by dsveddy on Wed Apr 12, 2023 4:12 pm
Some updates on my frame gremlins:
I’ve had issues with a creaky/slipping/wiggling seatpost. I’ll claim some responsibility for causing creaking issues by over-torquing my grub screw, causing it to partially strip, and in turn making it wiggly, causing creaking. I’ll also take responsibility for being a fat American, coming in at nearly 80kg, I’m guessing Seka wasn’t designing their bike with that much heft in mind considering this is a Chinese-market frame.
That being said, fundamentally I don’t think Seka are using a sound seatpost design.
The small plastic seatpost compressor is the most marginal design you can imagine. All your lateral stability on the Seka comes from the precision of the seatpost fit into the seat tube, and then grub screw itself is responsible for providing the rest of the stability. The small plastic design of the compressor also makes it highly dependent on the precision of fit to provide contact area for good vertical grip force, as well as to eliminate the elasticity of the interface, which would undermine grip force. Unfortunately, the precision of the fit is bad. There is something like 0.2mm margin all-around the perimeter between the seatpost and seat tube. As a result, the seatpost can laterally wag side-to-side a degree or so, and the grub screw creaks whenever the post moves. Lastly, like I (and others) have said before, the spec seatpost is way too long, the bottom hits the seat tube cutout, even when inserted at fairly normal heights (mine is at the 11.5cm mark). This means that the seat tube will push your seatpost back, further undermining the interface between the compressor and seatpost. There is nothing inside the seat tube to catch a slipping post, so if your post slips down, it actually gets trapped in the taper of the seat tube cutout, and you will need to use force (like a rubber mallet) to pound out your seatpost, and pray you haven’t broken anything in the process.
So, here’s what I’ve done to remedy my issues and what my recommendations are.
For starters, if you’re building a new frame: 1) DONT RIDE IT UNTIL you’ve cut away enough post to avoid hitting the cutout at your seatpost height. 2) DONT RIDE WITHOUT CARBON GRIP PASTE. Park tool’s has worked well for me. 3) grease the grub screw well, and seriously, don’t play around with overtorqueing the grub screw until you’ve tried suggestions 1 & 2. Mine started to shred around 8nm.
I’ve remedied my seatpost slop by putting a few layers of paper soaked in carbon grip paste (paper is in fact incompressible once compacted!) around the back half of the post to take up this slop, which has more or less fixed this issue.
I say this really comes down to Seka because when you look at other manufacturers’ seatpost designs, it’s clear they’ve designed to avoid these issues. Specialized and Giant are using metal compresor wedges with 2-3x as much contact area, and they wrap around the post profile much more than the Seka. They also use an expander design that wedges between the frame and the post, which provides more lateral stability across a larger contact area. This means that even if their seat tubes/seatpost interfaces are sloppy, the compressor wedge effectively takes it up and stabilizes the post.
At the very least, Seka could be speccing a larger compressor wedge, and a larger diameter grub screw that has more lateral/axial stiffness than that measly m6 screw. I think Seka could even easily issue larger, metal replacement wedges as a “bug fix”, which would go a long way to addressing these issues.
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