.
The flex stay will offer an extremely small amount of undamped spring I think, but so does the front triangle, and the rear triangle of bikes that don't have flex stays, it's just so small that is not really something that you would generally say would introduce significant changes to the feeling of the suspension.
I think what is happening is that generally flex stays are used to reduce weight, but weight reduction won't just be applied to the stay but the rest in the bike as well, and the wheels etc. plus they are shorter travel bikes so the suspension will have to be stiffer, all of that leads to a bike that can feel quite "chattery" but I don't think it's due specifically to the flex stay.
Literally the entire bike will flex when you put load on it. Stress and strain go hand in hand. It’s just a question of how much and that entirely comes down to how you design the frame. You could make something that has no flex stays or suspension and is an absolute noodle very easily.
I have seen other people get frustrated in trying to talk to you on forums and now I understand why. You still aren’t addressing any questions I’m asking you!
Of course you can make the bike flexy or stiff! But as the name implies, flex stays have an especially flexy member. So, at least one part of the frame is in the noodle category.
But I don’t think you’re actually interested in having a real convo about this.
Look at Pivot’s DH bike. It’s largely considered to be quite smooth yet has flex stays. A flex stay is not flexible in all directions, but you are making the assumption that it is.
You are right about almost everything. But specifically I have been talking about the stumpy alloy vs stumpy carbon. One is 4 bar one is flex stay. The flex stay had a chatter or a “twang” that is just not enjoyable. I believe it is this small amount of undamped spring. It is more flex than would be found on a non-flex stay design, and I believe this twang or “chatter” is an unintended consequence. Some engineers have said it is negligible, some say it doesn’t exist. I just havent seen proof but I am definitely trying to understand how the design of these bikes avoids this problem.
Beautiful example! Given how much more damped a DH bike is I doubt that the twang/chatter would be as apparent. I also don’t see a thin leaf spring where the seat stay belongs. Looks like flex stay design that avoids the pitfalls of the layout