Fortunately, this is usually an easy problem to fix.
Clean and apply new grease to the seatpost
|Bicycle mechanic applying|
grease to a new seatpost
Periodically, you should remove your seat post from the frame, and wipe the old grease off with a rag. Insert the rag into the frame with your finger and try (as much as possible) to clean the old grease from the inside of the seat tube as well.
Using your finger or a brush, apply a thin coat of new grease to the inside of the seat tube, or the outside of the seatpost. Re-insert the seat post to your preferred height, and clamp it in place.
Use standard bicycle grease if your frame and seatpost are both any kind of metal (steel, aluminum, or titanium). If either your frame or seatpost, or both, are carbon fiber, DO NOT use grease. Instead, use a carbon assembly paste. This performs the same function as grease, but is specially formulated so that it does not chemically react with the carbon fiber material.
Some frame-seatpost combinations might be especially stubborn and prone to slippage. You might even try carbon assembly paste on these even if they are metal. If you do try using carbon assembly paste on metal parts, clean and re-apply the paste even more often (two to three times per year) to prevent seizing.
Check and replace the seatpost clamp
The clamp that holds your seat post in place in the bike frame is either a plain bolt-on type, or a quick-release. The quick-release models are convenient, because they allow you to change your seat height as needed without using any tools. However, the bolt-on models do tend you keep your seatpost in place more reliably.
If you have a bolt-on clamp that's not holding, the bolt or clamp assembly may be stripped or cracked. Replacements can be found from around $8 to $25, depending on the quality.
|Bicycle with quick-release seatpost clamp|
|Bicycle with bolt-on seatpost clamp|
If you're not sharing your bike among family members, and you don't have any need to adjust your seat height on a regular basis, you should consider replacing your quick-release clamp with a bolt-on clamp. You can get a higher-quality replacement at a more reasonable cost, and it will hold your seat in place much better.
|Bicycle with integrated quick-release|
|Bicycle with integrated bolt-on|
Addressing tilt issues
You might not be having a problem with your seatpost slipping down, but maybe you're seat is spontaneously tilting forward or backward while you're riding. This is an issue with the clamp at the top of the seatpost that holds your seat.
|Cruiser-style seat rail clamp|
Many modern bikes have a clamp that holds the tops and bottoms of the seat rails between two wedges. This assembly is held in place by a single bolt, which can usually be adjusted with a 6mm hex wrench. Try tightening this bolt if you're having trouble with your seat angle slipping.
|Seatpost with single-bolt|
seat rail clamp
For aggressive off-road riding, the best style of seatposts have a dual-bolt clamp. The two bolts work in opposition from the front and rear of the clamp to keep your seat at the desired angle. This gives you the best of both worlds: micro-adjustment and secure holding power.
|Seatpost with two-bolt|
seat rail clamp
Of course, if you're not into the do-it-yourself thing, bring your bike on in to any Century Cycles store, and any of our expert mechanics can check it out and do what it takes to get you rolling again.
Seatpost and clamp sizes
Note that seatposts come in many different sizes. The diameter of the seatpost is measured in millimeters. This must match the INSIDE diameter of the seat tube on your bike's frame. There is no standard diameter, but some of the most common on modern bikes are 27.2 millimeters, 30.6 millimeters, and 31.6 millimeters. If you buy a seatpost that is too large by even 1/10 of a millimeter, it probably won't fit in your frame. Likewise, if your seatpost is too small by 1/10 of a millimeter, it WILL slip down, even after all of the above corrective actions are taken.
|Minimum Insertion Point|
markings on a typical
If you feel the need to raise the seat higher than the minimum insertion point, then the bike you're riding is likely too small for you. However, you may be able to address the issue by purchasing a longer seat post.
Seatpost clamps are also measured by the diameter in millimeters. Note that this must match the OUTSIDE diameter of the seat tube on your bike's frame. The seatpost clamp diameter is NOT necessarily the same for all bikes that use the same size seatpost, because the outside diameter of the frame will vary depending on the thickness of the material used for the frame tubes.