What is a Seatpost sleeve?
It’s a cylinder which is fitted in the frame of the bicycle, where the seatpost moves through the frame, to allow the seatpost to be lifted up and down smoothly. Most bikes use anti-seize grease here, but the Brompton design is made from smooth Nylon, which avoids all risk of getting grease on the rider. To find the sleeve, follow the seatpost down to where it goes into the bike’s frame. The top of the sleeve can be seen as a black ring around the seatpost.
Why must this part be replaced?
Through continued use, the part will wear and need replacing. It’s designed to be replaced. It will inevitably wear a little, every time the bike is folded or unfolded, due to friction from the steel post removing a minimal amount of the softer Nylon. If the seatpost sleeve were made from a non-wearing material, it would not be able to perform its frictionless function.
By the time the seatpost sleeve has become quite worn, you will already have noticed that raising and lowering the seatpost has become difficult. This is the sign that you need to have this replaced.
Sometimes when a rider experiences the seatpost has moved down by the end of a ride, they will tighten the bolt on the seat clamp to prevent this. This will work, but should not be tightened by more than a quarter of a turn and should not be tightened more than one or two times. Tightening further will squeeze the rear tabs on the frame too far towards each other, and is likely to result in irreversible damage to the frame. This would cost more to replace than replacing the seatpost sleeve.
How often will it need replacing?
As with most wearable parts, this can vary. However, the biggest factor here is how often the bike gets folded, the weight of the rider and the care taken to look after the seatpost. The average commuter rider will be expected to replace their seatpost sleeve every 2-3 years.