When a seatpost sinks under your weight, check four things in this order: clamp tension, a dry clamp bolt, post size, and slick surfaces between post and tube. Park Tool’s troubleshooting list for a post that keeps slipping follows the same sequence.
Don’t crank the bolt past its printed torque. Cervélo calls the figures on its seatpost wedges and in its manuals maximums, so a post that still slips at the limit needs another fix.
First, confirm what is moving
Start with the bike held upright against a wall. Draw a marker line on the post where it meets the top of the collar, sit on the saddle and shift your weight forward and back. If the line drops into the frame, the post is slipping; our saddle height guide helps you reset your height once it holds.
Then try Park Tool’s twist test: grip the saddle and try to turn it side to side. Park Tool says the binder is tightened so the saddle does not twist, and a post that does not twist will not move downward during use. A saddle that turns, or a line that creeps, means the clamp is not holding, so go to Step 1 before you ride.
If you still want a ride test after the post passes both checks, keep it short and slow, and stop at the first sign of movement. A saddle that tips or slides while the post holds points to its rail clamp, a separate fix. For a creak rather than a sag, see why your bike creaks.
This guide covers rigid, fixed-length posts, the scope of Park Tool’s guide. For a sinking dropper post, start with our dropper post explainer and the maker’s instructions.
How the clamp holds the post
On most frames with a round post, the top of the seat tube is slotted, and a collar bolt or a quick-release lever (a cam lever that closes without tools) squeezes that slot shut around the post, which is how Park Tool describes the usual round-post system. The clamp supplies the squeeze; friction between post and tube does the holding. Some frames clamp the post with an internal wedge instead, and for those Cervélo says the wedge’s internal gliding surfaces should be greased. Add the twist test to your pre-ride checks.
Step 1: Check clamp tension
Bolt collar or wedge bolt. Torque is twisting force on a fastener, measured in newton-meters (Nm); a torque wrench lets you stop at a set value. Tighten to the value printed on the collar or wedge, or given in your frame or post manual. Cervélo specifies a calibrated wrench and says you may not need to reach its printed figures, while Park Tool treats collar values as recommendations because the collar maker usually doesn’t make the frame or post. On a click-type wrench, stop at the release: turning past it over-tightens the fastener.
Quick-release lever. Park Tool says to set the tension with the adjustment nut so the lever starts to resist in the second half of its swing closed. On a lever that opens 180 degrees, that means resistance from about 90 degrees. Avoid over-tightening, and if the lever still won’t hold, lubricate its pivot.
Step 2: Fix dry threads on the clamp bolt
If the bolt’s threads and head are dry, Park Tool says they may keep the load from reaching the clamp. Its fix is to remove the bolt, grease the threads and head, and refit it. Makers do not all agree on what the clamp should get:
| Maker and clamp | What it says about the bolt or wedge |
|---|---|
| Park Tool, general guidance | Grease the threads and head of a dry bolt; light lubricant on a quick-release. |
| Cervélo, round clamp | Grease the threads of the fixing screw. |
| Cervélo, wedge clamp | Grease the internal gliding surfaces. The fixing screw has Loctite pre-applied, so it needs no grease. |
| ENVE, seatpost wedge | Do not grease the bolt threads or the sliding wedge surfaces. |
Follow the instructions for your own post and clamp, and don’t borrow one maker’s recipe for another’s. With no instructions, ask a shop.
Step 3: Confirm the post is the right size
A post that is too small cannot be properly secured, however tight the bolt. Park Tool says a post should be only about 0.1 mm smaller than the inside of the seat tube, and that some standards sit within 1 mm of each other. The diameter is commonly marked on the post, and a caliper can confirm it.
Check depth too. Without a minimum-insertion mark, Park Tool says a round post needs at least 2.5 times its diameter inside the frame, which is 68 mm (about 2.7 in) for a 27.2 mm post. Giant’s manual warns that under 80 mm (about 3.1 in) may cause frame damage or failure and serious injury, and ENVE’s wedge sheet specifies 75 mm (about 3 in). Never raise a post past its limit mark.
Step 4: Add assembly compound for grip
If the clamp, threads and size are all right and the post still slips at the recommended torque, Park Tool says an assembly compound may be applied. Its SuperGrip compound is silica grit in a greasy carrier, and the grit bites into the frame and post. It suits carbon fiber, aluminum and steel, but keep it off threaded parts, and its hard silica may blemish finished surfaces.
Grease and compound do different jobs: Park Tool presents grease as corrosion protection and brings in compound only for slipping. By material pairing:
- Steel or aluminum post in a steel or aluminum frame: grease or anti-seize compound, says Park Tool.
- Aluminum post in a carbon frame: grease, say Park Tool and Cervélo. Park Tool warns that a dry aluminum post can corrode in carbon.
- Carbon post in a carbon frame: Cervélo recommends carbon friction paste on the post and inside the seat tube. Park Tool says this pairing can generally be left dry, but to check the manufacturer first.
Treat the printed torque as a ceiling
Cervélo says the torque ratings on its seatpost wedges and in its bike manuals “are maximum values” and that it “may not be necessary” to torque a wedge to the maximum. Treat any maker’s figure the same way: a limit, not a target. If the post still slips at the stated torque, work through the rest of the list instead of adding force.
Under-tightening is not a safe default either. Giant’s manual says a loose saddle clamp or seat post binder can damage the seat post. Tighten to the stated value and confirm with the twist test.
When to stop and see a shop
- It still slips after correct torque, prepared threads, the right size and the right compound. Cervélo sends owners to an authorized retailer, with proof of purchase, to start a warranty claim.
- Any crack, deep scratch or discoloration on the post. Giant’s manual lists the seatpost among parts to check for these and says they indicate a part is at the end of its useful life and needs replacing. Do not ride a cracked post.
- After a crash. Giant says to have a dealer check the bike thoroughly, and that carbon components that have sustained an impact must not be ridden until a qualified mechanic has inspected them.
- A loose or damaged clamp. Giant’s manual says a loose saddle clamp or seat post binder can damage the seat post. If the clamp bolt keeps turning without ever firming up, or the collar is cracked, replace the part instead of tightening further.
Until it’s sorted, don’t ride a post that keeps dropping. Giant’s manual warns that an improperly tightened post can let the saddle move and cost you control.
Sources
- Park Tool: How to Remove and Install a Seatpost
- Cervélo Knowledge Base: My Cervélo seat post keeps slipping? What should I do?
- Park Tool: SuperGrip Carbon and Alloy Assembly Compound (SAC-2)
- ENVE: Seatpost Wedge Installation (PDF)
- Giant Bicycles: Owner’s Manual, English (United States) (PDF)
- Park Tool: Torque Wrench Use and Care
Last reviewed: October 2026.