I've got a 27.2mm post with a 30.9mm thermoplastic shim.
Post keeps slipping, I've greased the greasy bits, cleaned (with alcohol) the dry bits, got a better seat clamp, and used two different carbon pastes (one specifically anti-slip). The only proper improvement was switching the post to a Thomson I had spare, but it still slips a bit, and my expanding Topeak pump won't go inside the Thomson as the post is ovalised internally 🙁
So, do I size up the post to 30.9 (it's a rigid steel mtb) or replace the shim for another plastic one (thinking perhaps it's worn or contaminated with grease/carbon paste) or buy a metal shim?
I don't want to make the bike super harsh, but again I don't want to buy shims and still have things slip. The posts are fairly tight to install through the current shim and I'm putting more force into the QR and original clamp than I should need to.
Thanks 🙂
I've used metal shims and posts for 20 years without problems. I lightly sand the inside of the shim and outside of the post to remove any smooth finish and grease the outside of the shim. Never used carbon posts or plastic shims, but I think the same thing would apply - you don't want two smooth surfaces mated together.
Ages since nice I used a shim but I remember plastic ones being crap
Once always used the metal Superstar shins without any problems (no idea if they still make them)
I had a seat post fitted with a shim a few years ago. No matter how hard I cranked the quick release the post still slipped. Had a light bulb moment and realised I’d used the shim incorrectly, complete user error. I can’t quite remember what I did wrong but it was the location of the shim compared with the seat collar. Changed things around and it worked as you’d expect.
Worth checking the shim is fitted correctly?
I've ordered a metal one.
My current shim is installed after the seat clamp and the slot lined up with the frame slot. I did briefly try it with the slot at the back but seemed side e
My current shim is installed after the seat clamp
Put the shim in the frame first with the seat clamp on top of the shim.
Update time
I checked and my old shim (and the new shim) fit within the seat clamp so it doesn't make any difference which is fitted first.
I got a 30.9 and couldn't get either seatpost in more than about 40mm. Measuring the ID and OD of the various parts, it seemed about 0.3mm out, so getting a bit fed up and long weekend/holiday biking coming up, I ordered a 30.8 and a 30.6 (from Europe).
The 30.8 turned up and seems to have done the trick, although the seat clamp still needs a bit more pressure than I normally use (it's a new Hope QR).
Annoyingly the 30.6 was despatched the day before, I can return for free but if anyone wants a 30.6-27.2 let me know as there doesn't seem to be any UK stock! (It's not the more common 31.6)
I had this problem recently to sort on my brothers Pinnacle Ramin Plus which are notorious for slippage . I simply cut a small extra shim from an alu can and wrapped the plastic shim externally which tightened everything up nicely . Also degreased the post and inner shim area ( good idea re sanding btw ) then added carbon paste and to ensure no further issues bought a double non qr clamp for £8. Everything is solid as a rock now and keeps the more flexible post diameter.
