While this is not a topic that is breaking my brain or has any significant impact on any of my designs, I wanted to know if anyone has a rule of thumb or guide to selecting seat stay diameters for their steel and Ti designs?
I ask because of a recent post about creating a flex point in stays and an older post concerning MTB tubing has gotten my brain spinning a bit about details. This also comes on the heels of listening to Rob English talk about his thin stays on Cobra Joe’s podcast. He recalled talking to a race bike builder who said that the only reason his bikes have seat stays is because they were a UCI requirement. Rob did some basic stress testing of bikes with/without seat stays, plus how bridges affect stay stiffness, and seemed to conclude that current rear triangle design might be overdone. On the heels of that, Neuhaus does not use a bridge on their seat stays to great affect. On the flip side, Marin Team 2 bikes (aluminum) hit the market with rave reviews about the rear end, but are now known for cracking seat stays. Has anyone else overthought seat stays as well?
I have build two dirt jump frames one with a seat stay bridge and one without. And the one with a bridge flexes less to the side when you push a corner or a transition.
Rob English is right, most seat stays are too large in diameter for current frame designs. Disk brakes are the main reason, you no longer need a place to mount the brake or to stop them from flexing outward from cantis or v-brakes. The new driving parameter is how small can you make then without kinking from a dent or other sideways damage or impacts. Overall they’re over dimensioned for the compressive forces they encounter. And it’s harder to draw, flare, bend, etc smaller tubes. 8-10 mm seems like a practical lower limit.
Loose_Levin, that stands to reason. However, I’m curious to know which you prefer. Does the flex give rear-wheel-steering or just make it feel soft? I can see benefits to both having a soft rear triangle or stiffening it up with bridges.
alex.v, I have not seen 8-10mm stays off the shelf, so I’m assuming this is straight 4130. What wall thickness? Would you only apply this to road, or would you say this applies to a solid gravel bike as well?
I prefer the stiffer one, because you can push transitions harder wich meens more speed. But I think on a hardtail or Gravel flexing stays would be more comfortable
0.049” or 1,24 mm seems to be a standard wall thickness for both 5/16 and 3/8 straight wall. My gut says this should be fine, I’ve read English goes even thinner.
I built this with 0.035" (0.889mm) 3/8" 4130 which so far seems plenty stiff enough for a road bike under me at ~95kg… Would probably be ok on light gravel, but I don’t think I’d trust it as a proper MTB!
(I’d previously tried thinner diameter which was awful - although I’m not 100% convinced it was truly 4130, given how much easier it bent into the wishbone shape than the above tubes!)
