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I was working in the engineering department at Specialized during the whole "zertz" thing. They did absolutely nothing. We tested frames with them and without them on an MTS test bed as well as having strain gauges/etc and DAQ systems on for riding. They did not damp or mute anything, zero difference. The shape of the carbon stay itself did change how the rear end behaved when compared to a "normal" seat stay, but the zertz did nothing. It even got to the point where Specialized was ordering field test frames without them as the cost was a bit lower. Zertz was pure marketing. Same goes for the seat posts and forks with them.
I was also tasked with making the first flex stay mules for Specialized. I took 2 alloy Epic frames and removed the bearings from the Horst link and then welded the pivots solid and heat treated the frames. One was welded to be "relaxed" at top out, and the other at sag. So there was either an inflection point at sag, or it just acted as another spring for the entirety of travel. I believe they went with the sag version, and this is where the whole negative spring thing came from. The flex stay is basically trying to pull the frame into sag, then after sag it adds to the spring force.
I'll also add that FEA, even today, does not necessarily give you the same results as the real world. I saw this countless times, many hours of setting up FEA simulations and designing off of them, only to have the actual product not behave as predicted. It was used at a design tool, but also verified through lots of real world testing with DAQ and rider feedback.
A good example of this involves the press fit BBs on road bikes. When Specialized was moving from co-molded alloy cups that the bearings were pressed into (BB30), to having a carbon BB shell that bearing and plastic cup were pressed into (PF30), FEA was run and the results were basically the same. However, if you test rode two identical bikes (one being BB30 and the other PF30), you could instantly tell which one you were on. Very noticeable difference. Similar situation with Zertz; FEA showed that they did help damp the system, but this could not never be measured or verified in the real world with instrumentation or rider feedback.
This is some great intel. Thanks for sharing!
Wynn Masterd delaminated the chainstay back in the day, and at one point had a tube wrapped tightly around one side to keep it together.
This is a great point. I used to break shit all the time, mainly wheels, until I began racing XC and riding a single speed exclusively. While I no longer race, that experience still informs my trail/enduro riding style which I would describe as aggressively smooth. For the past 10 years or so my bikes were all longer travel trail or enduro bikes. A couple years back I got one of the flex stay Stumpjumpers after I bought an alloy Stumpy EVO. Both bikes are great in their own right, but more often than not, I am reaching for the flex stay bike. It's way lighter and way livelier, so to me, it's more fun to ride on my local trails. The trails have plenty of tech so like @sprungmass said you learn to hop and unweight through these parts. The EVO is now reserved for big backcountry rides or park days where it really shines.
Great discussion in here!
This is two rigid bars(the chainstay and the main pivot to shock link-your orange line) connected with a pivot. The chainstay and your orange line will move at the pivot and compress or extend the seatstay. The flex stay acts as a spring that can compress or extend to allow the axle to move.
This would flex, the axle will move and that motion will be undamped. I don't think that affects the ride but this set up would move like an undamped bike unless I'm missing something.
Again, flex stays are not flexible in all directions. It is not able to extend or compress axially any more than any other frame member including the chainstay.
extend or compress axially
I'm not sure what you mean. So you mean rotate? Because it does.that's why it replaced a pivot.
So imagine replacing it with a pivot and then driving the shock all the way to bottom of travel. Can the axle move beyond that point?
"extend or compress axially" means along its axis. So stretch or compress inline with its long axis. Likewise it won't buckle/bend under compression (which is what I thought was happening).
Yeah because the length of the bar(flex stay) changes.
There's no Side Side congruency theorem for triangles.
I think @Sheppy Wong is aiming at the picture with the banana seatstay. That would tend to bend like a bow a bit and give some more (undamped) travel.
You could make the rear triangle vertically compliant, but doing so would bring you jn a heap of problems elsewhere ride dynamics wise.
That would make more sense, but that's not how anyone does flex stays.
Here is a 2D representation of a Blur rear triangle where the flex stay is converted to standard pivot. The dimensions are meaningless because I didn't scale the image to match anything real. In one image, the shock length is not dimensioned. Because it is not dimensioned, the lines are blue because the sketch isn't fully constrained. In the second image, the shock has a length dimension associated with it. This fully constrains the sketch and nothing will move.
I think the point that @seanfisseli is trying to get across is:
In the video you can clearly see the Santa Cruz seatstays flexing up and down. This is probably because the flex stay is more flexible, by design, than most other chainstays out there that aren't intended to flex. That wobbbling up and down may or may not actually lead to the seatstay compressing meaningfully, but either way that wobble/flex/vibration is clearly there.
