I see a lot of metal bikes (steel and ti) using a flexing seat stay rather than adding another pivot point. What are some factors to consider with this design? Does anyone have a good rule of thumb for how far a stay can flex? I’ve drawn up a few flex stay designs and am starting to get serious about building one. I generally try to keep the change in angle (seat stay to chainstay) within one or two degrees. Is this an acceptable range of motion for a smaller diameter, thin walled tube?
Do this experiment. Put whatever tube you are considering into your vice and flex it holding the tube at the distance you will have from your drop out to connect at shock. I think you will find you can flex it a fair bit more befire feeling any real resistance in the tube. For example my XC rig goes through about 3 degrees in movement. I can easily flex it to 10 degrees and its only just stsrting to resist the bending force.
1-2° sounds okay. Like Sean says, it should be pretty easy to do some practical tests. Look at how much deflection you need over the length of your assembly and test it out.
Depending on the joining method it might be wise to try and keep the flex away from the joint.
Also think about the brake mount and whether that is affixed to the flexing member.
Thanks for the feedback, that is a great simple experiment to try. I’ll follow up or possibly do a full build thread on this bike, since the community always has such great tips and feedback
I would not be concerned about seat stay flexing. You should be clearly in the elastic part of the stress-strain curve.
My concern is the weld between the SS and dropout. Bend tests can’t show fatigue. That area is the perfect storm: highest stress concentration, weakened steel (heat affected zone), and stress riser.
This is why early steel disc forks cracked at the tip of the brake tab:
The fork undergoes much less flex an full suspension rear end would.
A clever way to control flex is to squish the tube in the desired “pivot” location, see the reeb sst chainstay. The idea is that your flex will be isolated to a non welded zone of your choosing, and the lateral stiffness will remain high.
The first example of Flexstays I remember was Swarf out of the UK. Check out their Contour model for some inspo if you haven’t already.
One really interesting thing I remember reading is they designed their flex stays so they were in zero tension at sag but slightly in tension unsagged.
The frame is 160mm of travel, I think this is one of the longest travel frames to rely on flex and certainly the longest travel to skip two pivots for flex.
The rear end is at zero tension at roughly 40% travel so just past intended sag. With this it’s splitting the total flex distance to a + and - from zero tension. I can easily cycle the rear end through most all of it’s travel with a light finger push (might actually be all it’s travel never actually measured it).
So far it’s been abused pretty good, raced a few enduros on it and the Psychosis DH including the massive road gap. I was hoping to get some Whistler bike park days on it this year but didn’t pan out.
Only issues I’ve had has been with my cs to drive side yoke, pretty sure this is my crappy brazing skills and not from the flex, I also should have used a thicker gauge tube in the cross braze between CS’s at the yoke.
I plan to develop this further and move to a flex CS to mimic a Horst layout on the next frame.
@earle.b that bike is very cool. Really inspiring me to try flexing members on more bikes as well. What is the chainstay length on that bike? it looks quite long, but thats not always a bad thing
