Bike Theory Lesson: My Take On Seat Post Shims

from Rookie’s keyboard

Hello, friends

Today, I am going to talk about a somewhat obscure cycling accessory – seat post shims (SPSs).

Those are cylindric adapters that wrap around your seat post so that it can fit in a seat tube that would otherwise be too wide. Without the shim, the seat collar won’t be able to hold the seat post in place.

Normally SPSs are made of PVC, thin aluminum, or steel. SPSs have a cutoff along the body that makes the unit more flexible and easier to install. 

Installation tips:

First, you have to take into consideration the following variables:

  • The diameter of your seat post
  • The diameter of the seat tube

If you have a 27.2mm seatpost, but your seat tube is designed for 31.8mm seatposts, you’ll need a “27.2mm ->31.8mm” shim.

After acquiring the right shim for your bike, follow the steps below:

  1. In case you are installing a shim on an aluminum or steel seat post, put a coat of grease between the shim and the seat post.

If you’re installing a shim on a carbon seat post, put carbon paste on the inside walls of the shim.

2. Put the seat post into the shim and slide until the top of the shim reaches the part of the seat post that will be sticking outside of the frame.

  1. If you have a metal frame, put a light coat of grease on the shim to avoid a seized seat post due to corrosion.

If you’re installing the seat post in a carbon frame, cover the outside of the shim with the lubrication recommended by the frame manufacturer.

Important: Some carbon frame manufacturers won’t respect the warranty if you use a shim.

4. Slide the seat post into the seat tube. Align the saddle. Tighten the collar to the setting required by the manufacturer.

If you have a carbon frame and/or seat post, you will need a torque wrench as carbon can’t deal with too much compression (it may crack.)


It’s really simple, fellas. Anyone can do it. Just take your time.


Don’t forget that seat posts have a minimum insertion length that protects the seat post and the frame from cracks.

If the shim is as long or longer than the minimum insertion length required by the seatpost, it will work just fine.

The frame doesn’t know the difference between a shim-less seatpost and one with a shim.

Of course, frames have a minimum insertion point too. But it’s shorter than that of a seat post. 

The requirement is to have the bottom of the seat post past the location where the seat tube and the top tube meet.

Common Issues

The most common problem is corrosion. Hence the need to use grease (or compound paste for carbon). 

Alternatively, you can also get a PVC shim since plastic doesn’t corrode. You could also make your shim from a PVC pipe.

Dropper Post?

In the past, when dropper posts were highly limited and thus unable to cover all seat post sizes, shims were widely used.

DIY Shims Made From Aluminum Cans

If you want to stop a seat post from slipping down the frame, and there’s nothing else available, a common solution is to make a small shim from an aluminum can (all you need are basic scissors).

This method has a downside, though. Regular STPs have a “stop lip” that prevents them from falling into the seat tube.

The DIY version doesn’t. You could, however, fight this downside by securing the shim to the seat post with duct tape. Don’t wrap the entire body. Two points, top and bottom, are sufficient.

The other two cons of the DIY method are that the shim has sharp edges (you can file them off) and that it’s a bit difficult to apply lubrication.

To be honest, I am running precisely this shim on my road bike, and it works fine. However, if you frequently change the position of the seat post, I wouldn’t recommend it.

The Negatives

  • More parts, more problems

A shim is another bike part that you have to deal with. This could be particularly annoying if you often change the height of your seat post.

  • No Warranty 

As mentioned, some carbon frame manufacturers would declare a warranty invalid if a shim is used.

  • Looks

Some people hate the looks of a shimmed post.

  • Extra Maintenance

If you have a shim, you have to lubricate it on both sides whereas a shim-less seat post requires only one layer of lubrication.

The Positives

  • Budget-friendly

A seat post shim will save you money by eliminating the need to buy a new post.

  • Quick Seat post Swapping

If you have two bikes with dissimilar seat tube diameters, a shim would allow you to use the same seat post on both bikes.

  • Unusual Seat Post Sizes Are No Longer a Problem

If you have a seat tube or a seat post of unusual diameter, a shim could save the day.

More Seat Posts To Choose From

A shim boosts the number of seat posts that your frame can accept. As a result, you can try more exotic models and benefit from discounts.

Until next time

– Rookie






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