Sean is hypothesizing that this wobble/flex/vibration can be felt on the trail, even it if doesn't lead to meaningful compression of the seatstays, and this degrades the bike's performance.
I am skeptical, but I don't think the question is stupid (even if the question did "evolve" over time)
Of course not, thus the bad design I mentioned. But this is what @seanfisseli was trying to get across with the undamped talk.
@hamncheez2003 adding a pivot will cause it to shake even more. As I said before, it's not a flex-stay thing, it's a triangular construction with compression loaded sections thing.
This has somehow morphed from talking about undamped spring force and rebound from a flex stay to transmission of vibrations. Very different things. Any frame will transmit vibrations to varying degrees. To figure that out you have to look at natural frequency and then also the frequency of impacts on whatever trail you're riding. I would guess that most flex stay frames have a higher natural frequency.
Santa Cruz chose to design the Blur with a very thing seat stay. This is not something that is a requirement for the functionality of a flexure. In the huck to flat video it would appear the seat stay actually nearly buckled. You could do away with the flex stay and it would still nearly buckle. Actually might require increased strength from the seat stay since a beam pinned on both ends buckles at a lower force than a beam that's fixed at one end and pinned at the other. @Primoz posted a picture a little bit back that depicts this.
Im assuming that any flex of the stay changes its length. Unless I'm missing something obvious I can't picture a flex system that doesn't change lengths wihout a pivot. That's why pivots work, they put the bar in straight compression or tension between the pivots.
the key word was "meaningfully"
If the amount of force needed to compress the shock is more than the amount of force needed to flex the seat stay, I still think there is a chance that the SS flexes instead of compressing the shock. I know that’s not in accordance with the axle path, but the axle path is attempting to redirect the forces exerted on the rear wheel into the shock, and if something interrupts that (shock getting stiffer than a member of the linkage) the force may not follow the engineered path. God I love this thread.
Also, if the direction of the force is especially rearward then these flex stay bikes (with their forward axle paths) might have more trouble converting that force into linkage movement, again opening up seat stay flex as a place for the energy to move into.
These are hypotheses btw, and yelling and people on the internet is not a valid scientific way to prove or disprove their hypothesis. Some of y’all are acting like science on your side with ZERO experiments demonstrating your own hypothesis is more valid that what I am positing.
This is a nothingburger. What are you trying to say here?
I said it’s an undamped spring. If a spring is oscillating and vibrating without anything to damp said vibrations/oscillations, it is an undamped spring, no? This was always a major point of mine. How much said spring compresses and extends is another story entirely, but a story I’m still open to exploring!!!
Your tires are an undamped spring, your handlebar is an undamped spring. Your dropper post fully raised flexing forward and backward while pedaling is an undamped spring.
For the flex in a flexstay design to be undamped would mean that the force required to move vertically the wheel a significant amount of travelis higher than the stiction force of the seal of the air spring. Otherwise it's just one part of a series of spring from your tires, rim, spoke, wheel axle, rear triangle, linkage and shock. And since the shock is at the end of this and is damped than the full series of spring is damped.
Also carbon fiber don't really lose energy as heat when it flex so most of the energy accumulated by the rear triangle flexing is transferred back further to the shock which damp it all.
But as other have mentioned, a flexstay design will require a higher force at the wheel axle for the system to move vertically instead of a 4bar setup they typically replace since a bearing require much less force to rotate than a seatstay/chainstay to flex. So your suspension damping tune must account for those higher forces.
It’s funny how many of us have zero evidence. It’s all theory/conjecture/conventional thinking. I love coming up with ideas!
If it is a bar with two pivot points and all the loads going into the two pivots (so faux bar seatstay), yes. If you have three pivots (faux bar chainstay, Horst link seatstay, etc.) or have a brake mounted to it, no. You went from a bar to a truss which is also loaded by side loads and torque (moment) loads and the dimensioning and calculation of it becomes completely different.
No. Time and and time again it has been explained to you that the flex does not mean additional axle travel.
Add a pivot. It will be EXACTLY the same.
Some of us have years of education and experience from the field...
FWIW carbon fibre damps a lot more than metals. And a handlebar can also be somewhat damped 😉
Aaaand I'm out. I think we ran the course of the debate and are spinning in circles.